Ecological specialization and population size in a biodiversity hotspot: how rare species avoid extinction

Abstract

Species with narrow environmental niches typically have small geographic ranges. Small range size is, in turn, often associated with low local abundance. Together, these factors should mean that ecological specialists have very small total populations, putting them at high risk of extinction. But some specialized and geographically restricted species are ancient, and some ecological communities have high proportions of rare and specialized endemics. We studied niche characteristics and patterns of distribution and abundance of terrestrial vertebrates in the rainforests of the Australian Wet Tropics (AWT) to identify mechanisms by which rare species might resist extinction. We show that species with narrow environmental niches and small geographic ranges tend to have high and uniform local abundances. The compensation of geographic rarity by local abundance is exact, such that total population size in the rainforest vertebrates of the AWT is independent of environmental specialization. This effect would tend to help equalize extinction risk for specialists and generalists. Phylogenetic analysis suggests that environmental specialists have been gradually accumulating in this fauna, indicating that small range size/environmental specialization can be a successful trait as long as it is compensated for by demographic commonness. These results provide an explanation of how range-restricted specialists can persist for long periods, so that they now form a major component of high-diversity assemblages such as the AWT.

Fig. 1.

Relationships of environmental specialization to geographic range size (A) and local abundance (B) among rainforest vertebrate species of the AWT. Specialization is measured as the first principal component from a factor analysis of three primary measures of niche dimensions (climate specialization, climate marginality, and vegetation specialization). Range size (ha) and abundance (/ha) are measured as residuals after controlling for effects of phylogeny (see Methods and Dataset S1).

Fig. 2.

Total population sizes of rainforest vertebrates in the AWT bioregion. (A) Distribution of total population size for 159 vertebrate species in rainforests of the AWT. The bars are empirical data, and the line shows a normal fit to these data; the empirical distribution departs only slightly from lognormal (Shapiro–Wilk W = 0.9699; P = 0.044). (B) Total population sizes of vertebrate species in relation to the degree of environmental specialization as measured by values on the first principle component derived from a factor analysis of climate specialization, climate marginality, and vegetation specialization.

Fig. 3.

Relationship of abundance to range size of vertebrates in the AWT. Symbols distinguish species that are restricted to the rainforest and associated wet sclerophyll forest (filled circles) and forest generalists (open circles). The relationship is significant only in the first category of species (F_1,93 = 11.57, P < 0.001). Range size (ha, × 100,000) and (log10) abundance (per ha) are measured as residuals after controlling for effects of phylogeny (see Methods).

Fig. 4.

Distribution of specialization over the phylogeny of AWT vertebrates. The proportion of lineages composed of specialists is shown from the root (rank 0) to the tips of the phylogeny. Species were classified as specialists if they were above the median value of the specialization index used in Figs. 1 and 2. “Rank” refers to the number of branch points away from the root and was used as the metric for the analysis because we did not have information on branch lengths or a time scale.