Background information fact; by edgardowelelo@yahoo.com, Master of the Game
Around the borders of the OCEAN, where the LAND meets the SEA, lie some of the most challenging of the OCEAN HABITATS. From sandy beaches to rocky shores and river estuaries, waves and weather constantly erode COASTAL AREAS. While many creatures come here to breed, there are few permanent residents. Those that do live here must be hardy and adaptable enough to survive the daily ebb and flow of the TIDES, which transform their habitat on a daily basis.
THE DYNAMIC BORDER
The coast is a dynamic border between LAND and SEA, shaped by the wind and the waves and dominated by the cycles of the tide. Of all the OCEAN HABITATS, it is the COAST that is always on the move, being constantly shaped by waves and weather. The meeting of LAND and SEA has produced a wide variety of environments, from sandy beaches to rocky shores, from river estuaries to mangrove forests, from sand dunes to salt marshes. For the animals and plants that try to survive here, life is arduous; salt spray fills the air and waves buffet the coast. The constant ebb and flow of the tides transforms the HABITAT on a daily basis. But contending with the elements has made the inhabitants hardy and opportunistic. Scavengers come to pick through the FLOTSAM of life thrown up by the waves, and plants have evolved ways of coping with salt spray. During the breeding season, quiet coastlines are thronged with countless birds, turtles and mammals. The constant activity along the coastline means that its only predictable feature is its unpredictability.
THE SHAPE OF THE COAST
COASTAL SCENERY is as varied as it is spectacular. Anyone who has driven south from SAN FRANCISCO along the coastal highway will testify to the breathtaking views from the massive cliffs of the CALIFORNIAN COASTLINE. In the GALAPAGOS ISLANDS off the coast of ECUADOR the young volcanic shore is constantly battered by massive Pacific rollers crashing into the blow – holes they have eroded, sending great plumes of surf hundreds of metres into the air. In the BAHAMAS the azure blue sea laps quietly against gently sloping white sand beaches. On the east coast of England the wide skies and flat expanses of mud and salt marshes welcome thousands of geese and wading birds every autumn. Despite this apparent variety, there are basically just two different types of coastline – rocky and sandy.
HIGH – ENERGY COASTLINES
If you want to see massive waves crashing against the LAND, the most spectacular places are usually ROCKY COASTS. These are areas of high wave energy, where sediment quickly washes away and erosion dominates. Eroding shorelines often have high cliffs and dramatic rocky features, such as arches, blow – holes and sea stacks.
The Old Man of Hoy in the Orkneys, a tall pinnacle of rock sticking straight out of the ocean, or the stepped basalt of the Giant’s Causeway on the Irish coast are both good examples. ROCKY SHORES tend to be steep coasts that have usually been recently uplifted, or are still rising as a result of geological events. These uplifted coasts, of which the west coast of NORTH AMERICA is a good example, have not had much time to be eroded or accumulate sediments. During the last ice age, huge sheets of ice covered much of the north of that continent. These scraped the sediments from the continental shelf, leaving bare rock beneath. When the ICE melted, the coast slowly rose, leaving the rocky exposed coastline for which CALIFORNIA is noted. Geology and topography are important factors in determining the landscape’s ability to resist the erosive force of waves as they hurl the sand, pebbles and gravel against the cliff face. Most erosion takes place along weaknesses in the rock, such as faults and joints, and eventually the cliff face will collapse into the sea. If the sea currents are not too strong, an offshore terrace of eroded material may form. Usually made up of pebbles, cobbles or even boulders, these beaches are very transitory, changing their form and shape with the seasons. Any fine material or sediment is washed away, moved to more sheltered coastlines where sandy beaches can form.
LOW – ENERGY COASTLINES
Being protected from the full force of the ocean, low – energy coastlines have less dramatic wave action, so the material eroded from rocky shores can be deposited there. These depositional, or sandy coasts, which made up 75 per cent of the WORLD’S ICE-FREE COASTLINE, are dynamic environments, changing shape under the influence of wind and water. Where wave action is minimal, the result tends to be MUDDY SHORES: greater wave action produces a SANDY BEACH. In high latitudes sandy beaches are made up of minerals, mostly quartz. The white sand beach of the TROPICS has a biological origin, coming from the eroded skeletons of CORALS and other marine animals. In fact, much of that beautiful white sand has emerged from the guts of creatures which specialize in eating CORAL.
Just as with ROCKY SHORES, the shape of sandy beaches depends entirely on the forces of wind and water. In the few places where more sediment is being deposited than eroded, the beaches march steadily out to sea. Hastings in the south of England is a good example. In 1066, when the Normans invaded, the town was right on the coast: now it lies several kilometres inland. Normally the cycles of erosion and deposition roughly balance each other through the year. A big winter storm may wash away a lot of sand, but it will gradually return as conditions get back to normal.
In this way the shape of sandy beaches is constantly changing, with sand moving up and down the beach and being transported along the coastline. Sand brought ashore by the waves will eventually get pushed right up the beach until it comes under the influence of the wind. Then it may be blown into ripples, which gradually build into dunes. Vegetation, such as marram grass, often helps to trap the wind – blown sand and stabilize it. These dunes can grow to huge proportions. The highest in the world, on Moreton Island in Australia, have reached 250 m (820 feet), while the Alexandria dune field in Algoa Bay, South Africa, covers over 120 sq km (46 sq miles) and is still advancing.
ESTUARIES (Intersection of ocean and river, where fresh water and salt water meet)
The other key coastal habitats where sediment is being deposited faster than it is being eroded are estuaries – the places where rivers meet the sea. These vast, flat expanses of salt marsh and mud – flats are among the most productive coastal habitats. Their rich waters provide nurseries for millions of young fish and shellfish, and migrating birds use them as refuelling stations on their long journeys.
Human beings have also taken advantage of the sheltered conditions they provide and built ports in them. Many estuaries were formed at the end of the last ice age when rising sea levels drowned river valleys. The Thames estuary in England and Chesapeake Bay near Washington DC, were both shaped in this way. Others, such as the Waddenzee in the Netherlands, are formed by a bar of sediment that gradually built up across the mouth of the river, eventually forming a barrier. The spectacular fjords of Norway, New Zealand and Chile are third type of estuary.
Retreating glaciers cut deep valleys which were partly submerged when sea levels rose. At their mouths a sill or step of rock remains, partially isolating the fjord from the sea. Finally, as in the case of San Francisco Bay, volcanoes or movements of the Earth’s crust can cause the depression of an area of coastline, which floods to become an estuary. As all estuaries have relatively narrow mouths, they are usually sheltered from the force of waves. They are, none the less, strongly influenced by the tides. The meeting of seawater and fresh water creates a special coastal habitat where the key challenge is dealing with changing salinity.
NOTE AND REMEMBER
LUGWORMS are probably the most common large invertebrates on sandy shores in EUROPE; up to 70 creatures can be found per square metre (just over a square yard).
IN THE GRIP OF THE TIDES
More than any other factor, it is the cycle of tides that dominates life along the coast. This is well illustrated in places such as the Bay of Fundy in Canada or The Wash on the east coast of England. At low tide, it is possible to walk huge distances across their expanses of mudflat with only wading birds and the occasional seal for company. Suddenly, though, the tide will turn and the ocean that seemed so far away comes rushing in. Like a galloping horse, the tide reclaims the land, and within just a few hours, the habitat is completely transformed. The rise and fall of the tide varies a great deal around the world. In the Mediterranean, for example, the difference between high and low tide is as little as a metre, while in the Bay of Fundy, which has the world’s largest tides, the vertical drop can be as much as 15 m (50 feet). The Severn Estuary in the west of England comes a close second, with a 13-m (40-foot) fall. The tide’s ability to totally transform the physical appearance of a habitat presents real challenges for the animals and plants that try to live there.
