Interactions between exotic and native lady beetle species stabilize community abundance.

A 23-year time-series of abundance for 13 lady beetle species (Coccinellidae) was used to investigate community stability. The community exhibited persistence in ten habitats, no overall trend in abundance, and low temporal variability quantified as Population variability (PV) = 0.33 on a scale from 0 to 1 that declined to 0.16 in the past 8 years. This high level of stability occurred as exotic lady beetles disrupted populations of the native species. For hypothetical communities of pairs of species (with randomly generated annual abundances in the range for lady beetles), PV increased linearly with the correlation coefficients between individual time series, illustrating a "portfolio effect". PV for the real community and the negative correlation between the abundance of exotics and natives fit this relationship precisely. A gradual decline of natives matched by an equal gradual rise in the abundance of exotics contributed to the negative correlation that stabilized the community. The abundance of the dominant species, an exotic, was negatively correlated with other exotics and most natives, and its stability increased over time, helping to stabilize the community. The community was most stable in habitats where beetle abundance was high (crops, particularly perennial crops) and, unexpectedly, was least stable in habitats with high diversity and stability of vegetation cover (forests). These data are consistent with the hypothesis that competition between exotic and native species, with release from competition for natives in some years, stabilized the abundance of this community. Stability may not last if populations of native species continue declining.

Predator identity and the nature and strength of food web interactions.

1. Most trophic interaction theory assumes that all predators are an abstract form of risk to which prey respond in a quantitatively similar manner. This conceptualization can be problematic because recent empirical work demonstrates that variation in the responses of prey to different predators can play a key role in structuring communities and regulating ecosystem function. 2. Predator identity - the species specific response of prey to a predator - has been proposed as an ultimate mechanism driving the relative contribution of indirect effects in food webs; however few studies have explicitly tested this hypothesis. 3. This study explores the impact of predator identity on direct consumptive (CE) and non-consumptive effects (NCEs), and on the relative contribution of indirect, density and trait-mediated effects in trophic cascades within host-parasitoid communities. 4. We systematically compared the individual, host-parasitoid-plant interactions of two actively foraging parasitoid species with disparate foraging styles, one aggressive and one furtive, a common aphid host and plant. Our results demonstrate that the degree of risk aversion by prey to each particular predator species (i.e. predator identity) is a key factor driving the nature and strength of direct and indirect transmission pathways. 5. Both parasitoid species, in general, had a negative impact on plants. The magnitude of the aphid anti-predator dispersal response was positively correlated with plant infestation and plant damage. The qualitative effect of predator-induced infestation of new plants superseded the quantitative effects of predator-mediated reductions in aphid numbers. 6. The greatest indirect impact on plants was generated by the aggressively foraging parasitoid, and the strength of the aphids anti-predator response (a NCE) antagonistically traded-off with CEs due to an increased investment in attempting to capture risk-sensitized prey. In contrast, the furtive parasitoid did not elicit a strong anti-predator response, had little indirect impact on plants, but generated very high CEs due to the advantage of ovipositing into a sedentary prey population. 7. Our data suggest the responses of prey to different predatory cues may be an important mechanism driving the relative contribution of transmission pathways in trophic cascades. We conclude that predator identity is a key factor influencing the nature and strength of food web interactions.