Population Dynamics of Magpie Geese in Relation to Rainfall and Density: Implications for Harvest Models in a Fluctuating Environment

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Abstract
The population dynamics of magpie geese during 1984-87 are described in relation to variations in seasonal rainfall and density in the wetlands of the Northern Territory, Australia. These relationships are used to model the probable effects of harvesting goose populations in a fluctuating environment. Rate of increase was used as a direct summary of population processes (births, deaths and dispersal) and was estimated for five geographic regions between 4 consecutive years. Antecedent seasonal rainfall was used to index resource availability, and antecedent density was used to index possible density-dependent processes. Rate of increase was correlated positively with rainfall and negatively with density, with time lags of 8 and 6 months, respectively. These relationships were combined into a joint regression equation to predict the relative contributions of each effect. The combined regression was homogeneous between geographic regions and between years, suggesting that dispersal effects do not necessarily mask the effects of fecundity and survival. The population and harvesting dynamics of magpie geese were modelled with and without the effects of variable rainfall. The models predict that environmental variability lowers mean density and increases mean fluctuation. Under these conditions, maximum harvest rate and maximum sustained yield of magpie geese were 25% less than predicted by a simple logistic model. The results reported are only exploratory, and are treated with caution. Compensation to a reduction in density can only be demonstrated by experimentation or detailed lifetable studies, and resource relationships can only be ascertained by direct study.