HIGHS AND LOWS
Animals and plants living between the high and low – tide marks find their environment completely changed by the daily ebb and flow. Spending part of the time under water and part of it exposed to the air makes great physical demands. For a start, there is the constant risk of drying out when exposed to the desiccating effects of sun and wind. Then there is the variation in temperature to contend with. While the ocean’s temperature, in any one place, tends to remain relatively constant, air temperatures can vary enormously with the seasons, rising and falling by several tens of degrees.The salinity of a habitat can also change drastically with the tidal cycle. In estuaries, for example, the rising tide pushes a wedge of denser salty water into the fresh water twice or even four times a day. No other ocean habitat undergoes such drastic physical changes on such a regular basis, and any creatures hoping to survive there must be able to deal with such a dynamic environment. Feeding opportunities along the coast also vary enormously with the tides. As the tide retreats, for example, the BARNACLES, MUSSELS and SEA ANEMONES that encrust the rocks along the coasts of Europe and North America are exposed and no longer able to filter their food out of the water. They also run the risk of drying out in the air.
On sandy shores many animals face similar problems. The sediment is full of filter – feeding animals, such as cockles and razorshells, which live in burrows to avoid drying out at low tide. They can collect their food only when the tide is in. The tidal cycle on sandy shores and estuaries is rather like the opening and closing of a larder door. The low tide allows terrestrial animals to exploit food sources hidden in the sediment: massive flocks of wading birds fly in to feed on WORMS and SNAILS. But the larder door closes for them when the tide returns. Now it is the turn of marine animals to eat their fill.
Worms living in burrows can start filter – feeding again and BIVALVE MOLLUSCS can extend their siphons out into the water column to catch passing food particles. But they have to be careful, because those same siphons make a tasty meal for flatfish that swim back into the estuaries with the returning tide. These fish are, in their turn, the favourite food of ospreys. The advance and retreat of the tides is the dinner going in the lives of most of the animals living along shallow coastlines.
THE BAY OF FUNDY
THE HIGHEST TIDES ON PLANET EARTH occur in the MINAS BASIN, the eastern extremity of the Bay of Fundy in Nova Scotia, Canada. Twice a day 100 billion tonnes of sea water – the combined daily flow of all the rivers on Earth – pour in and out of the bay with dramatic effect. At low tide the fishing boats in the small harbour at Wolfville rest high and dry on the mud, but just six hours later they have 14 m (45 foot) of sea beneath their keels. At high tide, the Hopewell rocks look just like any other small rocky islands covered with a handful of pine trees. But at low tide these same trees are left almost suspended in mid – air supported by tall pillars of rock that tower over the, now exposed, mud way below. The currents entering the funnel – shaped bay from the Atlantic exceed 8 knots, and as the water squeezes its way through the 5 – km (3-mile) wide channel, it is pushed to incredible heights. Today a tiny part of this energy is trapped in the only commercial tidal power plant in the western hemisphere. The peak output of this generator is 20 megawatts, about 1 per cent of Nova Scotia’s electrical needs.
NOTE AND REMEMBER
Low – pressure weather systems suck up water beneath them producing bigger tides, while with high pressure systems the reverse is true.
LIFE ON THE EDGE
One of the pleasures of going to the seaside is swimming in the sea and riding the surf. For the creatures that live in this environment, however; life is not so much fun. The COAST is the only part of the marine world to face regular exposure to air and consequently is one of the most demanding habitats. Each day, waves and weather gradually erode the cliffs and the ebb and flow of the tide regularly transform the habitat. From rocky coasts, to sandy shores and muddy estuaries, all of the creatures that live in these habitats must be hardy and opportunistic, timing their feeding and breeding to fit in with the coming and going of the tide.
TIME AND TIDE
If you visit the same beach at different stages of the TIDAL CYCLE, you will notice a range of marine life appearing at different times. When the TIDE is out, creatures such as seabirds and raccoons come to the beach to search for mussels, worms and clams, which hide in burrows beneath the sand or mud. When the TIDE returns, the animals that have been taking refuge from the sun come into their own: mussels open their shells, seaweed becomes moist again and moves in the water and sea anemones unfurl and wait for prey to approach. Influenced by the Moon and Sun, however, TIDES vary in height from day today and creatures, particularly those that are static, have had to adapt.
If you look on the side of a rocky cliff you will notice that marine life forms distinct stripes or zones all the way up the cliffside. By spacing out in this way, each individual species finds a position where their needs are met. Filter feeders, for example, cluster where they will be covered by the TIDE for at least part of the day.
INTERTIDAL ZONE
The INTERTIDAL ZONE is the area between the high and low – water mark. Marine life is distributed between these two extremes in a series of distinct bands – a phenomenon known as ZONATION.
SANDY COAST
SANDY SHORES are unstable and constantly shifting. As a result, many creatures burrow for protection. This also prevents them from drying out at low tide. Millions of micro-organisms live hidden between the sand grains on the beach. The surf zone attracts a great variety of fish and diving birds.
ESTUARY (Intersection of ocean and river, where fresh water and salt water meet)
At low tide, massive flocks of shore birds and waders arrive to feed on the multitude of creatures burrowing under the mud. At high tide, fish take their turn to feed, nipping any siphons left at the surface by molluscs, while birds swoop down to snatch fish from the shallows.
ROCKY COAST
At low tide, many creatures are exposed and at risk of drying out. Some e.g. mussels, have hinged shells that close to seal in moisture. Many sea anemones withdraw their tentacle, turning into jelly-like blobs. In summer, rocky cliffs are filled with nesting sea birds such as guillemots. Rock pools provide a haven for many animals that suffer from exposure at low tide. At high tide, mussels and anemones open up.
ZONES ON THE BEACH
The TIDAL CYCLE is more complex than its daily retreats and advances suggest. At different times of the month and at certain times of year the strength of tides can vary enormously, and this is the key factor in determining the distribution of life along the coasts. Racoons hoping to search for mussels have to be careful to avoid the highest spring tides. Those mussels, in turn, tend to live in the middle of the shore, because they are filter-feeders and need to spend at least part of their day under water. When exposed at low tide, their shells form such a perfect seal that they are able to survive without drying out. Few mussels are found at the bottom of the shore because there they are eaten by marine predators, such as dog whelks and starfish. These in their turn are restricted to the water’s edge because they cannot survive long out of the water, and need to live below the lowest spring tides.
On COASTS WORLDWIDE it is the fascinating interaction between the physical forces of the tide and the biological factors of competition and predation that has produced different communities of animal and plants at different levels on the shore. This vertical zonation is particularly obvious on steep, rocky coasts where the different animal communities leave clearly visible bands of different colours on the rocks.
TIME TO BREED
Many of the plants and animals that live between the HIGH – and LOW – TIDE MARKS use the sea to fertilize and distribute their eggs, and they are careful to reproduce only when the tidal conditions are at their best. A high spring tide may well be the time to spawn for static coastal creatures, such as CORALS and some SEA WEEDS, who want to wash their larvae well out to sea. Others who return to the land to breed are also careful to synchronize with the tides. The Californian grunion, for example, is a small, silvery fish that arrives in thousands during the spring and summer, literally riding in on the surf. By timing their spawning to coincide with the highest tides, the grunions ensure that their eggs are laid on the uppermost parts of the beach, where they are less likely to be eaten by intertidal animals. In addition, because the tides are just starting to recede, the grunion can be sure that their eggs will not be washed away. In a final act of synchronization, the eggs hatch out of the sand just in time for the nest high spring tide, which reaches far enough up the beach to wash them out to sea.
Olive Ridley turtles also seem to coordinate their egg – laying with the tidal cycle. On just a few nights each year, tens of thousands of females emerge together on just a few beaches worldwide in a breeding spectacle called an ARRIBADA. These arribadas always start when the moon is in its first or last quarter. This is the time of neap tides, when the sea tends to be calmer and more of the beach remains exposed, making egg – laying easier.
