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.