Abstract
Adaptations for efficient performance are expected to shape animal morphology based on selection for microhabitat use and ecological forces. The presence of competitor species is predicted to cause niches to contract and enhance trait divergence. Therefore, increased species richness is expected to lead to greater trait divergence, and to result in reduced overlap and similarity between morphologies of sympatric species. We examined patterns of morphospace occupancy and partitioning in the skink fauna of New Guinea, the world's largest tropical island. Because skink species richness is largely decoupled from elevation in New Guinea, we could examine the effects of both factors (as proxies for competition and abiotic conditions), on morphospace occupancy and partitioning. We measured 1,860 specimens from 79 species of skinks throughout Papua New Guinea, and examined their morphospace occupancy in a spatial context. We calculated, for each assemblage within equal-area cells, the volume of morphospace occupied by all skinks, the mean volume occupied per species, and the mean distance and overlap between all species pairs. We then examined whether these metrics are related to species richness and elevation. Elevation is a stronger predictor of morphospace occupancy than species richness. As elevation increases, intraspecific variation decreases and morphologies become more similar to each other such that overall morphospace occupancy decreases. Highland skinks are, on average, smaller, thinner and shorter limbed than lowland species. We hypothesise that harsh climates in the New Guinea highland habitats impose strong selection on skinks to occupy specific areas of morphospace that facilitate efficient thermoregulation in suboptimal thermal conditions. We conclude that the effect of competition on trait divergence on a community and assemblage scale is eclipsed by abiotic selection pressures in these harsh environments.
Highlights
Niche partitioning is among the most fundamental processes in ecology, and is a major force generating phenotypic diversity
Because skink species richness is largely decoupled from elevation in New Guinea, we could examine the effects of both factors, on morphospace occupancy and partitioning
We examined morphospace occupancy and partitioning in the skink fauna of Papua New Guinea (PNG)
Summary
Niche partitioning is among the most fundamental processes in ecology, and is a major force generating phenotypic diversity. Competition between taxa occupying similar niches could be reduced via partitioning of microhabitat use, activity times and diet—all of which are thought to manifest in morphological adaptation (Brown & Wilson, 1956; Grant & Grant, 2006; Pianka, 1974; Slatkin, 1980). That said, such community-wide character displacement is not always manifested (Meiri et al, 2011; Simberloff & Boecklen, 1981; Stuart & Losos, 2013), the importance of competition in shaping community structure has long been contested (Connell, 1983), and the degree to which competition-driven processes are more important than abiotic selective pressures is still unclear. Habitat filtering will result in sympatric species being more similar to each other than to allopatric ones
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