Population Dynamics of Eastern Grey Kangaroos in Temperate Grasslands Don Fletcher BA Institute of Applied Ecology University of Canberra A thesis submitted in fulfilment of the requirements of the degree of Doctor of Philosophy in Applied Science at the University of Canberra 2006 ii ACKNOWLEDGMENTS I am deeply appreciative of the support and guidance of my principal supervisor Prof. Jim Hone, throughout the project. His commitment to principles of scientific method in applied ecology, his knowledge of the literature, the breadth of his interests, and his patience, were inspiring. I also thank Stephen Sarre, my second supervisor, for his successful effort to complement what Jim provided, for constructive and friendly advice and criticism and for broadening my knowledge of the application of molecular biology to ecology. My employer, Environment ACT, supported the project in various ways throughout, especially in tolerating my absence from the office when I was undertaking fieldwork or working at the University of Canberra. I particularly thank the manager of the research group at Environment ACT, Dr David Shorthouse, for dedicated and patient support, and for ably defending the project at the necessary times, and my colleague Murray Evans who carried an extra workload in 2002–03 in my absence. I also thank the former manager of the ACT Parks and Conservation Service, Tony Corrigan, for enlisting support at a crucial time when the project was commencing. The former Marsupial CRC, particularly its leader, John Rodger, supported the project handsomely with a cash grant without which it would not have commenced. I want to thank the Applied Ecology Research Group (now the Institute of Applied Ecology) for friendly support. It has been an excellent research environment with a broad range of topics to learn about and a suitable range of activities, both academic and social, to encourage a friendly and worthwhile learning environment. The denizens of the postgrad room and environs at UC added much to my enjoyment of the project – especially Anna McDonald, Nicci Aitken, Kiki Dethmers, Michelle Walter, Alex Quinn, Megan McCann, Marion Hoehn, Wendy Dimond, Erika Alacs, and John Roe. Thank you for sharing your inevitable trials and tribulations, and of course enthusiasm and high spirits. I thank Peter Bayliss for prompting and encouraging me to attempt postgraduate study. I am indebted to Peter for introducing me to the theory and practice of aerial survey methods, for facilitating the commencement of the study and for providing me with Caughley’s (1976a) paper as an introduction to consumer-resource models. Most of all, it was Peter Bayliss, with input from Jim Hone and Steve McLeod, who designed a larger kangaroo research project from which I copied the main elements of the design reported. iii I thank Steve McLeod and John Druhan for training with the walked line transect method and for demonstrating their method of vehicle-based line transect survey at night, of which my kangaroo counting method was an extension. I also want to thank John for a copy of his database for entering kangaroo survey data, and Steve for apt advice about managing the research project. Damien Woolcombe and Mark Dunford helped me when I had difficulties using the computer programs Access and Arc View. For helping me learn how to access scientific literature using the internet I am grateful to Pat Tandy, Tricia Kelly, Erika Alacs, and Anna McDonald. The work crew at Tidbinbilla prepared the site where the temporary kangaroo yards for the grazedown were erected, and helped me remove the carpet and other debris at the end. After the yards were burnt in the 2003 bushfire, ACT Fencing and Metalwork allowed me to retain, for an extra year, the security fencing panels from which they were made, until grass returned and I could complete the grazedown. Some parts of this project would have been impossible without the support of many volunteers. More than 200 names are listed in an appendix, but no doubt I have missed a few, for which I apologise. The dedicated support of those who returned time and again is especially appreciated. Even these are too many to list here but I particularly thank Peter Hann, Mel White and Angelina McRae for many hours of companionship and assistance. Finally, but in first place, I thank my wife Kerry Moir for tolerating my absence, either physically or mentally, from many occasions when she wanted a more complete family, and my children for tolerating less interaction with their dad than may otherwise have occurred. iv ABSTRACT This thesis is about the dynamics of eastern grey kangaroo (Macropus giganteus) populations and their food supplies in temperate grasslands of south-eastern Australia. It is based on the study of three populations of eastern grey kangaroos inhabiting ‘warm dry’, ‘cold dry’, and ‘warm wet’ sites within the Southern Tablelands climatic region. After a pilot survey and methods trial in early 2001, the main period of study was from August 2001 to July 2003. The study populations were found to have