SURVIVING THE COAST
The different coastlines – be they rocky, sandy or muddy – present the animals and plants that live there with different challenges. The coast is one of the most demanding ocean habitats. Though many come here to visit, there are very few permanent residents. To live here animals and plants have to be able to withstand the constant battering of waves and weather and cope with the continual cycle of change imposed by the tides. All the residents of the coast originally came from either the land or the sea. For them, living in this borderland has meant compromise, forcing them to adapt to life out of their normal element. Temperature and salinity fluctuate constantly and opportunities to feed come and go. The animals and plants that cannot move easily are carefully arranged into different zones from the top to the bottom of shore, depending on how well they can cope with the continual change imposed by the tidal cycle. And as each type of coastline – rocky, sandy or muddy – presents a different set of survival challenges, the adaptations are many.
THE ROCKY SHORE
The Muckle Flugha Lighthouse on SHETLAND is the most northerly point in the British Isles. In the winter, its lonely beam illuminates a wild and rugged coast of steep cliffs and bare sea stacks that stand like massive gravestones in the sea. It is difficult to imagine a less welcoming place, and in winter it seems almost completely devoid of life. Yet come the summer, thick clumps of pink thrift burst into flower these same cliffs fill with hundreds and thousands of sea birds. Gannets turn the sea stacks white, guillemots and razorbills crowd every suitable rocky ledge, puffins pop cheekily in and out of rabbit burrows, and peregrine falcons return to prey on the other sea birds. For a few short months, it is all noise and action, but as autumn approaches, the activity abates almost as quickly as it started. Cliffs like those in the Shetlands make a sudden transition between land and sea. Their steep sides make them inaccessible to most animals, but it is the salt spray more than anything else that makes life difficult. Breaking waves and strong winds continually douse the cliffs with salt and few permanent residents can put up with such conditions. Most of those that do cling precariously to cliff – side life, rely on the return of the summer visitors for some crucial part of their existence. Where soil accumulates in cracks and crevices, small – tolerant plants, such as sea campion and thrift, may cling to the cliff all year round. The occasional dropping from a summer nesting sea bird is vital in enriching this soil with nutrients. A few small snails or insects will survive among the cracks, and some ingenious ticks that feed off the blood of sea birds will spend a whole winter without a meal, waiting on the cliffs till their hosts return to breed in the summer.
SMASHED BY THE WAVES
At the top of rocky beaches, where spray from breaking waves is thrown above the high tide levels, is a habitat called the SPLASH ZONE. Life forms here must be particularly tough to survive, and a good place to observe the challenge is CAPE DOUGLASS on the western edge of the GALAPAGOS. Uninterrupted by any land, great swells develop out in the Pacific Ocean and come crashing on to the GALAPAGOS SHORES. The islands’ marine iguanas feed off algae that grow on the rocks just below the tideline and they have no option but to brave the pounding waves. As cold – blooded reptiles, they have to wait for the heat of the morning sun to warm them up and give them energy to venture down to the SPLASH ZONE. Special claws on their feet allow them to cling to the rocks even when it seems as if a massive wave must completely wash them away. Most of the females cannot withstand life in the ocean because it is too cold, but the larger male iguanas can survive under water for 10 minutes or so. Knowing that there are richer algae pickings on the submerged rocks, the males launch themselves out through the massive surf to the slightly deeper water below the SPLASH ZONE: they know it is worth the risk. The iguanas share the SPLASH ZONE with the beautiful Sally light foot crab, another species unique to the GALAPAGOS. These brightly coloured crabs can scamper ahead of the breaking surf at surprising speed, and their highly flattened bodies seem to protect them in the waves. But the real master of the SPLASH ZONE has to be the aptly named surfbird. This migratory wader specializes in feeding on tiny invertebrates along the tideline. One moment it is calmly feeding on the rocks and then, just as an enormous wave is about to break, it flies straight up into the air like a tiny jack – in – the – box, avoiding the impact. But many splash zone residents are not able to run or fly away from the breaking waves. Instead they do everything possible to stay firmly attached. BARNACLES have developed a glue so strong that many commercial companies have failed to synthesize it.
Mussels hold on with a “beard” of strong protein fibres produced by a special gland in their feet. More mobile molluscs, such as LIMPETS and CHITONS, have developed a powerful muscular foot, which they use like a suction cup to fix them to the rocks.
There are even fish, such as GOBIES and CLINGFISH, which use a similar technique, having modified their pelvic fins to act as suction cups. The few seaweeds that cling on in the splash zone have a network of strong roots called a HOLDFAST with which they clamp themselves to the rocks. Staying put is an essential survival technique on the rocky coast, particularly below the tideline.
EXPOSED BY THE TIDE
Venture a little further down the steep sides of a rocky coast and you are soon below the tideline, the highest point reached by the tide. Here, it is not just waves that are a problem, animals and plants have to deal with regular exposure and submersion for many hours, sometimes even days. With the exception of a few bivalve molluscs that can bore into rock, most animals and plants have nowhere to hide, and those living near the top of the beach are subject to the greatest physical stress. The first big problem is temperature change. While temperatures remain relatively constant and mild in the water: they can vary enormously on land, ranging from intense heat during the day to freezing conditions at night. Fish living in tide – pools have had to become much more tolerant of changing temperatures than their relatives below the tides. So long as the pool does not actually freeze up, the temperature will not drop below – 10C (300F).
The problem is overheating. One species of PERIWINKLE is known to have survived temperatures of up to 490C (1200F). In the TROPICS some snails living on the rocky coast have developed special ridges along their shells that act like cooling ribs on a car radiator. Colour also helps to prevent overheating: the dog whelk, for example, has evolved a white form as well as a brown one because white better reflects heat from the sun. The white dog whelk also predominates on sheltered Atlantic coasts where the risks of overheating are greater.
THE DANGER OF DRYING OUT
Creatures exposed on the rocky shore at low tide run the risk of drying out in the sun and wind. To avoid this, many tidal animals have developed a protective shell. BARNACLES and MUSSELS have shells and can seal in moisture by simply closing them. Others, such as LIMPETS, have a single shell. To get a watertight seal, they clamp themselves on to the rock and even dig a little depression to make the seal more effective. Animals that cannot clam up, such as CRABS, choose to run and hide. Moist cracks under rocks will suffice, but the perfect havens are rock pools. These oases are vital at low tide for a wide range of animals as they wait patiently for the sea to return. If running and hiding are not options, some animals and plants simply allow themselves to dry out. Chitons, which are small molluscs with eight overlapping plates on their backs, can survive losing 75 per cent of the water in their body tissues, and certain seaweeds can lose as much as 90 per cent of their water, becoming completely dry and crunchy. When the tide returns, they quickly recover.
HIGH – RISE HOMES
The massive cliffs of the NORTH ATLANTIC are home to literally millions of birds, each species having its particular preference for which level to live on. Near the bottom of the cliffs are the broad ledges and flat-topped stacks favoured by shags and cormorants. These birds have short, ineffective wings and prefer not to venture too high. On the main flat cliff – face are guillemots, razorbills and kittwakes, who nest together, seeming to find safety in numbers. The top of the cliffs, where the gradients flatten out and there are often areas of soil and grass, is the favoured nesting site for puffins and petrels which dig burrows into the soft ground. While auklets, such as those that nest in crevices amongst the boulders.
The thugs of the cliff-face are gannets, who will nest at any level so long as there is sufficient space. In fact, they will happily prod smaller guillemots out of the way or throw their eggs over the cliff. The only birds they will not mess with are fulmars, who defend themselves by spitting out an oily substance that damages the gannet’s feathers, reducing their water resistance.
THE SANDY SHORE
Walk along a sandy beach and the apparent absence of life is striking – nothing but the occasional wading bird chasing the surf and a handful of shells washed up on the beach. First impressions, however, are misleading. Although sandy shores lack the variety of animals and plants found on rocky coasts, those that do survive there often occur in very large numbers. The challenge is finding them. Many of the animals are too small to be visible to the naked eye. Hidden between the sand grains, are literally millions and millions of microscopic organisms. Practically every group is represented, including DIATOMS, PROTOZOA, ALGAE, NEMATODES (Round worms), COPEPODS and GASTROPODS.
