Testing the reliability of pellet counts as an estimator of small rodent relative abundance in mature boreal forest
Small rodents form an important group of prey for a number of predators in Canadian boreal forests, and monitoring their abundances annually is of prime interest for several research goals. We analyzed whether fecal pellet counts â€” used as an index of small rodent abundance (mainly voles) â€” are an accurate, rapid and inexpensive method for population monitoring in the boreal black spruce forest. Snap-trapping was used as a proxy to assess the relative abundance of small rodents, while counts of fecal pellet groups were conducted on circular plots superimposed to snap trapping grids. Captured rodents were mainly the southern red-backed vole (Myodes gapperi). Surveys were conducted during the high (2003) and declining (2004; intermediate level) phases of its abundance cycle. Surveys were carried out in mature forest only. Using generalized linear model (with Poisson error) for count data, we found a significant
relationship between the abundance of fecal pellet groups and rodents captures per 100 trap-nights in 2003 and in 2004. However, in both cases, relationships were weak (pseudo-R2 < 0.1). We hypothesize that such a lack of reliability in explaining variance in small rodent abundance is due to: (1) the high variability in rodent abundance, especially within season but also within and between years; and (2) the heterogeneous distribution patterns of fecal pellet groups. Our results thus suggest that fecal pellet count alone is a highly variable and inaccurate index of small rodent captures per 100 trap-nights (and ultimately of abundance) in the boreal black spruce forest on an annual basis.