Invasion of introduced conifers is shaped by mammalian herbivores: Evidence from a literature review

Background/Question/Methods

Invading conifer species, originally introduced for forestry, are increasingly recognized as serious problems in many parts of the southern hemisphere, where they are causing vast ecological and economical impact. Understanding the factors that can trigger or control invasion is key for conifer management. Grazing by mammalian herbivores has been proposed as one mechanism for halting conifer invasions. However, contrasting effects have been also reported (e.g. positive, negative, or no effect of grazing on invasion). Positive effects can happen when herbivores prefer to consume competitive vegetation releasing the trees. In this study, we attempt to identify situations where these different effects occur and aim to find factors that may explain these different responses. We searched for articles that focused on the spread of conifer species (introduced and native) under broad range of grazing situations (domestic cattle and wildlife). From this general search, we obtained 268 articles. After selecting for articles with enough detailed data to be analyzed for comparisons we kept 18 records. We used a discriminant function to see which variable(s)—identity of the conifer species, the recipient plant community, and herbivore attributes—best predicted conifer invasion response.

Results/Conclusions

The results show that species’ identity is the best predictor of invasion response (positive, negative or no effect), while herbivore density also has high explanatory power. As expected, some conifer species tend to be more invasive than others, and this was also moderated by herbivory. At intermediate levels of herbivory, invasion by conifers seems to be very aggressive, and at high levels of herbivory invasion did not occur. These results suggest that herbivores can mediate the invasion of invasive trees. In some locations, this information may be used by managers for controlling the spread of exotic trees. For example, managers may decrease the probability of invasion by simply increasing the density of herbivores (e.g., sheep or cattle) in areas near the source of invasive trees, such as in areas adjacent to pine plantations