Study design and interpretation of mammalian carnivore density estimates

Abstract

Ecological theory and wildlife management often depend on reliable comparison and interpretation of population density estimates. A synthesis of 1,772 mammalian carnivore population estimates (713 unique to reference, species, site, and size of study area) from 74 species revealed global patterns among aspects of study and interpretive design that undermine the reliability and usefulness of density comparisons. The spatial extent of the study area could explain most of the variation in density, probably because study areas are typically delineated around population clusters. We related the scale-defined density estimates (regression residuals) to 28 other variables measured from the published literature, but none provided convincing biological explanation of the variation in density. Many aspects of study and interpretive design were possibly ill-suited to identifying the factor(s) influencing density. Study attributes and findings were reported inconsistently, and were subject to ideological motivations. Descriptions of vegetation were most difficult to relate to density. More intensive sampling and estimation methods produced above-average density estimates, but the differences were slight and the evidence lacking for concluding whether these more intensive methods were also more accurate. The first underlying factor extracted from principle-components analysis described the growing recognition of population declines and range reductions among large-bodied carnivores, which has also influenced study design. Another factor described an increasing trend for density to be compared and extrapolated to larger areas, but without adjusting for the effect of scale. To understand the factors influencing carnivore distribution and abundance, sampling and reporting methods (e.g., site description with maps) will need to represent the available pool of species, locations, and ecological conditions at larger-than-conventional spatial and temporal scales.

Grizzly Bear Density in Glacier National Park, Montana

“…However, comparisons of density across studies must be made cautiously due to differences in age groups included in the estimates and bias caused by lack of geographic closure Shenk 2001, Schwartz et al 2003). Furthermore, mammalian carnivore density estimates tend to increase with decreasing study area size (Smallwood and Schonewald 1998), presumably because smaller studies tend to target areas where animals are known to occur rather than marginal habitat or areas where populations are sparse (Miller et al 1997). Larger study areas include more habitat heterogeneity, which is typically associated with substantial variation in animal abundance.…”

Demography and Genetic Structure of a Recovering Grizzly Bear Population

“…Climate, topography, vegetation, and land use were highly variable and likely influenced bear density patterns. Further complicating comparison with other populations, mammalian carnivore density estimates tend to vary inversely with study area size (Smallwood and Schonewald 1998). Table 6.…”

Factors influencing the variation in capture rates of shrews in southern California, USA

“…Because of various methodological and biological reasons, detailed density estimates of shrews are rare, and usually do not include analysis of the variation in densities (Smallwood and Smith 2001). Since shrews typically die rapidly in traps, their density estimates are usually based on removal trapping.…”

“…Since shrews typically die rapidly in traps, their density estimates are usually based on removal trapping. In general, removal methods with shrews have generated higher density estimates than live-trapping (Smallwood and Smith 2001). However, without separate assessment using trap lines to detect shrews immigrating into the trapped area, the effective trapping area cannot be defined (Sarrazin andBider 1973, Bury andCorn 1987), making it problematic to convert numbers to density.…”

“…However, without separate assessment using trap lines to detect shrews immigrating into the trapped area, the effective trapping area cannot be defined (Sarrazin andBider 1973, Bury andCorn 1987), making it problematic to convert numbers to density. The use of trapping methods other than the preferred pitfall techniques (Smallwood and Smith 2001) usually underestimates the abundance of shrews. One often neglected factor affecting spatial and temporal density estimates of shrews is their activity level.…”