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Abstract Aim To assess the effect of habitat fragmentation and isolation in determining the range-size frequency distribution (RFD) of the shorefish fauna endemic to a discrete biogeographical region. Location The Tropical Eastern Pacific (TEP). Methods
Habitat isolation represents the separation between oceanic islands and the continental shore of the TEP and habitat fragmentation the degree of spatial continuity of habitats (i.e. reefs, soft bottom, nearshore waters) along the continental coast of the TEP. The effects of habitat isolation
and fragmentation were quantified by comparing the RFDs of (1) the species found on oceanic islands vs. the continental shore, and (2) species on the continental shore that use different habitat types. Results The RFD of the entire TEP fauna was bimodal, with peaks at both small-
and large-range ends of the spectrum. The small-range peak was due almost entirely to island species and the large-range peak due mainly to species found in both the continental shore and oceanic islands. RFDs varied among species using different habitats on the continental shore: reef-fishes
had a right-skewed RFD, soft-bottom species a flat RFD, and coastal-pelagic fishes a left-skewed RFD. Main conclusions Variation in dispersal capabilities associated with habitat isolation and fragmentation in the TEP appears to be the main mechanism contributing to differences
among RFD structure, although variation in tolerances arising from the dynamic regional environment may contribute to some patterns. Because diversity patterns are strongly affected by RFD structure, it is now evident that the insular and continental components of a fauna should be treated
separately when analysing such patterns. Furthermore, contrasts in RFD structure among species using different habitats demonstrate that a full understanding of the causes of diversity patterns requires analyses of complete regional faunas in relation to regional geography.