Reproductive interference explains persistence of aggression between species

Abstract

Interspecific territoriality occurs when individuals of different species fight over space, and may arise spontaneously when populations of closely related territorial species first come into contact. But defence of space is costly, and unless the benefits of excluding heterospecifics exceed the costs, natural selection should favour divergence in competitor recognition until the species no longer interact aggressively. Ordinarily males of different species do not compete for mates, but when males cannot distinguish females of sympatric species, females may effectively become a shared resource. We model how reproductive interference caused by undiscriminating males can prevent interspecific divergence, or even cause convergence, in traits used to recognize competitors. We then test the model in a genus of visually orienting insects and show that, as predicted by the model, differences between species pairs in the level of reproductive interference, which is causally related to species differences in female coloration, are strongly predictive of the current level of interspecific aggression. Interspecific reproductive interference is very common and we discuss how it may account for the persistence of interspecific aggression in many taxonomic groups.

Causes and Consequences of Behavioral Interference between Species

“…Despite their connections, aggressive interference and reproductive interference (see Glossary) have largely been studied by different researchers in relation to different theoretical frameworks [2,4,7] and in different study systems [3], even though many closely related species interfere with each other in both ways (see Table S1 in [8]). We do not believe that these two categories of interspecific interactions should be synonymized, because this would obscure important differences between them.…”

“…), species differences in female coloration directly affect reproductive interference because males use coloration to distinguish between conspecific and heterospecific females [8]. Variation in reproductive interference among species pairs strongly predicts levels of aggressive (male-male) interference in this genus, in accordance with the hypothesis that interspecific mate competition maintains interspecific territoriality [8]. Macroevolution, Macroecology, and Community Ecology Most research on behavioral interference has focused on how local conditions affect pairs or small groups of related species, yet interference can potentially shape biodiversity over large spatial and temporal scales.…”

“…Evolved responses to aggressive interference are classified as agonistic character displacement (ACD; Figure 1) [1]. ACD can cause species to diverge or converge phenotypically, depending on whether they compete exploitatively for resources (or mates) at a sufficient level for interspecific resource (or mate) defense to be adaptive [1,8]. In theory, both RCD and divergent ACD (ACDd) can drive species apart in signals and signal perception ('species recognition' traits), habitat preferences, and temporal activity patterns, including the timing of migration and breeding [1,4,27].…”

“…Finally, learning that increases assortative mating between partially diverged populations can contribute to speciation [90,93,96,98], whereas learning that contributes to stochasticity in mate choice would erode assortative mating and promote gene flow that could limit adaptive divergence and speciation, such as might occur when mate choice is flexible [99]. [1,8]. Regardless of whether the outcome is stasis maintained by selection or convergence, the process is classified as convergent ACD (ACDc; Figure 1) [1,7].…”

“…Both types of behavioral interference (Box 1) have been hypothesized to: (i) arise as a byproduct of intraspecific interactions (Box 2); (ii) cause local extinction as well as temporal and spatial habitat partitioning; (iii) prevent species from coexisting that otherwise would be expected to coexist; (iv) enable coexistence between species that otherwise would not be expected to coexist; (v) promote or prevent species range shifts and the spread of invasive species; (vi) cause sympatric species to diverge or converge through character displacement processes; (vii) cause populations within a species to diverge from each other due to character displacement in areas of sympatry; and (viii) contribute to reproductive isolation and speciation ( Figure 1). Despite their connections, aggressive interference and reproductive interference (see Glossary) have largely been studied by different researchers in relation to different theoretical frameworks [2,4,7] and in different study systems [3], even though many closely related species interfere with each other in both ways (see Table S1 in [8]). We do not believe that these two categories of interspecific interactions should be synonymized, because this would obscure important differences between them.…”