Introduction

Population size is one of the basic attributes of populations, but it is often difficult to estimate. Reliable population size estimation is essential for understanding the demography and conservation status of a population, especially if the species under investigation is rare or cryptic, or may be required in the context of pest control (Macdonald et al. 1998, Frantz et al. 2004).

The Eurasian beaver (Castor fiber), a keystone species, has been reintroduced over much of its former range following near-extinction. Evidence has repeatedly shown that beavers can provide important riparian biotic and abiotic ecosystem services.Theircapacitytomodifytheirenvironment can cause conflict, especially in intensively-used agricultural landscapes (Campbell-Palmer et al. 2016). Eurasian beavers originally inhabited rivers and lakes in most countries of Europe and North Asia until the Middle Ages. Due to excessive hunting, however, their populations have dramatically decreased (Nolet & Rosell 1998). Since the 1900s, when Eurasian beavers were nearly extinct, they have since shown recovery (up to ∼ 1.5 million) throughout much of their former range as a result of protected natural recolonization along with active reintroductions (Halley et al. 2020). In Poland, beavers were formerly close to extinction, but currently their population is estimated at over 100,000 animals (Central Statistical Office 2016). The observed increase was triggered by the introduction of the Active Beaver Protection Plan (Żurowski 1979). As a species, the beaver is protected by Polish and international law. Restoration of the beaver population was so effective that the species' status was changed from full to partial protection (Giżejewski & Goździewski 2016).

The recovery of the Eurasian beaver throughout western and central Europe is considered a major conservation success (Frosch 2014). In parts of Europe, beavers have returned to heavily-populated and intensively-managed river catchments, leading to human-beaver conflict and management issues. Beavers inhabit various habitats, which include agricultural and urban landscapes (Campbell-Palmer et al. 2016, Auster et al. 2019). Beavers live in family units, typically made up of an adult breeding pair with offspring from the current and previous years; such a unit ranges, on average, from two to seven individuals in total (Wilsson 1971). Due to the continued expansion of the beaver population (Batbold et al. 2016), places in which these animals exist should be constantly monitored, and the number of individual families should be systematically examined.

Most rodent species are difficult to observe due to their biological characteristics, strong anthropogenic pressure over time, and/or destruction of habitat. As a result, they inhabit remote areas, shift their activity to dusk or night hours, or live underground. Studying rare and elusive species inhabiting patchy environments is always a challenge for researchers. The data collected about their behaviour and activity is often indirect (mainly tracks and other signs of animal presence), and can be subjectively interpreted (Cutler & Swann 1999). Direct methods that relay on counts of the animals themselves (e.g. capture-mark-recapture, radio-tracking, spotlight, counts) are generally inadequate because of cost, staffing and permission requirements, are not readily applied to all habitats, and cannot easily be used by volunteers (Sadlier et al. 2004). Consequently, only limited areas can be surveyed with limited personnel. Therefore, the study of such animals using non-invasive genetic tools appears to be well-suited for this purpose.

The European beaver is an example of such a hard-to-access species. Finding beaver habitation is not difficult due to their visible and characteristic building activity. However, direct counting of individuals living in dens or lodges may be difficult, especially because of their secretive lifestyle and nocturnal activity.

Recently, glue or barbed-wire hair traps have been used to remotely pluck hair from terrestrial animals such as brown bears (Ursus arctos) (Woods et al. 1999), European badgers (Meles meles) (Frantz et al. 2004, Scheppers et al. 2007, Balestrieri et al. 2010), European wildcats (Felis silvestris) (Anile et al. 2012), martens (Martes Americana) (Mowat & Paetkau 2002) and Eurasian lynx (Lynx lynx) (Davoli et al. 2013), as well as from beavers and river otters (Lontra canadensis) (e.g. Depue & Ben-David 2007, Herr & Schley 2009). These findings showed that hair traps, when constructed according to the specific characteristics of the studied species, are an excellent alternative to invasive DNA sampling methods. According to studies on other species, DNA from a single guard hair may be sufficient to carry out genetic analyses, such as microsatellite-based genotyping (Scheppers et al. 2007). Moreover, DNA samples may be used as a material to determine species, sex and the genetic structure of the population, as well as the genealogy of specific individuals (Woods et al. 1999). Hair traps can also provide material required for stable isotope analysis (Magioli et al. 2018).

We were interested in learning whether the modified field methods presented by Herr & Schley (2009) were effective for beavers in Poland. We studied different passive hair trap constructions illustrated by Balestrieri et al. (2010) and Scheppers et al. (2007).

Hair traps used by Herr & Schley (2009) consisted of ordinary, two-stranded barbed wire with four-point barbs, with 15 cm between barbs, with the wire was attached to the stems of woody vegetation. Balestrieri et al. (2010) and Scheppers et al. (2007) constructed traps that consisted of two iron rods (5-6 mm in diameter and 70-80 cm long), supporting a single strand of barbed wire with four prongs per set of barbs and 10 cm spacing between barbs; these traps were placed about 20 cm above ground level.

In this study, we report the results of a trial of using passive hair traps as a tool for non-invasive and cost-effective hair sampling, for later genetic analysis of material obtained from European beavers coloniing the area of Stobrawa Landscape Park (southwestern Poland).

Fig. 1.

A passive hair trap suspended on a wooden stem, near the River Stobrawa (photo Marta Sobkowiak).

