Vulnerability of a focal plant to browsing by generalist mammalian herbivores : relative importance of self and neighbours
Creators: name: Miller, Alison
Publisher: name: University Of Tasmania
Year: 2023

description: In a heterogeneous environment, where food occurs as patches, herbivores have the opportunity to select at many scales, including vegetation patches, and individual plants within patches. The probability that a focal plant will be browsed may therefore depend not only upon its own physical and chemical characteristics, but also on those of the surrounding vegetation. Consumption of focal plants has been related to their own characteristics and to the quality of the neighbouring vegetation, but the relative importance of the two has received little attention, and as such, forms the focus of this thesis. I used a combination of captive animal and field trials to examine the relative importance of characteristics of a focal plant and the surrounding vegetation to browsing of the former by generalist mammalian herbivores (primarily the red bellied pademelon, Thylogale billardierii). This involved manipulating the palatability of focal plants, in this case Eucalyptus seedlings, by either fertilising or applying a chemical repellent. Consumption of seedlings was then examined, over a series of trials, in vegetation patches of varying height, palatability and density, and where herbivores had either a choice or no choice of vegetation patches. Results were considered in light of various hypotheses predicting plant vulnerability, including the associational plant refuge, short-term apparent competition, and apparency hypotheses. I found that both focal seedling and patch characteristics were important factors influencing browsing of seedlings. Seedlings of higher palatability were browsed more than those of low palatability. The relative quality, abundance and height of neighbouring vegetation all influenced browsing of seedlings through their influence on apparency, availability and relative palatability to herbivores. The influence of the surrounding vegetation depended on the scale at which herbivores were able to, or chose to, make foraging decisions. Seedling and vegetation effects were often additive. Many behaviours exhibited in captive animal trials were tested and confirmed in the field. Results demonstrate that animals can select at both the patch scale and at the scale of individual plants within patches. They demonstrate the need to consider characteristics of both the focal plant and its neighbouring vegetation when predicting the vulnerability of the former to browsing by generalist herbivores. Although I tested a number of hypotheses about plant vulnerability, most of these could be predicted from the same general foraging theory, i.e. herbivores maximise foraging efficiency based on available choices. As such, I demonstrate how many of these hypotheses can be combined into a broader, more general foraging model. In addition to the fundamental importance of these results, they have applications to the forestry industry. Tree seedlings growing in forestry plantations are often damaged by mammalian herbivores during the early stages of establishment. This damage can reduce plantation productivity and, as such, is typically controlled using lethal methods to reduce herbivore populations. Results show that both seedling quality and vegetation on plantations can be manipulated to reduce browsing of tree seedlings. Seedling and vegetation management therefore has the potential to reduce reliance on lethal methods for managing browsing in plantations.