Are climate-change-induced range shifts indeed so pronounced?-A modeling approach

One of the common consequences of recent climate changes are range shifts of animals and plants. Many works show poleward shifts of ranges as a consequence of global warming. However, many of these studies fail to use rigorous statistical tests. Here, we formally modeled the edges of a spatial distribution along a north-south axis and tested changes in species ranges. We modeled the spatial distribution of a species based on observed presence/absence grid-cell data, using the bounded Beta distribution. The distribution’s parameters were estimated using a likelihood optimization procedure that determined the best fitting model. Likelihood ratios were used to conduct a statistical test between the null hypothesis -no shift in range between two time frames - and alternative hypothesis - possible shift in range. We tested the model on simulated data and then applied it to data on British bird distributions in two time frames (early 1970s versus early 1990s). Our analysis gave different indications of actual shifts in distribution compared to previous works analyzing the same dataset without formal statistical modeling. This work emphasizes the importance of carefully formulated statistical models when estimating distributions of organisms in space, as is needed in testing the effects of climate change