Decomposition of invasive species in aquatic and terrestrial invaded systems display additive and nonadditive effects: insights using the EREN research model
The introduction of woody invasive species has been shown to significantly alter nutrient dynamics within the areas they invade, yet the impact of these invasions may extend beyond the terrestrial habitat into local streams. Additionally, the impact of the invasion in both these ecosystems may depend on the relative abundance of the invasive in unpredictable ways, given that litter decay in mixtures containing different species may significantly increase or decrease when different litter qualities are mixed (nonadditive decay). Our objectives for this study were to (1) experimentally test whether decomposition rates of invasive species are faster than the decomposition rates of native species across a wide range of environments and (2) explore how invasive abundance affects decomposition rates. Using the EREN (Ecological Research as Education) model, which supports collaborative research projects that include faculty and undergraduate students at Primarily Undergraduate Institutions, we simulated different levels of invasion by filling litterbags with leaf litter from woody species using five invasive:native ratios. Ten replicates of each treatment were deployed into a stream and also into a nearby upland area at each site, and then collected at intervals throughout the following year to assess mass loss.
Results varied in important ways between years. In the first year of the study, invasive litter did often decay faster than native counterparts, though the pattern was not universal. For example, Rhamnus cathartica litter decayed faster than native counterparts but Ailanthus altissima decayed similarly to its native counterparts at all sites, reinforcing the importance of species-specific litter quality. In contrast, all invasive litter decayed faster than native counterparts the second year of the study. Nonadditive decay was also observed both years, but the pattern was idiosyncratic the first year while in the second year of the study, only litterbags that had been placed in aquatic sites demonstrated nonadditive decay. These results reinforce the importance of litter and site quality, and suggest that more studies are needed that evaluate both terrestrial and aquatic response to invasions.