Geographic distribution change and climatic niche change of Odonates in Great Britain

Species are largely thought to maintain broadly static niches over time, an assumption underpinning much theoretical ecology including the implementation of ecological models to project species’ current and future distributions. Here, we assess niche conservatism in odonates in Great Britain over the past six decades by simultaneously quantifying changes in species geographic distribution and evaluating temporal trends in species realised climatic niche.

Methods
These results are based on an analysis of species occurrence records for Odonates in Great Britain that were downloaded from the Global Biodiversity Information Facility and the National Biodiversity Network Atlas for the period 1961 and 2020. Odonates occupancy trends throughout this period were extracted using the Frescalo method (see Hill’s 2002 paper ‘Local frequency as a key to interpreting species occurrence data when recording effort is not known’).

Occurrences were subsequently divided into two time periods, T1 (1961-1980) and T2 (2001-2020) and species geographic distribution change as well as species climatic niche change compared between these two periods. Species geographic shift was analysed in terms of the change in the geographic centre of species’ distribution between periods – the distance and direction between species centre at T1 and T2 are provided.

Species climatic niche change was based on 19 bioclimate variables and 6 climate extreme indices, following a Principle Component Analysis. Schoener’s D index of niche overlap was calculated which provides a measurement of the overlap between the climatic niche experienced by species at T1 and T2 ranging from 0 (no overlap) to 1 (compete overlap). This index is calculated by quantifying, for each grid cell in the climate space, the difference in smoothed kernel densities of species occurrence between periods. Niche expansion gives the proportion of species’ niche present at T2 only and niche contraction, the proportion of species’ niche no longer present at T2.

Statistical tests for niche similarity between periods were used to determine whether observed climatic niches at T2 were statistically similar to observed climatic niches at T1. The hypothesis for niche conservatism was tested by comparing observed climatic niches at T1 with random simulations of species’ niches at T2 within the available climate space with the same kernel density distribution as T1. By repeating these simulations 100 times, a null distribution of species overlap values were generated to determine if observed species niche overlaps are more statistically similar (niche conservatism) or not (niche divergence) than random, based on a significance threshold of 0.05.