Although tiny, these animals are very mobile, constantly migrating up and down through the sand with the tides or changing temperature and light. Of the slightly larger animals at the top of the shore, crustaceans seem to have adapted particularly well. If you have ever had a picnic on a sandy beach you may remember that sand is not the only thing that can get into the sandwiches. Sand hoppers – amphipods just a few millimetres long – can reach plague proportions and specialize in feeding on the rotting vegetation that often washes ashore. On tropical beaches, exactly the same job is done by ghost crabs, which emerge every evening from their burrows in sand to scavenge. They get their name from their white colour and their rather spooky habit of appearing from nowhere whenever a potential food source appears.They will eat practically anything, and will even drag unfortunate turtle hatchlings down into their burrows. Some types of crab occur in really spectacular numbers. Australian soldier crabs, for example, march across stretches of sand in organized groups hundreds strong, while the activity of swarms of sand bubbler crabs can cover the entire surface of a beach with thousands of tiny balls of sand. They specialize in sieving for microscopic life forms that live between the sand grains. Having passed the sand through their mouths, they leave the waste product as distinctive calling cards’ – perfectly round balls of sandy waste.
WITHIN THE SURF
Along the water’s edge, where the waves break, is the surf zone – a much more active area. Among the wading birds that forage here is the small sanderling which chases along the very edge of the surf like a tiny clockwork toy. Each year sanderling migrate to and from their nesting grounds in the high Arctic right down to the very tips of South Africa and South America where they spend the winter. As many fish establish their nurseries among these shallows, the surf zone also attracts a variety of diving birds. Little terns, for example, hover for minutes over the breaking waves before diving in to grab a fish with their dagger – like bills. Much larger pelicans ride the breeze created by the waves before suddenly diving into the sea with their massive breaks wide open.
Because food is so abundant at the water’s edge, a number of animals have taken to surfing up and down the beach with the tide. Whelks, beach clams and plough snails all use their expandable mollusk foot as a kind of surfboard. Real masters of the art are the surfing snails of South Africa.If a dead fish or other tasty meal washes up, these snails emerge from their burrows having scented the odour of the rotting food on the current. They then ride the waves up the beach before the fish is washed beyond their reach by the rising tide. The wealth of flotsam and jetsam deposited on these low – energy coasts makes them a surefire attraction to all sorts of scavengers.
WITHIN THE SAND
Unlike rocky coasts, sandy shores are very unstable and constantly shifting in response to waves, tides and currents. Below the tide – line, there are no solid places for attachment. With the exception of seagrasses, no plants have solved this problem, and animals that live here usually burrow into the sediment for protection. The variety of life depends very much on the type of sediment, and that in turn depends on the power of the waves. In calm, sheltered areas, where fine sediments can settle, the shoreline tends to be muddy: where the waves are strong, the beach will be sand. Burrowing in the sand has the advantage that you are less likely to dry out than in the open. But grain size is very important.
Coarse sand tends to drain water faster than mud which is one of the reasons sandy beaches have relatively little animal life. Grain size also affects the amount of oxygen that filters through. The smaller particles found in muddy sediment pass water and thus its dissolved oxygen less well.
That is why many animals living in the sediment have developed ways to bring the oxygen to them. Cockles that hide just below the surface extend a siphon up above the mud. Water is inhaled on one side and expelled out the other. Another bivalve mollusc, the thin tellin, has a long inhalant siphon, which resembles an elephant’s trunk. When the tide is up, the tellin extends this trunk over the surface of the mud, using it to suck up particles of food. While the annelid worms like ragworms dig u – shaped burrows in the sand, maintaining a constant circulation of oxygenated water with the array of bristles on their bodies.
FINDING FOOD IN THE SAND
The main source of food for all animals living in sand is dead material washed in with the tide. There are various different ways of collecting it. Some like the sea cucumbers and the worms simply act like hoovers, sucking up the sediment. As it passes through their guts they digest any valuable particles or small organisms in the sand. The worm castes you find on sandy beaches at low tide are the final product of this process as the worms expel the processed sand from their burrows.
Others are more selective, taking only certain particles. Sand dollars browse the surface of the sand, picking up tasty particles with their tube feet, while the bivalve molluscs extend their siphons out of their burrows to make their selection. Others still choose not to wait until the potential food has settled. These suspension feeders use a variety of techniques to pick out particles from the water column. An olive shell from Central America makes a mucus net it suspends just above the bottom, while the mole crab has a large pair of bushy antennae that it can hold up in the water to catch food. Cleverly, the mole crab also uses these antennae to catch a ride on the surf when it wants to move up or down the beach.
WHALE AND DOLPHIN STRANDINGS
Every year a number of whales and dolphins seem to lose their way and get washed up on beaches worldwide. Sometimes mass strandings occur, when tens, even hundreds, of animals wash up together. The most common victims of stranding are TOOTHED WHALES, such as sperm, killer and pilot whales, and bottle – nosed dolphins. Exactly what causes whales and dolphins to end up on the beach remains a mystery. Individual strandings may occur simply because an animal is injured or old.Mass strandings seem to arise from errors in navigation made by the whole group. This might explain why TOOTHED WHALES, which tend to travel together in tightly knit social groups, are the usual victims. But what causes the navigational error? Storms, reduced visibility and even noise pollution may all be factors that interfere with the whales’ sonar. It has also been suggested that whales may follow magnetic contours and occasionally blunder into dangerously shallow waters. It is even possible that parasitic ear infections could damage the whales’ sonar. Whatever the cause, it is a distressing sight to find groups of ocean – going animals trapped in this way. Often, people try to help by attempting to re – float the whales, but sadly the physiological stress is so great that most die within hours.
SCAVENGERS ALONG THE COAST
You can never be certain what will wash up on the tide: it might be clumps of seaweed, the bodies of jellyfish or even, now and again, a stranded whale. As all beachcombers know, the strand line can be a very profitable place to search, and a range of different animals has learnt to exploit this unpredictable bounty.
TREASURES OF THE SURF
Along the water’s edge on almost every beach in the world there is always an enormous quantity and variety of washed – up material, most of it biological in origin. Common finds are whelks’ yellow egg cases and fragile black mermaid’s purses. Sometimes an old piece of marine timber containing hole – boring ship – worms may wash up. But the real prizes, which regularly appear on beaches in the TROPICS are sea beans. These incredibly hard and beautifully smooth black seeds are dropped into rivers by tropical lianas and travel out to sea. There they can survive for months, even years, before they wash up and germinate on a distant beach.
STRAND LINE UNDERTAKERS
Many different animals have learned about the rich pickings to be had from the water’s edge, and regularly trawl the strand line. Scavenging sandhoppers will eat almost anything, but are particularly fond of seaweed. Many land – dwelling beetles and flies also help to clean up the debris. Larger scavengers also join the feast. North American racoons, for example, have learnt that the tideline can provide unusual snacks, while South African baboons will venture right down to the waves in their search for the egg cases of sharks and dogfish.
WHALE OF A PRIZE
The ultimate find for nature’s beachcombers has to be a stranded whale or dolphin. So many tonnes of meat dumped on the beach at once are a real bonanza. Exactly why whales end up stranded above the tideline remains something of a mystery. Injury, parasitic infection, disease, social factors and errors in navigation are all among the possible explanations. Each year a number of humpback whales wash up on Alaska’s remote beaches and their bulk provides a feast lasting several months for local scavengers. Bald eagles and ravens fly in to pick at the carcass, while black bears are strong enough to bite through the tough skin. Even packs of wolves may emerge from the forest to dine on rotting whale.
