Cumulative stressors impact macrofauna differentially according to sandy beach type: A meta-analysis

Many studies have demonstrated human impacts on sandy beach macroinvertebrates. However, little is known about causative drivers among multiple co-occurring stressors and how these interact with natural habitat conditions to yield specific faunal responses. We performed a global meta-analysis to shed light on how interactions between human disturbances and beach morphodynamics shape macroinvertebrate populations. We found that supralittoral forms (Talitridae and Ocypodidae) responded more negatively to the Human Modification Metric (a proxy for urbanization) on dissipative beaches, whereas intertidal organisms (Hippidae and Cirolanidae) showed more negative responses on non-dissipative beaches. Based on these findings we propose the Cumulative Harshness Hypothesis (CHH), which predicts higher sensitivity of beach macroinvertebrates to human disturbances when inhabiting a harsher physical environment according to their life histories. Secondly, we compared the response of macroinvertebrates to urbanization levels from local to larger scales (from 500 to 50000 m). Supralittoral families responded more negatively to local urbanization, which leads to habitat loss due to removal or reduction of upper beach zones. Conversely, intertidal organisms with planktonic larval stages were more affected by urbanization at the largest spatial scales, which we hypothesize disrupts metapopulation dynamics by impacting the supply of larvae that could colonize human-disturbed beaches. The differential effects of human disturbances on macroinvertebrates according to beach morphodynamics suggest that the efficiency of these ecological indicators for beach monitoring is context-dependent. Focusing on multiple stressors rather than on a single one is also critical to mitigate human impacts on these threatened ecosystems.