Thermophilization: Concepts, Methods, and Implications of Changes in Community Composition due to Global Warming

With global warming, many species will shift their geographic ranges coldward (i.e., towards higher latitudes and elevations) to remain within suitable thermal conditions. At the community level, these range shifts result in "thermophilization"-the increasing relative abundance of warm-adapted ("thermophilic") species and the concomitant decrease of cold-adapted species over time. Quantifying rates of thermophilization provides a powerful approach to assessing responses to rising temperatures at the community level, particularly in high-diversity systems where the detailed spatial data needed to calculate species-level responses to warming for thousands of individual species are often unavailable. This review synthesizes our current understanding of thermophilization across terrestrial, marine, and aquatic systems. We detail the most common methods used to quantify thermophilization, primarily through the calculation of the Community Temperature Index (CTI, °C), which aggregates species' thermal affiliations into a single community-level metric that can then be tracked through time to calculate an annualized thermophilization rate (TR, °C year^-1). We evaluate the advantages of this approach, such as its ability to partition the contributions of underlying demographic processes (i.e., recruitment vs. growth vs. mortality, or colonization vs. extinction) and/or individual species and species groups, its flexibility in using diverse data sources like herbarium records and citizen science, and its ability to test relationships between TR and site-specific conditions such as elevation, landcover, and anthropogenic disturbances. We also discuss some of the standing challenges and limitations of the thermophilization approach, including a lack of standardization that hinders large-scale synthetic analyses, the challenge of propagating estimation errors from the species- to community-level, and difficulties in interpreting and communicating results. We then discuss the implications of thermophilization for ecosystem stability and function, including the potential for biodiversity losses and altered ecosystem services. Finally, we highlight several important and exciting avenues for future research on climate-driven shifts in community composition.