A matter of proportion? Associational effects in larval anuran communities under fish predation

Abstract

In Batesian mimicry, a species lacking defences against predators benefits from mimicking the aposematic signal of a defended species, while the model may incur the costs of reduced defensive efficacy. Similar reciprocal indirect effects may emerge even when the signal is not mimicked; termed associational effects, such interactions are well known in plants sharing herbivores but have received little attention in animal studies. We investigated associational interactions in a system where unequally defended prey (chemically defended Bufo bufo and undefended Rana temporaria tadpoles), sharing general morphology but not an aposematic signal, were exposed to predation by the carp Cyprinus carpio along a gradient of relative prey abundance. In the absence of fish, the assemblage composition had no effect on the survival of Rana, while that of Bufo decreased with increasing abundance of Rana. Fish reduced the survival of tadpoles from both species. However, increased relative abundance of Bufo in the community led to enhanced survival in both Bufo and Rana. Increasing relative proportions of heterospecifics reduced metamorph mass only in Bufo, indicating greater sensitivity to interspecific competition compared to Rana; the effect was reduced in the presence of fish. Our results show that undefended non-mimetic prey enjoy reduced predation with increasing relative abundance of chemically defended prey, which in turn suffer greater mortality with an increasing proportion of the undefended species. Associational resistance/susceptibility, driven by current assemblage composition, not by selection for resemblance, can shape the dynamics of mixed communities of defended and undefended prey in the absence of mimicry.

Keywords: Associational resistance; Bufo bufo; Indirect effects; Mimicry; Rana temporaria.

Fig. 1

Proportion of undefended Rana temporaria tadpoles surviving to metamorphosis (mean ± SE) along the gradient of the initial proportions of chemically defended Bufo bufo in experimental tadpole communities. Empty circles indicate treatments in the absence of fish, filled circles indicate treatments with fish. Each treatment combination was replicated four times, except for the single-species treatment (initial B. bufo proportion = 0) replicated twice

Fig. 2

Proportion of chemically defended Bufo bufo tadpoles surviving metamorphosis (mean ± SE) along the gradient of their initial proportions in experimental tadpole communities. Empty circles indicate treatments in the absence of fish, filled circles indicate treatments with fish. Each treatment combination was replicated four times, except for the single-species treatment (initial B. bufo proportion = 1) replicated twice

Fig. 3

Metamorph mass (mean ± SE) of Rana temporaria (N = 470) along the gradient of the initial proportions of chemically defended Bufo bufo in experimental tadpole communities. Light-shaded bars indicate treatments without fish, dark-shaded bars indicate treatments with fish (for the number of replicates, see the caption of Fig. 1). In cages only stocked with R. temporaria, no tadpoles survived metamorphosis in the presence of fish (initial B. bufo proportion = 0; indicated with dagger)

Fig. 4

Metamorph mass (mean ± SE) of Bufo bufo (N = 719) along the gradient of their initial proportions in experimental tadpole communities. Light-shaded bars indicate treatments without fish, dark-shaded bars indicate treatments with fish (for the number of replicates, see the caption of Fig. 2)