The powerhouses – freshwater
Estimates from studies in Wales suggest that
there are well over 250,000 invertebrates for every square metre of
river bed – and this figure doesn’t include the phenomenal number
of micro-organisms that also occur.
Although we hardly notice them, these are the
powerhouse of our rivers, lakes and ponds, turning matter such as
living plants and dead leaves into food for fish and frogs, otters
What kinds of invertebrates?
Because there is little calcium in freshwater
in Wales, insects are our most common freshwater invertebrate
groups, while molluscs – creatures with shells – for example, are
less diverse here than in other parts of lowland Britain. In Wales,
insects that need clean water with high levels of oxygen thrive -
groups such as dragonflies, stoneflies, mayflies and caddisflies do
Specialities of Welsh rivers
include the stonefly Isogenus nubecula, which only occurs
in the River Dee and is under threat there. The beautiful
mayfly Potamanthus luteus lives in sandy reaches of the
River Wye, which is also home to the native white-clawed crayfish
Austropotamobius pallipes. A few rivers still
support small populations of the freshwater pearl-mussel
Margaritifera margaritifera, although this is sadly now on
the verge of extinction.
Mountain lakes contain rare
species, such as the pea-mussel Pisidium conventus and the
diving beetle Dytiscus lapponicus that are more typical of
high-altitude lakes in northern England and Scotland. In lowland
pools, the medicinal leech Hirudo medicinalis and the
curious fairy shrimp Chirocephalus diaphanus occur.
Even ditch systems, like
those of the Gwent Levels, contain important invertebrates,
including the great silver water beetle Hydrophilus
piceus, Britain’s largest insect.
Freshwater invertebrates react quickly to
changes in their habitat and to pollution events, so are good
indicators of the health of water quality. From our studies we know
that many species are still declining, despite overall improvements
in quality. We can only be content with water quality when we know
that the invertebrates that live in the water are thriving.