Continental‐Scale Assessment of Climate‐Driven Marine Species Range Extensions Using a Decade of Citizen Science Data

ABSTRACT Aim Climate‐driven extensions of species distributions have serious consequences for human wellbeing and ecosystems. The recent growth of citizen science data collection represents an underutilised resource for the early detection of marine species range extensions (i.e., expansion of species' distributions at the poleward edge) that can enable proactive conservation and management. Here, we present a framework for the systematic assessment of evidence for marine species range extensions along a continent's coastlines from observations collected by different citizen science programmes. Location Australia's coastal oceans. Methods Observations of 200 marine species on a pre‐registered target list from around Australia during 2013–2022 were sourced from the citizen science databases Redmap Australia, iNaturalist, and Reef Life Survey. We established historical (circa 2012) poleward distribution limits for populations of target species and identified out‐of‐range (poleward of distribution limit) observations, which underwent expert validation. We assessed the likelihood that each species underwent range extension using a decision tree informed by citizen science observations and species traits. Results In total, 73 species (39%) were observed out‐of‐range, comprising 76 range extensions along different coastlines. Twenty‐five range extensions were assessed as high confidence, five with medium confidence, and 46 with low confidence. Range extensions were concentrated in Australia's southwest (Western Australia) and southeast (New South Wales and Tasmania), which are influenced by warm boundary currents and considered ocean warming hotspots. The mean extent of range extensions was 318 km (max. 1250 km). Main Conclusions As most (91%) range extensions identified were not previously described in the scientific literature from other data, we demonstrate that opportunistic citizen science monitoring can provide early detection of marine species range extensions at the continental scale. Given the varied consequences of range‐extending species for recipient ecosystems, effectively harnessing citizen science would critically enhance the capacity for needed targeted research and anticipatory management efforts.