Hierarchical drivers of cryptic biodiversity on coral reefs

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.