Functional response and potential predatory impact of Tortanus dextrilobatus, a carnivorous copepod recently introduced to the San Francisco Estuary

摘要

Despite a dramatic increase in the introduction of non-indigenous estuarine zooplank-ton in recent decades, the trophic implications of such introductions have rarely been quantified. Here we investigate predation rates of Tortanus dextrilobatus, a carnivorous copepod recently introduced to the San Francisco Estuary that achieves peak abundances in excess of 1000 ind. m~(-3). The functional response of T. dextrilobatus feeding upon 2 copepod prey types — the non-indigenous cyclopoid Oithona davisae, and the 'native' calanoid Acartia (Acartiura) sp. — was described by a type II functional response (Ivlev function) at 2 experimental temperatures. In 3 of these 4 treatments, 90% I_(max) was achieved within a naturally occurring range of prey densities. Taxon-specific seasonal size variation was identified, and carbon-based consumption values were used to determine temperature-dependent rates of predation on both prey types. These empirically derived consumption rates and temperature dependence values were then applied to broadscale surveys (1997 to 1999) of zoo-plankton community composition in order to estimate the predatory impact of T. dextrilobatus upon prey populations in the San Francisco Estuary. Predatory impact estimates (% population consumed d~(-1)) greater than 1% occurred on a regular basis when T. dextrilobatus was abundant, with maxima exceeding 20, 65, and 25% for O. davisae, Acartia (Acartiiura) sp. and all Copepoda, respectively. These observations support the hypothesis that non-indigenous invertebrate zooplanktivores can play a significant role, at least seasonally or episodically, in the secondary production dynamics of aquatic ecosystems.