Accounting for multiple stressors in regional stream ecosystem analysis: A demonstration with riparian invasive plants

Background/Question/Methods

Large cross-sectional data sets allow testing of hypotheses about how one part of an ecosystem relates to other parts. Tests such as these are of interest for many reasons, one of which is to gain insight into the role of stressors, such as land cover, nutrient loading, alterations of riparian habitat, or the presence of invasive species, on valued ecosystem components, such as in-stream biotic integrity. When these tests are constructed without accounting for known interactions among ecosystem components, the results may not provide a meaningful or actionable understanding of the relationship between stressors and valued ecosystem components.  Structural equation modeling (SEM) supports an approach that incorporates expected interactions among ecosystem components. We applied this technique to a probability sample of wadeable streams in the U.S.  mountain west. We developed a set of expected relationships of 6 invasive riparian plants (Cirsium arvense, Bromus tectorum, Rubus armeniacus, Phalaris arundinacea, Elaegnus angustifolia and Tamarix spp.), landscape disturbance, riparian structure, stream chemistry and other elements of stream ecosystems with in-stream faunal biotic integrity. We evaluated these expectations in two SEM models, one for biotic integrity of stream vertebrates and another for biotic integrity of stream macro-invertebrates.

Results/Conclusions

Simple tests of the relationship between the presence of each invasive plant and biotic integrity often show a significant and substantial reduction in biotic integrity associated with the presence of the riparian invasive.  This result suggests that these invasive species can be indicators of decreased in-stream biotic integrity. In contrast, SEM models that account for the interrelationships among stream ecosystem components and account for the effects of multiple stressors suggest that of the six invasive plants only Tamarix spp. presence has a significant and substantial effect on one measure of biotic integrity. The analysis also suggests that this effect is direct and indirect – mediated especially through stream chemistry and wood in the stream channel.