The emergence of an alternative phenotype following species loss: ecological, behavioral, and physiological evidence from spotted antbirds

Abstract

Current loss of species will make ecological processes increasingly dependent on how remaining species respond to the change in opportunities caused by local and global 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. Evolutionary implications of extinctions for leftover species, however, have largely been ignored. They can now be studied by taking advantage of detailed natural history data collected decades ago when field ethology was more widely practiced. I have explored the impact of species loss in a guild of ant following birds that was extensively studied in the 1960s prior to the loss of their dominant competitor. Here, I show that although one guild survivor, the bicolored antbird (Gymnopithys leucaspis), did not change its trophic behavior, individuals of another survivor, the smaller spotted antbird (Hylophylax naevioides), adopted new behaviors allowing them to exploit foraging opportunities once monopolized by the ocellated antbird (Phaenostictus mcleananni), the former dominant. In order to capitalize on the rich resources of arthropods now available at ant swarms, these spotted antbirds evidently relaxed their territoriality and now roam, an alternative phenotype. The others remained territorial and only forage at ant swarms passing through their territories. As a result, spotted antbird numbers doubled, while total biomass consumption of ant-following antbirds increased beyond pre-extinction levels. Roaming appears to be a good strategy: these individuals had lower circulating levels of corticosterone and produced more young. Territorial counterpart's do not appear to fledge fewer young than before, however, suggesting a stable resource polymorphism could develop between roamers and territorials. Such intraspecific diversification can precede and facilitate speciation. Behavioral changes in light of opportunities that arise from the loss of competitors should thus be considered an important diversifying process and one that is particularly relevant in today's changing world.