Habitat assessment of a relict brown bear Ursus arctos population in northern Spain

ELSEVIER P I I : S 0 0 0 6 - 3 2 0 7 ( 9 6 ) 0 0 0 8 l - X

Biological Conservation 80 (1997) 17-22 © 1997 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0006-3207/97 $17.00 + .00

HABITAT ASSESSMENT OF A RELICT B R O W N BEAR Ursus arctos P O P U L A T I O N IN N O R T H E R N SPAIN

A n t h o n y P. Clevenger , a F r anc i s co J. P u r r o y b* & Migue l Ange l C a m p o s b

aDepartment of Forestry, Wildlife & Fisheries, University of Tennessee, Knoxville, TN 37901 USA and Departamento de Biologla Animal, Universidad de Ledn, E-24071, Lern, Spain

bDepartamento de Biologla Animal, Universidad de Lern, E-24071, Ledn, Spain

(Received 2 August 1995; accepted 23 March 1996)

Abstract We assess the habitat quality of a small remnant population of Eurasian brown bears Ursus arctos in northern Spain based on a habitat evaluation procedure and analysis of bear distribution and road density. Habitat evaluation was based on our knowledge of bear habitat requirements using a combination o f radiolocation and sign data and analysis o f four variables (forest cover, elevation, distance to nearest village, distance to nearest roadway). Herein, we use those results to calculate habitat suitability indices

for 1 km 2 U T M cells over 3500 km 2 of bear range, by summing scores (from 0 to 1) for each variable. Roughly 15% of the study area was classified as high-quality bear habitat. In secondary bear range (sporadic presence) the average unpaved road density was more than twice as high compared to that in primary, year-round, range, and was highest in areas outside o f the bear range. The applicability of our habitat evaluation procedure to other small remnant Eurasian bear populations is proposed. © 1997 Elsevier Science Ltd. All rights reserved

Keywords." bear sign, Cantabrian Mountains, conservation, habitat assessment, road density, travel corridors.

INTRODUCTION

The Cantabrian Mountains in northern Spain harbour one of four remnant populations of Eurasian brown bears Ursus arctos in southern Europe; the others are in France, Italy and Greece. The Cantabrian population is composed of two, similar-sized nuclei (Fig. la) which together cover an area of nearly 6000 km:, and numbers around 50-70 individuals (Servheen, 1990; Clevenger & Purroy, 1991; Naves & Palomero, 1993). Habitat within the bears' range is fragmented, varied in structure and quality, subject to intense pastoral activity, and recently affected by development of large reservoirs and forest roads. The way brown bears utilize the available habitats

*To whom correspondence should be addressed. 17

and the factors that may influence their selection are important for the future conservation and management of the species in Spain.

The Cantabrian population has been diminishing steadily during the last 100-150 years (Clevenger et al., 1987), and despite the Spanish government affording legal protection for brown bears since 1973 a recent study has shown that they are still declining in numbers (Clevenger & Purroy, 1991). Since 1989, governments from the four Autonomous Communities (Asturias, Cantabria, Castile-Lern, Galicia) within the Cantabrian bear range have passed local decrees which increase the protection for bears and involve recovery plans for the conservation and restoration of suitable habitat, including identification of 'critical areas'.

During the last decade, research in North America has focused on the effects of human activities on brown bear behaviour and habitat use (Zager & Habeck, 1983; McLellan & Shackleton, 1988, 1989; Mattson, 1990; Titus & Beier, 1991) so that these activities can be effectively managed. Habitat evaluation and GIS- generated models for North American bear ranges have also received much attention (see Contreras & Evans, 1986; Clark et al., 1993; Rudis & Tansey, 1995). In Europe, the few studies documenting brown bear habitat selection and the effects of human activity are limited to Croatia and Spain (Cicnjak, 1991; Clevenger et al., 1992), but procedures for habitat evaluation based on empirical data have not been developed.

Many resource managers in European countries with small, relict bear populations are faced with the problem of how to obtain this information. Few radio-tracking studies have been carried out, and non-intrusive data collection techniques can present logistical and methodo- logical obstacles for such a rare and wide-ranging species (Clevenger, 1994a).

The purpose of this study is to assess brown bear habitat in the Cantabrian Mountains, Spain. We are interested in: (1) using information obtained by a combination of intrusive (radio-telemetry) and non-intrusive (sign) methods to determine habitat suitability indices; (2) analysing the effect of road density on bear distribution;

18 A. P. Clevenger, F. J. Purroy, M. A. Campos

Bay of Biscay

~ ~ ~ . ~ t I A s t u r i a s ' " • • " . - ~ : . "L--~ ~ m b a a

(b)

r . J v -? ,~. F- _/ - -~ - ~ ' \_._. f~

Castlle-L~n

Fig. 1. (a) Brown bear population nuclei in the Cantabrian Mountains, Spain. (b) Bear distribution in the eastern nucleus

(R, Riafio core area; P, Palencia core area).

and (3) discussing how our habitat evaluation method might be applied to other small, relict Eurasian bear populations.

