Predation impact of Acartiella sinensis, an introduced predatory copepod in the San Francisco Estuary, USA

摘要

The San Francisco Estuary (SFE), USA, is a highly invaded ecosystem where most of the zooplankton assemblage is exotic. Acartiella sinensis was introduced from Asia in 1993 and has become abundant (mean adult abundance similar to 500 ind. m(-3)) in brackish water during summer. The morphology of Acartiella species suggests a predatory habit, but predation by this genus has never been quantified. The introduction of A. sinensis to the upper, brackish region of the SFE coincided with several other introductions, so its predation impact could not be determined from time-series data. We determined functional responses of A. sinensis feeding on 2 other introduced copepod species, including several life stages of the highly abundant cyclopoid Limnoithona tetraspina and nauplii of Pseudodiaptomus forbesi, and extrapolated predation rates to the field to determine predation impact on copepod populations. Predation rates of adult female A. sinensis were higher on L. tetraspina nauplii than on adults or copepodites, and highest on P. forbesi nauplii, although prey selection experiments did not show a difference in consumption rate between nauplii of the 2 species. Mean clearance rates on nauplii at low density were 0.3 l d(-1) on L. tetraspina and 0.6 l d(-1) for P. forbesi. Predatory impact based on long-term monitoring data averaged 1% d(-1) for adults and copepodites, and 4 and 11% d(-1) for nauplii of L. tetraspina and P. forbesi, respectively. These predation losses are high relative to the population growth potential of these species in this unproductive region.