Separating ecological effects of habitat fragmentation, degradation, and loss on coral commensals

Little is known of the separate effects of habitat fragmentation and degradation, which are typically confounded by habitat loss. We examined the separate effects of habitat availability, fragmentation, and degradation on the species richness and abundances of commensal species occupying coral colonies on Australia's Great Barrier Reef. Species richness, total abundance, and the abundances of the two most common commensals, a crab (Trapezia cymodoce) and a shrimp (Palaemonella sp.), increased with coral colony volume. However, relationships between coral colony volume and abundances of these two species differed. The minimum size of coral colonies inhabited by the shrimps was considerably smaller than that inhabited by the crabs. Furthermore, while abundances of these shrimps increased with colony volume, the crabs tended to occupy their hosts as male: female pairs. We manipulated two aspects of the habitat provided by these corals to their commensals. We fragmented habitats while controlling for habitat loss, and we degraded habitats by killing corals without affecting the physical structure of the habitat. Habitat degradation caused rapid declines in species richness, total abundance, and the abundances of the crab and shrimp species. While these declines were rapid they were not instantanceous, suggesting that some secondary degradation of these habitats caused these declines. Habitat fragmentation, on the other hand, had no effect on species richness, total abundance, or the abundance of the shrimp species. In contrast, abundances of trapezid crabs increased in response to fragmentation, most likely as a result of the liberation of space from territory holders that was then colonized by additional individuals. The patch sizes used in this study were considerably smaller than those used in many terrestrial fragmentation studies. Also, marine populations tend to be more open than terrestrial ones and composed of species with greater dispersal capabilities. Therefore, the application of these results to practical conservation problems will require extreme caution. Nevertheless, disentangling the effects of habitat fragmentation and degradation from habitat loss made effects of fragmentation per se visible. Because these results are novel, such effects of habitat fragmentation could be more widespread than is currently appreciated, and relevant to the development of new management tools for biological conservation.