The geographic distributions of marine species are changing rapidly, with leading range edges following climate poleward, deeper, and in other directions and trailing range edges often contracting in similar directions. These shifts have their roots in fine-scale interactions between organisms and their environment—including mosaics and gradients of temperature and oxygen—mediated by physiology, behavior, evolution, dispersal, and species interactions. These shifts reassemble food webs and can have dramatic consequences. Compared with species on land, marine species are more sensitive to changing climate but have a greater capacity for colonization. These differences suggest that species cope with climate change at different spatial scales in the two realms and that range shifts across wide spatial scales are a key mechanism at sea. Additional research is needed to understand how processes interact to promote or constrain range shifts, how the dominant responses vary among species, and how the emergent communities of the future ocean will function.

Many species in terrestrial and aquatic systems are shifting where they live in response to climate change (Lenoir & Svenning, 2015). Marine species are particularly sensitive to temperature changes associated with climate change, in part because they have evolved in the relatively stable thermal conditions characteristic of the ocean (Pinsky et al., 2019). This high sensitivity, coupled with higher dispersal potential and limited biogeographical barriers have led marine species to track isotherms poleward six times faster than their terrestrial counterparts (Lenoir et al., 2020). In addition, there is evidence that marine species are moving deeper to maintain their thermal niche (Dulvy et al., 2008;

Aim: Although species richness globally is likely to be declining, patterns in diversity at the regional scale depend on species gains within new habitats and species losses from previously inhabited areas. Our understanding of the processes associated with gains or losses remains poor, including whether these events exhibit immediate or delayed responses to environmental change. Location:The study focuses on nine temperate marine ecosystems in North America. Time period:The study period varies by region, but overall encompasses observations from 1970 to 2014.Major taxa studied: We identified regional gains and losses for 577 marine fish and invertebrate species.Methods: From a total of 166,213 sampling events from bottom trawls across North America that informed 17,997 independent observations of species gains and losses, we built generalized linear mixed effects models to test whether lagged temperature can explain instances of gains and losses of marine fishes and invertebrates in North American continental shelf habitats. Results:We found that gains were less likely in years with high seasonality, consistent with seasonal extremes as a strong constraint on species occurrence. Losses were also negatively associated with high seasonality, but the response was delayed by 3 years. Main conclusions: Environmental conditions play a role in species occupancy across diverse temperate marine ecosystems. Immediate gains paired with delayed losses can drive transient increases in species richness during times of environmental change.Identifying the dynamics behind regional species gains and losses is an important step towards prediction of biodiversity changes across ecosystems.

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