A tale of two studies: Detection and attribution of the impacts of invasive plants in observational surveys

Abstract

Short‐term experiments cannot characterize how long‐lived, invasive shrubs influence ecological properties that can be slow to change, including native diversity and soil fertility. Observational studies are thus necessary, but often suffer from methodological issues. To highlight ways of improving the design and interpretation of observational studies that assess the impacts of invasive plants, we compare two studies of nutrient cycling and earthworms along two separate gradients of invasive shrub abundance. By considering the divergent sampling strategies and statistical analyses of these two studies, and interpreting their contradictory results in the context of other studies, we also aim to better describe the impacts of the focal invader, Rhamnus cathartica. In a new study of a single site in Minnesota, we observed positive correlations between buckthorn abundance and soil pH, soil nutrient pools, nutrient fluxes through leaf litterfall, earthworm abundance and root biomass. Multiple regression models showed these relationships persisted after accounting for variability in soil texture and tree species composition. For a separate, more expansive study in Illinois, other authors reported little to no correlation between buckthorn abundance and 10 soil properties, including earthworm abundance, pH and nutrient concentrations. However, like many other studies, their regression models only assessed predictors related to invader abundance. R2‐values for models of ecosystem properties ranged from 0 to .79 (adjusted‐R2) for our study in Minnesota and from <.05 to .16 (unadjusted) for the prior study in Illinois. Differences in sampling error and use of predictor variables between the two studies likely explain the contrasting results. Synthesis and applications. To reduce the uncertainty of conclusions from observational studies of invasive plants, future studies must ensure that heterogeneity of soils and vegetation is adequately accounted for in the sampling strategy and statistical analyses (e.g. analysis of covariance, multiple regression). Particular attention should be given to ecosystem properties with variability that likely pre‐dates the invader (e.g. geophysical features and tree community composition). In our study, effects of buckthorn on ecosystem properties were not only robust to the inclusion of potentially confounding predictors, but also consistent with expectations based on ecological stoichiometry and mass balance of element flow.

To list or not to list: using time since invasion to refine impact assessment for an exotic plant proposed as noxious

“…The potential utility of our approach also extends beyond woody invaders, as time-since-invasion data can be obtained from aerial imagery (Mitchell et al 2011, Dost al et al 2013. Observational studies addressing these questions through chronosequences are far more practical than experimental studies, particularly for long-lived woody invaders, but conclusions must be tempered by the correlational nature of the data (see Mueller et al 2018). While invasion ecology is increasingly recognizing the need to consider time since invasion in studies of invader impact, research examining local-scale patterns, including linkages to invader abundance over time, will be particularly informative (Mitchell et al 2011, Sofaer et al 2018.…”

“…Not surprisingly then, correlations between invader and native abundance are used to infer impact in some studies (reviews in Vil a et al 2011, Hulme et al 2013, Stricker et al 2015 and invasibility in others (Stohlgren et al 1999, Knight and Reich 2005, Mills et al 2012). More generally, variation in ecological factors affecting invader success (e.g., disturbance, resource availability) may confound, obscure, or alter relationships attributable to invader impact (Didham et al 2007, Kumschick et al 2015, Sokol et al 2017, Mueller et al 2018. More generally, variation in ecological factors affecting invader success (e.g., disturbance, resource availability) may confound, obscure, or alter relationships attributable to invader impact (Didham et al 2007, Kumschick et al 2015, Sokol et al 2017, Mueller et al 2018.…”

“…Yet relationships between invader and native abundance may be the product of either or both processes (MacDougall and Turkington 2005, Ortega and Pearson 2005, Bauer 2012, Sokol et al 2017). More generally, variation in ecological factors affecting invader success (e.g., disturbance, resource availability) may confound, obscure, or alter relationships attributable to invader impact (Didham et al 2007, Kumschick et al 2015, Sokol et al 2017, Mueller et al 2018.…”

“…However, it remains a challenge to distinguish patterns of invader impact from those reflecting invasibility, even if care is taken during study design to select invaded and uninvaded locations with similar background conditions , Kumschick et al 2015. When examining localscale relationships between invaders and native species, insight may be gained by accounting for ecological covariates potentially affecting abundance of both taxa (Bauer 2012, Mueller et al 2018. For example, the presence of multiple invaders and their potential interactive effects on native abundance can be accounted for in statistical models (Pearson et al 2016).…”

“…In the interim, our objective was to gather field data and conduct a robust assay of buckthorn's potential ecological impacts for use in the hearing. To do so, we used the recommended approach of examining localscale native plant abundance across gradients of invader abundance (Barney et al 2015, Mueller et al 2018, Sofaer et al 2018, while exploiting the woody nature of buckthorn to age plants and thereby consider temporal signatures associated with time since invasion. We also built on recent work to account for the potential impact of other invaders present in plots (Pearson et al 2016).…”