M E T H O D S

Study area Our study of habitat quality was conducted in the eastern population nucleus covering an area of c. 3000 km 2 and situated nearly exclusively (> 95%) on the south-facing slope of the Cantabrian Mountains (Clevenger et al., 1992). The area encompasses three Autonomous Com- munities, but predominantly lies in Castile-Le6n (Fig. 1 a). Elevations range from 600 to 2500 m. Vegetation of the area consisted of three broad communities: scrublands (Genista, Cytisus, Erica, Calluna spp.), grazing lands, and fragmented stands of mixed-deciduous forest of durmast oak Quercus petraea, Pyrenean oak Q. pyrenaica, and beech Fagus sylvatica.

Human activity in the area consists of livestock raising (the main economic activity in the region), small and large game hunting, small-scale coal mining in the lower elevations, and tourism which has recently been promoted by local governments. Human density in the core area of the brown bear range is roughly five people/km a.

Habitat selection by bears Data used in this paper were collected over a 6 year period. We investigated habitat selection by brown bears from 1985 to 1988 and detailed our findings in Clevenger et al. (1992). The methods and results of our habitat evaluation procedure are briefly discribed here.

Location data (n = 400) from one radio-marked, adult male bear and sign (tracks and scats; n = 105) from other non-marked bears were used to analyse habitat selection. We tested the null hypothesis that bears utilize habitats in proportion to their availability (Neu et al., 1974; Alldredge & Ratti, 1986), i.e. in relation, here, to vegetation type, elevation, distance to nearest village, distance to nearest roadway.

We found no significant difference between habitat use by the radio-marked bear and other bears in the study area. Throughout the year they showed a strong preference for forested habitats using mature beech and durmast/Pyrenean oak forest in greater proportion than their availability while other vegetation types were avoided. Bears also favoured areas with lower elevation disproportionately and tended to keep above and away from villages (mean =4-5 km) and roadways (mean = 3.9 km).

Habitat suitability measurement On the basis of these brown bear requirements in the eastern nucleus, we assessed habitat quality in the 3511 km 2 of the whole Castile-Le6n study area. The evaluation procedure uses a grid network and calculates a suitability index for each 1 km 2 UTM cell within the study area based on the four habitat variables described above. For each of the four variables, values from 0 to 1 were calculated in the following manner.

A forest cover value was determined by the proportion of deciduous forest occupying the cell. Pine Pinus sylvestris plantation cover received half the proportional value due to lowered food resource opportunities there compared to deciduous forests.

Mean cell elevation (minimum + maximum/2) within a preferred altitudinal band of l100-1400m was assigned a value of 1 while elevations outside this range received 0.

The distance to closest village and roadway were measured from the cell centre in two ways. If a village or roadway was present in the same valley as the cell centre, then we measured the distance directly; if not, then we measured the down-drainage distance to the nearest village or road, i.e. not across intervening ridges. We believe this procedure most accurately measures the effects of human activity on bear habitat choice in our mountainous study area since nearly all human-related intrusions (hunting drives, vehicles, etc.) originate from below and proceed up-drainage. Distances greater than 4-5 and 3.9 km to nearest village and roadway, respectively, were assigned values of I. Distances between zero and 4.5 km for villages, and zero and 3.9 km for roads, were each assigned proportional values between 0 and 1.

Brown bear habitat assessment 19

Roadways were defined as being all paved roads, including highways and rural roads.

For each cell, values were summed for the four variables to yield a total score and classified as one of five habitat suitability index categories (poor = <0.5; sub- average =0.6-1-5; average = 1.6-2-5; good =2.6-3.5; opt imum =>_3.6) to provide an overall habitat suitability index.

Travel corridors From information gathered while monitoring the movements of the radio-marked bear and others during the last 10 years, we were able to identify important travel corridors in the eastern nucleus. Habi ta t suitability indices were calculated for cells encompassing travel corridors.

Brown bear distribution and road density We analysed the influence of road density on bear distribution within a 5650 km 2 study area centred on the eastern Cantabrian nucleus (Fig. lb) based on 25 years' combined experience of bears in the region. Two range types were recognized: pr imary range consisting of areas used by bears all year round, and secondary range for areas only sporadically used. The rest of the study area was unoccupied. The pr imary range was composed of two disjunct areas, the Riafio and Palencia core areas, surrounded by secondary range. Roads passable by two-wheel-drive vehicles were classified as paved or unpaved; other tracks were disregarded. The mean road density was calculated for each road type within 5 km 2 cells in all three areas.

