Top-down control disrupts facilitation cascade in a mangrove ecosystem

Background/Question/Methods

Facilitation cascades refer to the indirect positive effect of one species on another that is mediated by an intermediate facilitator. While this recently described concept has already been shown to have important effects on biodiversity, species invasions, species abundance, and habitat structure, we know little about the factors that predict when facilitation cascades should be important. Mangroves are a classic example of a biogenic habitat created by the ecosystem engineering of a foundation species exemplified by numerous well-described positive associations. We made preliminary observations of strong and predictable patterns of spatial variation in the occurrence of a facilitation cascade in a Caribbean mangroves ecosystem that involved mangrove roots, oysters, and a suite of sessile invertebrates. We conducted surveys to quantify those patterns, and several experiments to elucidate the factors that control where the facilitation cascade occurs. 

Results/Conclusions

We found that a facilitation cascade in which mangroves roots provide attachment substrate for oysters which in turn create habitat for a diverse suite of sessile invertebrates was consistently observed outside semi-enclosed pond areas, whereas roots were frequently bare of epifauna inside ponds. Our experiments to examine the relative importance of physical and biological controls that could explain the spatial limitation of the mangrove facilitation cascade revealed that predators eliminated bivalves on roots transplanted inside ponds and precluded the existence of the epifaunal community in those areas. The facilitation cascade remained on roots transplanted inside ponds only if they were protected by predator exclusion cages, suggesting that physical factors are unimportant in limiting the occurrence of facilitation cascades in this mangrove ecosystem. Our results reveal that top-down control by consumers can have strong impacts on biodiversity and community structure by determining where facilitation cascades occur.