Intermediate habitat fragmentation buffers droughts: How individual energy dynamics mediate mammal community response to stressors

Biodiversity is threatened by land-use and climate change. Although these processes are known to influence species survival and diversity, predicting their combined effects on communities remains challenging. We here aim to disentangle the combined effects of drought-induced resource shortage and habitat fragmentation on species coexistence. To understand how both fragmentation and droughts affect individual movement and physiology, and ultimately influence population and community dynamics, we use an individual-based metabolic modelling approach to simulate a community of small mammals. Individuals forage in the landscape to ingest energy, which they then allocate to basal maintenance, digestion, locomotion, growth, reproduction, and storage. If individuals of several species are able to balance their energy intake and needs, and additionally store energy as fat reserve, they may overcome stress periods and coexist. We find that species recover best after a drought when they live in moderately fragmented landscapes compared to those with low or high fragmentation. In low fragmented landscapes, high local competition during resource shortages is problematic, while in highly fragmented landscapes, low energy balance and storage often lead to high mortality during drought. Intermediately fragmented landscapes balance these effects and show the least impact of droughts on species richness, a pattern that holds also when integrating observed drought time series from monitoring data in the model simulations. Due to the interacting negative impacts, we suggest that with ongoing global change, it is increasingly important to understand stressors simultaneously to identify measures that support species coexistence and biodiversity. Including individual energy dynamics allowed us to conflate the different global change effects through energy storage and energy allocation to different processes. Our presented community model, which integrates metabolic and behavioural reactions of individuals to different stressors and scales them to the community level, offers valuable insights with great potential to support nature conservation.

Methods
In this study, we used a relatively novel dynamic individual-based metabolic simulation model for a mammal community (see also https://doi.org/10.5061/dryad.4qrfj6qjf). The model is based on allometric relationships and movement in home ranges, allowing for a variety of species. The model was thoroughly tested and validated using real-world patterns from the literature. An extensive model development description is available in the format of a TRACE document with the publication. We used the model to simulate scenarios of different habitat fragmentation in combination with drought events and analyzed the resulting data.