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Considering sampling approaches when determining carnivore diets: the importance of where, how, and when scats are collected. Robin Steenweg & Michael ...

Mamm Res DOI 10.1007/s13364-015-0222-4

ORIGINAL PAPER

Considering sampling approaches when determining carnivore diets: the importance of where, how, and when scats are collected Robin Steenweg & Michael P. Gillingham & Katherine L. Parker & Douglas C. Heard

Received: 9 October 2014 / Accepted: 17 February 2015 # Mammal Research Institute, Polish Academy of Sciences, Białowieża, Poland 2015

Abstract Understanding the diets of carnivores is often required to inform their management, to conserve their prey, or to minimize depredation of domestic animals. Scat analysis is one of the oldest and most commonly used methods for determining carnivore diets and many accuracy issues associated with scat analysis have been addressed in other studies. Little attention, however, has been given to the questions of where, how, and when scats are sampled in the field and how these factors can affect conclusions about diet. Based on a review of 64 articles, the two most common sites for collecting scats from grey wolves (Canis lupus) are from wolf homesites (i.e., dens and rendezvous sites) and from along roads. Collections from such areas are often combined and interpreted as depictions of a seasonal diet, with no acknowledgment of the inherent problems associated with sampling at different sites. Rather than combining these two samples of scat, however, we were interested in comparing them. In our field study, we supported our hypothesis that the frequency of prey types differs significantly between wolf scats collected along roads and scats collected at wolf homesites and, therefore, would tell different stories about summer wolf diet if

scats had only been collected from one of these sites. This difference is likely due to a combination of three interactions of wolf ecology with our sampling design: local prey availability, timing of scat deposition, and movement behavior. This difference is especially of interest when considering three findings from our review: descriptions of sampling design are often omitted, scats collected from different sites are often combined into a single estimate of diet, and scats are often collected opportunistically. Given the results from our field sampling, these three common practices may significantly affect how carnivore diet data should be interpreted. Furthermore, using our own idiosyncratic study as an example, we discuss other common assumptions in the scat diet literature, including assuming that diet is constant across packs and years. To increase the reliability of diet estimates from scats, researchers should clearly articulate where, how, and when they collected scat samples and should discuss the assumptions of the chosen sampling design so that potential biases and inaccuracies can be assessed. Keywords Canis lupus . Carnivores . Dens . Diet . Roads . Scat analysis . Wolf

Communicated by: Dries Kuijper R. Steenweg (*) : M. P. Gillingham : K. L. Parker Natural Resources and Environmental Studies Institute, University of Northern British Columbia, Prince George, BC V2N 4Z9, Canada e-mail: [email protected] D. C. Heard British Columbia Ministry of Forests, Lands, and Natural Resource Operations, Prince George, BC V2N 4W5, Canada Present Address: R. Steenweg Wildlife Biology Program, University of Montana, Missoula, MT 59812, USA

Introduction Scat analysis is commonly used to document diets of carnivores (Litvaitis 2000), with the goals of increasing understanding of prey selection (Kumaraguru et al. 2011), quantifying predation on livestock, gaining insight into humanwildlife conflicts (Woodroffe et al. 2005), or managing predation on threatened species (Latham et al. 2011). Scat analysis is an appealing method for determining diets because collection is non-invasive, in contrast to other methods such as

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stomach-content analysis, and because samples are often abundant at predictable locations (Marucco et al. 2008). Furthermore, scat analysis is not as expensive as marking individuals to determine kill sites of prey (Peterson and Ciucci 2003). Despite the increasing sophistication of other non-invasive methods for diet determination (e.g., stableisotope analysis; Urton and Hobson 2005), scat analysis remains popular because scat samples are often easier to collect than hair and tissue samples for isotopic diet analysis, it is low-cost, and the detection of individual species and age classes of prey is possible (Nilsen et al. 2012). Many problems with scat analysis have been identified and addressed in the literature. For example, the ratios of digestible and indigestible portions of food classes can be calculated using conversion factors (Reynolds and Aebischer 1991), but scat analysis may still over represent small-bodied prey and under represent large-bodied prey due to differences in digestibility (Carbyn 1974; Mech 1966). This bias can be reduced by converting data from frequency occurrence to biomass consumed, which is also more ecologically relevant (Klare et al. 2011). Another concern with scat analysis is differentiating among scats from different species. Protocols have been developed to distinguish between similar-looking scats such as those of wolves (Canis lupus) and coyotes (C. latrans) (Weaver and Fritts 1979) and wolves and foxes (Vulpes vulpes) (Marucco et al. 2008). The reliability of observers’ abilities to distinguish among prey species during microscopic analysis also has been questioned, and blind testing to verify abilities is recommended (Ciucci et al. 1996; Fritts and Mech 1981; Spaulding et al. 2000). Sample-size requirements to achieve a desired level of precision or power can vary across species due to differences in diet diversity (Trites and Joy 2005; Williams et al. 2002). Regardless of how scats are sorted and analyzed, however, little has been documented about how the location of where scats are collected may affect results of dietary estimates. Ideally, when collecting scats, the sample should be representative of the greater population of scats on the landscape, which can be attempted by using a random sampling design. Two ways to randomly collect scats is to walk transects (e.g., Darimont et al. 2008) or following snow tracks in winter (e.g., Marucco et al. 2008), provided the process of selecting transects or tracks is random. Such practices, however, are often time intensive, and samples can be difficult to obtain because the distribution of scats on the landscape is usually clustered. It is often desirable, therefore, to focus scat collection in areas of higher scat concentration. Such areas include dens (e.g., Meriggi and Lovari 1996) and roads (e.g., Barja 2009). Scats also can be collected during visits to concentrated GPS locations of radio-collared animals (e.g., Latham et al. 2011) or opportunistically when performing other research. Many studies of carnivore diet implicitly assume that their chosen scat sampling design will produce a representative

