SUMMARY

Among the gravest threats to natural ecosystems and the maintenance of ecosystem services are the more potent non-native species. The massive invasions of Spartina alterniflora, an Atlantic cordgrass, into Pacific estuaries, are a window to the general ramifications of alien species in ecosystems. The proposition is that these invasions cause forceful feedbacks driven by sediment accretion that elevates the marsh, and greatly increases primary productivity in a change away from algal dominance in the process. As well, the phenology of productivity, the nature of nutrient cycling, and decomposition are shifted. The basal food web changes should propagate through herbivores and carnivores both trophically and through the structural and chemical changes brought on by these invasions, with concomitant threats to endangered plants, birds, and mammals, and other species of concern. Finally, Atlantic cordgrasses decrease human commercial, traditional, non-commercial, and esthetic values of Pacific estuaries. The final product of this proposal will be a truly integrated study of the dynamics of an invasive species, including a core mathematical/conceptual model, physical and biological feedbacks, and a careful, justified study of impacts on non-commercial human values.

The hub of this proposal is mathematical modeling of S. alterniflora spread, feedbacks with the environment, and impacts upon Pacific estuaries. The core model is termed “local-state, regional-state,” modeling, as in physiologically structured models of demography. The crucial innovation of this nonlinear stochastic approach is the use of discrete time and continuous states and explicit inclusion of stochasticity based on integro-difference equations, yielding substantial mathematical and computational advantages over reaction diffusion models. The state of the invasion will be the function p(x,z,a,n₁,n₂;t), where x is position along the shore transect, z is tidal height, a is age of Spartina, and n_1, n₂ are densities of other species or human valuations in the system; t is time. ENSO fluctuations, which probably affect these invasions, feedbacks, and non-reciprocal effects are readily included in this framework. Maximum likelihood will estimate the parameters in distributions of cordgrass dispersal. Hypotheses concerning the biological bases of positive feedbacks, Allee effects, and density dependence will be tested experimentally and results integrated into the model.

Parameterization will be from a rich set of historical records as well as from remotely-sensed visible, coarse to fine-scale IR and LIDAR images, referenced with GPS. Mixture analysis with data from plant spectral-feature matching of litter, canopy density and architecture, leaf pigments, water, and dry mass will inform the model of biochemical condition of cordgrass. Images from transects repeatedly-flown over the tidal cycle combined with on-ground electronic distance measuring (EDM) will give the fine scale topography of the clones and channels during the invasions. Experiments will give data on cordgrass demography, clonal growth and seed set, tide flow profile, sediment erodability and shear strength. Sediment accretion will be quantified with Pb‑210 and Cs‑137 and feldspar tracers. Intensively-studied sites will be extrapolated to the entire estuary and other estuaries using NASA’s new ASTER sensor on the Terra satellite, simulating ASTER spectra with high resolution AVIRIS from the local sites. A map will be built of Pacific estuaries using daily MODIS satellite images, delineating seasonal and interannual trends in terms of ENSO events, coastal surface temperatures, and ocean color. Regressions of plant measured biomass on remotely sensed data, from change-detection methods, will provide coarse and mid-scale cordgrass dynamics over time. Spartina impacts on benthic microalgae will be measured in control and experimentally-clipped areas.

Food web and community effects will be studied by pigments (HPLC) and stable isotope (d¹³C, ¹⁵N and ³⁴S) experiments of signature of benthic algae and cordgrass passing through infaunal and epifaunal invertebrates and the diets of other animals. Nutritional experiments will evaluate the value of S. alterniflora detritus relative to native detritus. The ultimate faunal effects of the invasion will be measured in marsh-nesting passerines and rallids in terms of nesting territory size and density, nesting success, foraging behavior, and incidence of egg-breaking. These observations will be combined with the overall model to develop long term predictions of the impact of Spartina on birds.

Finally, these invasions are ideal for studying non-commercial values lost to the changes caused by Atlantic cordgrasses. Avenues satisfying both to biologists and economists will be pursued using the Travel Cost and the Stated Preference methods. Integrating the valuation with the model will provide one of the first rigorous studies of invasive species on the value of ecosystem services.