Every year, about 100,000 cubic km (24,000 cubic miles) of water evaporates from the world’s oceans, condenses, and then falls as rain or snow. Most of this water disappears back into the atmosphere to continue this cycle, but about a third returns to the oceans by flowing either over ground or beneath the land surface.
This steady supply of fresh water sustains all the world’s land-based life, as well as creating highly diverse habitats — from streams, rivers, and lakes to reedbeds. marshes, and swamps — in which a wide range of different animal and plant life can thrive.
LAKES AND RIVERS
For permanent Water dwellers. life in lakes and rivers is shaped by many different factors. One of these is the Water’s chemical make-up. which is often dictated by the type of rock that forms the bed of a river or lake. Hard water. for example is good for animals that grow shells because it contains calcium that can be used as shell-building material. while water that is especially rich in oxygen is important tor highly active predatory fish. such as salmon and trout.
Water that is very deficient in oxygen, on the other hand. provides a poor environment for animal life as a whole because relatively few aquatic species. apart from specialized worms. can survive in it. In lakes and rivers. aq uatic animals usuaily occupy clearly defined zones. The brightly lit water close to the surface often teerns with water-fleas, copepods. and other microscopic forms of animal life.
They live here in order to feed on phytoplankton- the microscopic algae that becorne extremely abundant during the sumrner months. Feeding in the middle zone and near the water’s surface are larger animals. such as fish. that are able to hold their own against the strength of the current.
Weak swimmers live near the banks. where the current is slower, or among stones and sediment on the bottom. In still and slow-flowing water, surface tension supports insects that hunt by walking or running over the water.Some animals that are associated with freshwater habitats are not necessarily permanent water dwellers: instead, they divide their time between water and the adjacent land, entering the lake or river to hunt or breed, or using it as a nursery for their young.
A wetland is any waterlogged or flooded area with a covering of water plants. In some wetlands — reedbeds and bogs, for example — the plants hide the water completely. However, in most wetland areas of open water and dense vegetation are mixed, creating a ricn and complex habitat that can be exploited by animals of almost every kind.
Biologically, wetland is among the most productive inland habitats, sometimes surpassing even rainforest in the amount of food that it generates for animals. In temperate parts of the world, this productivity reaches a peak during spring and summer, but in the tropics and subtropics, it is more affected by the water supply.
Some tropical wetlands – South Americas Pantanal, for example — largely dry out during the dry season, but then look like vast lakes once it has rained. In many wetlands, the water is no more than a metre (3 1/4 ft) or so deep, which means that bottom-living animals and surface dwellers are rarely far apart.
This kind of environment is ideal for air-breathing swimmers, such as snakes and turtles and also for land-based animals that use water as a temporary refuge from danger. Unlike large lakes, wetlands have an extra dimension in the form of emergent plants which grow up through the waters surface and into the air.
These plants range in size from small grasses and rushes just tall enough to keep insects clear of the water, to water-loving trees that grow to over 35m (115 ft) high. Trees act as important roosting and breeding sites for waterbirds, providing them with shelter and relative safety from predators as well as keeping them close to the source of their food.
LIFE IN FRESH WATER
Fresh water is an essential resource for all land-dwelling animals, but it is also an important habitat in its own right. Freshwater habitats vary immensely, from temporary pools to giant lakes, and from tiny streams to rivers thousands of kilometers long.
As a result, the problems that animals must overcome in order to survive in their environment can be very different from one freshwater habitat to another. Strong currents, periodic drought, and intense competition for food are some of the difficulties for which animals must find solutions.
STAYING IN PLACE
Animals living in fast-flowing rivers face a constant battle with the current, and they cope with it in one of 2 main ways. The first is to avoid the problem by staying close to the river bed, where the current is relatively slow.
Many invertebrates, such as stonefly and mayfly larvae, never venture into open water; and to improve their staying power they often have a flat profile, so that if they position themselves with their heads facing upstream, the current presses down on their backs helping them to stay in place.
Dippers are the only songbirds that feed underwater, and can stay submerged even when moving against the current. They use their flapping wings to keep themselves in position while their feet grip the substrate of the river bed.
The second solution is to compete with the current by swimming against it. Species that cannot avoid the current match their swimming speed to the water flow, enabling them to stay in place — and they keep swimming even when they are asleep.
