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Facilitative interactions between the Eurasian badger (Meles meles), the red fox (Vulpes vulpes), and the invasive raccoon dog (Nyctereutes procyonoides) in Białowiez˙a Primeval Forest, Poland R. Kowalczyk, B. Je˛drzejewska, A. Zalewski, and W. Je˛drzejewski
Abstract: Based on radio-tracking of Eurasian badgers (Meles meles (L., 1758)), red foxes (Vulpes vulpes (L., 1758)), and raccoon dogs (Nyctereutes procyonoides (Gray, 1834)) and observations at burrows conducted in Białowiez˙a Primeval Forest (eastern Poland) in 1996–2002, we addressed the hypothesis that facilitative interactions between a native (badger) and an alien (raccoon dog) species contributed to the invasion success of the latter. In winter, 88% of badger setts were occupied by both badgers and raccoon dogs, 4% by badgers and red foxes, and 4% by all three species. In summer, only 20% of badger setts were cohabited by other carnivore species (10% by raccoon dogs and 10% by foxes). Duration of occupation of badger setts by raccoon dogs averaged 117 days (SE = 21 days). Seasonal variation in raccoon dog use of badger setts was explained by changes in ambient temperature: the lower was the temperature, the higher was the rate of sett occupation by raccoon dogs. When wintering in the same sett, badgers and raccoon dogs used different parts of the sett. We conclude that facilitation by badgers (through habitat amelioration and refuge from cold and predation) makes the realized niche of raccoon dogs larger than predicted from their fundamental niche. The facilitating role of badger is stronger in winter, which is a critical period for raccoon dog survival in the temperate and boreal zone. ` l’aide de donne´es obtenues par pistage radioe´lectrique de blaireaux (Meles meles (L., 1758)), de renards roux Re´sume´ : A (Vulpes vulpes (L., 1758)) et de chiens viverrins (Nyctereutes procyonoides (Gray, 1834)), et d’observations des terriers dans la foreˆt ancienne de Białowiez˙a (Pologne orientale) en 1996–2002, nous avons teste´ l’hypothe`se selon laquelle les interactions de facilitation entre une espe`ce indige`ne (blaireau) et une espe`ce e´trange`re (le chien viverrin) ont favorise´ le succe`s de l’invasion par cette dernie`re. En hiver, 88 % des terriers de blaireaux sont occupe´s a` la fois par des blaireaux et des chiens viverrins, 4 % par des blaireaux et des renards roux et 4 % par les trois espe`ces ensemble. En e´te´, seulement 20 % des terriers de blaireaux sont utilise´s aussi par d’autres espe`ces de carnivores (10 % par des chiens viverrins et 10 % par des renards). L’occupation des terriers de blaireaux par les chiens viverrins dure en moyenne 117 jours (ET = 21 jours). La variation saisonnie`re de l’utilisation des terriers de blaireaux par les chiens viverrins s’explique par les changements de tempe´rature ambiante; plus la tempe´rature est basse, plus de taux d’occupation des terriers par les chiens viverrins est e´leve´e. Lorsqu’ils passent l’hiver dans un meˆme terrier, les blaireaux et les chiens viverrins utilisent des sections diffe´rentes. Nous concluons que la facilitation par les blaireaux (par ame´lioration de l’habitat et fourniture d’un refuge contre le froid et la pre´dation) rend la niche re´alise´e des chiens viverrins plus large qu’on ne pourrait le pre´dire a` partir de leur niche fondamentale. Le roˆle de facilitation des blaireaux est plus important en hiver, qui est une pe´riode critique de survie des chiens viverrins dans les zones tempe´re´e et bore´ale. [Traduit par la Re´daction]
Introduction Historically, mainstream vertebrate ecology focused on the roles of competition and predation in structuring animal communities (e.g., Wiens 1989; Crawley 1992; Je˛drzejewska and Je˛drzejewski 1998; Krebs et al. 2001). Positive interactions (mutualisms that benefit both species, and Received 26 June 2008. Accepted 8 October 2008. Published on the NRC Research Press Web site at cjz.nrc.ca on 3 December 2008. R. Kowalczyk,1 B. Je˛drzejewska, A. Zalewski, and W. Je˛drzejewski. Mammal Research Institute, Polish Academy of Sciences, 17-230 Białowiez˙a, Poland. 1Corresponding
author (e-mail:
[email protected]).
