Abstract
Invasive planktonic crustaceans have become a prominent feature of aquatic communities worldwide, yet their effects on food webs are not well known. The Asian calanoid copepod, Pseudodiaptomus forbesi, introduced to the Columbia River Estuary approximately 15 years ago, now dominates the late-summer zooplankton community, but its use by native aquatic predators is unknown. We investigated whether three species of planktivorous fishes (chinook salmon, three-spined stickleback, and northern pikeminnow) and one species of mysid exhibited higher feeding rates on native copepods and cladocerans relative to P. forbesi by conducting `single-prey’ feeding experiments and, additionally, examined selectivity for prey types with `two-prey’ feeding experiments. In single-prey experiments individual predator species showed no difference in feeding rates on native cyclopoid copepods (Cyclopidae spp.) relative to invasive P. forbesi, though wild-collected predators exhibited higher feeding rates on cyclopoids when considered in aggregate. In two-prey experiments, chinook salmon and northern pikeminnow both strongly selected native cladocerans (Daphnia retrocurva) over P. forbesi, and moreover, northern pikeminnow selected native Cyclopidae spp. over P. forbesi. On the other hand, in two-prey experiments, chinook salmon, three-spined stickleback and mysids were non- selective with respect to feeding on native cyclopoid copepods versus P. forbesi. Our results indicate that all four native predators in the Columbia River Estuary can consume the invasive copepod, P. forbesi, but that some predators select for native zooplankton over P. forbesi, most likely due to one (or both) of two possible underlying casual mechanisms: 1) differential taxon-specific prey motility and escape responses (calanoids > cyclopoids > daphnids) or 2) the invasive status of the zooplankton prey resulting in naivety, and thus lower feeding rates, of native predators feeding on invasive prey.
Highlights
Worldwide, human impacts on freshwater and estuarine ecosystems facilitate the introduction and establishment of aquatic invasive species (AIS), resulting in significant economic and ecological impacts [1,2]
Our results indicate that diverse taxa of native predators in the Columbia River estuary (CRE) are capable of feeding on the invasive copepod, P. forbesi, some of these predators feed on native zooplankton at higher rates
While the precise reasons for differences in feeding rates or selection of specific prey types may be unknown —as it was in our study—whether or not predators select for or feed at higher rates on one type of prey over another, or adapt in any way to the presence of invasive prey (e.g. [75,76]) is important to understanding the impacts of zooplankton invasions on native predator populations, and the invaders’ effects on native food-webs
Summary
Human impacts on freshwater and estuarine ecosystems facilitate the introduction and establishment of aquatic invasive species (AIS), resulting in significant economic and ecological impacts [1,2]. In the Great Lakes (U.S.), loss of ecosystem services due to ship-borne AIS invasions was recently estimated at between $138 million and $800 million [3]. Aquatic species invasions generally occur as a result of human activities, including habitat alteration, aquaculture, exotic species trade or ship ballast-water releases, and they are a leading driver of global biodiversity loss [4,5,6]. Investigations of the interactions between native biota and AIS are crucial to gain better understanding of AIS impacts and to inform effective management strategies in the future
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