BREEDING BONANZA
Scavengers come into their own when sea creatures visit the land to breed. In Costa Rica the mass nesting of thousands of Olive Ridley turtles supports hundreds of black vultures which patrol the beach each morning. As the rising tide washes turtle eggs out of the sand, the vultures snap them up. Sea bird colonies also provide good opportunities for the scavenger. The burrows of many sea birds contain blood – sucking ticks that feed only when their hosts return to nest. In ANTARCTICA the only permanent land – based bird is the SHEATHBILL, which lives by scavenging round the colonies of breeding penguins and seals. Although it prefers to steal penguin eggs or food, the sheathbill is famous for eating everything and is quite happy surviving on penguin droppings. Antarctica’s most impressive scavenger though has to be the giant petrel. These vultures of the south often emerge from the carcass of a dead seal with their heads bright red with blood. Sea mammal colonies worldwide have their own particular scavengers. The sea lion colonies on Peru’s Pacific coast attract Andean condors that land and take dead pups. And fur seals breeding along Namibian’s Skeleton Coast suffer the attention of hyenas, jackals and even the occasional lion.
WHERE RIVERS MEET THE SEA
With their vast expanses of salt marsh and mudflat, ESTUARIES can seem dull and lifeless. But there is a certain magic in the maze of tidal creeks, the wide horizons of a wet mudflat reflecting the pattern of clouds and the constant threat of the chasing tide. Life is everywhere. Pick up a handful of mud and it is full of weird worms and shapely snails. Little fiddler crabs chase along the surface and massive flocks of wading birds darken the sky. Of all the coastal habitats, ESTUARIES are among the most productive – vital feeding and breeding grounds for many fish, birds and other animals. Although estuaries are formed in various ways, they all consist of a more or less enclosed area where a RIVER enters the SEA.
Large amounts of SEDIMENT are carried into estuaries by their rivers and, sheltered from the forces of currents or waves, this fine sediment settles as MUD. As the sediment is rich in organic material, estuaries are tremendously productive places, home to large numbers of organisms. However, life there is not easy and the key problem is changing salinity. At the top of an estuary, where fresh water from the river washes in, there is far less salt than at the mouth, where the daily rhythm of the tides introduces much more salt. Life in an estuary depends on adapting to these fluctuations in salinity.
Most of today’s estuary animals originally came from the sea, so they have difficulty coping with fresh water. With their high internal salt concentration, there is a constant threat that, due to osmosis, their bodies will flood with the surrounding water. Different animals have come up with different answers to this problem. Soft – bodied molluscs and polychaete worms, for example, actually change the constitution of their own body fluids to match the salinity of the water around them. Many fish and crabs, on the other hand, regulate the amount of water or salts in their bodies using gills, kidneys and other special adaptations. Salmon and sea trout, which migrate from oceans to rivers in the breeding season, are masters of this technique, regulating their body fluids with their kidneys.
Unlike the animals that came from the sea, estuary plants tend to have invaded from the land, so the problem for them is too much salt. Cord grass commonly found on temperate salt marshes in the Northern hemisphere, can excrete excess salt with special glands on its leaves. Samphire, on the other hand, has fleshy stalks, which are designed to accumulate large quantities of water and dilute any salts they take up.
LIFE AT THE TOP
Just like rocky and sandy coasts, estuaries are made up of a series of different communities of animals and plants that change as one walks nearer and nearer the sea. And being flat means that they are enormously influenced by even a small rise or fall in the tide. In temperate regions, estuaries are bordered by large grassy areas of salt marsh, which are usually dry being covered by water only at the highest spring tides. They are particularly extensive in places where the coastline is gently sloping, such as the Atlantic and Gulf coasts of North America and the east coast of England.
Exploring these marshes, leaping across the little mud creeks and searching all the hidden places, can be a great deal of fun. Skylarks fill the air with song on a summer’s day and, if you are lucky, you might spot the tracks of an otter that has spent the day hunting for flatfish. In the winter, skeins of grey geese arrive from their breeding grounds in the ARCTIC. At the top of the salt marsh the tide is a rare visitor, so this less changeable habitat is often invaded by land animals, such as rats, snakes, insects and birds. Among the many plants that like this environment are SAMPHIRE or GLASSWORT, SEA MEADOW GRASS, SEA ASTER and THRIFT. Any attempt by these plants to move further down the beach is thwarted in a constant battle with the tide. Being submerged by sea water makes it hard to survive and only CORD GRASS achieves this, living right down to the edge of the mudflats. This plant has an extensive system of stems that form a horizontal network under the mud to hold it secure. Even when the tide comes in, the tops of the leaves remain exposed to the air. Eventually, though, it gets too deep even for CORD GRASS, and the vegetation gives way to the stark expanse that lies at the heart of every estuary.
MUDFLATS
Below the tideline, where it is very difficult for plants to get a foot hold, are vast bare expanses of mud. Just as on the sandy shore, the key problem here for all life forms is the enormous variation in physical conditions caused by the tides. To escape these extremes, and to avoid the dangers of life on the surface, most animals burrow downwards, but this poses problems of its own. Compared with sandy sediment, mud is rich in organic material but, because it is dense, it is also very short of oxygen. Get your boot stuck on a mudflat and it is difficult to forget the smell of rotten eggs released when your foot finally escapes the mire. That smell is hydrogen sulphide, typical of oxygen – poor sediments. Dead matter is the main source of food.
Dig up the mud and you will discover a range of worms, molluscs and crabs hiding in their burrows. The sheer numbers of animals living on MUDFLATS is enormous. LUGWORMS are probably the most numerous large invertebrates inhabiting the sandy shores of northwest Europe, with densities of up to 70 individuals per square metre (just over a square yard). These numbers pale into insignificance, however, next to the tiny snail Hydrobia, which can reach population peaks of over 100,000 per square metre (just over a square yard).
This wealth of food attracts numerous predators. Carnivorous worms, snails and crabs hunt within the mud itself, while others burrow down in search of CLAMS, before drilling into their shells to eat the flesh within. On the returning tide, FLATFISH, such as flounders, slip in and nip off any siphons left at the surface by unwary molluscs. When the tide retreats, even bigger predators venture on to the MUDFLATS. Alaskan brown bears like nothing better than to dig CLAMS out of the mud with their enormous paws.
The most important predators on the MUDFLATS are probably BIRDS. Ospreys dive down to snatch FLATFISH from the shallows, while HERONS and EGRETS benefit from the enormous numbers of fish that use ESTUARIES as nurseries. DUCKS and GEESE are also regulars, but the real spectacle has to be the millions of wading birds that rely on ESTUARIES for food. Massive flocks, containing hundreds and thousands of BIRDS, come and go from the MUDFLATS following the cycle of the tides. Each wader species comes equipped with a different shape of bill, perfectly designed for feeding on their particular choice of food.
THE NEED TO BREED
Most of the animals found along the coast are only visitors, returning each year to breed. Above the tideline, there are relatively few permanent residents. Most of the animals we find along the coast are only visitors, returning each year to breed. The constraints of their evolutionary biology force them back to the element from which their ancestors came. The ancestors of today’s sea turtles, for example, were originally land – based reptiles, and as they still lay eggs with shells, they have no option but to deposit them on DRY LAND.
In just the same way, millions of sea birds have to find a firm surface to bring up the next generation. Similarly, many sea mammals must also leave the water to have their pups. By contrast, land crabs, which have adapted so well to a TERRESTRIAL existence, cannot breed without returning their eggs and larvae to the sea. All these animals attract predators and this pressure, and often the lack of suitable breeding sites, encourages the coast’s visitors to gather in enormous breeding aggregations – some of the greatest spectacles in nature.
REPTILES RETURN
The tiny island of ASCENSION, which lies in the middle of the ATLANTIC OCEAN, is just 11 km (7 miles) wide. While most of it is an inhospitable volcanic wasteland, it has a few sandy beaches that provide the only nesting sites for thousands of green turtles. These creatures, who spend most of their lives in a solitary pursuit of food along the coast of Brazil, make an annual 2400 km (1500 mile) migration out into the Atlantic Ocean. Exactly how they navigate their way to the tiny pinprick of rock that is ASCENSION still remains something of a mystery. However, year after year the females return to exactly the same rookery at which they themselves were hatched.
