Trait-by-environment interactions determine facilitation of rocky shore biodiversity by a habitat-forming algaMacroalgal Traits

Sigrid Iredell¹, Nina Schaefer¹, Melanie Bishop¹

¹Macquarie University, Australia

The frequency and intensity of extreme heat events are increasing, threatening the survival of rocky intertidal species already living near their thermal tolerance limits. Habitat-forming intertidal seaweeds, such as the fucoid Hormosira banksii, can provide cool and moist microclimates beneath their canopy that can protect turfing algae and invertebrates from extreme heat and desiccation at low tide. The capacity of habitat-forming seaweeds to provide protective microclimates may vary according to the interaction of local environmental conditions with their structural traits, such as density, thallus length, and vesicle size – which may display considerable intraspecific variation. This study assessed spatial variation across 9 sites, spanning 3 bioregions in (1) structural traits of H. banksii, (2) microclimates provided by H. banksii, and (3) facilitation of intertidal biodiversity by H. banksii. Additionally, through a cropping experiment replicated at 6 sites spanning 3 bioregions, this study assessed how thallus length of H. banksii interacted with local environmental conditions to influence persistence of associated species. We expected structural traits of H. banksii to vary across bioregions and, in combination with environmental variation, would lead to spatial variation in temperature amelioration by H. banksii and its facilitation of associated communities. We expected that cropping of H. banksii thalli would limit its protective function of reducing temperature extremes, leading to biodiversity loss, particularly at warmer sites. Identifying how particular structural traits of habitat-forming species mitigate heat and desiccation stress is critical to designing conservation strategies for a range of intertidal communities, preserving biodiversity in a warming climate.