Detectability of the Eurasian otter by standard surveys: an approach using marking intensity to estimate false negative rates.

False negative detections may bias the surveys for rare species and reduce the reliability of models based on the proportion of occupied patches. We assessed the detectability of the Eurasian otter Lutra lutra through the standard survey method by analysing the detection history of 28 sampling stretches surveyed monthly between March 2001 and January 2003. Each survey negative for otter spraints was considered as a false negative if the otter had been recorded in the previous and/or following month (respectively, cFN and FN). Otter marking intensity (MI) (MI=N° of spraints per kilometre) was calculated and assumed to represent an index of its relative abundance. Spraints were found in 81.7% of all surveys. Yearly MI ranged from 1.02 to 101.4 spraints per kilometre. In 2002, mean MI was significantly lower than in the previous year, while no clear seasonal trend could be outlined. The minimum number of surveys required to establish the occurrence of the otter, as estimated by a probability model, was 2.6 and was inversely related to MI. For a sub-sample of 18 sampling stretches, the relation between the frequency of both cFN and FN and five variables of potential interest for otters was tested by means of stepwise linear multiple regressions, yielding two highly significant models, which both included only MI as the explanatory variable. The frequency of both FN and cFN was correlated to MI and the resulting equations used to assess the percentage of surveys positive for otters in both years. After the correction for non-detections, otter site occupancy did not vary between the 2 years, except for one river when applying the more conservative estimate of false negatives (cFN). Multiple visits and the assessing of MI should become standard components of otter surveys. This approach has broad applicability and may be applied to assess the large-scale distribution of other rare or elusive mammalian carnivores.

Exploitation of food resources by the Eurasian badger (Meles meles) at the altitudinal limit of its alpine range (NW Italy).

Food availability has been suggested to be the main factor shaping the altitudinal limits of species distributions. We analyzed the badger (Meles meles) diet in the western Italian Alps and, particularly, at the altitudinal limit of its range, with the main aim of highlighting any reduction in earthworm availability with altitude which could act as a limiting factor for badgers. Earthworms were by far the main food resource of badgers, followed by fruit. The two-month importance of these two items in badger diet was inversely correlated. Earthworm consumption was negatively correlated with air temperature. The seasonal pattern of earthworm use by badgers seemed to be influenced by the reproduction and estivation times of some epigeic species, rather than by climatic conditions per se. The eating of fruit by badgers was at least in part independent from the availability of earthworms. The badgers' efficacy in preying upon earthworms also in adverse conditions and their reliance on a wide variety of food resources suggest that worm availability might play a minor role in shaping the altitudinal limit of the species on the Alps.