RESULTS

Habitat evaluation Opt imum brown bear habitat was found in only 43 cells (1.2% of total area), whilst 434 cells (12.3%) contained good habitat (Table 1). Nearly half of the study area (1652 km2; 47%) received an average suitability index, while sub-average and poor habitat categories occupied 30.3 and 9-0% of the study area, respectively. The mean habitat suitability index for the entire area was 1.7, which was considered between sub-average and average bear habitat.

Travel corridors Corridors were situated in high mountain passes between forested valleys. Most of the corridor cells had below-average habitat suitability indices because they were at high elevations and in non-forested habitats. However, since effective travel corridors are basically extensions of core reserves they merit the same protection status as critical habitat. All cells containing travel corridors were therefore assigned an opt imum habitat suitability index so that they could be included as habitat vital to bear conservation.

Table 1. Frequency of brown bear habitat suitability index categories in the Castile-Le6n eastern nucleus, Cantabrian

Mountains, Spain

Assessment category No. of 1 km 2 cells Percentage

Optimum 43 1.2 Good 434 12.3 Average 1 652 47-0 Sub-average 1 065 30-3 Poor 317 9-0 Total 3 511 100-0

Table 2. Density (km/km 2) of roads in various areas of the occupied and potential brown bear range in the eastern

Cantabrian Mountains, Spain, 1992

Range Area Mean Mean Total (km 2) paved road unpaved mean road

density road density density a

Occupied 2 175 0-22 0.48 0.71 Primary 575 0-26 0.28 0.55 Riafio 450 0.27 0.26 0.53 Palencia 125 0.24 0-34 0.58 Secondary 1 600 0.22 0-57 0-79 Not occupied 3 475 0.21 0-63 0-85

~Paved + unpaved road density.

Brown bear distribution and road density The area of occupied brown bear range totalled 2175 km 2 and the mean density of both road types together was 0.71 km/km 2 (Table 2). The two disjunct parts of the primary range were disparate in size and the Riafio and Palencia core areas had similar mean paved road densities though the mean unpaved road densities differed considerably. The secondary range represented c. 74% of the total range and had a 44% higher total road density than the primary range. The area uninhabited by bears was about the same as the total occupied area and had 20% higher road density. However, the most important result was that mean unpaved road density was more than twice as high in secondary range compared to pr imary range, and even higher in unoccupied ranges.

We also measured road densities in the Riafio area from maps prepared between 1942 and 1944 (1:50,000; Instituto Geogr~ifico y Catastral) and compared them with present densities. The paved road density was substantially lower 50 years ago (0-18 vs 0.27 km/km2), but more importantly, unpaved road densities have increased more than five times since then.

D I S C U S S I O N

Habitat evaluation Our approach to evaluating Cantabrian brown bear habitat provides a landscape analysis of the present condition and location of key areas within the eastern nucleus. The procedure is simple, hand-generated, inexpensive, and may be a useful tool for resource


managers in need of a rapid assessment of habitat condition within bear territory.

In our analysis, location data from bear sign were treated in the same way as radiolocation data. We believe that this is applicable to other populations of bears, or other terrestrial carnivores, if some important data collection requirements are met. Data collection must follow a sampling design which assures that the entire study area is sampled, and sampling effort is distributed in proport ion to habitat type. It is also crucial to maximize independent observations of sign, i.e. minimize repeated observations from the same animal by spacing out samples over time or space (see Alldredge & Ratti, 1992; Kendall et al., 1992; Morrison et al., 1992). Finally, bear sign must be sufficiently abundant to meet the requirements for habitat analysis and assumptions for subsequent statistical testing (Alldredge & Ratti, 1986).

Historically, resource managers responsible for the protection and management of relict bear populations in southern Europe have been reluctant to carry out studies using radio-telemetry for various reasons: mortality risks due to handling, elevated costs involved, or lack of experience and direction. The use of sign rather than capture and radiomarking, or a combination of the two, may provide a viable alternative (see Hamer & Herrero, 1987). Furthermore, when sign surveys are properly designed, and sample sizes sufficiently large, they can also provide reliable estimates of changes in population size over time (Kendall et al., 1992; Zielinksi & Stauffer, in press) as well as information on habitat and diet characteristics (Clevenger, 1994a,b).

