Parallel responses of functional niche features to natural and anthropogenic stressors

The interest in studying the functional component of biodiversity has increased during recent years. Functional measures show clear advantages in comparison to conventional taxonomic-based approaches, e.g. they can be directly linked to ecosystem services, the better across taxon comparability or the lower biogeographic influence. In addition, functional diversity can be a better driver to understand changes in ecological niches than taxonomic diversity because biological traits are directly related to environmental filters.  Ecological niche can be easily extended if we use species' biological traits to construct the multidimensional space in which the functional niche is based. This framework may be useful to predict changes in ecosystem integrity in the current context of global growing human pressures. Moreover, different studies have revealed that functional and taxonomic diversity could respond differently to stressors, which may result in different rates of diversity loss with increasing anthropogenic pressures. This has clear implications in conservation and biomonitoring programs, which have been broadly based in taxonomic criteria so far.

Here, we compiled a database of aquatic insects occurring in western Mediterranean rivers, containing taxonomic (presence/absence data) and functional (body size, life cycle, locomotion, physiology, reproduction, resistance strategies, feeding habits, and shape) information, to test the general hypothesis that similar responses of functional niche features are expectable against stress, irrespectively of its origin (i.e. natural or anthropogenic). We measured different functional niche proprieties to assess taxon (individual niche breadth, functional dispersion, relative redundancy) and community (community niche breadth, absolute redundancy) responses to gradients of salinity, flow temporality, and land use. Additionally, we partitioned the changes in community trait beta-diversity along these gradients into turnover and nestedness-resultant components, expecting the emergence of nested subsets of traits over such gradients. The relationships between dissimilarity matrices based on beta diversity components and the matrices of environmental distance due to stress intensity differences were examined.

Our results showed that individual niche and relative redundancy increased with stress intensity in the three gradients studied. Conversely, functional dispersion, community niche and absolute redundancy decreased with increasing stress intensity in all cases. The beta-diversity partitioning revealed that nestedness-resultant dissimilarity increased significantly with the environmental distance in all cases, explaining larger amounts of variance in comparison to turnover dissimilarity matrices. By introducing functional information, our results can be helpful to elucidate the historical process in which species colonised naturally stressed habitats and to improve biomonitoring and conservation programs.