Controls over native perennial grass exclusion and persistence in California grasslands invaded by annuals

Abstract

Despite obvious impacts of nonnative species in many ecosystems, the long-term outcome of competition between native and exotic species often remains unclear. Demographic models can resolve the outcome of competition between native and exotic species and provide insight into conditions favoring exclusion vs. coexistence. California grasslands are one of the most heavily invaded ecosystems in North America. Although California native perennial bunchgrasses are thought to be restricted to a fraction of their original abundance, the eventual outcome of competition with invasive European annual grasses at a local scale (competitive exclusion, stable persistence, or priority effects) remains unresolved. Here, we used a two-species discrete time population growth model to predict the outcome of competition between exotic annual and native perennial grasses in California, and to determine the demographic traits responsible for the outcome. The model is parameterized with empirical data from several field experiments. We found that, once introduced, annual grasses persist stably with little uncertainty. Although perennial grasses are competitively excluded on average, the most likely range of model predictions also includes stable coexistence with annual grasses. As for many other perennial plants, native bunchgrass population growth is highly sensitive to the survival of adults. Management interventions that improve perennial adult survival are likely to be more effective than those that reduce exotic annual seed production or establishment, reduce competition, or increase perennial seedling establishment. Further empirical data on summer survival of bunchgrass adults and competitive effects of annuals on perennials would most improve model predictions because they contribute most to the uncertainty in the predicted outcome for the perennial grass. This work demonstrates how demographic approaches can clarify the outcome of competition between native and exotic species, identify key targets for future empirical work, and predict the effectiveness of management interventions. Such studies are critical both for understanding the impacts of invasion and for targeting management responses that maximize the benefit to native species.

Positive litter feedbacks of an introduced species reduce native diversity and promote invasion in Californian grasslands

“…Findings of this study have important implications for understanding historical spread of invasive species in Californian grasslands, and point to cascading effects in invasion with an exotic species (Avena) replacing native species through positive litter feedback effects and successively promoting the expansion of a new invader (Elymus). Previous models of plant invasion principally focused on competition between native and exotic species (Godoy & Levine 2014;Mordecai et al 2015), but our results demonstrate that direct and indirect effects of litter may also play a major role (Foster & Gross 1998;Suding & Goldberg 1999). Evaluating litter feedback effects is particularly important because increasing drought events related to climate change are expected to slow down litter decomposition rates (Henry et al 2008;Santonja et al 2015) and might strengthen the relative importance of litter accumulation (Dukes & Mooney 1999;Bardgett et al 2013) in the next few decades.…”

Where and how to restore in a changing world: a demographic‐based assessment of resilience

“…However, a disturbance that is severe enough to kill Stipa adults and allow the establishment of Avena can shift basins of attraction towards an Avena state where Stipa is not able to reinvade, as Stipa populations rely primarily on adult survival than an intact seedbank to buffer population size (Mordecai et al . ). These dynamics likely occur during type conversions following severe fires (Larios, Aicher & Suding ), and suggest that practices that protect remnants from severe disturbances should be high priority for these areas.…”

“…Because we are simulating population size for two life stages of Stipa – seedlings and adults – we created a matrix describing the transition between seedling and adult and calculated GRWR as the dominant eigenvalue of the matrix of transitions rates between these two life stages (Mordecai ; Mordecai et al . ). The communities would be considered separate states if the invader could not increase when rare, while the other species is at equilibrium.…”

“…Levine & HilleRisLambers ; Mordecai et al . ), which adjusts growth rate by competitive interactions with other species. Below we outline, how we applied this approach to the life history of these species.…”

“…There is a rich body of literature using demographic modelling to evaluate when systems – generally characterized by the dominant species or a key functional group – may resist or be susceptible to invasion by another species or functional group (Levine & HilleRisLambers ; Mordecai et al . ). This approach can similarly be applied to infer whether systems are stable, have low resilience, or given the right starting conditions, can convert to an alternate state.…”