A large sample size of radio-marked bears does not have to be a prerequisite for obtaining valuable information for habitat management and conservation planning. Moreover, large sample sizes are generally unattainable when working with low density or remnant carnivore populations. The habitat assessment technique reported herein combined radiotelemetry and sign data from a small population of widely dispersed bears. Despite any inadequacies of our technique, we feel that it is important for resource managers in the Cantabrian Mountains to act quickly and begin making management decisions based on existing information rather than waiting until improved methods or additional data become available. By adopting an adaptive management philosophy, resource managers can improve recovery and habitat management plans when that information does become available (Waiters, 1986).

Brown bear habitat in the small, remnant southern European populations is still under increasing pressure from development (e.g. road construction, large reservoirs, open-pit mining, etc.) which seriously threatens to accelerate fragmention and the potential for the recovery and long-term survival of the species. We urge others working in these populations to begin bear habitat relationship and evaluation studies, with the aim of

developing land management plans in occupied and potential bear ranges.

The preservation of travel corridors between and within populations to minimize local extinction and genetic isolation is critical to any effective wildlife conservation plan (Harris, 1984; Harrison, 1992; Beier, 1993). Brown bear movement patterns need to be monitored and travel routes identified in order to protect travel corridors and maintain interchange within increasingly fragmented and isolated populations like these in the Cantabrian Mountains. Harris (1984) suggested that to enhance the connectivity of a habitat island system one might strategically locate smaller patches of old-growth (in the Cantabrian case, deciduous) forest to serve as 'stepping stone' islands between larger centres of dispersal. Failure to do so can lead to genetic isolation and local exinction (Simberloff, 1988; Sharer, 1990; Noss & Cooperrider, 1994). This approach should be applied in the Cantabrian Mountains as remnant deciduous forests are highly fragmented in both nuclei and continuity is tenuous. Obeso & Garcia Manteca (1990) reported that the mean patch size of beech forest in Asturias (western nucleus) was 232 ha (total coverage =63,085 ha), whereas for oak forest it was 196 ha (total coverage =35,737 ha). Distances also were greater between oak patches than beech patches. These are extremely small forest fragments for the western nucleus and are most likely the same or even smaller in the eastern nucleus (personal observations).

Brown bear distribution and road density Human activity associated with roads has been shown to have detrimental effects on brown bear populations in North America (Mattson et al., 1987; McLellan & Shackleton, 1988, 1989; Titus & Beier, 1991) and in Europe (Elgmork, 1978; Zunino, 1981; Cicnjak, 1991). On Chichagof Island, Alaska, Titus & Beier (1991) showed a positive correlation between autumn mortality of brown bears and cumulative kilometres of road construction between 1978 and 1989. In a heavily hunted western North Carolina black bear U. americanus population, Brody & Pelton (1989) showed that bears selected areas with low road density and vehicle traffic, and suggested that as road density increases so does the probability of bear-human encounters and increased mortality rates. Schoen et al. (1994) came to the same conclusion regarding brown bear use of salmon Oncor- hynchus spp. streams in uncut and logged watersheds with varying road types and densities.

Although our findings are similar and seem convin- cing, they are not conclusive and should be considered preliminary until the same analysis is conducted over more of the Cantabrian range, including the unoccupied region between the two nuclei. Roads probably do not prevent brown bears from inhabiting an area but they certainly increase the risk of deliberate or accidental killing of bears by humans.

Brown bear habitat assessment 21

C O N C L U S I O N S

The scarcity of high-quality brown bear habitat in our study area is evidence of a highly fragmented and widely dispersed deciduous forest component in addition to an important road network. Conservation goals should be to minimize further forest fragmentation and road development. Strategic travel corridors between core areas should be promoted by regeneration of deciduous forests, and efforts should be made to protect existing stepping-stone habitats within these corridors. Survival of brown bears in the Cantabrian Mountains will depend heavily on cooperation and coordination of recovery plans among the four Autonomous Community governments.

A C K N O W L E D G E M E N T S

Research on Cantabrian brown bear habitat relationships was funded by the US Spain Joint Committee for Scientific and Technological Cooperation, the Wildlife Conservation Society, and a grant from Spain's Ministry of Education and Science (DGICyT), Project PB91- 0680. The Consejeria de Medio Ambiente of Castile- Ledn financed the habitat evaluation project, during which the senior author was supported by a grant from the Spanish Ministry of Education and Science (CAI- CyT). We thank Dr B. N. K. Davis and two anonymous referees for their helpful comments and suggestions for improving the paper. This paper was originally presented at the 9th International Conference on Bear Research and Management, Grenoble, France.

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Habitat assessment of a relict brown bear Ursus arctos population in northern Spain