The Effect of Habitat-Forming Bryozoans on Biodiversity

Abstract

Bryozoans form complex habitat when high densities of heavily-calcified colonies grow to > 5 cm in three dimensions. Such habitat is found on hard and soft substrata; at shallow subtidal and shelf depths; where currents are strong but wave action is low; where sedimentation is low; and in polar, tropical, but most commonly temperate waters. Habitat-forming bryozoans support diverse assemblages of macroinvertebrates, but are damaged by anthropogenic activities, particularly commercial fishing of the seafloor using mobile gear. Bryozoan-generated habitat is unusually common in New Zealand waters. Presence-only habitat suitability models were developed for a suite of habitat-forming bryozoan species that occur in New Zealand. These models require testing and refining, but provide a useful tool for generating hypotheses about the historical distributions of habitat-forming bryozoans. Comparisons between the models and existing literature raised the possibility of undiscovered areas of bryozoan-generated habitat. Data describing the distribution of commercial fishing suggests that if bryozoan-generated habitat was present it is likely to have been destroyed, highlighting the need to understand how changes to habitat complexity affect benthic biodiversity. A mensurative study of macroinvertebrate assemblages associated with bryozoan-generated habitat was carried out on Otago shelf, south-eastern New Zealand. Variables describing the amount of epibenthic habitat and the diversity of sediment components were particularly important in explaining the various biodiversity metrics. Habitat complexity resulted from the growth and taphonomy of several species, particularly Cinctipora elegans. The abundance of macroinvertebrates associated with habitat-forming bryozoans drove changes in the assemblage structure (i.e. the passive sampling hypothesis of the species-area relationship, or SAR). Additionally, complex habitats increased habitat diversity, providing for specialist taxa (the habitat diversity hypothesis). No thresholds were identified in correlations between habitat complexity variables and biodiversity metrics, suggesting any reduction in complexity would negatively affect benthic biodiversity. To investigate further the relative importance of the SAR and to identify which elements of habitat structure were most important to associated fauna, PVC mimics of Cinctipora elegans were deployed on Otago shelf for six months. There were significant differences in assemblage structure and composition in relation to mimic branch density. Rarefaction and estimated total richness illustrated the strong effect of surface area on richness, but a small residual effect of habitat complexity on assemblage structure was apparent. A threshold for branch density was identified, below which actual richness and assemblage structure changed markedly. Macroinvertebrate assemblages associated with habitat-forming bryozoans on Otago shelf became more similar with increasing amounts of habitat, indicating a limited pool of mainly facultatively associated organisms, but diversity is enriched by the presence of specialists and organisms which occur only as epi- and infauna. Reconsidering the intermediate disturbance hypothesis indicated the richness of epifauna and infauna will be highest when disturbance is low and intermediate, respectively. Research necessary to support better management of bryozoan-generated habitat includes quantifying their importance as fish habitat, the effect of patch size on epi- and infaunal biodiversity and developing ecosystem indicators to allow monitoring of impacts and recovery.