Invertebrates are animals without backbones. Many invertebrates are herbivores and therefore low down the food chain – such as daphnia or snails. Carnivorous invertebrates include spiders, dragonflies or crayfish. Almost all invertebrate species can be food sources for vertebrates such as frogs, fish and birds
Because invertebrates form such a huge portion of the food chain, their presence or absence determines the overall composition of the river wildlife and is sensitive to the chemical composition and purity of the water. The invertebrate species that populate a clean river where dissolved nutrients, poisons and suspended particles are at a low level will be quite different from a polluted, murky stream. Also, different invertebrate species will be found in the main stream of the river from in the ponds and the brooks and ditches that feed into it.
Riverflies live most of their lives as larvae on the bed of rivers and still waters, emerging as short-lived adult flies mostly in spring and summer. Riverflies include stoneflies, alder flies, caddisflies and mayflies. Many of these are highly susceptible to pollution and as a way to monitor the health of a river, riverflies are surveyed by volunteers on a monthly basis all over the country as part of the National Riverfly Partnership.
The River Thame Conservation Trust are part of the Riverfly Partnership and hold annual riverfly training days for anyone wanting to help us monitor the health of the River Thame. If you would like to get involved then please get in touch here.
There are many other insects that reside in our rivers and surrounding habitats, ranging from those that live in the water, such as water scorpions, to the 1000s of species that live on the bankside vegetation. These insects may full depend on the river, or may just happen to be living next to it, but form an incredibly important part of our food chains and webs.
Zooplankton are tiny, usually microscopic, animals which live in water. In rivers, they drift with the water column because they are not strong enough swimmers to move against the current. Zooplankton are an important link in riverine food webs because they are prey to species at higher trophic levels, such as fish and aquatic insects. Their abundance and species composition can be threatened by changes in water quality including variations in nutrient levels, temperature, and pH. Native zooplankton are also at risk from invasive species. Zooplankton are generally found close to the water surface because they eat phytoplankton which need light for photosynthesis.