Testing the ‘abundant centre’ hypothesis on endemic reef fishes in south-western Australia

The abundance of a species is generally expected to peak at the centre of its distribution range and decline towards the range limits. Empirical evidence for such ‘abundant centre’ patterns is, however, limited, particularly for subtidal species. We tested the ‘abundant centre’ hypothesis for 8 fish species endemic to the shallow rocky reefs of south-western Australia. Fish abundances were quantified at 6 locations (18 reefs) across ca. 1700 km of coastline, and patterns of distribution were determined by testing the goodness of fit of 3 biogeographical models: (1) ‘normal’ (i.e. ‘abundant centre’ distributions), (2) ‘ramped’ (increase in abundance towards one of the distributional limits), and (3) ‘skewed normal’ (skewed ‘abundant centre’ distributions). Two species had their maximum abundances at the centre of their ranges (‘normal’ patterns), while one species had its highest abundance near its south-eastern range limit (‘skewed normal’ pattern). Two species increased progressively in abundances towards their south-eastern range limits (‘ramped’ patterns), and 3 species showed no apparent patterns across their distribution ranges. Consequently, the expectation that species are most abundant at the centre of their ranges was not supported as a general model by the present study.