Trade-off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community.

Joseph Waterton,Elsa E Cleland

doi:10.1002/ece3.2610

Abstract

Ecological trade‐offs are fundamental to theory in community ecology; critical for understanding species coexistence in diverse plant communities, as well as the evolution of diverse life‐history strategies. Invasions by exotic species can provide insights into the importance of trade‐offs in community assembly, because the ecological strategies of invading species often differ from those present in the native species pool. Exotic annual species have invaded many Mediterranean‐climate areas around the globe, and often germinate and emerge earlier in the growing season than native species. Early‐season growth can enable exotic annual species to preempt space and resources, competitively suppressing later‐emerging native species; however, early‐emerging individuals may also be more apparent to herbivores. This suggests a potential trade‐off between seasonal phenology and susceptibility to herbivory. To evaluate this hypothesis, we monitored the emergence and growth of 12 focal species (six each native and exotic) in monoculture and polyculture, while experimentally excluding generalist herbivores both early and later in the growing season. Consistent with past studies, the exotic species emerged earlier than native species. Regardless of species origin, earlier‐emerging species achieved greater biomass by the end of the experiment, but were more negatively impacted by herbivory, particularly in the early part of the growing season. This greater impact of early‐season herbivory on early‐active species led to a reduction in the competitive advantage of exotic species growing in polyculture, and improved the performance of later‐emerging natives. Such a trade‐off between early growth and susceptibility to herbivores could be an important force in community assembly in seasonal herbaceous‐dominated ecosystems. These results also show how herbivore exclusion favors early‐active exotic species in this system, with important implications for management in many areas invaded by early‐active exotic species.

Highlights

Ecosystems with Mediterranean-type climates harbor exceptional plant diversity and are among the most at risk to biodiversity loss in the coming decades due to multiple factors including changes in climate and land use, as well as invasion by exotic species (Sala et al, 2000; Underwood, Viers, Klausmeyer, Cox, & Shaw, 2009)
This is demonstrated by experiments in which the arrival time of late-germinating native species is experimentally advanced, leading to competitive suppression of later-planted exotic species (Cleland, Esch, & McKinney, 2015; Grman & Suding, 2010), with these effects sometimes persisting over multiple years (Vaughn & Young, 2015)
The results of such experiments raise an important question: Why do certain species, including many natives, display later emergence within the growing season despite the competitive advantage associated with earlier activity? Selection for early phenology has been hypothesized to be limited by exposure to stressful abiotic conditions before the onset of consistent growing season conditions (e.g., Anderson, Inouye, McKinney, Colautti, & Mitchell-Olds, 2012; Augspurger, 2013), and could expose early-active individuals to a greater risk of herbivory (Hanley, 1998)

Summary

| INTRODUCTION

Ecosystems with Mediterranean-type climates harbor exceptional plant diversity and are among the most at risk to biodiversity loss in the coming decades due to multiple factors including changes in climate and land use, as well as invasion by exotic species (Sala et al, 2000; Underwood, Viers, Klausmeyer, Cox, & Shaw, 2009). Priority effects mediated by such differences in arrival time have been shown to exert strong effects on community structure and function (Fukami, 2015) This is demonstrated by experiments in which the arrival time of late-germinating native species is experimentally advanced, leading to competitive suppression of later-planted exotic species (Cleland, Esch, & McKinney, 2015; Grman & Suding, 2010), with these effects sometimes persisting over multiple years (Vaughn & Young, 2015). Exclusion experiments have shown that herbivores, small mammals, exert strong control over plant productivity in herbaceous communities in Mediterranean-climate regions (Peters, 2007) In these ecosystems, there is often little herbaceous vegetation suitable for consumption prior to the arrival of seasonal rains, meaning individuals that emerge first may initially represent the only herbaceous vegetation available to consumers and are likely to be highly apparent and accessible in the landscape (Wainwright et al, 2012). This study tested two hypotheses regarding the relationship between emergence timing and mammalian herbivory on species performance: (1) earlier-emerging species are more susceptible to herbivores, and this susceptibility is greatest early in the growing season; and (2) in polyculture, the competitive advantage of early-arriving exotic species is reduced by herbivory, and this effect is greatest early in the growing season

| METHODS

| DISCUSSION

Findings

| CONCLUSIONS