Are Cantabrian brown bears recovering?

The Cantabrian brown bear (Ursus arctos) population of northwest Spain has been monitored since 1982. Population trends have been estimated using counts of females with cubs-of-the-year (hereafter, F(CUB)). A population viability analysis found a mean annual decrease of 4-5% for 1982-95, but with a stabilizing or even slightly increasing trend in the early 1990s. A recent paper in Ursus concluded that the population was recovering, with a 7.5% annual increase, based on the F(CUB) index for 1994-2004. We show several factors limit the interpretation of an increasing trend based on the F(CUB) data. First, data collection was not systematic, nor were spatial sampling and sampling effort sufficiently accounted for, leading to an arbitrary election of the period to estimate F(CUB) trends. Second, data sets did not meet probabilistic analytical requirements. Third, the assumption that the F(CUB) trend, albeit positive, directly reflects the population trend was not justified. In addition, we argue that alternative hypotheses explaining F(CUB) trends should have been presented, particularly because of the absence of a correlation between population and range increases. Altogether, we call for caution when analyzing data about critically endangered populations like that of brown bears in the Cantabrian Mountains.