In contrast to plants, an animal can change its environment quickly by moving location, and it may even move between aquatic and terrestrial environments. This may be because of physical parameters such as temperature or humidity change, or because of a search for resources such a food, shelter, or a mate.
Similarly, an animal can move to escape a competitor or a predator. Plants do not have this option but have evolved other mechanisms of protection. For instance a plant cannot run away from the animal which wants to eat it. But, it survives by having thorns which stop the animal completely finishing it; it may taste nasty; or it can store energy in its roots and grow new parts above ground later, when the animal has finished its meal.
Aquatic and terrestrial animals have different requirements in terms of what components of the physical environment are the most important to them. For aquatic animals, water is their medium; without it they die. However the condition of the water is also important: dissolved salts and oxygen, currents and levels of light may be critical. Aquatic animals evolve to suit specific hydrological conditions: if these conditions change the animals may die.
Terrestrial animals need water just as much as aquatic animals—without it they also die. However, for terrestrial animals climatic conditions are especially important, and particularly temperature. The great annual migrations of birds, for instance, between Europe and Africa, are the result of seasonal climatic changes.
3.8 THE DYNAMICS OF ECOSYSTEMS AND VIABILITY
Whilst Ecosystems can be described simply in terms of just plants (a mangrove forest) or just animals (the Andoni elephants), plants and animals have a dynamic ecological relationship.
However it is a relationship where animals are particularly dependent upon plants, and nowhere is this more obvious than in obtaining and using energy. All living things need energy and all energy comes from the sun; but only plants are able to convert the sun's energy into carbohydrates, which can be stored and used as energy. To obtain energy animals must consume plants or eat other animals which have already done so.
AUTOTROPHS AND HETEROTROPHS
Autotrophs are Primary Producers, being the photosynthesising (see 4.5.3) plants and plankton which convert water, the sun's energy and inorganic substances into the living matter upon which heterotrophs depend.
Heterotrophs are Consumers and include:
- Herbivores, animals which eat only plants;
- Carnivores, animals which only eat other animals;
- Omnivores, animals which eat both plants and animals; and
- Decomposers, animals and plants that live on dead and decaying material (Detritus). This group includes the:
- Primary Decomposers, those organisms (such as bacteria and fungi) that mineralise detritus thus releasing nutrients into the soil.
In the African savannah, grass converts the sun's energy into carbohydrates, antelopes eat grass to get their energy and lions eat antelopes to get theirs. It is a dynamic system:
