Temporal shifts in key mangrove crab species linked to biotic and abiotic factors at a latitudinal range limit

Mangroves in South Africa occur at a southern latitudinal limit (33°12′51″ S, 27°34′54″ E) where distribution is influenced by global climate change. Mangroves are experiencing poleward expansion, affecting the distribution of associated macrofauna. Monitoring surveys have been conducted for South African mangrove macrofauna, with research focusing on drivers of ocypodid crabs rather than grapsoid crabs, despite the key role of grapsoids in this ecosystem. Using uni- and multivariate models, we investigated (i) changes in brachyuran community over time (2016 vs 2023), (ii) the potential biotic (food availability, brachyuran diversity and abundance) and abiotic (sediment, water and geographic parameters) factors influencing occurrence and abundance of three co-occurring mangrove crabs (Austruca occidentalis, Neosarmatium africanum and Cristarma eulimene) along the east coast of South Africa, and (iii) the potential distribution of these three species under climate change scenarios. Brachyuran community structure changed significantly over time (2016–2023) at all mangrove sites sampled (df = 1, pseudo-f = 13.95, p < 0.05). Overall, the presence and abundance of all three species were influenced by total brachyuran abundance and diversity, with latitude also influencing the occurrence of these crabs. Specifically, A. occidentalis occurrence was correlated with sediment organic matter and importance values of mangrove tree species to forest structure, while temperature, salinity and sediment moisture influenced their abundance. The presence and abundance of both sesarmids were driven by microphytobenthos and salinity, while temperature influenced abundance. Both climate change scenarios (SSP245 and SSP585) projected future shifts in distribution of all three crab species, with warmer temperatures and increased rainfall patterns expanding habitat suitability of sesarmids in a poleward direction while contracting fiddler crab ranges. This study highlights the ongoing and future impacts of climate change in South African mangroves and their associated macrofauna, suggesting a projected poleward expansion in response to climate change. Additionally, this study highlights future distributional shifts of mangrove associated crabs that can likely alter coastal ecosystem dynamics.