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

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.

Synthesizing the effects of large, wild herbivore exclusion on ecosystem function

“…For example, removal of large herbivores often results in dramatic reductions in plant defenses, making them less resistant to future herbivory (Palmer et al, ; Ward & Young, ; Young & Okello, ). Large herbivore exclusion can also lead to increases in exotic plants (Ender et al, ; Seabloom et al, ) suggesting that wild herbivores help ecosystems resist exotic plant invasions.…”

Herbivory enhances the resistance of mangrove forest to cordgrass invasion

“…, Ender et al. ). Although the relative importance of competition and consumption has long been hypothesized to vary along environmental stress gradients (Menge and Sutherland ), we know little about how the relative importance of competition and consumption in resisting plant invasions varies along environmental gradients.…”

“…The relative importance of competition and consumption in biotic resistance has also been studied, although conclusions remain inconsistent (Levine et al 2004, Kimbro et al 2013, Alofs and Jackson 2014. Past studies indicate that the relative importance of competition and consumption varies with community structure (Stotz et al 2017), species traits (Grutters et al 2017, Zerebecki et al 2017, invasion stage (Levine et al 2004, Vil a andWeiner 2004), and habitat characteristics (Kimbro et al 2013, Alofs and Jackson 2014, Byun et al 2015, Stotz et al 2016, Ender et al 2017. Although the relative importance of competition and consumption has long been hypothesized to vary along environmental stress gradients (Menge and Sutherland 1987), we know little about how the relative importance of competition and consumption in resisting plant invasions varies along environmental gradients.…”

Cascading effects of mammalian herbivores on ground‐dwelling arthropods: Variable responses across arthropod groups, habitats and years

“…The fencing that surrounds each exclosure plot is 2.5 m tall and effectively excludes elk, but not other small‐ or mid‐sized herbivores such as deer or hares (J. H. Cushman, personal observation ). Other studies using this exclosure experiment have shown that elk exert major influences on the plant community (Ender, Christian, & Cushman, ; Johnson & Cushman, ; C. M. Lee, M. J. Spasojevic, & J. H. Cushman, unpublished data; C. Richter, M. J. Spasojevic, & J. H. Cushman , unpublished data), small mammals (Ellis & Cushman, ), plant functional traits (C. M. Lee, M. J. Spasojevic, & J. H. Cushman, unpublished data) and soil characteristics (V. Dodge, V. Eviner, & J.H.Cushman, unpublished data).…”