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facilitations that benefit at least one of the participants and cause harm to neither) were largely neglected or reported as anecdotal observations (e.g., Sludsky 1964; Selva et al. 2003). Yet the recent accumulation of empirical data shows that positive interactions are ubiquitous (Bronstein 1994) and may play a critical role in ecological communities by reducing physical or biotic stresses in existing habitats and by creating new habitats on which many species depend (Stachowicz 2001). Bruno et al. (2003) emphasized the need for including facilitation into ecological theory, for instance with the problem of community resistance to biological invasions. Indeed, recent reviews have documented that numerous exotic plants and animals are dependent on facilitation by native species (Simberloff and Von Holle 1999; Richardson et al. 2000).
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The spectacular raccoon dog (Nyctereutes procyonoides (Gray, 1834)) invasion of Europe in the 20th Century has so far brought concern about its possible competitive interactions with native medium-sized carnivores such as Eurasian badgers (Meles meles (L., 1758)) and red foxes (Vulpes vulpes (L., 1758)) (Kauhala 1995, 1996) and its potentially detrimental predation on native fauna (Kauhala et al. 1993; Kauhala 1996; Sidorovich et al. 2008). Raccoon dogs were introduced to the European part of the former Soviet Union between 1928 and 1955, and successfully expanded into central and northern Europe (Nowak and Pielowski 1964). They rarely dig burrows or dens, but frequently cohabit the setts of badgers for reproduction and wintering (Goszczyn´ski 1999; Kauhala and Holmala 2006). As burrows are an important resource for breeding, wintering, and day-time resting (Kowalczyk et al. 2003b, 2004; Roper 1993), the shared use of burrow systems may lead to negative (predation, interference competition) or positive (facilitation) interactions among the two species. The coexisting native species, namely the badger and red fox, show a notable level of tolerance, including communal denning (van Wijngaarden and van de Peppel 1964; Neal 1986). However, little is known about the interactions between raccoon dogs and badgers. In this paper, we explore the interactions among raccoon dogs, badgers, and red foxes manifested in their cohabitation of badger setts in a well-preserved temperate woodland in eastern Poland. Based on concurrent radio-tracking of badgers and raccoon dogs in 1997–2000 and observations at burrows in 1996–2002, we addressed the hypothesis proposed by Bruno et al. (2003) that facilitative interactions between a native (badger) and an alien (raccoon dog) species significantly contributed to the invasion success of the latter. Specifically, we proposed that badgers play the role of physical ecosystem engineers (sensu Jones et al. 1997), providing raccoon dogs with ameliorated habitats for a critical life stages, i.e., winter denning and reproduction.
Materials and methods Study area Białowiez˙a Primeval Forest (BPF) is one of the best preserved temperate lowland forests in Europe straddling the Polish–Belarussian border (52830’N–53800’N, 23830’E– 24815’E). The study was conducted in the 595 km2 Polish part of BPF. The relief of the area is generally flat (134– 197 m above sea level), with gentle hills and shallow depressions. Coniferous and mixed forests (mainly Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.), with admixtures of European white birch (Betula pendula Roth), European aspen (Populus tremula L.), and English oak (Quercus robur L.) cover 62% of the area, wet alder–ash (black alder (Alnus glutinosa (L.) Gaertn.), European ash (Fraxinus excelsior L.), rarely elms (genus Ulmus L.)) forest cover 18%, rich deciduous forests (English oak, European hornbeam (Carpinus betulus L.), littleleaf linden (Tilia cordata P. Mill.), Norway maple (Acer platanoides L.)) cover 14%, and open habitats cover 6% (Je˛drzejewska and Je˛drzejewski 1998). The climate of BPF is transitional between Atlantic and continental types with clearly marked cold and warm sea-
