The latitudinal gradient in European ant dark diversity: patterns and potential mechanisms

Abstract

Observed richness has long been the basis for studies of species distribution patterns and community ecology, but dark diversity (i.e. the group of species in the regional pool that can live under the specific local environmental conditions but which are not part of the local species richness) has recently gained prominence for providing complementary ecological information. In this study we aim to determine the role of latitude on shaping ant dark diversity in Europe, with special attention to the effects of elevation and biotic pressure. We computed dark diversity from observed and expected richness in 415 ant communities distributed through Europe. We found that ant dark diversity in Europe shows a negative latitudinal pattern (with greater values where there are more favourable environmental conditions), and this trend is more pronounced at higher elevations. Moreover, dark diversity increases more strongly with decreasing latitude than observed richness, suggesting the existence of a larger species reservoir at southern latitudes. Our results also show that dark diversity tends to increase with biotic pressure (i.e. abundance of dominant species). This suggests an effect of behaviourally dominant ants in structuring ant communities, an effect that is hardly detected in classical studies of ant community structure. Despite the fact that different mechanisms might shape the latitudinal pattern of dark diversity, our results suggest that in southern latitudes the important role of interspecific competition is prominent, while in northern latitudes the environmental filter is more important. At the local level, biotic pressure tends to increase dark diversity, although small-scale species dispersal limitation and habitat quality may also play important roles. The analysis of dark diversity patterns in ants allowed us to better understand the mechanisms that shape ant diversity, which might not be detectable only by analyzing observed richness.