Teasing apart fine- and coarse-scale effects of environmental heterogeneity on tree species richness in Europe

The environmental heterogeneity–biodiversity relationship is generally hypothesised to be positive, with greater heterogeneity leading to greater biodiversity. However, the generality of positive environmental heterogeneity–species richness relationships is often debated, with some studies finding non-significant or even negative relationships. Negative relationships have primarily been found at fine spatial scales. Both negative and positive relationships have a basis in ecological theory. Environmental heterogeneity at coarse scales opens up niche space to allow more species to coexist; whereas high local heterogeneity, for instance in topography, may lead to increased local extinction due to micro-fragmentation, or dominance of species suited to heterogeneous conditions. However, it is difficult to attribute how much of the variance is explained at different scales within the same modelling framework.

Here, we use a new data-aggregation method which enables us to include both fine- and coarse-scale environmental heterogeneity within the same analysis. Using this method, we were able to tease apart the fine- and coarse-grain effects of topographic heterogeneity on European tree species richness. At the coarse scale (0.5 degrees), we found a positive effect of range in elevation on tree species richness. However, when measuring range in elevation using a fine-scale moving window of radius 500 m, we found a negative relationship with tree species richness. This supports existing research that has shown negative relationships between environmental heterogeneity and species richness at finer spatial grains. Because we were able to include a measure of both local and landscape-scale topographic heterogeneity in the same model, for the first time we could fully capture the effects of both scales on coarse-grain species richness while accounting for the effect of the other scale.