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sons. Mean temperatures during the study period (1996– 2002) were –3.0 8C in January and 19.6 8C in July. The range of daily temperatures varied from –33.4 to 33.4 8C. The number of days with temperature below 0 8C ranged from 50 to 104 days (mean 78 days). Snow cover persisted for a mean of 86 days per year (range 61–137 days) with a maximum recorded depth of 37 cm. Mean annual precipitation during the study period was 548 mm. In BPF, the community of carnivores consists of 11 species (Je˛drzejewska and Je˛drzejewski 1998), including 4 species of denning predators: wolf (Canis lupus L., 1758), badger, red fox, and raccoon dog. The last one colonized BPF from the east and was first recorded in the early 1950s (Bunevich and Dackevich 1985). Population densities of the three species of medium-sized carnivores averaged 1.7– 5 raccoon dogs, 2.5–3.5 foxes, and 1.3–2.1 badgers per 10 km2 (Je˛drzejewska and Je˛drzejewski 1998; Kowalczyk et al. 2000, 2003b). In terms of biomass, raccoon dog has been the unchallenged dominant member of the community of carnivorous mammals of BPF (Je˛drzejewska and Je˛drzejewski 1998). All radio-tracked badgers spent winters in their main setts (Kowalczyk et al. 2004). Of 12 wintering sites of radio-tracked raccoon dogs, 9 (75%) were found in the main badger setts and 3 in hollow trees (R. Kowalczyk, unpublished data), which suggests the importance of badger setts as a winter refuge for raccoon dogs. Data collection Occupancy of the main setts in 15 territories of badger groups was recorded from June 1996 until June 2002. The setts were identified in the study on badger ecology (see Kowalczyk et al. 2000, 2003a, 2003b, 2004). They were large burrows with 3–12 active entrances and visible spoil hips at entrances. Each sett covered an area from 10 to 550 m2 (measured as MCP around sett entrances). The setts were usually inspected 1–4 times a month for 1–7 years, although some setts were visited more frequently (up to 25 times per month) during trapping sessions, radio-tracking, or direct observations of animals (compare Kowalczyk et al. 2003b, 2004). Irrespectively of the number of visits, the occupancy of a sett in a given month was recorded as 1 (occupied) or 0 (unoccupied). Occupation of a sett by a given species of carnivore was recorded based on (1) presence of radio-collared animals in the sett, (2) direct observations of noncollared animals or their cubs, and (3) other signs of recent activity (e.g., footprints, scats and latrines, hair and prey remains at the sett entrance). The occupation was defined as continuous if a sett was used for reproduction, winter sleep, or as a day-time rest for consecutive days. Breeding in setts was recorded if cubs were observed or found dead at the sett. For radio-tracking, badgers, raccoon dogs, and red foxes were captured in foot-snare traps and box traps placed near setts and equipped with radio alarms (which shortened the time the animals spent in a trap to 1–2 h) or net trapped (compare Kowalczyk et al. 2003b). Captured animals were immobilized and fitted with radio transmitters (Advanced Telemetry System, Inc., Isanti, Minnesota). In total, 18 raccoon dogs, 13 badgers, and 3 foxes were radio-collared. Radio-fitted animals were located during the day (usually between 0800 and 1500), 2–7 days a week. If a collared #
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1391 Table 1. Occupancy of 15 main Eurasian badger (Meles meles) setts by other species of medium-sized carnivores (raccoon dogs (Nyctereutes procyonoides) and red foxes (Vulpes vulpes)) in Białowiez˙a Primeval Forest in 1996–2002. Occupancy of badger main setts Species of carnivores Badger only Badger + raccoon dog Badger + red fox Badger + raccoon dog + red fox Total
Sett-winters
Sett-summers
n 3 62 3 3 71
n 39 5 5 — 49
% 4 88 4 4 100
% 80 10 10 — 100
Fig. 1. Seasonal pattern of occupation of main badger (Meles meles) setts by raccoon dogs (Nyctereutes procyonoides) and red foxes (Vulpes vulpes) in Białowiez˙a Primeval Forest in 1996–2002. Data averaged for all setts and years.
Fig. 2. Rate of utilization of Eurasian badger (Meles meles) setts by raccoon dogs (Nyctereutes procyonoides) in bimonthly periods in relation to ambient temperature in the Białowiez˙a Primeval Forest. J–A, July–August; J–F, January–February.
animal was located in the sett during the day, we precisely recorded its underground position as described by Roper and Christian (1992). To obtain precise underground locations, using squelch and fine tune, we reduced the strength
of the signal on the receiver. If the signal was still strong, we removed the aerial from the receiver. Then, by holding the aerial or receiver close to the ground, we moved it systematically over the area until the signal strength was maxi#
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Can. J. Zool. Vol. 86, 2008 Table 2. Wintering time of raccoon dogs (Nyctereutes procyonoides) in Eurasian badger (Meles meles) setts compared with the duration of winter sleep of badgers in the Białowiez˙a Primeval Forest in 1996–2002.
