Resistance, resilience, and community dynamics in mediterranean-climate streams

Streams and rivers in mediterranean-climate regions (med-rivers) are subjected to sequential, yet contrasting hydrologic disturbances of drying and flooding. Although seasonally predictable, these disturbances can vary in intensity and duration within and among mediterranean-climate regions (med-regions). Consequently, med-rivers differ in the permanence of their aquatic habitats. To persist, species have acquired matched resistance and resilience adaptations. They gain resistance either by enduring the stress or avoiding it. Community recovery (or resilience) is achieved with cessation of hydrologic stress that permits maximization of re-colonization and reproduction. Endurance strategies are usually disturbance-specific, but avoidance enables organisms to cope with both drying and flooding, and is the prevalent resistance strategy. Correspondingly, community persistence depends to a large extent on the integrity of refuges, an aspect that has so far been little explored. Existing information suggests that seasonal community succession becomes more pronounced with increasing aridity and declining water permanence. The invertebrate community in semi-arid med-rivers can therefore undergo succession through three to four identifiable assemblages, whereas in perennial streams the difference between wet and dry period assemblages is smaller. Community turnover is influenced by the intensity of the hydrologic disturbances and varies between wet and drought years.