Distance-dependent survival and distribution of juvenile corals: Janzen-Connell effects do not operate on two brooding Indo-Pacific corals
Gibbs, David A.
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The Janzen-Connell hypothesis proposes that species-specific enemies promote species coexistence through distance- and density-dependent survival of offspring near conspecific adults. I tested this hypothesis experimentally by transplanting juvenile-sized fragments of two species of brooding corals varying distances from conspecific adults, and observationally by assessing the spatial distribution of those two species in the field. Small fragments (as proxies for ?6 month old juveniles) of Pocillopora damicornis and Seriatopora hystrix were transplanted 3, 12, 24 and 182 cm upstream and downstream (relative to the prevailing current) of conspecific adults and their survivorship and condition (bitten off, overgrown by algae, or bleached) checked every 1-2 d. I also characterized the spatial distribution of P. damicornis and S. hystrix within replicated plots on three Fijian reef flats and measured densities of small colonies within 2 m of larger colonies of each species. Contrary to the Janzen-Connell hypothesis, juvenile-sized transplants exhibited no differences in survivorship as a function of distance from adult P. damicornis or S. hystrix and P. damicornis and S. hystrix were aggregated rather than overdispersed on natural reefs. Survival unaffected by distance from focal colonies as well as certain recruitment processes could generate the observed aggregation. I did observe predation of P. damicornis that was spatially patchy and temporally persistent due to feeding by the territorial triggerfish Balistapus undulatus. This patchy predation did not occur for S. hystrix. Thus, I found no support for the Janzen-Connell hypothesis, but did document hot-spots of species-specific corallivory that could create variable selective regimes on an otherwise more uniform environment, and help maintain the high diversity of corals typical of Indo-Pacific reefs.