How nutrition influences the distribution and abundance of chital deer (Axis axis) in northern Queensland

Abstract

Chital deer (Axis axis) have been present in the Burdekin district of northern Queensland for more than 130 years, but increases in abundance and distribution have been modest compared with other species of deer introduced to Australia. This population, originating from four founder animals in 1886, had grown to approximately 32,000 by 2014, located within about 100 km of their original release. Within this range chital are patchily distributed in the landscape, with localised densities of g 170/km2 within a broader environment where their density is very low (l 1.5/km2). The tropical environment is highly seasonal in terms of rainfall and forage resources with annual nutritional bottlenecks as well as forage shortfalls due to multi-year droughts. The annual rate of increase in chital abundance of l 7% suggests a suboptimal match between the introduced species and its new environment, which is the tenet of this study. The specific research questions addressed in this thesis were:1.nnnnn What factors have limited the increase in chital numbers since 1886?2.nnnnn What factors contribute to the observed patchiness of chital in the environment?This study focuses on the spatial and temporal influences of nutrition which are central to the success of ungulate populations. Strategies that maximise nutritional outcomes positively influence body condition, reproduction and drought resilience. This study estimated dietary intake by plant species over the wet and dry seasons of two consecutive years using macroscopic examination of rumen contents of 162 chital. Food plants consumed were assigned a preference by comparing their proportional intake with pasture composition estimated using step point surveys over wet and dry seasons. Chital were principally grazers during the wet season at a time when the nutritive value of grass was high but altered their preference to forbs and shrubs during the dry season according to plant phenology. The change in strategy from wet season grazer to dry season browser according to diet quality was not sufficient to maintain a stable nutritional intake. Body condition measurements of kidney fat indices and bone marrow fat percentages taken from the 162 chital indicate body condition declined during dry seasons.Reproduction in chital was found to be essentially aseasonal in northern Queensland at 19d S, which aligns with reports from equatorial regions where they originate. Although 77% of births occur during the five months from November to March when nutrition is optimal, the remainder are apparently born during a period when nutrition is both less available and poorer in quality. This aseasonal reproductive strategy is common to ungulates that evolved close to the equator and appears to subject calves born during the dry season in northern Queensland to increased risk of starvation due to lactational failure of dams.Variations in chital density are stark within the Burdekin region and my findings indicate animal densities align with mineral concentrations in soils and food plants. Differences in soil phosphorus and sodium in plants between areas chital utilise in lhighr and lnegligibler densities suggest that these may be determinants in habitat selection by chital. Low concentrations of zinc in both food plants and chital sera suggest that zinc may be a limiting nutrient for chital.The study period of 2014 and 2015 coincided with below-average rainfall sufficient to cause reductions in the populations of cattle and free-living macropods. The combined average density of the three principal species of macropod fell approximately 45% from 2014 while the decline in chital on the two study sites fell by approximately 80%. Animal density fell in accordance with body condition of chital and modelled predictions of available forage. It is proposed that periodic drought mortality may be one of the factors limiting the expansion of chital herds in northern Queensland.Chital in northern Queensland are perceived by landholders to compete with cattle for grass, and limit the dry season fodder available to cattle. The present study uses an lenergy modelr to predict the seasonal intake of grass by both cattle and chital, and quantify the impact of free living chital on cattle production. The grass consumed annually by 100 chital could support an additional 25 cattle during the wet season and an additional 14 cattle during the dry season.Findings within this study support an assertion that nutritional shortfalls have contributed to both the slow rate of increase in chital abundance and the observed variation in their density.nn