The roles of productivity and diversity in mediating dominance of an invasive freshwater snail

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1 The roles of productivity and diversity in mediating dominance of an invasive freshwater snail Leslie A. Riley* Department of Biological and Allied Health Sciences, Ohio Northern University Mark F. Dybdahl Washington State University According to long-standing theory, native diversity should limit the ability of species to invade and dominate. However, the importance of biotic resistance in repelling invasions remains controversial and it appears that high resource availability (i.e. high productivity) has the ability to neutralize competitive effects from native community members. Here, we tested whether production, diversity or both, influenced dominance of an invasive freshwater snail. The New Zealand mud snail, Potamopyrgus antipodarum, dominates macro-invertebrate communities in some western U.S. streams, but constitutes only a small portion of the community in other streams. Thus, we quantified relative abundance of Potamopyrgus across eleven streams in northwestern Wyoming. In addition, we measured primary production, native macroinvertebrate diversity and other supplementary variables. We found that primary production and macro-invertebrate diversity were both highly variable across streams. In addition, the dominance of Potamopyrgus was also highly variable, with the abundance (relative and absolute) of Potamopyrgus highest in the most productive stream. These results suggest that primary production might be more important than native community diversity in predicting success of Potamopyrgus.

2 Methods. We will conduct sampling within 12 streams, where Potamopyrgus is present (Kerans et al. 2005; Gustafson, Within each stream, we will measure primary productivity and algal standing stock and collect macro-invertebrates for measures of diversity and invasive dominance. We will examine the influence of productivity on Potamopyrgus dominance and macro-invertebrate diversity at three scales for each sampling time: (1) patch-scale patterns (0.9 m 2 ) within each stream (n = 8 for each stream), (2) patch-scale patterns across streams (controlling for streams; n = 96) and (3) combining patch samples within each stream to compare stream-scale patterns (7.2 m 2 ; n = 12). Reviewer comment: one-time sampling will not get at the trajectory of the spread of the snail Samples will be collected twice (May and August). 100 m Measurements and collections will occur within a 100-m reach in each stream (Fig. 5). Eight haphazardly chosen, independent patches will be marked to collect samples of macroinvertebrates (0.9m 2 ) (Fig. 5). These will each serve as a replicate for patch-level analyses. Within each of the 8 patch samples, we will measure chlorophyll a, ash-free dry mass (AFDM) and gross primary production (GPP). Fig. 5. Hypothetical stratified sampling design for one of 12 streams. Squares represent haphazard patch locations for macroinvertebrate samples and patch measures of chlorophyll a, AFDM and Productivity. Standing stocks of benthic algae will be estimated by measuring chlorophyll a and AFDM according to standard methods (APHA 1995). We will estimate GPP by incubating rocks in clear and dark PVC chambers with water continuously pumped through closed circuits to mimic stream conditions. Oxygen concentrations are recorded at the start of the incubation and one hour later. The decrease in oxygen in the dark chamber represents community respiration (CR), while the change in oxygen in the light chamber represents net primary production (NPP). GPP is calculated as: NPP - CR. Macro-invertebrate Diversity. We will collect benthic macro-invertebrates using a surber sampler (area = 0.9 m 2, mesh size = 500 µm) in cobble areas. All samples will be preserved in a 70% ethanol solution and returned to the lab for sorting and identification. Insects, crustaceans and mollusks will be identified to family and genus and species when possible (Merritt and Cummins 1996). We will calculate a variety of measures to quantify native macro-invertebrate diversity and dominance by Potamopyrgus at patch and stream levels. Diversity measures will include 1) total taxa richness and evenness, 2) taxa richness and evenness within the scraper functional group and 3) dominance of Potamopyrgus. Taxa richness is the total number of taxa (identified to the lowest taxonomic category possible). We will use a rarefaction method to control for varying abundance among patch samples and across streams (Gotelli and Colwell 2001). Evenness measures will be calculated using Simpsons diversity index:

