Abstract
Species-specific enemies may promote prey coexistence through negative distance- and density-dependent survival of juveniles near conspecific adults. We tested this mechanism by transplanting juvenile-sized fragments of the brooding corals Pocillopora damicornis and Seriatopora hystrix 3, 12, 24 and 182 cm up- and down-current of conspecific adults and monitoring their survival and condition over time. We also characterized the spatial distribution of P. damicornis and S. hystrix within replicate plots on three Fijian reef flats and measured the distribution of small colonies within 2 m of larger colonies of each species. Juvenile-sized transplants exhibited no differences in survivorship as a function of distance from adult P. damicornis or S. hystrix. Additionally, both P. damicornis and S. hystrix were aggregated rather than overdispersed on natural reefs. However, a pattern of juveniles being aggregated near adults while larger (and probably older) colonies were not suggests that greater mortality near large adults could occur over longer periods of time or that size-dependent mortality was occurring. While we found minimal evidence of greater mortality of small colonies near adult conspecifics in our transplant experiments, we did document hot-spots of species-specific corallivory. We detected spatially localized and temporally persistent predation on P. damicornis by the territorial triggerfish Balistapus undulatus. This patchy predation did not occur for S. hystrix. This variable selective regime in an otherwise more uniform environment could be one mechanism maintaining diversity of corals on Indo-Pacific reefs.
Highlights
The processes maintaining high numbers of species in tropical rainforests and coral reefs have long been investigated (Connell, 1978)
We focused on two brooding coral species (Seriatopora hystrix and Pocillopora damicornis) whose planulae recruit over short distances, the latter of which is known to be a preferred prey for some coral consumers (Neudecker, 1979)
Bleaching (47 and 46 fragments out of 320 for P. damicornis and S. hystrix, respectively) of neither species was affected by distance or direction (Figs. 2C and 2D)
Summary
The processes maintaining high numbers of species in tropical rainforests and coral reefs have long been investigated (Connell, 1978). Some tree species may experience Janzen–Connell effects (Johnson et al, 2012) but the generality of the pattern has been difficult to document (Hyatt et al, 2003). In part, this may be because numerous other processes (habitat heterogeneity, spatial patterns of competitors, etc.) could obscure Janzen–Connell effects. This may be because numerous other processes (habitat heterogeneity, spatial patterns of competitors, etc.) could obscure Janzen–Connell effects This makes experimental tests difficult in field settings, especially when spatial scales over which they may be relevant are unclear
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