Herbivore Recolonization Rate Influences Light and Nutrient Effects on Algal Based Stream Ecosystems

The dynamics of algal based ecosystems are influenced by both resource availability and herbivory. Following a disturbance, the relative importance of top down versus bottom up regulation of algal dynamics in a particular system depends on both herbivore immigration rates and local resource availabilities. The effects of herbivore recolonization and resource availability on the recovery dynamics of algal ecosystems following a disturbance were investigated during two field experiments conducted in 24 in situ stream channels in Convict Creek, California. In each experiment, light and nutrients were cross-classified in a 6x2 factorial design, with 2 replicates per treatment. Initial algal and invertebrate densities were low. Using upstream drift nets of different mesh sizes, herbivore immigration was restricted in one experiment and unrestricted in a second experiment. The relative importance of herbivore versus resource regulation of algae was influenced by herbivore immigration. When immigration was restricted, as might occur following a severe disturbance, algae were more closely regulated by resource availability. In contrast, when herbivore immigration was not restricted, algae were regulated by both resource availability and herbivory. Finally, the effects of light and nutrients on algae were interactive when immigration was restricted, but not when immigration was unrestricted.