Deciphering the Patterns and Correlates of Zooplankton Functional Diversity in Mountain and Lowland Ponds

Biodiversity studies have usually been conducted considering only the taxonomic dimension of ecological communities. However, diversity measurements taking into account the functional traits of species may be more sensitive in capturing aspects of community functioning that are not apparent with the more traditional taxonomic approach. Functional diversity (either based on alpha or beta diversity levels) is, therefore, a facet of diversity that provides a link between ecosystem functioning and community structure and may be an important metric for assessing the impact of global change in the context of applied conservation schemes.

Zooplankton is a key element of lentic ecosystems due to its linkage between primary producers and secondary consumers. Here, we tried to disentangle the spatial patterns and correlates of zooplankton functional alpha and beta diversity along a relatively wide altitudinal gradient (700–2100 m a.s.l.) across central and northwestern Spain. We also identified key environmental controls of zooplankton functional alpha diversity across an extensive set of lowland (48) and mountain (28) ponds sampled in 2004–2005 and 2007–2008, respectively. We also assessed whether replacement or richness differences drove overall functional beta diversity patterns and evaluated the relative contributions of geographical and environmental distances to spatial variation in functional diversity.

Our findings highlighted the importance of environmental filtering with increasing elevation, leading to a potentially concomitant decrease in functional alpha diversity values. However, altitudinal variation was not related to changes in functional beta diversity patterns, which were mostly the result of the pure loss or gain of species traits both in lowland and mountain ponds. We suggest that eutrophication associated with agricultural development was the main factor underlying the functional homogenisation of zooplankton communities in lowland ponds. Local environmental filtering was more strongly associated with functional beta diversity and its dominant trait richness difference component than with geographical distances.

This study supports the prediction that elevation fosters changes in different aspects of the variation of zooplankton community functions. Our results highlighted the importance of local conditions determining the functional architecture of zooplankton communities, both in physiologically extreme environments (mountains) and in lowland areas suffering from agricultural pressures and associated eutrophication processes. However, the fraction characterising relationships between functional diversity and environmental correlates was generally low. This suggests the prevalence of idiosyncratic responses and random stochastic events in structuring variation of trait composition in freshwater zooplankton communities.