Open Access

The success of invading species can be restricted by interspecific interactions such as competition and predation (i.e. biotic resistance) from resident species, which may be natives or previous invaders. Whilst there are myriad examples of resident species preying on invaders, simply showing that such an interaction exists does not demonstrate that predation limits invader establishment, abundance or spread. Support for this conclusion requires evidence of negative associations between invaders and resident predators in the field and, further, that the predator-prey interaction is likely to strongly regulate or potentially de-stabilise the introduced prey population. Moreover, it must be considered that different resident predator species may have different abilities to restrict invaders. In this study, we show from analysis of field data that two European predatory freshwater amphipods, Gammarus pulex and G. duebeni celticus, have strong negative field associations with their prey, the invasive North American amphipod Crangonyx pseudogracilis. This negative field association is significantly stronger with G. pulex, a previous and now resident invader in the study sites, than with the native G. d. celticus. These field patterns were consistent with our experimental findings that both resident predators display potentially population de-stabilising Type II functional responses towards the invasive prey, with a significantly greater magnitude of response exhibited by G. pulex than by G. d. celticus. Further, these Type II functional responses were consistent across homo- and heterogeneous environments, contrary to the expectation that heterogeneity facilitates more stabilising Type III functional responses through the provision of prey refugia. Our experimental approach confirms correlative field surveys and thus supports the hypothesis that resident predatory invertebrates are differentially limiting the distribution and abundance of an introduced invertebrate. We discuss how the comparative functional response approach not only enhances understanding of the success or failure of invasions in the face of various resident predators, but potentially also allows prediction of population- and communitylevel outcomes of species introductions.

A majority of the plant species that are introduced into new ranges either do not become established, or become naturalized yet do not attain high densities and are thus considered ecologically and economically unproblematic. The factors that limit these relatively “benign” species are not well studied. The biotic resistance hypothesis predicts that herbivores, pathogens and competition reduce growth and reproduction of individual plants and so suppress population growth of non-native species. We explored the effect of insect herbivory and surrounding vegetation on growth and fitness of the non-native biennial plant Verbascum thapsus (common mullein) in Colorado, USA. Mullein is widespread in its introduced North American range, yet is infrequently considered a management concern because populations are often ephemeral and restricted to disturbed sites. To evaluate the impact of insect herbivores on mullein performance, we reduced herbivory using an insecticide treatment and compared sprayed plants to those exposed to ambient levels of herbivory. Reducing herbivory increased survival from rosette to reproduction by 7%, from 70–77%. Of plants that survived, reducing herbivory increased plant area in the first year and plant height, the length of the reproductive spike, and seed set during the second year. Reducing herbivory also had a marked effect on plant fitness, increasing seed set by 50%, from about 48,000 seeds per plant under ambient herbivory to about 98,000 per plant under reduced herbivory. Our findings also highlight that the relationship between herbivory and performance is complex. Among plants exposed to ambient herbivory, we observed a positive relationship between damage and performance, suggesting that, as predicted by the plant vigor hypothesis, insect herbivores choose the largest plants for feeding when their choice is not restricted by insecticide treatment. In contrast to the strong effects of experimentally reduced herbivory, we found that cover of other plants surrounding our focal plants explained relatively little variation in performance outcomes. Overall, we found that herbivore-induced impacts on individual plant performance and seed set are substantial, and thus may help prevent this naturalized species from becoming dominant in undisturbed recipient communities.

Ecologists are increasingly encouraged by funding agencies and professional societies to communicate their research with the public. However, most receive relatively little training in how to do this effectively. Furthermore, evaluation of whether such an investment by ecologists actually achieves conservation objectives is rare. We created an education program, involving print, television, radio, and internet media, to increase awareness about earthworm invasions and to discourage anglers from dumping earthworm bait. Using pre- and post-surveys, we evaluated our program’s success in reaching its target audience and in changing knowledge and behavior. Few participants (4.1%) recalled seeing the program material and knowledge of the fact that earthworms are non-native in Alberta remained low (15.8% before, 15.1% after). Further, after being told about the negative effects of earthworms in forests, 46.7% of the anglers surveyed stated they would not change their bait disposal behavior in the future, with many commenting that they did not believe earthworms could be harmful. These results highlight the importance of evaluating education programs, rather than assuming they are successful. Given many participants' doubts that earthworms have negative effects, both regulations and education may be needed to reduce earthworm introductions.