Advancing stewardship of stream ecosystems altered by aquatic invasive species (AIS) can better ensure the continuity and long-term sustainability of Department of Defense (DoD) activities on Pacific islands. Because eradication of AIS is not always feasible, consideration must be given to alternative management approaches, including control efforts that can promote conditions favorable to at-risk native species. It is not clear how this can be achieved, however, because little work has been done on AIS control in oceanic island stream ecosystems. The project team addressed this deficit by evaluating genes-to-ecosystem outcomes of experimental AIS removals across a stream flow gradient in concordance with the development of a model-based decision tool to better evaluate AIS management under different hydrological and climate regimes.
Experimental AIS removals were conducted in 13 watersheds spanning a hydrological gradient across the Ko‘olau Range on the Hawaiian island of O‘ahu, which is home to a dense array of DoD installations and at-risk native species. Following a before-after, control-impact study design, AIS removals were executed following a protocol tailored for Hawaiian streams that included two tiers of precautionary steps to prevent collateral injury to electro-sensitive native species. Responses to removals were tracked for a period of up to two years, with adjacent upstream and downstream reaches serving as references. The project team examined whether removals yielded conditions favorable to native species by conducting snorkel surveys of population densities and community composition, as well as mark-recapture demographic assessments of Awaous stamineus, a native species of conservation concern. Additionally, the project team examined A. stamineus otolith microchemistry to determine how life-history varies according to AIS densities and hydrology, and whether removals influenced the balance of variation in local populations. The project team also assessed whether removals elicited an increase in genomic diversity by single nucleotide polymorphism genotyping Awaous stamineus in each study reach. The project team also conducted in-stream and stable isotope assays to determine whether removals influenced nutrient availability and trophic structure. Data from this work was used to parameterize a coupled ocean-watershed biophysical model to evaluate local-to-archipelago scale outcomes of AIS removal for prioritizing interventions that would maximize return on investment.
Experimental removals showed that (1) AIS can be controlled with nominal collateral injury and mortality of native species. Removals also (2) resulted in significant and sustained reduction of AIS densities, with the magnitude of reductions varying by target species and stream discharge. AIS removals also (3) triggered a pulse in recruitment and sustained increases in growth and body condition in A. stamineus. Though island-wide trends in A. stamineus life history (4) tracked discharge and densities of invasive Poeciliid live-bearers, AIS removals only led to a modest shift in the balance of local variation. Nonetheless, (5) genomic diversity increased following AIS removals, likely reflecting shifts in recruitment and life history. While AIS removals (6) did not elicit sweeping changes in ecosystem processes, some conditions did become more favorable for native species, like reductions in total suspended sediment (i.e., clearer water). Finally, model simulations illustrated functional trait differences can be strategically utilized to not only restore local populations but also to achieve metapopulation-wide benefits through demographic spillover.
Development of innovative approaches and actionable information for managing AIS can substantively improve DoD stewardship of stream ecosystems that cross DoD lands, especially on islands like O‘ahu where installations harbor native species under federal or state protection. This project not only empirically demonstrated that modified electrofishing can be a valuable tool for AIS control in Hawaiian streams, it also illustrated how managers can maximize benefits of AIS control through careful consideration of target species and climate-driven hydrological conditions. Likewise, the project delivered a model-based decision support tool that can serve as a defensible and transparent analytical framework for prioritizing interventions according to local, regional and archipelago-scale gains.