Of olives and carp: Interactive effects of two invaders on linked stream-riparian food webs
Stream-riparian ecosystems are influenced by multiple invasive species that may interact, influencing one another and generating synergistic effects on food webs and ecosystem processes. However, studies of multiple invaders are relatively rare. We investigated the interaction between two nonnative species widespread in the western USA: common carp and Russian olive (RO), an invasive riparian tree. Located in southeast Idaho, Deep Creek was an International Biological Program site in the early 1970’s; at that time carp were rare. Subsequently, RO was introduced and now forms a dense stand that we have previously shown caused increases in allochthonous inputs, benthic organic matter, and due to associated dinitrogen-fixation, may increase nitrogen-inputs, reduce nitrogen-limitation of primary producers, and possibly increase phosphorous demand. We examined and compared historic and current data of carp abundance and biomass, estimated trophic basis of their production, and determined effects of subsidized carp (via an exclosure experiment) on periphyton, macrophytes and detrital standing stocks. Additionally, because carp consumed olives, we conducted excretion experiments to investigate potential for stoichiometric imbalance between the two invaders to indirectly influence stream nutrient dynamics.
Since 1971, there has been a ~8-fold increase in carp biomass in Deep Creek, and our analyses suggest this size of carp population could not have been sustained by food resources available pre-RO. This contrasts to the native speckled dace that dominated pre-invasion, but whose population has declined by ~80% for unknown reasons. We found nearly two-thirds of the gut contents of carp, on average, consisted of RO material. These patterns are consistent with commensal facilitation of carp by RO. Results of a small-scale, experimental manipulation of carp suggest this subsidized population may also directly consume and limit in-stream biomass of algae and macrophytes; e.g., chlorophyll-a concentration increased ~3X when carp were excluded. Our excretion measurements revealed that when carp, whose bodies are phosphorous-rich, consume nitrogen-rich olive material, they excrete ~4X more N than when they have not consumed olives, and this may amplify the recycling and export of N from streams invaded by both species. The invasion of RO and subsequent facilitation of carp is characteristic of an ‘invasional meltdown,’ and the combination of interacting invaders appears to be driving changes in food webs and ecosystem processes.