Prey selection by an apex predator: the importance of sampling uncertainty

The impact of predation on prey populations has long been a focus of ecologists, but a firm understanding of the factors
influencing prey selection, a key predictor of that impact, remains elusive. High levels of variability observed in prey
selection may reflect true differences in the ecology of different communities but might also reflect a failure to deal
adequately with uncertainties in the underlying data. Indeed, our review showed that less than 10% of studies of European
wolf predation accounted for sampling uncertainty. Here, we relate annual variability in wolf diet to prey availability and
examine temporal patterns in prey selection; in particular, we identify how considering uncertainty alters conclusions
regarding prey selection. Over nine years, we collected 1,974 wolf scats and conducted drive censuses of ungulates in Alpe
di Catenaia, Italy. We bootstrapped scat and census data within years to construct confidence intervals around estimates of
prey use, availability and selection. Wolf diet was dominated by boar (61.563.90 [SE] % of biomass eaten) and roe deer
(33.763.61%). Temporal patterns of prey densities revealed that the proportion of roe deer in wolf diet peaked when boar
densities were low, not when roe deer densities were highest. Considering only the two dominant prey types, Manly’s
standardized selection index using all data across years indicated selection for boar (mean = 0.7360.023). However,
sampling error resulted in wide confidence intervals around estimates of prey selection. Thus, despite considerable variation
in yearly estimates, confidence intervals for all years overlapped. Failing to consider such uncertainty could lead erroneously
to the assumption of differences in prey selection among years. This study highlights the importance of considering
temporal variation in relative prey availability and accounting for sampling uncertainty when interpreting the results of
dietary studies.