One potential biological explanation for this pervasive pattern is that evolutionary radiations are facilitated by ecological opportunity (Schluter 2000
), whereby speciation is most likely when resources are abundant and potential competitors are scarce. As a radiation progresses, ecological ‘niche space’ becomes increasingly saturated, resulting in fewer opportunities for speciation (Walker & Valentine 1984
; Valentine 1985
). Under such a model, speciation rates are predicted to show density dependence (Nee et al. 1992
), because the rise in species diversity through time would be mirrored by a corresponding decline in the speciation rate. In a meta-analysis of 45 avian radiations, Phillimore & Price (2008)
found widespread evidence for temporal slowdowns in diversification rates and speculated that ecological constraints on clade growth resulted in density-dependent speciation. Weir (2006)
reported that neotropical avian clades with greater numbers of sympatric species showed more pronounced declines in diversification rates through time, which suggests that clade diversity within a particular ecological or biogeographic theatre might itself limit the rate of clade growth. Although these studies are consistent with density-dependent diversification, there have been no formal tests of the process in any evolutionary radiation, because methods have not been available that can discriminate between density dependence and other processes that might also result in temporal declines in diversification rates.
Here, we develop a novel conceptual framework for testing whether diversification rates show density dependence, and we explore the role of this process during the radiation of continental North American wood warblers (Parulidae) in the speciose genus Dendroica
warblers are an ecologically appropriate group in which to test for density dependence diversification, as this process is most likely to be driven by interspecific competitive interactions. Dendroica
species diversity is high in many local North American assemblages, but the composition of those assemblages is variable among sites (Lovette & Hochachka 2006
). The matrix of potential species interactions is even more complex when integrated across the history of this group, as most Dendroica
species have persisted through climate cycles that would have further scrambled their geographical ranges and spatial associations. These warblers are a classic example of behavioural niche differentiation, in which co-occurring species differ in subtle aspects of their foraging and breeding behaviour (MacArthur 1958
; Price et al. 2000
). Our previous studies based on a time-calibrated mitochondrial DNA (mtDNA) phylogeny found that the Dendroica
group underwent an explosive burst of diversification early in its history, followed by a pronounced decline in the rate of lineage accumulation (Lovette & Bermingham 1999
). Taken together, these observations suggest the possibility that broad-scale patterns of diversification in the Dendroica
warblers might be related to ecological interactions among species across evolutionary time-scales.
Our statistical approach extends the birth–death model that has been used previously for inference on diversification rates (Nee et al. 1994
; Barraclough & Vogler 2002
; Nee 2006
; Rabosky 2006b
) to speciation rates that vary continuously through time. We apply the method to an improved phylogeny of Dendroica
warblers that is complete at the species level and which is based on both mtDNA and nuclear sequence loci. Our results indicate that the observed pattern of speciation in Dendroica
is best approximated by a density-dependent diversification process and suggest that ecological interactions among species can leave an imprint on evolutionary history that can be reconstructed from molecular phylogenies alone.