Aquatic invertebrates and dry ditches

24th November 2010, Wednesday

A few weeks back I refered to a land-holding at East Guldeford where almost all of the ditches dried out completely last year. A number of surveys were undertaken this summer to advise on management which also give an indication of how various groups coped with this serious challenge.  So how did the ditch invertebrates cope?

Dried out ditch bed

The answer is surprisingly well.  This summer the ditches were sampled in a systematic way that allowed the results to be quantified and they were found to support an outstanding fauna, in fact to my surprise this turned out to be the most productive area the surveyor, Dr Martin Drake, has sampled on Wallad Marsh (which is renowned for having excellent habitat for these species).  

Perhaps the most attractive species found was the soldier fly Odontomyia ornata , a Red Data Book (RDB) 2 species that was thriving amongst the stands of umbellifer flowers in the ditches.   For some reason these waterbodies had a rich assemblage of rare weevils such as Bagous longitarsus pRDB1, Bagous puncticollis RDB1, Bagous tubulus RDB2, and several other scarce species.  Another pRDB1 species was the ground beetle Badister collaris which is found on damp mud around drying wetlands.  The numerous scarce water beetles included the great silver beetle Hydrophilus piceus which was breeding in several of the ditches.  Medicinal leech were in many of the ditches in good numbers.

Clearly these aquatic invertebrates were thriving despite the hostile dry conditions in 2009.  One of the reasons for the outstanding quality of the site was that many of the ditches had a salt content high enough to make them brackish, and this feature is well known for supporting rich assemblages of aquatic invertebrates.  Another reason was that the ditches margins were shallow, and therefore warm – encouraging rapid development of these insects.

To maintain the quality of the site it will be important to continue grazing the site, allowing the stock to access the ditches, and to avoid reducing the salinity.  So, if you had the opportunity to pump fresh water into the ditches in late summer to prevent them drying out you would do more harm than good.  Better to let them dry out and stay saline, perverse though that might seem. 

More about the response of the aquatic plants in a later post.