THE IMPACT OF SIZE-SELECTIVE PREDATION ON COMPETITION BETWEEN 2 DAPHNIA-SPECIES - A LABORATORY STUDY

(1) This study examines the influence of size-selective predation on competition between two rockpool Daphnia species, D. magna (Straus) and D. longispina (O.F. Muller). Two different predator profiles were used, one simulating a gape-limited (vertebrate) predator, and the other simulating a size-dependent (invertebrate) predator sensu Zaret (1980). Predation was executed by manually removing fixed percentages of populations in susceptible size-classes in accordance with the profiles, irrespective of species. Thirty-six two-species and seven one-species experiments were performed with predation rates from 0 to 90% per 8 days. (2) Under gape-limited (vertebrate) predation, the time to extinction of the larger D. magna decreased with increasing predation rates. In the size-dependent (invertebrate) predation experiments, coexistence of both species was common up to c. 75% predation per 8 days, and then D. longispina became extinct at higher predation rates. Extinctions of the sub-dominant species did not occur when it was reared alone under high predation rates. Both interspecific competition and predation appeared to be important for the exclusion of the species most susceptible to predation. (3) The results support the idea that vertebrate predation can be important in regulating prey size distribution and species composition in rockpool and pond zooplankton communities. They also suggest that low rates of invertebrate predation may have little effect on zooplankton composition in rockpools and ponds. (4) Predation and competition interacted to produce differences in species composition between different predation rates and profiles. Interspecific competition was still of importance under high predation rates. It is therefore suggested that the view that either one or the other of these processes operates in a particular community may be too simplistic.