A few key species drive community thermophilization under experimental warming

Community thermophilization—the process by which communities are increasingly dominated by species from warmer biogeographic regions—is driven by immigration of warm-adapted species, shifts in relative abundances among resident species, and extirpation of cold-adapted species. Studies on thermophilization due to climate warming typically use observational data, with few studies using experimental data to causally attribute warming as the driver of thermophilization. Furthermore, knowledge of specific species that are key drivers of thermophilization may be especially relevant to natural resource management and restoration practices, but these key contributors are rarely identified in thermophilization studies. To address these gaps, we used plant community data from six warming experiments across the United States to assess community thermophilization, both within and across years, using the Community Temperature Index (CTI), a measure of the average thermal niche of a community. We then decomposed these CTI trends into individual species contributions to determine the top contributing species to thermophilization. We found that, on average, CTIs were higher in warmed treatments relative to ambient and, in four of the six experiments, CTIs showed a more positive trend over time under warming. In terms of species contributions, we determined that relatively few species drive the CTI changes. The Gini coefficients for species contributions, which measure inequality in a set of values, ranged from 0.49 to 0.85 across experiments, indicating high inequality—i.e., a small number of species account for most of the contribution. The identity of these species was also context-dependent: we did not find evidence for phylogenetic relatedness or shared functional traits among the top contributing species across experiments. These findings confirm warming as a driver for community thermophilization and highlight the key species responsible for the shifts in CTIs, which may be especially relevant for conservation initiatives aimed at increasing community resilience in a warmer climate.