Niches, body sizes, and the disassembly of mammal communities on the Sunda Shelf islands

Abstract

The rising sea level at the end of the Pleistocene that created the islands of the Sunda Shelf in Indonesia and Malaysia provides a natural experiment in community disassembly and offers insights into the effects of body size and niches on abundance, distribution, and diversity. Since isolation, terrestrial mammal communities of these islands have been reduced by extinction, with virtually no offsetting colonization. We document three empirical patterns of disassembly, all of which are significantly different from null models of random assembly: (i) a diversity-area relationship: the number of taxa is strongly and positively correlated with island area; (ii) nested subset composition: species that occur on small islands tend to be subsets of more diverse communities inhabiting larger islands; and (iii) body size distributions: species of intermediate body sizes occur on the greatest number of islands, and smaller islands have smaller ranges of body sizes, caused by the absence of species of both very large and extremely small size. These patterns reveal the role of body size and other niche characteristics, such as habitat requirements and trophic status, in the differential susceptibility of taxa to extinction.

Fig. 1.

Diversity-area relationships for nonvolant, terrestrial mammals of the Sunda Shelf islands for species (A) and genera (B). Lines are ordinary least-squares regressions fitted to log-transformed data. Statistics for the resulting power laws are given.

Fig. 2.

The nested subset structure of the faunas of the Sunda Shelf islands as revealed by ordered presence–absence matrixes. (A and C) Presence–absence of taxa was ordered vertically by number of occurrences and horizontally by island area for species (A) and genera (C). (B and D) We plotted the associated body size distributions of species (B) and genera (D) having differing frequencies of islands occurrences. Each island and taxon is labeled by a code that references data in Dataset S1. Bo, Borneo; Su, Sumatra; Ja, Java; Bk, Bangka; Na, Natuna Besar; Bg, Banggi; Je, Jemaja; Ka, Karimata Besar; Ti, Tioman; Sa, Siantan; Sr, Sirhassan; Re, Redang; Pn, Penebangan; Pr, Perhentian Besar.

Fig. 3.

Frequency distributions of body sizes of species and genera of the Sunda Shelf islands. (A and C) Log-binned histograms for species and genera comprising the entire pool of species occurring on all islands. (B and D) Cumulative frequency distributions for species and genera (circles and stars) and double power law fits to these distributions using maximum-likelihood estimation (bold and gray lines). Note that the cumulative distributions plot the proportion of species larger or smaller than a given body mass, resulting in curves that cross at the median size.

Fig. 4.

Body size–incidence and body size–area relationships for mammals on the Sunda Shelf islands. (A and C) Size–incidence relationship: body mass of species and genera as functions of number of island occurrences. (B and D) Size–area relationships: body mass of species and genera as functions of island area. The bold lines are regression fits to the maximum and minimum body masses of species or genera having a given number of island occurrences (A and C) or of species or genera found on each island (B and D). The gray lines are regression fits to the second-largest and smallest body masses on each island (B and D). Note that with increasing incidence and decreasing area the body size distributions converge toward an intermediate body size in the range of 122 to 454 g.