Time Starting date Ending date Duration (days)
Winter sleep of badgers in their setts
Wintering of raccoon dogs in badger setts
Mean 17 Nov. 14 Feb. 90
Mean 12 Nov. 9 Mar. 117
SE (days) ±1 ±6 ±6
Minimum to maximum 15–18 Nov. 15 Jan. – 25 Feb. 60–100
SE (days) ±12 ±10 ±21
Minimum to maximum 27 Sept. – 15 Dec. 3 Feb. – 21 Apr. 53–206
Note: Data collected by radio- and snow-tracking (n = 8 sett-winters in 5 setts).
mized. The point of maximal strength of the signal was assumed to be the position of an animal, and it was mapped with an accuracy of 1 m. To see if radio-collared individuals of different species sharing the sett avoided, ignored, or attracted each other, dynamic interactions were examined for selected pairs of animals occupying the same sett (Kenward 1992) using the program RANGES 6 version 1.08 (R.E. Kenward, A.B. South, and S.S. Walls, Anatrack, Dorest, UK). Based on observed distances (DO) between simultaneous underground locations of pairs of animals and expected distances (DE), calculated as the mean distance between each location from one animal and all locations from the other animal, the coefficient of interactions (CI) was calculated based on Jacobs’ (1974) index: CI = (DE – DO)/(DE + DO), where CI can vary from –1 (maximal avoidance) to +1 (the strongest attraction). During winter, badger and raccoon dogs did not show any signs of activity for several days or weeks (Kowalczyk et al. 2003a; R. Kowalczyk, B. Je˛drzejewska, and A. Zalewski, unpublished data). Therefore, in the setts not inhabited by any radio-collared individuals, we assumed the continuous presence of animals if we recorded their tracks going into the sett before winter sleep but did not see tracks going out afterwards. In total, data on 71 sett-winters and 49 settsummers (breeding seasons) were collected and analysed.
Results Setts investigated during the study were occupied by three species of medium-sized carnivores: badgers, red foxes, and raccoon dogs. Of 15 main setts permanently occupied by badgers, all were used by raccoon dogs (for 1–6 winters) and one (for 3 winters) by red foxes for wintering. Five of these were used by raccoon dogs (single use) and three (for 1–2 years, each) by foxes for reproduction. Cohabitation of setts by two species was very frequent in winter and rare in summer (Table 1). Of 71 sett-winters, badgers were the sole inhabitants of the setts in just 4% of seasons, whereas raccoon dogs were recorded cohabiting badger setts in 88% and red foxes in 4% of seasons. Simultaneous occupation of setts by all three species was recorded sporadically (Table 1). Of 49 sett-summers, reproduction by raccoon dogs in badger setts was found in 10% of seasons, and that by foxes in another 10%. In summer, the majority of setts (80%) hosted exclusively badgers (Table 1). Sett cohabitation rates by more than one carnivore species were significantly different between winter and summer (G test for homogeneity of percentages, G = 155.6, df = 3, p < 0.001). Occupation of badger setts by raccoon dogs and red foxes never led to their abandonment by the badgers. When analysed in more detail, the presence of raccoon
dogs in badger setts peaked in January–February (Fig. 1). Variation in raccoon dog use of badger setts was best explained by changes in ambient temperature (r2 = 0.79, F[1,23] = 88.272, n = 25, p < 0.0005): the lower was the temperature, the higher was the rate of sett occupation by raccoon dogs (Fig. 2). The relationship was also significant for the cold season only (November–April: r2 = 0.41, F[1,10] = 6.994, n = 12, p = 0.025). Red foxes were recorded in 3%–14% of badger setts from November to June, with the highest occupation rate in the breeding season (April–May; Fig. 1). Utilization rates of badger setts by foxes declined with warmer temperatures (entire year: r2 = 0.20, F[1,23] = 5.893, n = 25, p = 0.02), but the relationship for the cold season (November–April) was not significant (r2 = 0.02, F[1,10] = 0.188, n = 12, p = 0.67). Raccoon dogs usually moved into badger setts between late September and mid-December (12 November, on average), and used them for 53–206 days (mean 117 days) (Table 2). No significant differences were detected between the duration of raccoon dog wintering time and the duration of winter sleep by badgers (Mann–Whitney U test, U = 29, p > 0.5). Raccoon dogs did not delay their moving into a sett until badgers were inactive. They often moved into the setts when their hosts (badgers) were still active and they stayed there for some time after the badgers had resumed activity in spring (Table 2). When wintering in badger setts, both raccoon dogs and foxes utilized the same entrances as did the badgers. Badgers and raccoon dogs sharing one sett and located simultaneously used different sections of the sett (Fig. 3, Table 3). In two setts, we found that the same site (chamber) in the sett was used by a badger and then by a pair of raccoon dogs after 2 and 9 days, respectively. Distances between simultaneous underground locations of badgers and raccoon dogs varied from 2 to 23 m (Table 3). Expected and observed distances between within-sett locations of individuals of both species did not differ significantly (Mann–Whitney U test, U = 11.5, p = 0.31), and neither avoidance nor attraction between badgers and raccoon dogs wintering in the same sett was recorded (interaction indices approaching zero; Table 3). Badgers tended to utilize larger within-sett ranges when raccoon dogs occupied the sett (57 ± 25 m2, range 12–79 m2, n = 6) than during winters with raccoon dogs absent (23 ± 11 m2, range 8–34 m2, n = 4); however, the difference was not statistically significant (Mann–Whitney U test, U = 21, p = 0.055). We recorded only five cases of raccoon dog reproduction in active setts of badgers. There was no sign of concurrent reproduction by badgers in three of those cases, and there was no data available on badger breeding activity in the #
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Fig. 3. Use of within-sett space by simultaneously radio-tracked Eurasian badgers (Meles meles) and raccoon dogs (Nyctereutes procyonoides) in three large setts in the Białowiez˙a Primeval Forest.
