In western flood plains, species cope with bullfrogs and sunfish invaders

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Alien bullfrogs and sunfish species introduced for consumption and sport are known to alter ecosystems and hinder native amphibians and fish in the Pacific Northwest Highlands. However, there is little research on how these introductions affect native species in lowland floodplains.

A new study of a floodplain in southwest Washington shows that most native species are adapting well to the invaders by changing their food sources and feeding strategies.

Findings may apply to other lowland waters and other native species in response to bullfrog and sunfish invaders. Findings could also help wildlife managers develop appropriate action plans when these non-natives are resident.

“The study shows that native species, at least in this floodplain, can tolerate non-native bullfrogs and sunfish,” he said Meredith HolgersonAssistant Professor of Ecology and Evolutionary Biology at the College of Agriculture and Life Sciences and first author of the study, “Freshwater floodplain habitats buffer native food webs from adverse effects of non-native centrarchids and bullfrogs‘, published online March 28 in the journal Freshwater Science.

Diverse habitats — which create hiding spots — and plentiful alternative food sources are two key factors that allow native species to coexist with non-native invaders, Holgerson said.

“The good news is that we don’t have to worry about removing these non-native species from flood plains like we do in high mountain systems where bullfrogs and sunfish have adverse effects,” Holgerson said. “If we want to manage something, we should manage for the living space.”

This could include promoting available food resources and preserving emerging vegetation at water edges where fish or amphibian larvae can hide, she said.

Both bullfrogs and sunfish have been introduced to fresh ones by humans

water bodies worldwide. Bullfrogs, native to the Northeastern United States, were brought to the West Coast to breed frogs’ legs. Sunfish, also known as centrarchid fish, including bass, crappie, bluegill, and sunfish, were introduced to the West for recreational fishing.

In the study, the researchers looked at how native and non-native species coexist by analyzing what the different species ate and whether they competed for the same resources. Ideally, a perfect study design would have compared waters where only bullfrogs and natives were present; sunfish and natives only; both invaders together and natives; and bodies of water without invaders.

“Unfortunately, you often find bullfrogs and sunfish together in an infested landscape,” Holgerson said.

In waters with and without non-natives, the scientists took tissue samples from a range of predators and prey and measured their stable carbon and nitrogen isotopes naturally occurring in the landscape. The isotopic signatures for carbon (carbon-12 and carbon-13 ratios) in a consumer’s tissues can be traced to various food sources to understand what they are eating.

Similarly, isotopic signatures for nitrogen (ratio of nitrogen-14 and nitrogen-15) indicate an organism’s place in the food chain. Organisms higher up the food chain retain more of the heavier nitrogen-15 than nitrogen-14, Holgerson said.

Overall, the ecologists found that two species of native salamander larvae and native three-spined sticklebacks fed slightly deeper in the food web, shifting food resources in the presence of bullfrogs and sunfish. The data suggest that sticklebacks — which are known to be flexible diets — ate more open-water zooplankton and fewer bottom-dwelling invertebrates (crustaceans, worms and aquatic insects) when they competed with sunfish for food.

The isotopic data suggest that salamander larvae shifted from open water to hide more at pond edges, where they ate more bottom-dwelling invertebrates.

Frogs were less affected by non-native introductions. Since frog larvae are herbivorous, the data suggests that the algae they ate was plentiful enough to limit competition between non-native bullfrogs and native frog larvae.

“By shifting their dietary strategies, native species could potentially coexist with these non-natives rather than suffer population decline,” Holgerson said.

The study was funded by a David H. Smith Conservation Research Fellowship and Portland State University.

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