Assessing the sensitivity and resistance of communities in a changing environment

Assessing the sensitivity and resistance of communities in a changing environment

We propose that the ecological resilience of communities to permanent changes of the environment can be based on how variation in the overall abundance of individuals affects the number of species. Community sensitivity is defined as the ratio between the rate of change in the log expected number of species and the rate of change in the log expected number of individuals in the community. High community sensitivity means that small changes in the total abundance strongly impact the number of species. Community resistance is the proportional reduction in expected number of individuals that the community can sustain before expecting to lose one species. A small value of community resistance means that the community can only endure a small reduction in abundance before it is expected to lose one species. Based on long-term studies of four bird communities in European deciduous forests at different latitudes large differences were found in the resilience to environmental perturbations. Estimating the variance components of the species abundance distribution revealed how different processes contributed to the community sensitivity and resistance. Species heterogeneity in the population dynamics was the largest component, but its proportion varied among communities. Species-specific response to environmental fluctuations was the second major component of the variation in abundance. Estimates of community sensitivity and resistance based on data only from a single year were in general larger than those based on estimates from longer time series. Thus, our approach can provide rapid and conservative assessment of the resilience of communities to environmental changes also including only short-term data. This study shows that a general ecological mechanism, caused by increased strength of density dependence due to reduction in resource availability, can provide an intuitive measure of community resilience to environmental variation. Our analyses also illustrate the importance of including specific assumptions about how different processes affect community dynamics. For example, if stochastic fluctuations in the environment affect all species in a similar way, the sensitivity and resistance of the community to environmental changes will be different from communities in which all species show independent responses.

An intuitive approach for measuring the resilience of biological communities to environmental change

Research Highlight: Saether, B. E., Engen, S., & Solbu, E. B. (2023a). Assessing the sensitivity and resistance of communities in a changing environment. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.14003. In the face of global change, conservation strategies can be informed by understanding which biological communities are most at risk. Metrics that reflect the 'resilience' of communities to change could have great utility, but there is still no consensus on the most useful way to measure it. Saether et al. introduce an intuitive approach to thinking about and measuring resilience based on how variation in the total number of individuals within a community affects the number of species. By using dynamic species abundance distribution models, they also quantify the different sources of population-level variation that contribute to community resilience. Evenness emerges as an important predictor of resilience, with more even communities predicted to be more sensitive to abundance loss. An attractive feature of their approach is the ability to estimate the key parameters using commonly used generalized linear mixed effects models, which they illustrate with a case study on forest bird communities. The approach is ripe for comparison across different systems to explore how these proposed metrics complement existing biodiversity metrics and how they help understand the risk of communities from environmental change.