Resource partitioning among five sympatric mammalian herbivores on Yanakie Isthmus, south- eastern Australia Naomi Ezra Davis Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy September 2010 Department of Zoology The University of Melbourne i Abstract This thesis combines multiple approaches to improve our understanding of large herbivore ecology and organisation in a contemporary assemblage made up of species with independent evolutionary histories on Yanakie Isthmus, Wilsons Promontory National Park, Victoria, Australia. In particular, this thesis compares niche parameters among populations of five sympatric native and introduced herbivore species by simultaneously assessing overlap in resource use along two dimensions (spatial and trophic) at multiple scales, thereby providing insight into resource partitioning and competition within this herbivore assemblage. Faecal pellet counts demonstrated that inter-specific overlap in herbivore habitat use on Yanakie Isthmus was low, suggesting that spatial partitioning of habitat resources had occured. However, resource partitioning appeared to be independent of coevolutionary history. Low overlap in habitat use implies low competition, and the lack of clear shifts in habitat use from preferred to suboptimal habitats suggested that inter-specific competition was not strong enough to cause competitive exclusion. However, low overlap in habitat use between the European rabbit Oryctolagus cuniculus and other species, and preferential use by rabbits (and avoidance by other species) of the habitat that appeared to have the highest carrying capacity, suggested that rabbits excluded other grazing herbivores from preferred habitat. High overlap in habitat use was apparent between some species, particularly grazers, indicating some potential for competition if resources are limiting. In particular, the eastern grey kangaroo Macropus giganteus had a narrow niche, occurred at low densities and had low population metabolism relative to other species, consistent with competitive suppression. In contrast, the common wombat Vombatus ursinus appears to be the strongest competitor in this assemblage, being numerically dominant, utilising the greatest proportion of resources, and displaying a relatively broad habitat niche. Habitat modification by fire, including changes in vegetation composition and structure, altered fine-scale partitioning of habitat resources by sympatric herbivore species, and changed the composition of the herbivore community. Faecal pellet counts demonstrated a decrease in herbivore densities, particularly grazers, following the burn, probably associated with reduced ground layer ii vegetation cover. In contrast, there was an increase in the density of browsers, particularly the swamp wallaby Wallabia bicolor, probably associated with increased shrub cover. Fine-scale partitioning of habitat resources was evident through inter-specific differences in abundance, population metabolism and use of fine-scale vegetation strata prior to and after the burn. Inter-specific differences in feeding strategies and thus resource requirements appear to facilitate coexistence within this assemblage. Further, overlap in fine-scale habitat use appears to be greater between native and introduced species than between native species, suggesting that species with independent evolutionary histories have inherently less resource partitioning than species with long coevolutionary histories. Microhistological diet analysis showed that the diets of the relatively large-bodied introduced hog deer Axis porcinus and relatively small native swamp wallaby consisted mainly of dicots. The diet of the small, introduced European rabbit contained similar proportions of monocots and dicots. The diets of the native eastern grey kangaroo, intermediate in size, and large native common wombats, consisted mainly of monocots, but kangaroos also consumed moderate amounts of dicots. Overlap in food use by the five species was high, particularly between native and introduced species, but also between some native species. Despite a high potential for food resource competition, it appears that coexistence of herbivores on Yanakie Isthmus is facilitated by ecological separation. However, patterns of ecological separation, niche breadth and diet overlap in this guild did not conform well to body-size related predictions: the species with the narrowest and the broadest diet niches were intermediate in size, and the largest species consumed a greater proportion of dicots than did several smaller species. Interactions between intrinsic and extrinsic constraints on diet choice are likely to influence the diet of herbivores on Yanakie Isthmus. This study provides important preliminary insights