Rivers - Life in a Lowland River

The animals and plants living in an upland river are specially adapted to a life in fast-flowing often turbulent, cold, oxygen-rich water. They have to be strong swimmers or be able to anchor themselves securely so they are not washed away. However, the communities of animals and plants in a lowland river, where the flow of water is much slower, are much more like those of lakes and ponds. The main difference between a lowland river habitat and a lake or pond is that there is still some flowing movement in even the slowest of rivers. This difference has some important effects on the wildlife communities, which should be kept in mind when studying a river habitat, and, perhaps, comparing it to a pond or lake. The most important effects are:

• plankton (microscopic plant and animal life) and floating plants are absent, for the current carries them away. Still water habitats are usually rich in plankton
• river creatures deposit their eggs under gravel or stones, or attach them to a rock or leaf, so they do not get carried away. Still water animals produce eggs or young that drift about in the water.
• the flow enables some species to disperse, drifting downstream to colonise another part of the river. Large numbers of some species deliberately drift away at certain times of the year.
• some species find it easier to obtain food, as nutrients and small animals are continually carried down-stream by the current. Predators (hunting animals) in one section may feed on prey that have been washed into their habitat from further up the river.

These effects mean that when studying a river, it is wise to investigate more than one section. At any one place there will probably be fewer species than in a lake or pond, but over the entire river system there is a huge variety of animal and plant life.

As in any natural habitat, the animals and plants living there depend on each other for food. The water plants obtain their energy from sunlight, making their own food (a process called photosynthesis). The plants, also known as producers, are eaten by herbivores (plant eating animals) such as snails, water fleas and freshwater shrimps. These primary consumers, in turn, are eaten by a carnivore (meat eating animal) such as a roach. This secondary consumer is eaten by yet another carnivore such as a pike. The pike is a top predator, or tertiary consumer, and is not normally preyed upon itself. These plants and animals are linked together in a food chain:

In reality, of course, more than one species of predator eats the snails, fleas, shrimps and the roach. Within a habitat several different food chains interlink to form a food web.

FOOD FOR THOUGHT - Humans could also be included as a predator in a river’s food web; but we must be careful not to disturb the food chains too much and upset the natural balance of life. What would happen to the food chain above if fishermen caught all the pike in a river?