Prey Selection by an Apex Predator: The Importance of Sampling Uncertainty

PLoS One. 2012;7(10):e47894. doi: 10.1371/journal.pone.0047894. Epub 2012 Oct 26.

Abstract

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 (61.5 ± 3.90 [SE] % of biomass eaten) and roe deer (33.7 ± 3.61%). 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 = 0.73 ± 0.023). 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Choice Behavior / physiology
  • Data Collection / methods
  • Deer / physiology
  • Diet*
  • Feces / chemistry
  • Food Chain*
  • Italy
  • Models, Biological
  • Population Dynamics
  • Predatory Behavior / physiology*
  • Species Specificity
  • Uncertainty
  • Wolves / physiology*

Grant support

MD is a Ph.D. student funded by a Durham University Doctoral Fellowship. This funder had no role in the study design, data collection and analysis, decision to publish or prepration of the manuscript. Some funding for CC and EB (as students) and additional logistical support of field work for the study was provided by the Regional Government of Tuscany and Provincial Government of Arezzo. These institutions are responsible for managing the study area and are therefore interested in accurate unbiased metrics of the wildlife in the area. The authors worked with these institutions to design monitoring regimes (for wolves, boar, and roe deer) toward this end. These organizations had no other role in the analysis, decision to publish or preparation of the manuscript. Their participation in survey design and data collection has had no impact on the integrity of this study as their only goal was accurate, unbiased monitoring of the resident wildlife.