The tortoise and the hare: reducing resource availability shifts competitive balance between plant species

Abstract

Summary Determining how changes in abiotic conditions influence community interactions is a fundamental challenge in ecology. Meeting this challenge is increasingly imperative in the Anthropocene where climate change and exotic species introductions alter abiotic context and biotic composition to reshuffle natural systems. We created plant assemblages consisting of monocultures or equal abundance of the native community dominant bluebunch wheatgrass (Pseudoroegneria spicata) and the exotic spotted knapweed (Centaurea stoebe), a co‐occurring invasive forb that has overtaken grasslands across the western United States. We subjected these composition treatments to drought (20% of average precipitation vs. average) and herbivory on C. stoebe by its biocontrol agent Cyphocleonus achates to explore how reduced precipitation might influence the effects of competition and biocontrol herbivory on C. stoebe's abundance. At the end of 7 years, C. stoebe dominated mixed‐species plots under normal precipitation conditions, with biomass 50% greater than that of the native P. spicata. However, under drought stress, P. spicata's biomass was >200% greater than C. stoebe's. Interestingly, both species were impervious to drought in monoculture, indicating the importance of the drought by competition interaction. The biocontrol herbivore reduced C. stoebe abundance and indirectly increased P. spicata biomass in mixed‐species drought plots, but these effects were only marginally significant and relatively weak. Overall, C. stoebe abundance was primarily driven by the drought by competition interaction, with negatively additive but weak effects of the drought by herbivory interaction. The response of the exotic to the treatments was driven by rapid changes in population density linked to its fast life‐history strategy, while the native's response was driven by changes in per capita plant biomass linked to its slower life‐history strategy. Individual plant performance metrics did not predict overall population responses for the invader, indicating the importance of longer term population measures. Synthesis. These results demonstrate that reduced precipitation inputs linked to climate change can dramatically shift the balance of plant competition, even toggling the advantage from exotic to native dominance. They also illustrate the importance of biotic interactions in predicting species responses to abiotic change.

The fluctuating resource hypothesis explains invasibility, but not exotic advantage following disturbance

“…Although annuals increased following disturbance in this system, the shift was small, short-lived, and not biased by provenance. We attribute the differential provenance effects by system to the fact that in the harsher Montana environment, native perennials are largely stress-tolerators that are slow to respond to disturbances and resource influxes (see Maron et al 2012, Pearson et al 2017, but see Taylor et al 2017, whereas in the more hospitable and productive La Pampa system, many native perennials are highly competitive species that are quick to recover from disturbance, allowing them to preempt both native and exotic annuals. These results illustrate how extrinsic community assembly processes that may bias local species pools in favor of certain exotic traits may play out very differently depending on the traits of resident species and their efficiency in responding to resource fluctuations.…”

“…Both communities are semiarid grasslands dominated by native, perennial, caespitose grasses (Montana: Pseudoroegneria spicata , Festuca idahoensis , and Poa secunda ; La Pampa: Piptochaetiun napotaense , Nassella tenuissima , N. clarazii , and Poa ligularis ), with native forbs comprising >50% of species richness, and both communities have similar species abundance distributions (Pearson et al. : Fig. 3).…”

“…Cover (%) was estimated in each plot prior to initial treatments and in the first post-treatment year. We visually estimated cover by species to the nearest 5%, except species under 10% cover were estimated to the nearest 1% and species <1% were recorded as 0.1% cover (see Pearson et al 2017). For non-seed addition plots initiated in year one of the study, cover per species was also estimated in post-treatment years two and four.…”

Global change stressors alter resources and shift plant interactions from facilitation to competition over time

“…Climate change and biological invasions are rapidly altering ecological communities, in part through their dramatic effects on resource availability and plant competition (Allen et al 2010, Caldeira et al 2015, Pearson et al 2017. As temperatures increase and precipitation patterns change, resource inputs to ecosystems are entering novel ranges (Smith 2011).…”

Competition between co‐occurring invasive and native consumers switches between habitats

“…Here, O. edulis alters its niche size and M. gigas shifts its niche centroid in response to each other, which ultimately could allow the coexistence of both species (Gilbert, Srivastava, & Kirby, ). The effect of different ecological niches on species interactions has already been shown in other systems, such as grasslands and for avian communities (Carrete, Sanchez‐Zapata, Tella, Gil‐Sanchez, & Moleon, ; Pearson, Ortega, & Maron, ). Here, we suggest that competition success in benthic systems could also be determined by differences in ecological niches, represented as substratum topography, which underpin biological interactions.…”