Land-use changes influence climate resilience through altered population demography in a social insect

Biodiversity is threatened by both climate and land-use change. However, the synergistic impacts of these stressors and the underlying mechanisms remain poorly understood. This study seeks to bridge this knowledge gap by testing two competing hypotheses regarding the concept of the realized thermal niche. The Fixed Niche Breadth hypothesis suggests that a species' thermal niche remains constant despite fluctuations in population density resulting from land-use changes. This hypothesis links habitat loss directly to a reduced availability of suitable climate. Conversely, the Habitat Loss-Allee Effect hypothesis posits that land-use changes narrow the realized thermal niche by lowering population densities, which impairs individual fitness in unfavorable temperatures due to the Allee effect—the positive impact of higher population density on individual fitness. To investigate these hypotheses, we developed an individual-based model that integrates the Allee effect to examine how climate and land-use changes affect population density and the thermal niche in social organisms. We empirically tested our model predictions by studying the distribution and cooperative behavior of burying beetles (Nicrophorus nepalensis), which compete with blowflies for carrion resources, along two elevational gradients in Taiwan. These gradients serve as temperature gradients, one in an intact forest and the other in a human-altered landscape with substantial forest loss. Our results support the model predictions and show that landscape forest loss reduces beetle population densities and disrupts their dispersal dynamics, resulting in smaller cooperative groups. This, in turn, limits the beetles' ability to compete with blowflies in warmer environments, resulting in a contraction of the realized thermal niche. Together, our findings support the Habitat Loss-Allee Effect hypothesis while rejecting the Fixed Niche Breadth hypothesis. By highlighting the effects of habitat loss and fragmentation on both intra- and interspecific social interactions, our study improves understanding of species' vulnerability to the combined threats of climate and land-use change. Ultimately, our results underscore the importance of considering the demographic and behavioral consequences of land-use change when assessing species' vulnerability to climate-land-use synergies.