Assessing geographical range shifts of the California Gnatcatcher in relation to multiple environmental stressors through time
Ecological niche modelling is considered a valuable tool for predicting a species shift in geographical range. However, the typical use of this tool has been to predict shifts in species’ distributions based on expected future climatic changes, and the shifts in species’ distributions that have already occurred are often ignored. Past geographical range shifts in a species’ distribution can provide valuable information on the current and future state of a species, and we argue that there is a need to understand how a species has already shifted in range and what stressors are related with such range shifts.
Our study site took place in southern California’s Western Riverside County, and our focal species included the federally threatened California Gnatcatcher. In particular, we asked the following questions 1) how are species geographical ranges shifting through time and 2) which environmental stressors are related to past geographical range shifts? We first utilized ecological niche models to describe past changes in each species’ distribution. We then examined relationships between the shifts in suitable habitat to a suite of possible environmental stressors (temperature, precipitation, land use changes, exotic species, and nitrogen deposition) using generalized linear models.
We found that both the California Gnatcatcher have lost large amounts of suitable habitat through time. Land use change and nitrogen deposition have the strongest relationship with the reduction of suitable even with the inclusion of climatic variables. Our study suggests that non-climatic environmental stressors may be just as important, if not more, as climate change to incorporate into niche models. Our study also shows that niche models can be a useful tool for understanding the influence of multiple environmental stressors, and we urge future studies to incorporate multiple known environmental stressors into niche models in order to move towards more sustainable management plans.