two other cases. Raccoon dogs breeding in badger setts were recorded from early May until late June. Five cases of red foxes breeding in active setts of badgers were recorded, but only in one case did both badgers and foxes rear young in the same sett. There were no signs of badger reproduction detected in the remaining four cases. Foxes breeding in badger setts occupied them permanently in June–July and usually used one of the outermost entrances that was not utilized by badgers. A small sample of direct observations of interactions be-
tween badgers, raccoon dogs, and red foxes at badger setts (Table 4) showed that aggression of hosts (badgers) towards the other species did occur (killing of cubs and possibly adults), whereas we found no evidence of aggression by the two squatting species towards the sett host.
Discussion Our study showed that badger setts in a European temperate woodland were an important resource for two other den#
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Can. J. Zool. Vol. 86, 2008 Table 3. Behaviour of Eurasian badgers (Meles meles) and raccoon dogs (Nyctereutes procyonoides) sharing a sett in winter (spatial data shown in Fig. 3) in the Białowiez˙a Primeval Forest. Distances (m) between badgers and raccoon dogs inhabiting the same sett Sett no. 1a 1b 2 3
Percent overlap of within-sett ranges 28 42 61 2
Expected mean 7.6 10.8 10.7 7.3
Observed mean (minimum to maximum) 7.7 (5–11) 10.8 (2–18) 10.9 (2–23) 9.6 (4–13)
Interaction index (CI) –0.01 0 –0.01 –0.14
Note: Number of simultaneous radiolocations of the two species varied from 11 to 29. Interaction index calculated after Jacobs (1974).
Table 4. Interactions between Eurasian badgers (Meles meles), raccoon dogs (Nyctereutes procyonoides), and red foxes (Vulpes vulpes) recorded at badger setts in the Białowiez˙a Primeval Forest in 1997–2002. Type of interaction Killing of cubs
Date or years 4 May 2001
Possible killing of adults and disease transmission
5 May 2002 21 Feb. 2002
Scavenging
9 Mar. 1997 7 Feb. 1997
Mortality of raccoon dogs and foxes in badger setts (causes unknown)
12 Jan. 2001 1997–2002 1998
ning carnivores, especially the non-native raccoon dogs. Through ecosystem engineering, badgers ameliorate unfavourable habitats for raccoon dogs by providing them with a predictable, long-lasting refuge from a physical stress (cold) and predation. Indeed, raccoon dogs in Europe are frequently recorded in badger setts. For instance, in Slitere Nature Reserve, Latvia, 41% of active badger setts were occupied by raccoon dogs in winter (Zoss 1992). In Zˇemaitija National Park, Lithuania, 8% of badger setts were used by raccoon dogs for reproduction (Ulevicˇius 1997). As reported by Kauhala and Holmala (2006) from southeast Finland, radio-collared badgers and raccoon dogs were located wintering or resting together on 24% of simultaneous locations in winter and 12% of locations in summer. Because of their thick fur and fat reserves, raccoon dogs may survive even cold winters (Korhonen et al. 1983) as long as they have some form of den to provide thermal refuge. However, as raccoon dogs rarely excavate their own dens (Novikov 1956), their winter inactivity makes them an attractive and easy target for larger predators. In the Białowiez˙a Forest, predation (mainly from wolves and dogs) was one of the main factors of raccoon dog mortality, which was responsible for 30%–40% of deaths (Kowalczyk et al. 2009). However, wolves and Eurasian lynx (Lynx lynx (L., 1758)) rarely consumed the dead raccoon dogs (0%–2.7% in diets; Je˛drzejewska and Je˛drzejewski 1998). Deep and complex setts of badgers provided raccoon dogs with dependable pro-
Description Badger(s) killed two cubs of raccoon dogs and buried them in a latrine near sett entrance Badger(s) killed a fox cub and buried it near sett entrance Rabid raccoon dog (adult male) with numerous bites was found dying at badger sett Raccoon dogs dragged a carcass of an adult badger out of a sett and scavenged on it Fresh skull of an adult badger found on snow tracks of raccoon dogs near the active badger sett As above A total of six old skulls of raccoon dogs and two old skulls of foxes thrown away by badgers cleaning their setts Carcasses of two raccoon dogs found near active badger setts