As well as supporting freshwater residents, rivers and lakes receive visitors from the sea. These are migratory fish, which divide their time between fresh- and saltwater habitats.
Anadromous species, such as salmon, breed in rivers but spend most of their adult lives offshore. Catadromous species, such as some eels, do exactly the opposite: they live in fresh water, but swim out to sea to breed. For most fish, fresh water makes a much safer nursery than the ocean, and this justifies their long journey upriver to lay their eggs.
However, because the food supply in fresh water is limited, the young eventually make for the much more dangerous, but also more fruitful environment of the open sea. The advantages of this are apparent where some, but not all, members of a species migrate: those that swim out to sea usually grow much bigger than those that stay behind.
Like migrating birds, migrating fish are often remarkably accurate in pinpointing the place of their birth, returning there to spawn even though it might mean an inland journey of over 2,500 km (1,500 miles).
Each river has its own characteristic chemical fingerprint and, with their acute sense of smell, migratory fish are able to identify the estuary that they left as young fish. By monitoring the scent or the water as they progress upriver, they home in on their own spawning ground. Migrating fish often meet barriers to their progress.
Salmon are famous for jumping waterfalls and rapids, while eels tackle obstacles by slithering around them over land. Eels usually do this after dark, in damp conditions, when they can survive out of water by breathing through their skin.
LIVING IN AND OUT WATER
Most amphibians, such as frogs and toads, develop in fresh water habitats and then, as they approach adulthood, take up life on land, returning to water to breed. However, insects evolved amphibious lifestyles long before the first true amphibians appeared.
Today, almost every patch of fresh water, from the smallest puddle to the largest lake, is inhabited by insects. Mosquito larvae feed on microscopic fresh water life, where as the larvae of dragonflies and damselflies stalk larger prey, catching it with a set of extensible jaws called a mask.
These insects leave the water when they become adult, but water beetles and bugs remain in it for life, although their ability to fly makes it easy for them to spread from one pool to another. Some larger animals have developed lifestyles that straddle water and land.
Snakes are good swimmers and a number of species, such as the grass snake, specialize in catching aquatic animals. The anaconda, the heaviest snake in the world, uses water for cover and for support: despite its size, water buoys it up, reducing its effect weight to almost nil and enabling it to swim at considerable speed.
As well as providing food, fresh water is a valuable resource for some land-based animals seeking refuge from predators or daytime heat. At dusk, birds often roost on lakes and reservoirs while, during the day, hippos, capybaras, and beavers spend a lot of time in the water, emerging under the cover of darkness to feed on land.
In warm parts of the world – particularly in the tropics – rivers and lakes often dry out completely for several months each year. In these conditions, freshwater animals need some unusual adaptations to survive.
When a lakes water level drops, its temperature increases. Warm, stagnant water often contains so little oxygen that many fish suffocate, but lungfish – characteristics inhabitants of tropical lakes and wetlands – are experts at coping with drought.
They gulp air at the surface and, when their home starts to dry out, they burrow into the mud, sealing themselves inside mucous cocoons. Later in the year, when heavy rain falls and soaks the mud, the cocoon breaks down, and the fish wriggle away.
Caimans and turtles also hide away in this way although their scaly, waterproof skins mean that cocoons are not needed. Being “cold – blooded”. or ectothermic, they need relatively little energy to stay alive. So they can survive drought-induced food shortages for months. Animals that cannot survive drought often leave behind drought-resistant eggs, which hatch when water returns – an effective way of bridging the gap between one wet period and the next.
This survival strategy is used by water feas, rotifers, tardigrades, and many other “micro-animals”, as well as some kinds of fish, which live in temporary pools or in the film of fresh water that covers mosses and other plants.
COMING UP IN AIR
Freshwater animals need to breathe oxygen. Fish use gills to extract it from the water, but many invertebrates collect it from the air. These air-breathers include many water snails many insects, and the water spider — the only spider to have evolved a fully aquatic lifestyle. For animals living at the waters surface, air is easy to reach.
For fully submerged ones, breathing requires periodic trips to the surface to replenish air reserves, which are stored in or on the insects body. often forming a film-bubble that gives the animal a silvery sheen.
The water spider has an exceptionally elaborate storage system. It constructs a “diving bell” from strands of silk trapping a large bubble of air inside it. The bell acts as both air and nursery — a unique example of an animal creating a submerged habitat that resembles dry land.