sample of scats that permits inference to be made to the entire population of wolf scats, and thus, to the wolf population of interest. Our overall objective was to test this assumption and evaluate if estimates of carnivore diet were affected by where, how, and precisely when scats were collected in the field. Estimates of carnivore diet may differ, for example, when scats are collected from different sampling locations, even during the same season. We hypothesized that samples of wolf scat collected during summer from two different locations would be significantly different from each other. To test our hypothesis, we collected and analyzed summer scats for multiple wolf packs from the two most common sampling sites: along roads and from wolf homesites. Homesites are defined as dens and rendezvous sites where pups are kept in summer while members of the pack are hunting prey throughout their territory (Joslin 1967). Because true wolf diet is unknown, a difference between diet estimates indicates that one, or both, of our estimates of summer wolf diet is inaccurate. If the choice of sampling site leads to unrepresentative samples, then these biases should be acknowledged when making inferences about carnivore diets.

Materials and methods To understand where carnivore scats are most often collected, we reviewed articles, books, reports, and theses (hereafter, simply referred to as articles), which used scat analysis to characterize wolf diets. We included all articles found using Web of Science searches that we could access electronically or in hard copy. In addition, we included articles in the compilation of literature on wolf diets by Peterson and Ciucci (2003). We conducted fieldwork in the Parsnip River study area (PRSA), located in north-central British Columbia (BC), Canada (~55° N 122 °W), 100 km northeast of Prince George, BC. The Parsnip River bisects the study area. To the southwest is a plateau of rolling hills with mixed forests in the sub-boreal spruce (SBS; Picea mariana and Picea glauca) biogeoclimatic zone (Meidinger et al. 1991); elevations range from 600 to 1650 m. To the northeast lie the Central Rocky Mountains, with valleys that run down to the Parsnip River; this area is characterized by the SBS zone at lower elevation (~700–1100 m) and an Engelmann spruce-subalpine fir (Picea engelmannii, Abies lasiocarpa) zone at upper elevations (1100–2500 m; Coupe et al. 1991). The study area has a long history of logging, and much of the area is in early-seral growth stages, although little logging has occurred above 1100 m. In the past 40 years, ~4 % of the PRSA has been harvested for timber on the Central Rockies side of the Parsnip River, and 21 % has been harvested on the plateau side (Steenweg 2011). The PRSA has a full complement of large carnivores: wolves, grizzly bears (Ursus arctos), black bears (Ursus

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americanus), cougars (Puma concolor), lynx (Lynx canadensis), and wolverines (Gulo gulo). Approximately 35 wolves in 7 packs inhabit the study area. We expected the main prey for wolves to be moose (Alces americanus), mountain caribou (Rangifer tarandus caribou), and beaver (Castor canadensis). Moose densities were high in the study area, estimated to be 2600±1200 (mean±95 CI) animals in 2005 with a density of 1.18 moose/km2 (Walker et al. 2006) and declining to 1200 ± 300 or 0.47 moose/km 2 by 2009 (Gillingham et al. 2010). Mountain caribous occupy the mountainous terrain in the PRSA away from roads and cutblocks, selecting mid-to-high elevations and rarely descending below 1100 m (Jones 2007; Jones et al. 2007). Caribous were at low density (0.048 caribou/km2; Steenweg et al. 2009, updated as per D. C. Heard, unpublished data) during our study. Reports from local trappers indicated that beavers, which occupy ubiquitous riparian areas surrounding the Parsnip River and tributaries, are abundant (F. Booker, personal communication). Other ungulates available to wolves, such as elk (Cervus elaphus), deer (Odocoileus spp.), mountain sheep (Ovis spp.), and mountain goats (Oreamnos americanus) are uncommon. In the PRSA, we collected separate samples of wolf scats from the two most common collection sites: one sample from wolf homesites and one sample from along roads. We located homesites of GPS-collared wolves (see collaring details in Steenweg 2011) and visited them in summers 2008 and 2009 after wolves had vacated the areas; homesites consist of both dens and rendezvous sites (Joslin 1967) and are areas where pups are left while adult wolves leave to hunt during late spring and early summer. A pack’s choice of homesite location changes most years, therefore, we could only collect scats from packs that were currently collared and assumed that all scats had been deposited during the year they were collared. These areas were thoroughly searched by three researchers, and we collected all scats found. To obtain a sample of scats from along roads, two observers drove ≤25 km/h to visually locate and discard all scats on the roadbed of all major forestry roads in the study area (210 km) in early June 2009. In early- to mid-October 2009, we again drove these roads, but collected all wolf scats on the road for analysis. The length of road within each pack’s home range varied (~20–35 km; Steenweg et al. 2009); therefore, to spread out pack representation in the final road scat sample, we limited the maximum number of scats we believed to be from a single pack. Some studies suggest that a minimum sample size of 59 is required for adequate statistical power to detect differences between two carnivore diet samples (Trites and Joy 2005). Power also depends on the diversity of the species’ diet; for example, seals have higher diet diversity than wolves and thus require smaller sample sizes for the same level of precision (Williams et al. 2002). During preliminary analyses, a sample size of 40 scats was determined to be sufficient to identify all prey

species, including those that were rare (Steenweg 2011). We randomly selected 40 scats for analysis from all scats found on roads for each pack or at homesites for each pack. If fewer than 40 scats were collected for a pack, we analyzed all scats for that pack. We were not concerned with confusing wolf scats with coyote scats because coyotes were rare in the PRSA. We chose, however, not to collect small-diameter (

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