A SAFE PLACE FOR THE EGGS
Sea turtles produce the soft, leathery eggs typical of most reptiles. As they evolved on land, they are not able to withstand the rigours of salt water, so they must be laid on land where the warmth of the sand incubates them. The traditional nesting beaches are known as ROOKERIES, but why particular beaches are favoured is far from clear. It may be that current nesting choices reflect historic patterns that have existed for hundreds or thousands of years. Many suitable beaches may now be untouched simply because the local turtle population has been over – exploited or disturbed by man.
Certainly, tagging experiments and DNA analysis have shown that females not only return to the same beach every year, but, typically emerge within a few hundred metres of where they last nested. A perfect nesting beach for sea turtles needs to have open – water access, it must be free from the risks of flooding by tide or ground water and the sand must be just the right consistency for egg – laying. Too soft and it is almost impossible for the female to dig a nesting burrow without it collapsing. Too solid and there is not enough air for the incubating eggs. Gathering off the ROOKERIES to mate after weeks or even months of migration, the females are sexually receptive for just one week.
During this time they may be inseminated by several males, who are sexually active for about a month. At the end of this courtship period, the males return home, but the females will remain for months. Using stored sperm, they fertilize the eggs inside their body, then wait offshore for about two weeks until they are ready to lay. They then haul themselves on to the beach. Within minutes of leaving their natural habit, the turtles are exhausted by the struggles of returning to land. Their eyes clog with sand and every metre up the beach seems an effort. The final challenge is digging a hole in the sand and laying about 120 eggs within it. Just two weeks later, the ordeal must be repeated to lay another clutch of eggs. This process continues throughout the breeding season, with some turtles laying up to 11 clutches. During the long months at the ROOKERIES, the females spend little time feeding, living instead off their reserves of fat stored up before the migration began. They are unlikely to return the following year. It will be between two and eight years before this particular set of females undertake the breeding migration again.
THE HATCHLINGS EMERGE
Incubation of the eggs normally takes about eight weeks, but the temperature of the sand determines the speed with which the embryos develop. As turtles have no sex – determining chromosomes, temperature also determines the sex of the hatchlings. Below 280C (820F) and almost all the hatchlings will be male. Above 30.50C (870F) and nearly all the hatchlings will be female. Once they emerge from the eggs, the baby turtles face a real challenge in getting to the sea. In a blindly co-operative effort, the siblings get to the surface of the nest with a sporadic series of thrashings. This may take several days, and they usually emerge at night or during a rainstorm, as hot daytime sand could be lethal. Watching a clutch of turtles hatching is always a touching moment. The surface of the sands begins to twitch. A little black head appears, then a flipper, another head, and soon a gaggle of tiny clockwork toy – like hatchlings emerges. With no adults to guide them, the newborns rely on instinct to find the sea. Strongly attracted to light, they scurry towards the dim glow reflected off the ocean’s surface. As soon as they reach the surf, the tiny turtles instinctively dive down to the bottom, riding the undertow out to calm water beyond the breakers. Then, for 24 hours or more, the hatchlings swim frenziedly to reach deeper water. Those that survive will drift the ocean currents for several years before eventually finding their way to the traditional feeding grounds on the continental shelf.
MEAL IN A SHELL
A number of different predators have learnt that breeding turtles provide an easy source of food. CRAB ISLAND, off the tip of Australia’s remote Cape York Peninsula, seems like a perfect nesting place for the rare flat – backed turtle: it is isolated and free of human disturbance – but it is also the home of massive salt water crocodiles. These creatures, which can measure 7 – 8 m (23 – 26 feet) long, are gruesome foes, but even they find it difficult to crack open the shell of a metre – long turtle. For up to 30 minutes the crocodile thrashes the turtle in the surf. Eventually the carapace shatters and all that is left on the beach the following morning are a few scraps of shell and traces of blood. Despite this hazard, most of the females make it safety up the beach to lay their eggs, but eight weeks later the hatchings emerge to a different threat. Hundreds of night herons will suddenly appear and catch most of the hatchlings as they dash for the sea. Pelicans also join in, filling their beaks with sand in their haste to steal the hatchlings before the herons. The few newborns that do make it to the water must then get past the crocodiles, sharks and fish waiting in the shallows. Given this gauntlet of predators it is hardly surprising that probably fewer than one in a thousand hatchlings survive to adulthood.
Pressure from predators may be the reason why many turtles choose to breed on islands. Although birds may pose a problem, there are likely to be fewer ground – based predators. The Olive Ridley, however, is one turtle that often nests on mainland shores. There they face a range of extra predators including ants, vultures, ghost crabs, coatis, racoons and feral pigs, but their response has been to develop a different nesting strategy. Having discovered safety in numbers, the females return to breed in their tens of thousands for just a few nights each year. This synchronized mass nesting is called an arribada and probably plays a significant part in diluting the effect of predators.
OUT OF THEIR ELEMENT
On just a few beaches worldwide, and on just a few nights each year; thousands of animals leave the water and gather at the TIDELINE to participate in some nature’s greatest spectacles. Arriving suddenly, as if from nowhere, they seem to time their arrival with a particular phase of the moon, to ensure the tide is at just the right height to help with their breeding needs. This ritual, which lasts just a few days, attracts a whole variety of predators, who have learnt to exploit these occasional bonanzas of food.
FISH OUT OF THE WATER
There are just two species of FISH that will leave the SEA to breed on land – the CAPELIN and the GRUNION. Both look remarkably similar, being thin, silvery fish about 15 cm (6 in) long. During June and July each year certain sandy beaches in Newfoundland are literally covered with thousands and thousands of CAPELIN, which form a writhing silver mass all along the water’s edge. A similar spectacle takes place on beaches along southern California and northern BAJA CALIFORNIA when the GRUNION return. Males and females arrive together; and once the females have laid their eggs, the males compete to fertilize them.
Grunion spawning in particular is a precision exercise, which always takes place during the three or four days following the high spring tides. This allows the fish to lay their eggs at the very top of the beach and ensures that the lower tides on the following days do not wash them away. Why Grunion and Capelin alone of all fish choose to spawn out of the sea is not entirely clear. Possibly they are avoiding intertidal predators on their eggs, or perhaps their eggs may develop faster in the warmer conditions on land.
FOSSIL OUT OF WATER
Each year in May thousands of HORSESHOE CRABS appear on the beaches of DELAWARE BAY on the USA’s eastern seaboard. Looking rather like ancient frying pans, horseshoe crabs are the only living relative of the long – extinct TRILOBITES and have remained unchanged since the CAMBRIAN ERA. These ancient creatures are not in fact crabs: they belong in a group of their own, which is closely related to ARACHNIDS (the group of which spiders are members). They come to land to breed and time their return on the high spring tides, mostly under cover of darkness. The males, two – thirds the size of their mates, cluster around the water’s edge as the females arrive. Each male grabs a mate with the glove – like claws on his front pair of legs and pulls her up to the high tideline. The female then lays as many as 20,000 eggs and the male fertilizes them. With spawning complete, the crabs leave and the waves wash sand over the nest. These eggs provide a valuable source of food for migrant birds, and each year large flocks stop off at Delaware Bay to refuel as they journey north to their ARCTIC breeding grounds.
LAND CRABS RETURN
Today’s land crabs all evolved from ancestors that lived in the sea. By carefully preserving moisture, they can survive a more parched lifestyle on land, but they still return to the sea to breed. By far the most spectacular example of a breeding migration of land crabs to the coast occurs each wet season on Christmas Island, in the middle of the Indian Ocean. Around 120 million red crabs, each about 12 cm (5 in) across, wait for the rains to arrive, then march from their forest home down to the coast. They mate in burrows close to a few suitable beaches, and the females then wait for just the right conditions before releasing their eggs into the sea.
