Abstract
Parrotfish perform a variety of vital ecological functions on coral reefs, but we have little understanding of how these vary spatially as a result of inter-habitat variability in species assemblages. Here, we examine how two key ecological functions that result from parrotfish feeding, bioerosion and substrate grazing, vary between habitats over a reef scale in the central Maldives. Eight distinct habitats were delineated in early 2015, prior to the 2016 bleaching event, each supporting a unique parrotfish assemblage. Bioerosion rates varied from 0 to 0.84 ± 0.12 kg m−2 yr−1 but were highest in the coral rubble- and Pocillopora spp.-dominated habitat. Grazing pressure also varied markedly between habitats but followed a different inter-habitat pattern from that of bioerosion, with different contributing species. Total parrotfish grazing pressure ranged from 0 to ~264 ± 16% available substrate grazed yr-1 in the branching Acropora spp.-dominated habitat. Despite the importance of these functions in influencing reef-scale physical structure and ecological health, the highest rates occurred over less than 30% of the platform area. The results presented here provide new insights into within-reef variability in parrotfish ecological functions and demonstrate the importance of considering how these interact to influence reef geo-ecology.
Highlights
Coral reefs are built and shaped, both structurally and ecologically, by the organisms that inhabit them [1]
We address the following questions: (1) How does total grazing pressure and bioerosion rate vary among habitat types as a function of parrotfish assemblage? (2) What are the dominant species and size contributors to these geo-ecological functions? Empirical data on these issues are needed to understand how these ecological functions are likely to respond to ongoing environmental change [25,28,39]
Our findings demonstrate the extent to which parrotfish bioerosion rates and grazing pressure can vary across different reef habitats and that the spatial patterns of these ecological functions are not necessarily tightly coupled
Summary
Coral reefs are built and shaped, both structurally and ecologically, by the organisms that inhabit them [1]. Carbonate production (e.g., by scleractinian corals and coralline algae) and bioerosion (e.g., by fish and urchins) are especially important controls on reef growth potential and topographic complexity, thereby influencing wave energy regimes and habitat provision for many commercially important species [2,3,4,5]. Along with bioerosion, grazing by fish and urchins is important, because it impedes build-up of foliose algal biomass and conditions the composition of turf algae assemblages [6,7]. This can increase juvenile coral survival rates [8], reduce partial coral mortality and disease [9,10], and increase reef resilience [11]. There is strong evidence that parrotfish target protein-rich cyanobacteria living on and within the reef framework as their primary food source, at least in the Indo-Pacific [17,18]
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