The effects of size-selective predation and environmental variation on the distribution and abundance of a Chironomid, Paraborniella tonnoiri Freeman

Abstract

Larvae of the Australian chironomid P. tonnoiri live in temporary rainpools and aestivate in the dry mud during the summer drought. Over most of their range, they are restricted to large, deep, vegetated pools. A number of factors influence population numbers, but the most critical in determining the species' distribution is the presence of another chironomid, Allotrissocladius sp. Fourth-instar Allotrissocladius larvae kill P. tonnoiri when the latter are in first and possibly second instar. The deep, vegetated pools have ample food available when P. tonnoiri breeds, so young larvae grow rapidly through their early instars and soon cease to be vulnerable. In shallower, non-vegetated pools, with much less food, the larvae grow more slowly, are vulnerable for longer, and are consequently eliminated. P. tonnoiri is also absent from areas where low temperatures during early life may slow its growth, and it seems likely that the same mechanism is responsible. The study thus suggests that it may seldom be enough to know about the gross structure and physical characteristics of an ecosystem in order to predict whether or not a species will persist in it-it is also necessary to know some of the details of when and how it interacts with others.