The importance of competition as a driver of evolution remains controversial. Rare insight into this mechanism is provided by the “Great Speciators” of the Indo-Pacific. These lineages are known for their rapid evolution. Species from these lineages are often found in secondary sympatry with close relatives, with similar resource requirements. Species with such similar resource requirements are likely to compete. In competing species, traits which minimise resource competition are expected to experience positive selection. This process is known as ecological character displacement (ECD). ECD typically results in a greater difference in resource-exploiting traits between competing species. Conversely, if a competitor is absent from an ecosystem, the remaining species may experience character release. Here we demonstrate cases of competitively mediated selection in “Great Speciator” lineages of South-east Sulawesi, Indonesia, the Zosterops white-eyes and Todiramphus kingfishers. The effects of competitively mediated selection were analysed by comparing the morphological niche space of populations Zosterops and Todiramphus species in the presence of a sympatric competitor, with populations where that competitor was absent. Using traits which indicated feeding niche in birds (bill morphology) and body size (wing length and weight), Bayesian ellipses were constructed to analyse differences in morphological niche space between these populations.
Results/Conclusions
Zosterops and Todiramphus species pairs were assessed at sites throughout South-east Sulawesi. Where Lemon-bellied White-eyes (Zosterops chloris) and Pale-bellied White-eyes (Zosterops consobrinorum) were found in sympatry there was strict niche partitioning, with little overlap in morphological niche space. The Pale-bellied White-eye was the stronger competitor, dominating high-quality habitats where it was present. However, at sites where Lemon-bellied White-eyes had no competitors, they showed increased morphological niche space. This is clear evidence of ecological character release, with Lemon-bellied White-eyes expanding into the morphological niche space split between the species in sympatry. Similarly, Collared Kingfishers (Todiramphus chloris) and Sacred Kingfishers (Todiramphus sanctus) strictly partition habitat on mainland Sulawesi. Collared Kingfishers are found inland while Sacred Kingfishers are found in coastal mangrove, avoiding direct competition. However, on small oceanic islands this habitat partitioning does not occur as most available habitat is coastal. Here, Collared Kingfishers show an increase in bill and body size, allowing access to larger prey than Sacred Kingfishers, partitioning resources. This reduces direct competition, a clear example of ECD. These results provide rare empirical support for the theoretical framework of competitively mediated selection and illustrate the utility of using “Great Speciator” lineages for studying how competition drives evolution.