The interactive effects of fire and herbivory on understorey vegetation and its dependent fauna

Abstract

Interactions between multiple disturbances have been shown to have unexpected, and often undesirable effects on ecosystems and biodiversity. Improving our ability to predict and manage the outcomes of multiple disturbances is therefore an important research priority. In this thesis, I focused on the interaction between fire and grazing (or browsing) by large herbivores. Evidence of the individual effects of fire and large herbivores is substantial, but there has been little quantitative synthesis of the effects of native herbivores on biodiversity. Using a systematic review and meta-analysis, I found that high densities of large herbivores usually have negative effects on other animals. However, I found that interactions between large herbivores and episodic disturbances, such as fire, remain poorly understood. I therefore designed a field experiment to test the interactive effects of prescribed fire and large herbivores on forest flora and fauna. I used full and partial exclosure fences to create a gradient of herbivore pressure across both burnt and unburnt sites, and measured the responses of vegetation, spiders and small vertebrates. I found that fire and herbivory interacted strongly to affect vegetation, with herbivory limiting the recovery of vegetation from fire. In contrast to the vegetation response, small vertebrates responded to the individual, but not interactive effects of disturbance. I then focused on the mechanisms driving interactive effects on vegetation, and found that the interaction occurred through both numerically mediated (concentration of herbivores in burnt sites) and functionally moderated (stronger effect of herbivores post-fire) pathways. The differing responses of plants and animals to fire and herbivory was at odds with existing literature, where the effects of large herbivores on fauna are usually attributed to vegetation changes. I therefore tested to what extent vegetation mediated the effects of fire and herbivory on web-building spiders – a group sensitive to changes in habitat structure. Vegetation structure partially mediated the negative effect of fire on spider density, while negative effects of large herbivores on spiders were mostly independent of vegetation. Different x types of web builders differed in their responses, resulting in important changes to the spider community following disturbance. The results of my experiments highlight the importance of focusing on mechanistic pathways for understanding and managing disturbance interactions. However, in reviewing recently published fire-grazing studies, I found that most reported only net effects of interactions. I demonstrate how by failing to identify mechanistic pathways, or non-linear effects, such studies are limited in their management applications. I describe adjustments to disturbance interaction studies that would improve their ability to inform effective management and advance theory. Collectively, my research shows that fire and large herbivores can have strong interactive effects on forested ecosystems and their associated biota, and highlights the importance of considering large herbivores in forest fire planning. It also demonstrates the value of a mechanistic understanding of interactions for the management of disturbance regimes. Such considerations are of broad relevance to the management of multiple disturbances, particularly in the context of increasing global change. Show more