into herbivore community niche dynamics on Yanakie Isthmus. High diet overlap and overlap in habitat use at some scales between some species, coupled with resource limitation is likely to result in inter-specific competition, particularly given indications of resource limitation through diet niche adjustments, broad niches and high diet overlap between the native and introduced herbivores in this community. Experimental manipulation is required to obtain a mechanistic understanding of species interactions and conclusively demonstrate competition. iii Declaration This is to certify that i. the thesis comprises only my original work towards the PhD expect where indicated in the Preface; ii. due acknowledgement has been made in the text and all other material used; iii. the thesis is less than 100,000 words in length, exclusive of tables, maps, bibliographies and appendices. ___________________ ___________________ (Signature) (Date) iv v Preface The only data in this thesis that I did not collect myself were faecal pellet counts conducted by Parks Victoria staff after the ecological burn at Big Hummock between July 2004 and February 2005 (Chapter 4). Parks Victoria staff also contributed to this thesis by shooting animals used for the diet study (Chapter 5) and collecting stomach samples from animals on the sampling nights for which I was not present. Parks Victoria staff conducted ecological burn at Big Hummock that comprised the treatment for my study of the influence of fire on fine scale habitat use (Chapter 4). Ian Gordon, PhD Astat, Director of the Statistical Consulting Centre, University of Melbourne, provided advice on statistical analysis and in particular, undertook the Generalised Linear Modelling used to assess relationships between herbivore densities and vegetation attributes (Chapter 3) and made suggestions regarding analysis of pellet decay data (Chapter 2). As all chapters have been written as manuscripts for publication with Graeme Coulson and Dave Forsyth as co-authors, Graeme and Dave have provided editorial input on drafts and contributed to the ideas presented in each chapter. In addition, two anonymous referees provided constructive criticism that improved the methods and results sections of Chapter 5. These sections have been published as part of a paper that examines the diet data presented in this thesis from a management perspective: Davis, N. E., Coulson, G. and Forsyth, D. M. 2008. Diets of native and introduced mammalian herbivores in shrub-encroached grassy woodland, south- eastern Australia. Wildlife Research, 7:684-694. vi vii Acknowledgments Thank you to my supervisors, Graeme Coulson and Dave Forsyth. Your patience, wisdom, time, and ongoing support and encouragement throughout a challenging few years have been greatly appreciated. This thesis has come together thanks to your guidance. Many Parks Victoria staff offered support during this project, both practical, and more importantly, in the form of sharing of their extensive local knowledge of Yanakie Isthmus. Matt Hoskins, my project officer, supported me in all my field requests with a sense of humour (no matter how obscure he thought they were). During the early stages of this project, Elaine Thomas demonstrated a contagious enthusiasm for the restoration of Coastal Grassy Woodland on Yanakie Isthmus, and thus a passionate interest in my work – Elaine was always encouraging and never failed to deliver on requests of any nature, big or small! In the later stages of this project, Dan Jones, has been a major port of call within Parks Victoria, and like Elaine, has shown ongoing support and been keen to help out wherever possible. Jim Whelan’s great knowledge of Yanakie Isthmus has been invaluable. Thank you to John Wright, for making this project happen. Mick Keenan, Graeme Baxter, Scott Griggs, the late Stuart Judd and other Parks Victoria staff at Tidal River all provided support in various ways. Ian Gordon, PhD Astat, Director of the Statistical Consulting Centre, University of Melbourne, provided advice on statistical analysis, as did Jan Carey and Mick Keough, for which I am grateful. Ian Gordon also provided support and interest in my project above and beyond the call of duty: he was instrumental in convincing me that the depth and breadth of my project was worthy of conversion from MSc to PhD. Ron Mayze (Para Park Co-Operative Game Reserve) and the late Geoff Moore (Australian Deer Research Foundation) generously shared their deer expertise. In particular, Ron volunteered many nights attempting to catch hog deer at the Prom with me, however, despite his extensive experience, we never succeeded! Nevertheless, Ron’s passion for hog deer and selfless giving of time and knowledge has taught me a lot. Thank you