PhDA large predator, the nymph of the dragonfly Cordulegaster boltonii
(Anisoptera) (Donovan), has recently invaded Broadstone Stream, an acid headwater in
southern England. Because of its large size, the invader established itself as a new top
predator. The Broadstone Stream food web is exceptionally detailed and the community
has been studied since the early 1970s. The invasion of C boltonii, therefore, provided
a rare opportunity to investigate the effects of a potentially strong perturbation upon a
well-described system.
At the peak of the invasion C boltonii density exceeded seventy nymphs per
square metre, comparable to the abundance of the previous top predators. The invasion
appeared to part of a long-term trend, within an otherwise persistent community,
towards a fauna less tolerant of profound acidity.
Mobile, epibenthic prey were particularly vulnerable to C boltonii, due to high
encounter rate. In field experiments,t he invader depressedth e abundanceo f two such
species, a previous top predator and a detritivorous stonefly, whereas many other taxa
were largely unaffected. Predator impact was strongest during peak prey abundance in
the summer and autumn, and weakest in the spring when prey were scarce.
The diets of the resident predators and C boltonii overlapped extensively when
prey were seasonally abundant, but resource-partitioning increased as prey abundance
declined. The recent decline in the abundance of P. conspersa, which had the most
similar diet to C boltonfl, may be due to competitive and predatory interactions with the
larger predator.
Cordulegaster boltonii preyed upon virtually every animal taxon within the food
web. Consequently, the complexity of the web (e. g. linkage density, omnivory and
chain length) increased following the invasion. However, most taxa were rare and most
feeding links were weak when the web was quantified.Natural Environment Research Counci
