Abstract
Growing human-wildlife conflicts and legal conservation obligations increased the need for precise information on Eurasian otter (Lutra lutra) population parameters for species status assessment and wildlife management measures. Scat surveys have become the method of choice to monitor species distribution range, abundance and habitat use. Although methodological concerns exist, scat abundance is often used as main indicator for otter population estimates and trends. To evaluate whether scat counts serve as good proxy for otter densities in linear stream habitats, we modelled the relationship between old/fresh scat abundance and otter numbers on two spatial scales, including also seasonal effects. Actual otter densities and marking behaviour were ascertained by genotyping of otter faeces collected at 218 marking sites along 132.6 km of four salmonid streams. Otter densities in our study areas ranged from 0.16 to 0.28 otters per stream kilometre. The age of scats and the level of spatial scale were determining factors when evaluating the suitability of scat abundance as index for otter densities. Covering large parts of an otter’s territory, fresh scat abundance increased with otter density, irrespectively of season. On the spatial scale of single marking sites, the relationship was not well supported, and no relationship could be found when using old scats in the models. Otter marking behaviour and differences in marking site use intensity, as well as seasonal differences in traceability and accumulation of old scats were discussed to bias the relationship between scat abundance and otter numbers.
Highlights
IntroductionUnbiased estimators for distribution and abundance of animal species are fundamental to understand population ecology, necessary for legally required reporting obligations (e. g., according to the EU Habitats Directive 92/43/EEC), and critical to make sound wildlife management decisions (e. g., evaluation of the effectiveness of conservation measures or regulating plans)
Unbiased estimators for distribution and abundance of animal species are fundamental to understand population ecology, necessary for legally required reporting obligations (e. g., according to the EU Habitats Directive 92/43/EEC), and critical to make sound wildlife management decisions (e. g., evaluation of the effectiveness of conservation measures or regulating plans). Data on these population parameters are interesting for animal species that have suffered from persecution for a long period and are recovering due to increased conservation measures (Chapron et al 2014; Ripple et al 2014), and/or species which compete with humans for natural resources
For genetic analysis, 1039 fresh scats were collected on days 2 and 3 of each collection period, of which 592 revealed an unambiguous genotype resulting in 45 unique genotypes (Table 1; for individual detection rates see Table S1 in Online Resource 1)
Summary
Unbiased estimators for distribution and abundance of animal species are fundamental to understand population ecology, necessary for legally required reporting obligations (e. g., according to the EU Habitats Directive 92/43/EEC), and critical to make sound wildlife management decisions (e. g., evaluation of the effectiveness of conservation measures or regulating plans). The population recovery has led to a revival of conflicts between the feeding needs of this fish-eating mustelid, commercial fisheries and anglers, and nature conservation proponents (Klenke et al 2013) These human-wildlife conflicts and the conservation status of otters (Annex II and IVof the EU Habitats Directive) increased the need for accurate methods to get detailed knowledge on otter population parameters and status assessment. The concerns were mainly based on presumed individual differences in otter marking behaviour (Hutchings and White 2000; Kruuk 2006) as well as on seasonal and spatial differences in deposition and traceability of otter scats (Kruuk et al 1986; Reuther et al 2000) This methodological debate has never been solved and a standardised method how to extrapolate from scat abundances to otter densities is still missing, scat abundance is often used as a main indicator for otter population estimates and population trends It is overdue to evaluate this methodological issue in estimating otter abundance or density
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