The decline of a large yellow-footed rock-wallaby (Petrogale xanthopus) colony following a pulse of resource abundance

Abstract

The dynamics of a yellow-footed rock-wallaby (Petrogale xanthopus) colony in central-western Queensland were monitored between spring 1991 and winter 1994. The two years immediately before the study witnessed well above average rainfall, while average rainfall was recorded during the period of the study. Both trapping and standardised visual survey data were modelled using the Jolly-Seber-Cormack (JSC) mark-recapture estimator and the Minta-Mangel (MM) mark-resight estimator. The JSC population estimates were considerably lower than those derived from the MM estimator, indicating that the trapping program sampled only a portion of the total population. Nevertheless, a strong degree of correlation existed between both estimators, suggesting that the dynamics of the trappable subpopulation mirrored those of the total population. The colony declined markedly in size throughout the study (103 to 48 individuals, winter 1992 to winter 1994, JSC estimates; 175 to 116 individuals, summer 1993 to winter 1994, MM estimates). No significant correlations (P > 0.05) were found between seasonal and/or biannual exponential rates of population increase (r) and environmental variables. Nevertheless, a strong positive correlation was observed between biannual r and short-term rainfall (r = 0.90) and pasture conditions (r = 0.85-0.93), suggesting that the colony was influenced to some extent by fluctuations in available resources. Annual r was calculated at -0.29 (1992-93) and -0.49 (1993-94). The marked reduction in colony size suggested that it was declining towards its average carrying capacity, following a strong pulse of recruitment linked to the above-average rainfalls of 1990.