Predicting functional redundancy in Mediterranean riparian communities

The widespread impairment of natural habitats has a differential effect on the organisms depending upon the combination of traits of the affected species. At the ecosystem level, stress can reduce the ability to provide essential services when certain species are removed. In fact, some plant communities have been related to the provision of particular services. Riparian vegetation has a key role in the diverse ecotones between aquatic and terrestrial ecosystems, providing irreplaceable services and functions such as organic matter supply to many lotic habitats, sediment retention, and provision of food and shelter to numerous animals. Unfortunately, riparian ecosystems have been exposed to a high degree of anthropogenic pressure. In this sense, these areas are probably among the most biologically complex and highly impacted ecosystems. In this context, functional redundancy (FR; number of species contributing similarly to an ecosystem function) can be linked to the functional stability of the ecosystem, allowing a rapid assessment of the ecosystem impacts exerted by certain stressors. Here, we use a database of riparian plants occurring in a semiarid Mediterranean basin (Segura River basin) with contrasting stress gradients to explore the response of FR to different environmental factors, including natural (flow temporality, salinity) and anthropogenic stressors (habitat alteration, dam regulation and land use), as well as to their possible interactions. By using these data, we also aim to forecast the values of FR for the entire study area, which allows the identification of the areas most vulnerable to future disturbances. The computation of FR values for each survey unit was made following the approach proposed in Laliberté et al. (2010). We used General Linear Models (GLM) to study the relationship between the FR values and the environmental variables and to forecast FR values in the whole basin.

Results showed that FR in woody riparian vegetation was negatively affected by natural (duration of drought period, salinity) and anthropogenic stressors (physical habitat modification, dam regulation and percentage of agricultural land in the basin). The best model explaining the FR distribution pattern included an interaction term (duration of drought period x dam regulation x basin agriculture) that accounted for 65% of the variance. As a consequence, temporary streams flowing through an agricultural basin and/or impaired by dam regulation had reduced values of FR. On the other hand, the medium-sized rivers, streams and creeks with a perennial character placed in basins dominated by natural land-use and low hydrological alteration displayed higher values of FR and potentially greater stability against disturbances. Finally, the implications of the obtained results in the conservation and management of riparian areas and rivers are discussed.