Native herbivores and environmental heterogeneity as mediators of an exotic grass invasion.

Cody L Ender,J Hall Cushman,Caroline E Christian

doi:10.1002/ece3.2727

Abstract

Given that many exotic plant species throughout the world are having large ecological and economic effects, it is vital to understand the forces that mediate their success in novel landscapes. Both native herbivores and recipient ecosystems can have substantial effects on the performance of exotic plant species, and may interact with each other or vary in their effects over time. Unfortunately, few studies have evaluated the importance of these kinds of context‐dependent effects. Here, we use a 17‐year‐old exclosure experiment stratified across a coastal grassland in northern California to address the relative importance of a reintroduced mammalian herbivore, tule elk (Cervus canadensis nannodes), and environmental heterogeneity in mediating the growth, abundance, and recruitment of a problematic grass invader, Holcus lanatus. We found that elk reduced Holcus abundance, aboveground biomass, percent cover, frequency, and seedling recruitment, but that these effects often varied among habitat types, with effects being greater in open grasslands than shrub‐dominated grasslands. The performance of Holcus populations also varied significantly among habitat types, with the invader usually having the greatest success in Baccharis‐dominated grasslands. Our results suggest that environmental heterogeneity had much greater influence on Holcus success than elk, and that these effects were due largely to soil pH and moisture. The negative effects of elk on Holcus appeared after 4 years and did not intensify after an additional 13 years. Furthermore, despite their negative effects, these prominent herbivores did not prevent the spread of Holcus into previously uninvaded areas. Our research highlights the importance of assessing the individual and interactive effects of native herbivores and environmental heterogeneity on the success of invasive, exotic plant species. It emphasizes the reality that the negative effects of herbivores on exotic plant species will often vary across heterogeneous landscapes and may be insufficient to prevent the expansion of these invaders.

Highlights

Given that many exotic plant species throughout the world are having large ecological and economic effects, it is vital to understand the forces that mediate the success of invaders in their recipient landscapes
Our research addressed the following three questions: (1) What is the relative importance of native herbivores and recipient environments in mediating the growth, abundance, and recruitment of a dominant exotic grass? (2) Can the effects of a heterogeneous landscape on invader success be explained by abiotic soil differences? (3) Has the Holcus invasion changed over time and have herbivores, environmental heterogeneity, or their interaction mediated this trajectory? Answers to these questions will forward our general understanding of the drivers of plant invasions and will help guide more effective efforts to control invasive perennial grasses in altered landscapes
Using a 17-y ear-old exclosure experiment, we have shown that a reintroduced native herbivore and environmental heterogeneity both play important roles in affecting the dominance of an extremely invasive exotic grass

Summary

| INTRODUCTION

Given that many exotic plant species throughout the world are having large ecological and economic effects, it is vital to understand the forces that mediate the success of invaders in their recipient landscapes. Herbivores are an important biotic characteristic of recipient communities that can influence dominant plant invaders through their activities as consumers, disturbance agents, dispersers, and fertilizers (Maron & Vila, 2001; Vavra, Parks, & Wisdom, 2007) Given their potential to impact exotic plant species, herbivores could be useful in managing invasive populations. Answers to these questions will forward our general understanding of the drivers of plant invasions and will help guide more effective efforts to control invasive perennial grasses in altered landscapes Through their activities as herbivores and disturbance agents, we predict that elk will have negative effects on the growth and abundance of Holcus, but positive effects on recruitment by increasing the availability of safe sites. Holcus has become a problematic and widespread invader in California and the California Invasive Plant Council has designated it as having substantial ecological impacts due to its ability to form dense monocultures that reduce species richness of native grasses and forbs (Cal-IPC, 2006; Corbin & D’Antonio, 2010; Deck, Muir, & Strauss, 2013)

| METHODS

Findings

| DISCUSSION