Density- and trait mediated indirect effects facilitate coexistence and stability in experimental and natural insect communities.
van Veen, Frank*,1, van Holland, Peter 1, Brandon, Callum1, Godfray, Hugh1, 1 Imperial College London, Ascot, United Kingdom
ABSTRACT- The importance of density mediated indirect effects in shaping population dynamics has been acknowledged for some decades and it is becoming increasingly clear that so-called trait mediated indirect effects can have an impact of a similar magnitude. By using a combined approach of a laboratory experiment, a model, a field survey and a field experiment we show how the two types of indirect effects may interact and thus promote stability. We show that stable coexistence occurs readily in experimental laboratory communities consisting of two competing aphid species and a parasitoid which attacks the competitively dominant aphid. However, the competitive interaction and the host-parasitoid interaction on their own are both highly unstable and always result in extinctions. With a model we show that the stable coexistence in the more complex communities can be explained by the balancing effects of a density mediated indirect interaction on one hand and a trait mediated indirect interaction on the other. The first is initiated by the parasitoid which reduces the density of the dominant competitor thus reducing its effect on the host plant, allowing the weaker competitor to persist. The second effect is caused by interference by the weaker competitor with the foraging behaviour of the parasitoid thus preventing overexploitation of the host. If a similar natural enemy mediated effect also operates in natural communities then we would expect to find more mixed species colonies than expected by chance. Data from a field survey confirms this prediction. Finally we present data from a field experiment that shows that this non-random distribution is due to a predator mediated effect and not due to a host plant mediated effect.
Key words: coexistence, aphid, indirect effect, microcosm
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