Species loss results in territorial breakdown and emergence of alternative phenotypes in ant-following birds

Background/Question/Methods

Progressive loss of species will make ecological processes increasingly dependent on how surviving species respond to opportunities opened by these extinctions. The loss of a species can alter the costs and benefits of resource acquisition; for some – such as parasites and predators – it can mean a loss of resources while for others – such as competitors – it can mean ecological opportunity open for exploitation.  Ecological opportunity has been considered to be one of the prerequisites for adaptive radiations. Yet, we know little about the processes involved, or the degree to which diversification can occur in the face of new ecological opportunities in ‘saturated’ communities. Studying how remaining species respond to the loss of a dominant competitor may shed light on some of these processes. This approach was once difficult in the majority of vertebrate systems due to the long time lag for responses of surviving species to occur. Here, we capitalize on one of the many systems studied decades ago that has since lost a dominant competitor: a guild of ant-following birds. By comparing current day behaviors of remaining species in the absence of their dominant competitor, to (i) historical data prior to the loss of the dominant competitor, and (ii) to a nearby site where the dominant competitor remains as a reference, we have been able to draw inferences to the role of competition and ecological opportunity on the expression of diversity within populations.  

Results/Conclusions

Specifically, we show the emergence of an alternative phenotype in subordinate spotted antbirds (Hylophylax naevioides) associated with exploitation of the ecological opportunities opened by the extinction of the socially dominant ocellated antbird (Phaenostictus mcleananni). Some spotted antbirds relaxed their traditional territoriality and now roam in search of ant swarms, rich feeding sites. Traditionally, ocellated antbirds exclude spotted antbirds from profitable foraging locations at ant swarms. Roaming birds fledged more young than territorial individuals, suggesting that this alternative phenotype is profitable. However, territorial spotted antbirds in sympatry with roaming spotted antbirds do not appear to do worse than before; further diversification could result in a resource polymorphism between roaming and territorial individuals. We conclude that species loss that results in ecological opportunity, enhancing the success of alternative phenotypes, may be an important driver of diversification in surviving species. Further, the study of such quasi-experiments can shed light on the processes involved in adaptive radiations.