Janzen-Connell effects partially supported in reef-building corals: adult presence interacts with settler density to limit establishment

The Janzen-Connell hypothesis predicts increased survival of
early life stages with decreasing conspecific density and increased
distance from conspecific adults. Evidence of
Janzen-Connell effects in maintaining diversity is common in
plant communities, but its importance in sessile marine
invertebrates communities remains unclear. Under controlled
aquarium conditions, we examined the effect of density-dependence and
adult conspecific water treatments (absent/present) on propagule
settlement success and settler post-settlement survival, along with
associated spatial patterns, for six broadcast-spawning, reef-building
coral species from three families. We also tested if settlement success
was linked to increasing propagule species diversity for three coral
species from two families. We found that the probability of settlement was
density independent and not influenced by adult present water treatments
Yet, adult present water treatments and settler density did have a
synergistic negative effect on the probability of short-term settler
survival for all species examined. Settlers also showed greater spatial
aggregation as their numbers increased, but were less aggregated in adult
present water treatments compared to those in adult absent water
treatments. We further show evidence of significant species interactions
among propagules, as settlement in single-species trials was four-fold
higher compared to mixed-species trials. Our findings from controlled
experimental arenas indicate that the early establishment of corals was
predominantly limited by density-dependent settler-adult interactions
among conspecifics and propagule-propagule interactions among
heterospecifics. Thus, the proximity to established conspecific adults,
settler density and species diversity of propagules are relevant drivers
of local coral community diversity and structure. Based on these outcomes,
we suggest that the Janzen-Connell hypothesis, with demonstrated
importance for plants, is partially upheld for reef corals.