Why aren't warning signals everywhere? On the prevalence of aposematism and mimicry in communities

Abstract

Warning signals are a striking example of natural selection present in almost every ecological communityfrom Nordic meadows to tropical rainforests, defended prey species and their mimics ward off potential predators before they attack. Yet despite the wide distribution of warning signals, they are relatively scarce as a proportion of the total prey available, and more so in some biomes than others. Classically, warning signals are thought to be governed by positive densitydependent selection, i.e. they succeed better when they are more common. Therefore, after surmounting this initial barrier to their evolution, it is puzzling that they remain uncommon on the scale of the community. Here, we explore factors likely to determine the prevalence of warning signals in prey assemblages. These factors include the nature of prey defences and any constraints upon them, the behavioural interactions of predators with different prey defences, the numerical responses of predators governed by movement and reproduction, the diversity and abundance of undefended alternative prey and Batesian mimics in the community, and variability in other ecological circumstances. We also discuss the macroevolution of warning signals. Our review finds that we have a basic understanding of how many species in some taxonomic groups have warning signals, but very little information on the interrelationships among population abundances across prey communities, the diversity of signal phenotypes, and prey defences. We also have detailed knowledge of how a few generalist predator species forage in artificial laboratory environments, but we know much less about how predators forage in complex natural communities with variable prey defences. We describe how empirical work to address each of these knowledge gaps can test specific hypotheses for why warning signals exhibit their particular patterns of distribution. This will help us to understand how behavioural interactions shape ecological communities.

Mapping the evolution of accurate Batesian mimicry of social wasps in hoverflies

“…Figure 2 and our binary analyses highlight how wasp mimicry is more of a continuous spectrum than a binary, or categorical trait, which has important implications for how future studies define mimicry. It is also important to note the majority threshold for mimicry was still passed by 52% of the hoverflies studied here, suggesting that wasp mimicry could be a much more prevalent feature of natural communities than previously estimated (22%: Gilbert 2005; Kikuchi et al 2021). Even the vaguest resemblance to a noxious or abundant model can afford protection to a mimic, perhaps because the optimal predator behavior may be to avoid risks by not sampling even poor mimics whenever possible, resulting in relaxed selection on mimetic accuracy (Gilbert 2005;Pfennig and Kikuchi 2012;Sherratt and Peet-Paré 2017).…”

Factors determining the dorsal coloration pattern of aposematic salamanders

“…In the Alps, predatory vertebrates tend to attain lower abundances at high altitudes 52 , and in absence of direct quantitative data on the abundance of predators, altitude can be considered as an additional proxy of overall variation of biotic communities (i.e., lower predator abundance and prey availability). Although a recent study showed that the relative size of the yellow pattern affects the probability of being predated, with less predation on individuals with more yellow coverage 30 there is no evidence that as yellow increases also the toxicity of the animal raises 53 . Unfortunately, extensive, and accurate data on predation pressures on fire salamander population are missing, and our measures of predator presence were rough proxies, which were far to be comprehensive and likely dampened the robustness of predator effect assessment.…”

Chemical defense acquired via pharmacophagy can lead to protection from predation for conspecifics in a sawfly

“…This phenomenon is often seen in combination with automimicry, wherein undefended individuals (mimics) benefit from the unpalatability of defended individuals (models) [17,18]. Furthermore, unpalatability or unprofitability of organisms can be associated with bright or aposematic coloration that functions as a warning signal to predators [19,20]. There is usually positive density-dependence in aposematism, such that conspicuous warning signals are more effective when they are common [20,21].…”