Invasive Bromus tectorum alters natural selection in arid systems

Abstract

Summary While much research has documented the impact of invaders on native communities and ecosystem services, there has been less work quantifying how invasion affects the genetic composition of native populations. That is, when invaders dominate a community, can they shift selection regimes and impact the evolutionary trajectory of native populations? The invasion of the annual grass Bromus tectorum in the Intermountain West provides an opportunity to quantify the effects of invasion on natural selection in wild populations. The shift from a perennial‐dominated native community to one dominated by a highly competitive annual species alters the timing and intensity of competitive pressure, which has the potential to strongly shift selection regimes for native plants. To quantify traits under selection in contrasting environments, we planted seeds of two native perennial grasses, Elymus multisetus and Poa secunda, into three invaded, invaded but weeded and relatively uninvaded sagebrush systems. We quantified phenotypic traits of seedlings from separate maternal plants, describing differences in phenotypes among individuals. We then asked which traits were associated with survival and plant size in adjacent invaded and uninvaded sagebrush systems, following individual seed performance for 3 years. We found evidence for divergent selection between invaded and uninvaded sagebrush systems, with contrasting phenotypic traits associated with greater survival or plant size in these different growing conditions at all three field sites. For example, at one field site, P. secunda families with higher root tip production were more likely to survive in invaded and weeded environments, but this was not the case in uninvaded environments. Similarly, for E. multisetus, root mass fraction, seed mass and allocation to coarse or fine roots affected survival and plant size, again with contrasting relationships across invaded, weeded or uninvaded environments. Synthesis. Impacts of invasive species extend beyond ecosystem and community composition changes and can affect the evolutionary trajectory of native populations. By quantifying natural selection in invaded landscapes, we identified phenotypic traits that are potentially adaptive in these invaded systems. Importantly, these traits differed from traits associated with success in uninvaded communities. This insight into adaptive, contemporary evolution in native species can guide restoration and conservation efforts.

Combining active restoration and targeted grazing to establish native plants and reduce fuel loads in invaded ecosystems

“…Understanding how initial restoration approaches and repeated grazing treatments influence future densities of adult plants at this site requires longer‐term observations, and future assessments are planned. However, because mortality risk declines substantially after the first or second growing season (Leger & Goergen, ), second‐year results can provide important insights about how to address seedling establishment, which has been identified as a critical restoration bottleneck in dryland ecosystems (James et al, ).…”

“…Establishing native plants from seed in highly invaded settings remains challenging (Eiswerth & Shonkwiler, ). Existing work on restoration in drylands suggests that seedling establishment is a critical bottleneck in these ecosystems (James, Svejcar, & Rinella, ), with survivorship increasing substantially after the first or second growing season (Leger & Goergen, ). Our work explored the separate and combined efficacy of five approaches that could potentially be utilized to alter competitive dynamics and increase seedling establishment in dryland, fire‐prone restoration settings: using seed coatings to increase water availability (Madsen, Kostka, Inouye, & Zvirzdin, ), choosing species that can compete at the seedling stage with invasive annuals (Rowe & Leger, ), bolstering seed rates (Mazzola et al, ), using spatial separation to reduce competition among planted species (Porensky, Vaughn, & Young, ), and using targeted spring and fall grazing to reduce invasive plant competition and associated wildfire risk (Davies et al, ; Schmelzer et al, ).…”

“…Existing work on restoration in drylands suggests that seedling establishment is a critical bottleneck in these ecosystems (James, Svejcar, & Rinella, 2011), with survivorship increasing substantially after the first or second growing season (Leger & Goergen, 2017).…”

Long‐term dynamics and impacts of plant invasions

“…A poorly studied aspect of whether invader impacts change over time is the possibility of evolutionary responses of initially affected native species to the presence of the invader. In the final article of this special feature, Leger & Goergen () evaluated how the presence of an abundant annual grass invader over many decades in the Intermountain West of the United States has altered selection regimes and evolutionary trajectories of native species. While the invader, Bromus tectorum , has been well studied and shown to have competitive effects on native species, the authors demonstrate that persistent invasion by B. tectorum has selected for different phenotypic traits of resident natives and hence greater survival for some native perennial grass species.…”

Restoration islands: a tool for efficiently restoring dryland ecosystems?

“…Mazzola et al ), but once established, plant survival probability increases significantly (e.g. Chambers ; Meyer & Pendleton ; Leger & Goergen in press). The limited planting area of islands allows managers to increase the initial establishment through a combined approach of strategic site selection and targeted resource deployment.…”