This always occurs during the last quarter of the moon, when there is the least difference between HIGH and LOW TIDES, because it is safer for the females to approach the water’s edge at that time. If caught in a rising tide, these land crabs could easily be drowned. At night and precisely on the turn of the high tide, thousands of female crabs head down to the surf to spawn, turning the beach completely red. In three or four nights it is over and the normally blue tropical sea turns brown with crab eggs. The baby crabs will eventually return at exactly the same state of the tide.
SEA BIRD CITIES
Just like SEA TURTLES, the world’s sea birds have no option but to return to LAND to breed. While the ocean can supply all their day – to – day needs, it cannot provide the dry, hard surfaces on which to raise the next generation. In ANTARCTICA the emperor penguin is unique among birds in its ability to lay its egg on the FROZEN SEA ICE that surrounds that continent each winter. All other penguin species – indeed, all the other sea birds – can lay their eggs only on dry land. Some 95 per cent of the world’s sea birds nest together in colonies compared with only 15 per cent of the world’s other birds, and these huge sea bird cities are spectacular sights. STEEPLE JASON, for example, a remote rocky island on the western edge of the FALKLANDS ARCHIPELAGO, is the breeding ground for about half a million black – browed albatross, who normally spend their time scouring for food in vast wastes of the SOUTHERN OCEAN. Their chosen island is constantly battered by some of the wildest winds anywhere and their closely packed nests stretch in unbroken lines to the horizon. At the start of the breeding season there is a constant cacophony of noise as courting pairs call to each other and clack their bills in display, and the air is heavy with the stench of guano. This enormous concentration of life can come as something of a surprise to anyone who has spent time at sea. Albatross, and indeed most sea birds, normally live a solitary life, thinly spread over the oceans searching for food. It is only when they come together to breed that it is possible to appreciate the enormous numbers of birds out there.
Although sea birds make up just 3 per cent of the world’s 9000 or so species, there can be little doubt they are very numerous. Their life at sea makes them notoriously difficult to count but some species, such as the LITTLE AUK in the ARCTIC and the WILSON’S STORM PETREL in Antarctica, must number in their hundreds of millions. In fact, the Wilson’s storm petrel may well be the most numerous single species of bird in the world.
WHY SO MANY BIRDS?
One of the most striking features of SEA BIRD COLONIES is their huge variety in size. It is thought that as many as 2 million CHINSTRAP PENGUINS nest together on ZAVODOVSKI ISLAND, a remote volcano in the vast SOUTHERN OCEAN. The warmth from the volcano helps to melt snow and ice from the slopes of this ANTARCTIC ISLAND early in the breeding season, providing the BARE ROCK that the penguins need to lay their eggs. Further south, near the Antarctic continent itself, where conditions are considerably harder and the sea is FROZEN for much of the year, CHINSTRAP COLONIES are often much smaller, comprising just a few hundred birds.
The key factor in determining the size of any colony seems to be FOOD SUPPLY. Birds that can search for their food over a huge distance tend to form LARGE COLONIES. Sooty terns, for example, forage hundreds of kilometres from their nests on the islands in the TROPICS, and their colonies can contain several millions of pairs. By contrast, the little terns that nest on shingle banks along the coast of BRITAIN rarely number more than a hundred or so well – dispersed nests. They catch fish locally for their growing chicks and the supply of food is less reliable.
But why do many seabirds choose to nest together in such large colonies? Shortage of suitable nesting space is rarely an important factor. Certainly in the NORTH ANTLANTIC there is a wide choice of empty cliffs that would provide for an expansion in sea bird numbers. One of the possible advantages of nesting together is that birds can share information about the location of good sources of food. But probably the main benefits to be gained from breeding together are the social aspects of colony life. It is social stimulation that encourages gonad development and synchronizes the whole breeding cycle. By laying their eggs together at the same time; the birds reduce the risk of stepping on each other’s eggs.
When they start to feed their chicks, well-coordinated colonies might be better able to exploit the strongly seasonal food supplies typical of the oceans. Finally, by breeding together and ensuring that their chicks all leave the nest at the same time they are substantially reduce the impact of predators.
SAFE NESTING PLACES
SEA BIRDS have learnt to exploit a wide variety of different coastal habitats to find safe places to nest. In the NORTHERN HEMISPHERE many choose to nest on or near STEEP SLOPES or CLIFFS. This is particularly true in the NORTH ATLANTINC, where the coastline is well endowed with spectacular cliffs, stacks and rocky islands. The remote island of St Kilda, 130 km (80 miles) to the west of the Scottish mainland, is a perfect example of prime nesting territory. Its three (3) rocky stacks – BORERAY, STAC LEE and STAC an ARMIN emerge almost vertically straight out of the sea. For the 50,000 pairs of gannets that nest there they provide lots of wind and air space, vital for take -off and landing, and safety from ground predators. Avoiding such predators is a key issue for sea birds when they return to land. An agile Arctic fox, for example, can easily steal the eggs from FULMARS that nest too close to the top of the cliffs in ICELAND, and otters on the west coast of Scotland have learnt to raid the colonies of nesting terns to pick off vulnerable young chicks before they can fly. Antarctica has no ground predators, so penguins can safely nest on flat ground. By contrast, in the ARCTIC, POLAR BEARS and FOXES have driven most of the sea birds up on to the CLIFFS. While cliff – nesting eliminates the problem of predators, it represents other hazards. Nests are very exposed to the weather and there is a constant risk that eggs or chicks might fall hundreds of metres to a rocky death. Common guillemots, which nest on tiny rocky ledges on the steepest of cliffs, have developed a specially elongated egg shape that seems to protect the eggs from rolling over the edge. Another adaptation has been noted among kittiwake chicks: when compared with the offspring of similar gull species, kittiwakes are remarkably calm and careful – essential qualities for high – rise survival. The scree slopes or boulder fields found at the bottom of cliffs can provide more congenial nesting sites than sheer cliff – faces themselves. In Britain black guillemots and razorbills have both taken advantage of the protection offered by holes and crevices in the scree, while the high Arctic swarms with little auks, who nest in crevices among the boulders.
OUT IN THE OPEN
Many sea birds, particularly in the TROPICS and ANTARCTICA, have no option but to nest on flatland as there are very few suitable cliffs. Isolated islands, with few or no ground predators, are therefore particularly important nesting sites. Gulls, terns and skuas are typical ground – nesting species of the northern hemisphere. Being agile birds, they can take off quickly when in danger, but they have no hesitation in aggressively attacking any intruder who threatens their eggs or chicks. In the TROPICS and the SOUTHERN HEMISPHERE boobies and albatrosses can defend themselves by size alone. However, many sea birds, awkward and vulnerable on land, have taken to excavating burrows.
In the North Atlantic, the most familiar of these birds is the PUFFIN. Using their colorful bills, PUFFINS excavate tunnels a metre or so into the earth. Finding suitable places to dig, however, is not always easy, so these colonies are often large and very densely packed. BIRD ISLAND, off the western tip of SOUTH GEORGIA in the South Atlantic, was so named by Captain James Cook because of the staggering numbers of sea birds he found there. To this day there are many parts of the island where it is impossible to walk without collapsing the burrows of the literally millions of seabirds that honeycomb the hillside. On Nightingale Island, near Tristan da Cunha in the South Atlantic, great shearwaters breed in such numbers – estimated in millions – that the birds run out of burrow space. As a result, nearly 250,000 eggs each year are laid on the surface by birds that have failed to find a burrow.