3 D i S i 1 ni ( ) N 2 where n i is the total number of organisms of a particular species and N is the total number of organisms of all species. Maximum diversity occurs when D = 0. This index accounts for both richness and evenness of community members (Southwood and Henderson 2000). Finally, dominance of Potamopyrgus will also be calculated for patch and stream levels, using the Berger-Parker dominance index: d N max N T where N max is the abundance of Potamopyrgus compared to the total abundance (N T ). Supplementary variables. We will collect data on other variables within each patch that could potentially influence invasive dominance. First, stream size will be estimated by measuring stream width and depth along eight transects. Second, water velocity, measured with a flow meter, can be multiplied by stream width and depth to calculate discharge (Allan 1995). Third, water temperature, salinity and dissolved oxygen will be measured within each stream reach using a YSI electronic probe (Yellow Springs, Ohio). Finally, because substrate heterogeneity has been experimentally demonstrated to promote diversity in streams, we will also estimate substrate heterogeneity along the same eight transects using visual surveys as a potential factor affecting macro-invertebrate diversity (Schmude et al. 1998). Statistical analyses. We will utilize a multiple linear regression and partial regression approach to test the relationship between productivity and invasive dominance by creating a linear model that includes supplementary variables and algal productivity values (GPP, chlorophyll a, AFDM). This type of analysis allows for examination of linear and curvilinear relationships while also facilitating comparisons across levels of scale (i.e. patches within steams and comparisons across streams). Reviewer comment not completely clear how they would statistically separate the relative effects of biodiversity and primary production on the abundance and dominance of the mudsnail. We will also include measures of native invertebrate diversity (taxa richness and evenness) and a term for the interaction between productivity and diversity in statistical analyses. This approach removes the effect of supplementary variables on invasive dominance while examining the relationship between productivity, native diversity, and the interaction between productivity and native diversity in predicting invasive dominance. Next, we will eliminate unnecessary predictor variables. Multicollinearity among predictor variables will be avoided by removing redundant, non-significant predictor variables from the model. We will improve the fit of the model by including polynomial functions (quadratic terms) to account for any curvilinear relationships from examination of predictor residuals. This step is crucial, given the

4 likelihood that the interaction between productivity and diversity is curvilinear. The predictive value of the full model will then be compared to a series of reduced models using F-tests. Finally, we can test the significance of individual predictor variables using partial regression coefficients. We will test for relationships between diversity and productivity by testing for significant partial regression coefficients for this interaction in the model. We will then conduct partial regressions between dominance and productivity, holding diversity constant, and dominance and diversity, holding productivity constant. This approach will be repeated at three levels: (1) patch-scale patterns within each stream (n = 8 for each stream), (2) patch-scale patterns across streams (controlling for streams; n = 192) and (3) combining patch samples within each stream to compare stream-scale patterns (n = 24). Stream-scale patterns will be examined by calculating diversity and dominance indices from the addition of all eight patches together. All analyses will be performed using Systat 10 (Systat 2000). Expected Results. A significant positive relationship between benthic algal productivity and the Berger- Parker dominance index for Potamopyrgus supports the Productivity hypothesis. On the other hand, no relationship between algal productivity and Potamopyrgus dominance supports Environment-Matching. Finally, if native diversity measures and/or interaction terms between native diversity and productivity are significant predictors of invasive dominance, diversity also influences invasive dominance in this system. JNABS abstract: 2007: Grazing effects, reproductive life history traits and invasive impact of the New Zealand mud snail across a productivity gradient ABSTRACT. Understanding factors that influence variation in species interactions is important for predicting the impact of disruptors to communities, such as invasive species. Benthic primary production is one factor likely to influence consumer-resource interactions given that grazing impacts can be larger and primary consumer densities are often higher in productive areas. The New Zealand mud snail, Potamopyrgus antipodarum, an exotic in the western U.S., dominates macroinvertebrate communities in some streams, but densities vary widely in other areas. In field experiments across 9 streams, we tested the hypothesis that this invasive snail has larger grazing effects and reproduces earlier in more productive streams. Using in-stream cages with and without snails, we examined the response of periphyton to snail grazing and measured snail size at first reproduction. We found that in the most productive streams, Potamopyrgus decreased overall algal standing stocks, and to a lesser extent, gross primary production. Snails also reproduced earlier in the most productive streams. Together, these results suggest that highly productive streams experience the largest grazing effects and also support fast-growing populations of Potamopyrgus, eventually leading to greater overall impacts on native stream communities in productive habitats. Species introduced from outside their native range can detrimentally impact natural communities, yet their adverse effects vary in unpredictable ways. So, why would the impact of one species vary across the introduced range? Impacts are often greatest when a species becomes numerically dominant.

5 Dominance is expected to be mediated by aspects of the new community, such as diversity and primary production. According to long-standing theory, native diversity should limit the ability of species to invade and dominate. However, recent studies suggest this is not always the case and productivity (i.e. resource availability) might overwhelm effects of diversity. Increasing primary productivity is often positively related to plant invader abundance, regardless of native diversity. Diversity, alone, might not be enough to confer biotic resistance upon communities.