Woodland soil, a small ecosystem Animals, plants and other living things, together known as organisms, must survive together in the natural world. They depend on and relate to each other, such as by being plants and plant-eaters, predators and prey, or parasites and hosts. Organisms must also fit in with their non-living surroundings including air, water, soil and rocks, and cope with changing conditions such as weather, climate and seasons. Scientists think of the natural world as being divided up into ecosystems, distinct areas in which living things interact with their living and non-living surroundings: their environments. All ecosystems taken together form the biosphere, the living world. The study of ecosystems is called ecology.
An individual organism
The basic part or unit of ecology is an individual organism, such as an animal or plant. Individual organisms hardly ever live on their own. They exist and interact with others, satisfying their needs such as shelter and nourishment.
For example, the tiger beetle, a fierce hunting insect with large jaws to seize prey such as worms, is part of the soil community. It competes for small prey with others such as centipedes and spiders. It may be hunted itself by larger predators such as shrews or moles.
The part or role that an organism plays in its community, in this case as a small predator, is called its ecological niche. The community, of which the tiger beetle is a member, lives on the floor of a broadleaf woodland: its habitat. This is one small part of the temperate woodland biome.
Living things in an ecosystem that interact and rely on each other for survival, form a community. Their natural homes may be small like a garden pond or a rotting log, or extensive, like a lake or forest. These homes are all different types of habitat, places based on similar kinds of plants or physical features such as soil type. Examples are oak woods, salt marshes, coral reefs, sand dunes or the deep ocean floor. The largest habitats are vast areas known as biomes.
One of the most important activities living things do is feeding. Plants “feed” on sunlight and minerals from the soil and form the group known as primary producers. Animals that eat plants form another group, herbivores. They range from tiny insects to rabbits. Animals that eat animals are carnivores. When plants and animals are linked in this way, it is called a food chain.
The level on which an animal or plant stands in the food chain is called its trophic level. At the lowest level are the producers, which are high in number. At the middle levels are consumers. At the highest level are large predators, which are fewer in number.
Since animals eat more than one kind of food, food chains are part of more complex food webs. In a North American forest, for example, plant-eating animals such as rabbits are prey for lynx or bears. Shrews are insect eaters and they are hunted by these same predators. Bears also feed directly on plants and insects.
The animals and plants that live together in a pond are also linked by a food web, pictured here (the arrows point towards the feeding animal). Pond plants are eaten by herbivores, from tiny worms and grubs to ducks. Small carnivores like sticklebacks and diving beetles eat the grubs and worms. Larger carnivores such as perch eat the smaller ones. The heron hunts a number of herbivores and carnivores.
The animals and plants in a food web are interdependent: they depend on each other for their food. This helps keep the plant and animal populations within an ecosystem in balance.
Any changes to one part of the food web caused by, say, disease or human interference, tend to affect the rest of the web. If the plants (the producers) died, for example, the animals (the consumers) that feed on them, such as rabbits, voles and insects, would starve and die themselves. The other animals in the food web that fed on them would die, too—or migrate elsewhere.
If there were fewer insects there would be more grass for the rabbits and voles, so their populations would increase. However, there would be less food for the birds and shrews, so their populations would decrease. This would mean less food for the hawks. But since there would be more rabbits and voles for them to prey on, their populations might stay the same.
Toxins and food webs
Interdependence explain how toxins (poisons) can move through the food web. A good example is a marine food web. Phytoplankton, a type of algae that floats in surface waters, can absorb poisonous substances such as mercury (until recently, used in the manufacture of pesticides and paint products). Some species of phytoplankton can produce powerful toxins themselves. When they mass together in what are called algal blooms, the phytoplankton may turn the water red or brown, an occurrence known as a "red tide".
Phytoplankton are eaten by zooplankton, tiny, microscopic animals that live in vast swarms of billions of individuals. These are eaten in turn—along with phytoplankton—by a number of ocean creatures, ranging from tiny fish to the giant whales. Although the amount of toxins produced by an individual phytoplankton is minuscule, the levels can build up quickly as each animal higher up the food chain takes in more and more of the toxin. This is called bioaccumulation.
Krill, mussels and small fish are often unaffected by the toxins, but they pass them on further up the food chain—to larger fish, for example. The larger animals get larger and larger doses: these are potentially fatal. Toxins can travel all the way through the food web to humans through contaminated food fish or shellfish. They can cause respiratory conditions, diarrhoea, vomiting, numbness, paralysis and death.
The slow, natural change of an ecosystem is called succession. For example, if a woodland is destroyed, it may gradually grow back. During this process, plants and animals move to an area, altering the environment so that it becomes more suitable for other species. Eventually, the ecosystem reaches a stable point, known as a climax. Ecological succession in a broadleaf woodlandAt first, directly after a wildfire, there is bare ground with little plant life (1). Later, wind-blown seeds settle on the ground and start to flower. Insects come to feed on the plants (2). Larger plants and bushes grow, providing food and shelter for larger animals (3). Trees eventually become the dominant plants. A woodland ecosystem is formed (4).
Consultant: Chris Jarvis