Herbivores and Facilitation Mediate Plant Invasions
Exotic species have become an increasingly prominent feature of ecological landscapes throughout the world, and their interactions with native and exotic taxa in these novel environments may play critical roles in mediating the dynamics of such invasions. Ideas on this topic have been strongly influenced by two prominent hypotheses. The Enemy Release Hypothesis predicts that exotic taxa should thrive in their new ranges because they are freed from control by native predators, pathogens and herbivores. In contrast, the Biotic Resistance Hypothesis proposes that natural enemies and competitors in recipient communities will limit the success of invasive exotic taxa. Given these divergent views on invasion, the key is to understand how the losses and gains of biotic interactions affect the success of exotic species in their introduced landscapes.
Facilitation and herbivory are increasingly recognized as central forces in population and community ecology and these interactions may play important roles in mediating the success of exotic species. Assessing the relative importance of herbivory and facilitation jointly in the same system will greatly enhance our understanding of the factors that determine the success of plant invasions.
In Cushman et al. (2011), my collaborators and I explored the influence of herbivory by native mammals and facilitation by native and exotic plants on the success of an invasive South African grass, Ehrharta calycina, at a coastal dune in northern California. Using a 2-year exclosure experiment, we showed that a native herbivore, black-tailed jackrabbits (Lepus californicus), significantly reduced the height, shoot production, fecundity and aboveground biomass of this exotic grass. Data from two comparative studies and a neighbour-removal experiment revealed that Ehrharta frequently escaped herbivores by associating with three neighboring plant species – an exotic perennial grass, Ammophila arenaria, an exotic perennial succulent, Carpobrotus edulis, and a native perennial shrub, Baccharis pilularis. Ehrharta growing in association with neighbors was taller, had fewer grazed shoots, produced greater numbers of spikelets and had greater above-ground biomass than unassociated individuals. Furthermore, removing neighbors generally eliminated these benefits in 7 months, although effects differed among neighbour species. An additional neighbor-removal experiment conducted in the absence of jackrabbits indicated that neighbor removal did not have significant impacts on Ehrharta height, shoot production, spikelet production or above-ground dry biomass. These results suggest that the primary means by which Ehrharta benefits from neighboring plants is protection from herbivores – either because they are less apparent to herbivores or less accessible – and that Ehrharta likely incurred minimal costs from associating with neighbors. Ehrharta was more frequently associated with neighbors than expected due to chance, and less frequently found in open dune habitat. These results are consistent with the hypothesis that the effects of herbivory and facilitation have been sufficiently strong to shape the local distribution of this invader in the landscape.
Our research has demonstrated that herbivory and facilitation have jointly influenced the dynamics of a biological invasion, and highlights the importance of evaluating the effects of multiple interactions on invasions in a single system. Our work adds to a growing body of literature indicating that native herbivores commonly impact exotic plant invaders, which offers support for the Biotic Resistance Hypothesis and questions the prevalence of the Enemy Release Hypothesis. Our research also points to the important role that facilitation can play in promoting the success of plant invasions, although much more work needs to focus on this critical topic. Finally, our study highlights the value of evaluating the effects of multiple ecological interactions on invasions and the net outcome of these interactions in a single system. Indeed, we believe that understanding the impacts of various forms of global change – such as biological invasions – will require a more integrated approach that considers interactions among native and exotic species as well as positive and negative effects.
Exotic species have become an increasingly prominent feature of ecological landscapes throughout the world, and their interactions with native and exotic taxa in these novel environments may play critical roles in mediating the dynamics of such invasions. Ideas on this topic have been strongly influenced by two prominent hypotheses. The Enemy Release Hypothesis predicts that exotic taxa should thrive in their new ranges because they are freed from control by native predators, pathogens and herbivores. In contrast, the Biotic Resistance Hypothesis proposes that natural enemies and competitors in recipient communities will limit the success of invasive exotic taxa. Given these divergent views on invasion, the key is to understand how the losses and gains of biotic interactions affect the success of exotic species in their introduced landscapes.
Facilitation and herbivory are increasingly recognized as central forces in population and community ecology and these interactions may play important roles in mediating the success of exotic species. Assessing the relative importance of herbivory and facilitation jointly in the same system will greatly enhance our understanding of the factors that determine the success of plant invasions.
In Cushman et al. (2011), my collaborators and I explored the influence of herbivory by native mammals and facilitation by native and exotic plants on the success of an invasive South African grass, Ehrharta calycina, at a coastal dune in northern California. Using a 2-year exclosure experiment, we showed that a native herbivore, black-tailed jackrabbits (Lepus californicus), significantly reduced the height, shoot production, fecundity and aboveground biomass of this exotic grass. Data from two comparative studies and a neighbour-removal experiment revealed that Ehrharta frequently escaped herbivores by associating with three neighboring plant species – an exotic perennial grass, Ammophila arenaria, an exotic perennial succulent, Carpobrotus edulis, and a native perennial shrub, Baccharis pilularis. Ehrharta growing in association with neighbors was taller, had fewer grazed shoots, produced greater numbers of spikelets and had greater above-ground biomass than unassociated individuals. Furthermore, removing neighbors generally eliminated these benefits in 7 months, although effects differed among neighbour species. An additional neighbor-removal experiment conducted in the absence of jackrabbits indicated that neighbor removal did not have significant impacts on Ehrharta height, shoot production, spikelet production or above-ground dry biomass. These results suggest that the primary means by which Ehrharta benefits from neighboring plants is protection from herbivores – either because they are less apparent to herbivores or less accessible – and that Ehrharta likely incurred minimal costs from associating with neighbors. Ehrharta was more frequently associated with neighbors than expected due to chance, and less frequently found in open dune habitat. These results are consistent with the hypothesis that the effects of herbivory and facilitation have been sufficiently strong to shape the local distribution of this invader in the landscape.
Our research has demonstrated that herbivory and facilitation have jointly influenced the dynamics of a biological invasion, and highlights the importance of evaluating the effects of multiple interactions on invasions in a single system. Our work adds to a growing body of literature indicating that native herbivores commonly impact exotic plant invaders, which offers support for the Biotic Resistance Hypothesis and questions the prevalence of the Enemy Release Hypothesis. Our research also points to the important role that facilitation can play in promoting the success of plant invasions, although much more work needs to focus on this critical topic. Finally, our study highlights the value of evaluating the effects of multiple ecological interactions on invasions and the net outcome of these interactions in a single system. Indeed, we believe that understanding the impacts of various forms of global change – such as biological invasions – will require a more integrated approach that considers interactions among native and exotic species as well as positive and negative effects.