An evaluation of faecal pellet counts to index rock-wallaby population size

Abstract

This study assessed the efficacy of faecal pellet counts to index population size within yellow-footed rock-wallaby (Petrogale xanthopus) colonies. One hundred 1-m2 quadrats were permanently located in each of two colonies and emptied of pellets on a monthly basis between September 1996 and August 1998. The mean number of pellets (MNP) accumulated per season (quadrats as replicates) and a mean pellet presence/absence per quadrat index (MPAP) were calculated for each colony. Other population estimates and indices (direct counts, mark–recapture and aerial surveys) were also collected at these colonies or drawn from other studies, allowing comparison with the pellet data. The trends in abundance across seasons suggested by these additional estimates and indices were an overall increase at one colony with no change at the other. While these trends were reflected by the seasonal MNP trends across the eight seasons monitored, this was less often the case for the like-season MNP comparisons. The large degree of variation in seasonal MNP data may render it unsuitable for use in such short-term like-season comparisons. The MPAP seasonal data were also found to be unreliable as an index of population size for yellow-footed rock-wallaby. In Spring 1997, one pellet collection period was missed, resulting in a combined September–October 1997 count at each colony. This increased period between pellet counts appears to have resulted in increased pellet decomposition and/or loss. This disrupted sampling regime produced a marked discrepancy in the pellet indices, particularly in the colony with more individuals and thus more pellets to lose, and highlights the importance of a structured sampling regime.