the highest densities of any kangaroo populations, 450 to 510 km-2. Their density was the same at the end of the two year study period as at the beginning, in spite of a strong decline in herbage availability due to drought. The eastern grey kangaroo populations were limited according to the predation-sensitive food hypothesis. Fecundity, as the observed proportion of females with late pouch young in spring, was high, in spite of the high kangaroo density and restricted food availability. Age-specific fecundity of a kangaroo sample shot on one of the sites in 1997 to avert starvation was the highest reported for kangaroos. Thus, limitation acted through mortality rather than fecundity. Population growth rate was most sensitive to adult survival but the demographic rate that had the greatest effect in practice was mortality of juveniles, most likely sub-adults. The combination of high fecundity with high mortality of immatures would provide resilience to low levels of imposed mortality and to fertility control. The normal pattern of spring pasture growth was not observed in the drought conditions and few of the recorded increments of growth were of the magnitude considered typical for sites on the southern and central tablelands. Temperature was necessary to predict pasture growth, as well as rainfall, over the previous two months. The best model of pasture growth (lowest AICc) included negative terms for herbage mass, rainfall over the previous two months, and temperature, and a positive term for the interaction between rainfall and temperature. It accounted for 13% more of the variation in the data than did the simpler model of the type used by Robertson (1987a), Caughley (1987) and Choquenot et al. (1998). However this was only 63% of total variation. Re-evaluation of the model based on measurements of pasture growth in more typical (non-drought) conditions is recommended. Grazing had a powerful influence on the biomass of pasture due to the high density of kangaroos. This is a marked difference to many other studies of the type which have been conducted in semi-arid environments where rainfall dominates. v The offtake of pasture by kangaroos, as estimated on the research sites by the cage method, was linear on herbage mass. It was of greater magnitude than the more exact estimate of the (curved) functional response from grazedowns in high–quality and low–quality pastures. The widespread recognition of three forms of functional response is inadequate. Both the theoretical basis, and supporting data, have been published for domed, inaccessible residue, and power forms as well (Holling 1966; Noy-Meir 1975; Hassell et al. 1976, 1977; Short 1986; Sabelis 1992). Eastern grey kangaroos had approximately the same Type 2 functional response when consuming either a high quality artificial pasture (Phalaris aquatica), or dry native pasture (Themeda australis) in autumn. Their functional response rose more gradually than those published for red kangaroos and western grey kangaroos in the semi-arid rangelands, and did not satiate at the levels of pasture available. This gradual behaviour of the functional response contributes to continuous stability of the consumer-resource system, as opposed to discontinuous stability. The numerical response was estimated using the ratio equation, assuming an intrinsic rate of increase for eastern grey kangaroos in temperate grasslands of 0.55. There is indirect evidence of effects of predation in the dynamics of the kangaroo populations. This is demonstrated by the positive relationship between r and kangaroo density. Such a relationship can be generated by predation. A desirable future task is to compile estimates of population growth rate and simultaneous estimates of pasture, in the absence of predation, where kangaroo population density is changing, so that the numerical response can be estimated empirically. The management implications arising from this study are numerous and a full account would require a separate report. As one example, kangaroos in these temperate grasslands are on average smaller, eat less, are more numerous, and are more fecund, than would be predicted from other studies (e.g. Caughley et al. 1987). Thus the benefit of shooting each kangaroo, in terms of grass production, is less, or, in other words, more kangaroos have to be shot to achieve a certain level of impact reduction, and the population will recover more quickly, than would have been predicted prior to this study. Secondly, of much importance to managers, the interactive model which can readily be assembled from the products of Chapters 4, 5 and 8, can be used to test a range of management options, and the effect of variation in weather conditions, such as increased or vi decreased rainfall. For example, the model indicates that commercial harvesting (currently under trial in the region), at the maximum level allowed, results in a sustainable harvest of kangaroos, but does not increase the herbage mass, and only slightly reduces the frequency of crashes when herbage mass falls to low levels.