A host-specific habitat former controls biodiversity across ecological transitions in a rocky intertidal facilitation cascade

Abstract

Few studies have quantified facilitation cascades from rocky intertidal systems, across ecological transition zones, or where the secondary facilitator is an obligate epiphyte. Here, we address these research gaps. We first quantified distributions of the seaweed host Hormosira banksii and its obligate epiphyte Notheia anomala at different tidal elevations in summer and winter at Kaikoura, New Zealand. This analysis showed that the host and the epiphyte were, in both seasons, most abundant at ‘high’ and ‘low’ tidal elevations respectively, probably driven by contrasting responses to competitors and desiccation. We subsequently quantified richness and abundances of mobile invertebrates associated with Hormosira and various levels of epiphytic Notheia. Hormosira fronds were collected from different elevations, seasons, diurnal cycles, with different neighbouring algae and following an epiphyte-removal experiment. All tests showed positive density-dependent effects of Notheia-epiphytism on richness and abundances of invertebrates, with strongest facilitation occurring at the transition from intertidal to subtidal habitats. Our results support a growing number of facilitation cascade studies from different ecosystems and habitats, and suggest that habitat formation–driven facilitation cascades may be particularly common in marine benthic systems where epibiosis can be a dominant life form.