PREDATORS ON THE WING
Inevitably, dense colonies of nesting sea birds attract airborne predators, and this is amply illustrated on the island of TALAN, just off Russia’s far – eastern seaboard, in the Sea of OKHOTSK. Just 3 km (1.8 miles) square, Talan is home each summer to over 4 million sea birds of 14 different species. Among the many predators drawn to the island, the most impressive is the Steller’s sea eagle, the world’ largest eagle, with a wingspan a third as big again as that of the golden eagle. Throughout the breeding season these massive birds patrol the CLIFFTOPS, riding on the updrafts and looking out for kittiwakes as they come and go from their chicks. When the eagles fold away their giant wings and swoop down the cliffside, the kittiwakes explode from their nests, calling loudly to confuse the predators. Eventually the eagles are successful in snatching a meal from the air, and each summer up to twenty of them and their growing chicks make a very good living out of sea birds alone. But by nesting together and co – ordinating their breeding to ensure that their chicks fledge at roughly the same time: by keeping up numbers, the kittiwakes can ensure that the effect of predation is minimized. On the other side of Talan massive scree slopes provide nesting sites for another sea bird that has taken the numbers game even further.
Every evening during the breeding season, just before sunset, massive swirling clouds appear out low over the ocean. Gradually these clouds build, getting closer and closer, until suddenly the slopes are invaded by thousands and thousands of crested auklets, small black auks just 25 cm (10 in) tall. Their colonies are so large that the numbers are difficult to count, but some are believed to provide nests for half a million pairs. So much potential food attracts many predators, and the slopes are continually patrolled by ravens, peregrine falcons and even sea eagles. But, by returning en masse to their nests each evening, the auklets can swamp the predators.
For a peregrine falcon, which catches its prey by high – speed dives, a dense flock of swirling auklets provides a challenge. Unplanned air strike could seriously damage a peregrine, so it holds back, and thus for any individual auklet, the chances that it will be taken are greatly reduced by the simple force of numbers. The ultimate way to avoid airborne predators, though, is to leave and return to nesting sites under cover of darkness. During daylight hours some colonies seem completely deserted, the only evidence of breeding activity being the burrow entrances that pepper the hillside. Nothing starts happening until night falls, and even then overcast or misty conditions are preferred. Suddenly, as if from nowhere, the air is full of fluttering wings and calling birds. Like so many wind – blown snowflakes, hundreds of PRIONS descend on their nesting site. These tiny members of the petrel family spend most of their lives far out to sea, but during the breeding season they come to land en masse to lay their eggs or tend their chicks. Frantically busy all night, the majority are gone by dawn, having returned to their natural ocean habitat before first light. Only a few remain to keep the eggs and chicks warm during the day ahead.
MAMMALS RETURN
There can be few stretches of coastline that offer such stunning natural spectacles as St Andrews Bay on the island of South Georgia in the South Atlantic. For a start, the scenery is breathtaking; massive white rollers continually crash onto a 3-km (2 mile) beach left by retreating glaciers. In the distance glaciers are still there, tumbling down the sides of chain of impressive mountain peaks. Between the sea and mountains the flat glacial plain provides a perfect nursery. All year round, and throughout the worst of the Antarctic winter, 150,000 pairs of king penguins come to this beach to breed. The activity begins in September and October, with arrival of 4 – tonne bull southern elephant seals. The smaller females follow and soon the whole beach is packed with up to 5000 of the species. For six busy weeks the bay echoes to the roars of males as they battle for access to the females. These bloody, but rarely deadly, contests can last up to 20 minutes.
LURE OF THE LAND
Many, but not all, marine mammals need to leave the sea to breed. To understand why it is important to realize that all of today’s marine mammals have evolved from ancestors that originally walked the land and continue to share a number of mammalian characteristics: they ae warm – blooded, they breathe air with lungs and they give birth to live young who suckle milk secreted by the mother’s mammary glands. However, they have also made major adaptations for life at sea.
Life in the water involved two (2) key problems for the ancestors of sea mammals. In the first place, water is far more viscous than air, so marine mammals had to streamline their shape to became efficient swimmers. In addition, water conducts heat much faster than air, so sea mammals had to develop a number of ways to insulate their bodies and reduce heat loss. Of all marine mammals, the WHALES and DOLPHINS (cetaceans) are most specialized for life in the water and never need leave it. But pinnipeds (seals, sea lions and walruses) all leave the sea to breed. Seemingly it is simply too energy – sapping to give birth and suckle a warm – blooded pup in such cold water.
There are two different strategies for solving this problem. Many seals live at high latitudes near the poles where they can give birth and suckle their pups on ice floes. Harp seals in the Arctic and Weddell seals in the Antarctic are both examples. While suitable floes are abundant and offer easy access back to the sea, ice has distinct disadvantages: it affords little shelter, acts as a heat sink and can easily break up. For these reasons, many seals and all the sea lions choose to breed on land.
Although more predictable and providing better shelter, land poses its own problems. Seals and sea lions sacrifice their mobility on leaving the water, and it is not easy to find suitable terrain with easy access to the sea and an absence of predators. Consequently, sea mammals tend to breed on the COAST, often forming enormous colonies. On the PRIBILOF ISLANDS in the BERING SEA, for example, over a million northern fur seals come together in just a few separate colonies.
WHY BREED TOGETHER?
Shortage of suitable beaches is not the only reason for these large colonies. Animals used to leading relatively solitary lives searching for food find real social advantages in coming together to breed. All the seals and sea lions that come to land have developed mating systems where one male mates with several females during a breeding season. The exact degree of this POLYGYNY varies from species to species, but a successful bull elephant seal may fertilize up to a hundred females in just one season. As mammals, pinnipeds are predisposed to this breeding system. Because they lactate and the mother’s milk is the primary source of energy for raising young, the males are free to spend all their time fighting for females. This activity is fuelled by the thick blubber that keeps them warm in the water, as it also acts as a useful store of energy when they are on LAND.
The southern and northern elephant seal and they grey seal have a breeding system where males fight to defend females. Pressure to win these battles is so great in elephant seals that the males have grown to an enormous size compared to the females. A southern elephant seal bull weights 4 tonnes while the female weighs only 1 tonne. This is the largest size differential between the sexes in any mammal. Once a dominance hierarchy is established through the bloody fights, it is maintained with visual threats and roars.
Successful males aim to maintain exclusive access to the females in their group. The females, meanwhile, compete to be in particular groups, choosing central positions that tend to be held by dominant males. This all adds to the cohesion of the colony and encourages elephant seals to breed in dense groups. While a successful male may stay on top for up to four seasons and mate with over 400 females, a female can only bear one pup per season, producing only 12 pups in her lifetime.
However, by sticking with a strong male, she will at least ensure the genetic strength of all of her offspring. The North Atlantic grey seal, which breeds on isolated beaches on the BRITISH COAST, has only recently turned to breeding on land. That is known because their pups are white – perfect camouflage if you are on ice, but less good on rock. The move from ice to land is probably the grey seal’s attempt to extend its range further south. Interestingly, grey seal colonies are nowhere near as dense as those of elephant seals, and the size difference between the sexes is far less extreme.
However, male grey seals are gradually becoming larger than their mates, and this probably indicates that their colonies will grow denser too. Their breeding system, it seems, is becoming similar to the other two land-based species – the northern and southern elephant seals. All the fur seals and sea lions come to land to breed in colonies. These range from the loosely clustered groups of GALAPAGOS fur seals, with less than a hundred animals, to the extraordinarily dense colonies of Antarctic fur seals, where 150,000 animals will cram on to a beach 1 km (0.6 miles) long. The males do not fight directly for females but try to defend a resource that the females need, such as a safe place to have her pups or a good spot for keeping cool. Animals insulated for life at sea often find the heat on land uncomfortable, particularly if they live near the tropics. In SOUTH GEORGIA, the bull fur seals return first to establish territories, hoping to attract females when they return. At the start of the season, a male may control 60 sq m (650 sq feet), but as more animals return and competition hots up, this may shrink to just 22 sq m (235 sq feet). Females tend to prefer the territories near the top of the beach and the more successful males hold these.
NOTE AND REMEMBER
Southern Elephant seals have been recorded diving as deep as 1700 m (5600 feet) in search of food.
