How to estimate wolf numbers without snow

In areas with dense cover or little snow, the wolf is one of the most difficult animals to census. In the Iberian Peninsula and other southern regions of wolves’ global range, snow is rare in winter, so wolves must be surveyed without snow. Spanish researchers Blanco and Cortés described in a recent paper ¹ the diverse methods used in Spain and Portugal to census wolves and made a critical review on them, exploring the sources of error when estimating number of packs, the limitations of the simulated howling method, the impossibility of knowing the percentage of wolves living outside packs without using intensive radiotracking, and the difficulty of assessing average pack size.

Spain, Slovakia, Poland, and Finland have made an exception on wolf protection, shifting the wolf from the Bern Convention’s Appendix II (Convention on the Conservation of European Wildlife and Natural Habitat 1979), “totally protected species”, to Appendix III, “protected species that can be subjected to controlled hunting”. Since in Spain each Autonomous Region has the right to decide its own policies, the Spanish wolf is hunted in six of the nine regions in which wolves breed. Moreover, each autonomous region decide, among other issues, if they survey or not the wolf population and the interval between surveys.

In Spain there is a large wolf population (few hundred packs; see figure 2 and table 1), distributed over more than 100.000 km² in very different habitats —from well preserved mountain regions to agricultural and densely populated areas—. Apart from the first wolf survey carried out in 1987 and 1988 ², no other national survey has been conducted. Thus, the wolf population estimate in Spain is the sum of the most recent surveys carried out in every autonomous region.

Table 1. Number of packs detected in the Autonomous Regions of Spain and in Portugal. Credit: Blanco and Cortés (2012).

Almost all wolf counts are based on estimating the number of reproductive units, i.e. wolf packs. In some countries in Europe, such as Scandinavia and Poland, snow has been crucial to estimate wolf populations. In the small Italian Alpine population, although wolf census are mainly based on faecal genotyping, snow is needed to track wolves and collect their scats. In North America the most common method used during the last 30 years has combined estimates of occupied range and pack size —carried out in winter when snow is present—, with the aid of aerial radiotelemetry (following radio-collared individuals). However, in the southern parts of its range, where snow is absent in most of the regions, such as in Spain and Portugal, the number of wolf packs is estimated by locating the litters of pups in the summer or early autumn when they are at rendezvous sites. Wolf pups live in the den during their first 8 weeks, but during this period their mother might move them from one den to another. From about 8 to 20 weeks of age pups inhabit an area above ground that includes a “nest” or nests where they huddle together, a network of trails, and various play areas. They are called by wolf researchers rendezvous sites or loafing sites ³.

The field work that will eventually lead researchers to finding rendezvous sites involve three main activities: a) Interviews with outdoorspeople, such as shepherds, naturalists, wardens and hunters, in order to gather information on wolf presence; b) Carry out transects on foot or by driving along forest roads to detect wolf sings (tracks, scratches or scats); c) Conduct sit-and-wait sessions from vantage points and imitating wolf howling in order to confirm pup presence when pups respond by howling as well.

The estimate of the total number of wolves in an area is usually given by [(A x B) + C], where A is the number of packs, B is the average pack size (average number of wolves living in each pack), and C is the percentage of wolves living out of the packs (solitary and pairs). Although this formula seems very simple, several difficulties arise in Spain when estimating each variable.

Potential errors in estimating the number of packs

• In large study areas the main source of error is failing to detect some packs. The authors’ experience has shown that sometimes, even with several radio-collared individuals in one pack, it is difficult to discern where a pack finishes and where the next one starts.

• By counting wolf responses to human (or recorded) howling, the number and location of packs can be inferred and this is a good technique to locate wolf packs on a small study area. However, surveys carried out in Spain produced a percentage of packs which were unconfirmed (i.e. the presence of pups has not been confirmed). Thus, extrapolations of this index of wolf abundance to true wolf numbers may turn out inaccurate.

Potential errors in estimating pack sizes

• Estimating the average number of wolves per pack is a complicated endeavour because members of packs search for food alone or in small groups, and it is unusual to observe all of the pack members together. The main factors assembling the members of the pack are snow and large prey species in winter, but since in Spain snow is scarce, wolves in winter usually travel alone or in small groups, as they do in the summer.

• In the summer the restrictions imposed by dense vegetation, the mainly nocturnal activity of wolves and the rendezvous sites particular attendance patterns (variable and unpredictable), make it difficult to count all wolves in the pack. Therefore, pack size estimations in summer are usually considered as minimum numbers.

• The average pack size can suffer large variations as a consequence of changes in food availability, hunting pressure or the recovery of a population after heavy human-caused mortality.

Potential errors in estimating solitary wolves

• An important percentage of a wolf population can be solitary —dispersers or floaters individuals—. These solitary wolves are very difficult to estimate with the population surveys used in Spain and Portugal. In agricultural habitats of Castilla y León, for example, the authors found that the 14 wolves radio-collared spent 28% of the radio-days living out of the packs, alone or in pairs. Moreover, the presence of floaters and pairs settled in the interstices of packs’ territories may confound the pack delimitation.

In summary, in order to detect an accurate pack size and number of packs in a country with a climate and political idiosyncrasy such as Spain, the authors recommend more intense ecological studies with radiotracking and genetic analyses. When wolves are hunted, the impact of wolf removal varies a great deal between populations with large packs and a lot of dispersers ready to replace the missing pack members, and populations with small packs and absence of solitary wolves. Thus, by using the methods described above it is very difficult to assess wolf population changes derived from changes in food resources or an increase of wolf hunting.

In my opinion, while in scientific studies accurate estimates are always necessary, in management and conservation planning the population trends might sometimes be sufficient to decide on relevant conservation actions.