Trait correlates of distribution trends in the Odonata of Britain and Ireland

Abstract

A major challenge in ecology is understanding why certain species persist, while others decline, in response to environmental change. Trait-based comparative analyses are useful in this regard as they can help identify the key drivers of decline, and highlight traits that promote resistance to change. Despite their popularity trait-based comparative analyses tend to focus on explaining variation in range shift and extinction risk, seldom being applied to actual measures of species decline. Furthermore they have tended to be taxonomically restricted to birds, mammals, plants and butterflies. Here we utilise a novel approach to estimate occurrence trends for the Odonata in Britain and Ireland, and examine trait correlates of these trends using a recently available trait dataset. We found the dragonfly fauna in Britain and Ireland has undergone considerable change between 1980 and 2012, with 22 and 53% of species declining and increasing, respectively. Distribution region, habitat specialism and range size were the key traits associated with these trends, where habitat generalists that occupy southern Britain tend to have increased in comparison to the declining narrow-ranged specialist species. In combination with previous evidence, we conclude that the lower trend estimates for the narrow-ranged specialists could be a sign of biotic homogenization with ecological specialists being replaced by warm-adapted generalists.

Keywords: Climate change; Comparative analysis; Dragonfly; Range change; Species characteristics.

Figure 1. The distribution and density of sites (monads) from which the trend estimates were derived.

The shading represents the number of unique sites within the hectad that were included in the analysis.

Figure 2. The mean and 95 percentiles of the trait coefficients across 10,000 model iterations.

Each categorical variable had a reference category which had a parameter estimate set to 0. The reference categories were as follows: region, “southern”; breeding habitat, “lentic”; and for overwintering stage, “eggs.”