Material and Methods

Field sampling

Traps were set at two beaver sites. The first site was on the bank of the River Stobrawa (50°50′24.4″ N, 17°38′32.6″ E), and the second near a small pond adjacent to a cornfield (50°50′01.7″ N, 17°37′20.6″ E).

The traps were constructed using two-stranded barbed wire with a diameter of 1.7 mm, with groups of four barbs every 10 cm. The wire was cut to an appropriate length depending on where it was set. Two methods of attaching traps were used, depending on the situation. In the first the wire was attached to shrubs or trees at the trap site, as in Herr & Schley (2009). In the absence of suitable vegetation, another method was used; the wire was fixed to metal frames or stakes; as in Scheppers et al. (2007) and Balestrieri et al. (2010), 1-2 m from the water's edge. The trap was set about 20 cm above ground level, at such a height that only hairs were sampled. The barbed wire was sufficiently flexible so as to avoid scratching the animals (no signs of blood were observed during hair trapping) (Fig. 2). Thus, no beavers or other wildlife were harmed following the placement of the hair traps or in the process of collecting hair samples. In contrast to Herr & Schley (2009), we did not use bait to attract the animals. Beavers emerging from water onto the land, or returning to the water, passing under the trap, left a sample of fur containing both guard hair and undercoat (Fig. 3). The number of traps varied between individual trips (Table 1).

Fig. 2.

Location of beaver occurrence in the Stobrawa Landscape Park (southwestern Poland) (photo Marta Sobkowiak).

Fig. 3.

A beaver hair sample containing undercoat and guard hair (photo Marta Sobkowiak).

The first day of each trip was devoted to setting the traps, which were then checked during the following days. Traps were placed at the study areas for two days on each trip. Our research was not conducted on a continuous basis, but in the form of ten separate trips. A hair sample was defined as all the hair collected from a single barb, including both guard hairs and undercoat fur. Samples were placed in test tubes with ethyl alcohol, and stored at –20 °C.

Results

The research was conducted for 27 days and was divided into ten separate field trips. The total number of control days was 17, with the remaining ten days devoted to setting the traps on the first day of each trip.

The total number of samples obtained was 12 (Table 1), translating into a hair-trapping success of 0.7 samples per control day. Eleven samples were collected in the spring (from March to April), and one in the winter (from December to February). Seven fur samples were collected at the first site, and five samples at the second site. The majority of samples included both guard hair and undercoat; only one of them consisted only of undercoat, and one sample was a single guard hair. When only considering samples with guard hairs, trapping success was 0.65 samples per day (Table 1). All hair samples were collected using the modified hair traps presented by Herr & Schley (2009). Based on hair morphology, all collected hair samples were assigned to a target species (Pucek 1984).

Discussion

The results of this study demonstrated that obtaining hair samples from the European beaver in a noninvasive way is an effective method. As indicated by Anile et al. (2012), the effectiveness of hair traps depends on the studied species, duration of the research, and the ecological characteristics of the studied area. Taking into account the conditions in which beavers live, the trap model made by Herr & Schley (2009) seems to be the most effective for this species. Metal rod based hair traps, constructed like those used by Balestrieri et al. (2010) and Scheppers et al. (2007), turned out to be ineffective. The ground near the river bank was too unstable for a trap with such a structure to remain intact. In a few cases, beavers passing under the trap pulled it from the ground. In addition, periodically rising water levels sometimes flooded the traps located near the river bank. Each overturned or sunken trap was repaired or replaced with a new one. In all of these situations, we were able to retrieve the hair traps.

Table 1.

The summary of hair trapping data collected at two sites in the Stobrawa Landscape Park (SLP), Poland.

The average number of samples taken was 0.7 per control day, a similar number to that obtained by Herr & Schley (2009) (0.87 samples per control day). In addition to setting hair traps, in the previous study researchers used bait in the form of cut, suspended apples. Many researchers showed that first hair samples appeared a few days after the trap had been set (e.g. Herr & Schley 2009, Magioli et al. 2018). In our research, the first hair samples were collected on the day after the traps were set.

In this study we show that the best time for hair sampling from beavers is early spring, since almost all samples were obtained in March and April. This finding confirmed the fact that these animals show the highest activity at times of herbaceous vegetation growth. No samples were collected during winter; this might have been a result of a decrease in activity by beavers in trapping sites. According to Dzięciołowski (1996), the average daily activity of beavers significantly decreases after water freezes. During this time, animals stay in their burrows and consume stored supplies, without the need to go ashore. In spring, almost all of the studied area, at both sites, was abundantly grown with herbaceous vegetation, of which the most dominant species was nettle (Urtica dioica L.).

Traps located in sites covered with nettles did not provide any samples, and hair samples were obtained from places that was not covered in vegetation, possibly indicating that beavers may express aversion to nettles.

Studies on rodent populations are often difficult to perform due to their nocturnal activity (Green et al. 2013). Direct observations at night are inefficient, and live-trapping is an expensive and time-consuming process (Alasaad et al. 2011). In addition, trapping affects animals negatively, being a strong stress stimulus (Moncrief et al. 2008). As demonstrated in our research, in the case of protected and endangered species such as beavers, passive acquisition of genetic material is a cheaper and effective alternative. In Poland, the legal provisions regarding species protection stipulate that beaver live-trapping is only allowed between October 1 and March 15. Therefore, the use of hair traps not only limits negative impacts on animals (Woods et al. 1999), but also creates the possibility to conduct research for a longer period than the legally-permitted capture of these animals.