Invasion of a stream food web by a new top predator
A 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.