Spartina densiflora, an invasive species in the marshes of Humboldt Bay

The purpose of this study was to model the abundance of Spartina densiflora in Humboldt Bay. Ten marsh sites, with an average of twenty-eight plots per site, were surveyed for Spartina abundance and the environmental gradients that could potentially correlate to Spartina percent cover. Seventeen environmental covariates (gradients) were measured, and three of those covariates were found to correlate to Spartina abundance: available phosphorus, redox potential, and elevation. These three covariates were useful in describing and predicting Spartina abundance in each plot, based on the field (and lab) measurements of the covariates. It was found that differences between each site, which were not accounted for by plot covariate values, could be incorporated into the model and increase the descriptive and predictive power of the model. The covariates which describe differences between sites were calculated by taking the site average and standard deviations of the covariates phosphorus, redox potential, and elevation for each site. The phosphorus site averages for all ten sites were calculated, made into the variable PhosphorusSiteAvg., and used to create the fourth covariate. The standard deviation of elevation for all the plots at each of the ten sites was incorporated into the variable ElevNStDev, which became the fifth model covariate. The standard deviation of redox potential for all the plots at each site became the sixth covariate, RedoxSiteStDev. The equation of the model is: Spartina abundance = - 2.1051 + 0.0571*Phos. – 0.000352*Redox + 0.271*ElevN - 0.0493*PhosphorusSiteAvg - 1.28*ElevNStDev + 0.00676*RedoxSiteStDev with R-squared = 0.6089. An equation delineating Spartina habitat in Humboldt Bay was developed using logistic regression. The six covariates used in both the Spartina abundance model and habitat delineation were analyzed for both their relationships to Spartina abundance and their relationships to each other. This analysis was summarized as a list of site characteristics that make a salt marsh resistant or susceptible to invasion by Spartina.