Invertebrates
In high-quality chalk rivers, there is a great diversity of aquatic invertebrates, with a wide range of insects such as caddisflies, mayflies, shrimps and stoneflies. The abundance of these organisms has been shown to be closely related to river flow rate with drought conditions significantly reducing their abundance. Over the past few years, anglers have become increasingly concerned about declining fly life, citing low summer flows, high winter flows, silt and water quality problems.
Invertebrates play an important part in the food chain of chalk rivers by feeding on algae and detritus and providing a food source to fish and mammals. Monitoring the status of the invertebrate community therefore provides a useful measure of the health of the wider river ecosystem.
The invertebrate community of the River Itchen has been shown to be of exceptional diversity in recent years with over 300 species recorded from the main river channel. Many of these invertebrates are rare in a national context and their conservation requires the maintenance of high water quality and a diverse array of habitats in the river.
The southern damselfly is one of Europe's most endangered insects, and is nationally and globally rare; their distribution in England represents a significant part of the global resource. They can still be found in old water-meadow ditches and shallow channels, notably in the Test and Itchen valleys. The southern damselfly has very specialised habitat requirements including the presence of extensive marginal vegetation within permanently flowing, unshaded ditches. Suitable habitats are currently fragmented in the Itchen valley and if this species is to be conserved in the long term, it is essential that habitat management works are continued and supported in the future.
The white-clawed crayfish used to be extremely common in most chalk rivers but is now considered to be globally threatened and an endangered species in Europe. They have declined to very low levels in recent years and have been virtually lost from many rivers, including the Avon. They now only survive at a handful of sites on the headwaters of the Rivers Test and Itchen. The main reason for the decline is a fungal disease which is carried and transmitted by american signal crayfish which was introduced into the UK in the mid-1970s. The future of our native crayfish is dependent on the factors responsible for its decline being addressed.
Hampshire & Isle of Wight Wildlife Trust and the Environment Agency are currently working together to try address the decline of both the white-clawed crayfish and the southern damselfly though a partnership project. To find out more about this exciting new project, contact Ben Rushbrook on BenR@hwt.org.uk or call 01489 774400.
