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Dryad

Hierarchical drivers of cryptic biodiversity on coral reefs

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Jun 05, 2023 version files 304.13 KB

Abstract

Declines in habitat structural complexity have marked ecological outcomes, as currently observed in many of the world’s ecosystems. Coral reefs have provided a model for such changes in marine ecosystems, but our understanding has been centred on corals and fishes at broad spatial scales when metazoan diversity on coral reefs is dominated by small cryptic taxa (herein: ‘cryptofauna’). Given the paucity of studies and high taxonomic complexity of the cryptofauna, both of which limit a priori hypotheses, we asked whether hierarchical structuring theory provides a compelling framework to impose order and quantify pattern. In general terms, we explored whether cryptic communities are sufficiently described by broad seascape parameters or limited by a set of processes operating at their distinctly nested microhabitat scale. To address this theory and gaps in knowledge for the cryptofauna, we characterised community structure in coral rubble, an eroded coral condition where biodiversity proliferates. Rubble was sampled along a depth and exposure gradient at Heron Island on the Great Barrier Reef, Australia, to parameterise environmental and morphological indicators of sessile taxa and motile cryptofauna communities. We employed a hierarchical study framework from microhabitat to seascape scales, which were evaluated using non-structured multivariate analyses and Bayesian structural equation modelling. While the non-structured analyses showed the effects of seascape on the cryptobenthos and its community, this approach overlooked the finer hierarchical patterns in rubble ecology revealed only in the structured model. Seascape parameters (exposure and depth) influenced microhabitat complexity (i.e., rubble branchiness), which determined the cover of sessile organisms on rubble pieces, which shaped the motile cryptofauna community. Rubble is likely to be increasingly prevalent on coral reefs in the Anthropocene and is typically associated with low seascape-level complexity and reduced macrofaunal richness. Parallel with hierarchical structuring theory, we show a similar response operating at the microhabitat scale whereby low rubble complexity (i.e., branchiness) reduces cryptobenthic structure, diversity and size spectra. We expect there may be an initial increase in biodiversity and trophodynamic processes derived from branching rubble, but a delay in ecosystem-scale outcomes if coral, and thus rubble, generation and complexity cannot be sustained in a future ocean.