tection not only against cold but also, and perhaps even more importantly, against predation. In winter, badgers and raccoon dogs occupied different parts of a shared sett. Badger setts are usually large with numerous entrances, as well as a system of chambers connected by hundreds of metres of tunnels (Roper 1992). From spring to autumn, badgers utilized the majority of within-sett space (Kowalczyk et al. 2004). In winter, their underground activity was limited to a small part of the sett while they slept (Kowalczyk et al. 2003a, 2004). This restricted activity facilitates other species settling into unoccupied parts of the sett, as observed in Białowiez˙a Primeval Forest. The facilitating role of badger is stronger in winter, when the settling of raccoon dogs in badger setts removes the multiple stress resulting from cold and predation, and thus enhances the survival of the raccoon dogs. Ultimately this may affect populations densities of raccoon dogs. In spring and summer, the costs of sett sharing are probably higher than the benefits owing to the risk of intraguild predation. Generally, pup mortality is rather high in raccoon dogs. As found by Helle and Kauhala (1993), only 20% of young survived the first year of life. Observations from the Białowiez˙a Primeval Forest show that mortality of cubs during the first 3 months of their life may reach 60% (Kowalczyk et al. 2009). In our study, we observed killing of red fox and raccoon dog cubs by badgers. We have not #
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found any evidence for the opposite case, i.e., badger cubs being killed by either of the two other carnivores. However, remains of a badger cub in raccoon dog scats have been reported from the Białowiez˙a Primeval Forest (0%–4.7% in raccoon dog diet; Je˛drzejewska and Je˛drzejewski 1998). Furthermore, shared use of setts increases the contact rate among different species of carnivores and creates circumstances favourable for the transmission of diseases and exchange of parasites (Sargeant et al. 1982; Kauhala et al. 2006). Raccoon dogs are important carriers of rabies, sarcoptic mange, Echinococcus multilocularis Leuckart, 1863, and trichinellosis (Westerling 1991; Oksanen et al. 1998; Oivanen et al. 2002; Holmala and Kauhala 2006). Rabies creates a special risk in sett sharing, as it is an important source of carnivore mortality in Eastern Europe (Holmala and Kauhala 2006). The invasion of raccoon dogs, potential competitors of badgers, did not result in a decrease of densities of badgers in the Białowiez˙a Primeval Forest or in other European localities (Kauhala 1995; Kowalczyk et al. 2000). It appears that these two species are adapted to coexist and utilize available resources with minimal competition (Je˛drzejewska and Je˛drzejewski 1998). In conclusion, our study documented that the interactions between badgers and raccoon dogs, as well as between badgers and foxes, consist of facilitative and competitive components, and that their net effect varies seasonally. According to the conceptual model proposed by Bruno et al. (2003: Fig. 1 on p. 120), facilitation by badgers (through habitat amelioration and refuge from predation) makes the realized niche of raccoon dogs substantially larger than predicted from their fundamental niche. The net effect of facilitation and competition is positive in winter, but probably not in spring and summer when costs of interspecific predation on cubs override the benefits of facilitation. Since winter is a critical period for the survival of raccoon dogs in the temperate and boreal zone, we conclude that facilitative interactions between badgers and raccoon dogs contributed to the invasion success of the latter species, which is in line with the hypothesis by Bruno et al. (2003).
Acknowledgements We are grateful to E. Bujko and students M. Williams and J. Simpson from Farnborough College of Technology (England) for their help in radio-tracking animals. M. Hayward kindly corrected the English in the manuscript. The study was financed by Polish State Committee for Scientific Research (KBN) grant 6 P04C 057 12 and the budget of the Mammal Research Institute, Polish Academy of Sciences.
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