Direct and indirect effects of nursery habitats on coral-reef fish assemblages, grazing pressure and benthic dynamics

Abstract

Migrating species are common within seascapes, but the potential for these movements to alter the populations and functional roles of non-migrating species (e.g. by increasing predation) is rarely investigated. This study considers whether the presence of nursery habitats (mangroves and seagrass) simply enhances the abundance of nursery-using parrotfishes and piscivores on nearby coral reefs, or also affects other parrotfishes. Data from 131 reef sites and multiple seascape configurations across 13 degrees of latitude were used to model correlations between biophysical variables, including nursery habitat connectivity, and the abundance and grazing pressure of both nursery-using species and other parrotfishes and piscivore biomass. Connectivity to mangroves and dense seagrass was positively correlated with the biomass of nursery-using species, but was also negatively correlated with non-nursery parrotfish populations. This reduction may be caused indirectly by nursery habitats increasing confamilial competition and predation by nursery-using piscivores, particularly affecting small parrotfishes settling directly onto reefs. As key reef grazers, parrotfishes affect coral demographics. Consequently, a spatial simulation model predicted the impacts after five years of changes in grazing pressure because of nursery habitat connectivity. The model demonstrated that high nursery connectivity was correlated to changes in grazing pressure on nearby reefs that could potentially lead to differences in coral cover of ∼3–4% when compared to low connectivity reefs. However, the direction of this change depended on the seascapes’ characteristics. Historically, large-bodied, nursery-using parrotfish would have increased grazing in all nursery-rich seascapes. Overfishing means that nursery availability may have spatially variable impacts on coral cover, influencing reserve design. This study suggests that nursery availability may directly and indirectly modify an ecological process, and alter an ecological cascade (migrating species increase predator and competitor abundances, affecting other grazers and consequently corals). Therefore, elucidating the multi-species impacts of animal movements is required to better understand ecosystem functioning.