Habitat-forming species as microclimatic refuges for intertidal organisms-a review and meta-analysis

The persistence of species in stressful environments can be dependent on the availability of microhabitat refuges. On intertidal rocky shores, habitat-forming species, such as seaweeds and bivalves, can provide thermally stable and moist microhabitats that protect inhabitants from environmental extremes. It is, however, unclear how the protective functions of intertidal habitat-forming species vary among groups of habitat-forming species or across environmental gradients (e.g., shore height, latitude). We undertook a systematic review and meta-analysis of existing literature on effects of intertidal habitat-forming species (seaweeds, bivalves, barnacles, polychaetes) on microclimate (temperature, moisture, humidity), comparing effect sizes across taxonomic groups and environmental settings (temperature only). We identified 30 studies assessing effects of habitat-forming species on microclimates, including 29 studies measuring temperature, 6 measuring substrate moisture, and 3 air humidity. Studies were predominantly from mid-intertidal shore heights and temperate and sub-tropical regions, with only a single study from the tropics. The meta-analysis (based on 15 studies) showed that habitat-forming species significantly reduced maximum temperatures compared to bare plots, and that this effect size was similar between seaweeds and bivalves. There was a trend for greater effect sizes high on the shore, and at lower latitudes. Major gaps in the functional groups of habitat-formers and locations studied were found with limited identification of sources of variation across habitat-former effects. Advancing our understanding of how habitat-forming species shape microclimates across different environmental settings is critical to informing climate change adaptation strategies for biodiversity.