Invertebrate communities and drivers of their composition on gravel beaches in New Zealand

Gravel beaches are discrete, irregularly separated habitats along New Zealand’s coasts. They are one of a diverse range of small, disparate, naturally rare ecosystems that tend to occur in extreme environments, and provide critical habitat for threatened, rare and endemic species. New Zealand’s gravel beaches are threatened by urbanisation, weeds, adjacent agriculture, introduced animals and predicted sea-level rise. We studied 51 gravel beaches distributed along the New Zealand coastline to provide primary information on invertebrate composition, habitat patterns and threatened species, and how these relate to national (climate), landscape (surrounding habitat type and human influences), and site-level (geomorphology, vegetation) factors. Invertebrate abundance was mostly driven by beach-scale factors with little influence of the surrounding landscape. However, urbanisation and the presence of exotic plants were significant drivers of invertebrate community composition. A number of observations of interest (i.e. rare species, new localities, habitat specialists, threatened species and exotic species with incursion risk) were also recovered from gravel beaches. Our results demonstrate that vegetation surveys are not necessarily adequate indicators of other biotic components of gravel beach ecosystems and suggest that further ecological assessments of gravel beaches are warranted.