Assessing the ecological status in species-poor systems: a fish-based index for Mediterranean Rivers (Guadiana River, SW Spain)

Abstract

The assessment of the ecological status of freshwater ecosystems is a key issue for many international laws as the Water Framework Directive in light of the actual impoverished status of such ecosystems. Different multimetric approaches have been successfully developed all around the world in different freshwater environments. However multimetric indices are difficult to apply in Mediterranean rivers basins, where freshwater fish communities feature very low species richness per site and high number of endemics with generalist and opportunistic life strategies. A site-specific approach was followed to develop an adaptation of the multimetric concept in the Index of Community Integrity. The presence-absence of ten native freshwater fish species was modeled and used to assess the deviation of the observed and expected community composition at reference condition. These deviations were transformed into probabilities to belong to a reference site and species by species measures were then integrated in a final score. The use of presence-absence only data reduces the possible errors associated to incorrect estimations of species´ abundance and its seasonal changes. The index was sensitive to both habitat and biotic disturbances while irresponsible to natural sources of variation. To our concern, this is the first index specifically tested to be responsible to biotic perturbations, which traditionally have been forgotten pressures in Indices of Biotic Integrity.