Resource utilization and interspecific relations of sympatric bobcats and coyotes

OIKOS 94: 236 249. Copenhagen 2001 Resource utilization and interspecific relations of sympatric bobcats and coyotes Jennifer C. C. Neale and Benjamin N. Sacks Neale, J. C. C. and Sacks, B. N. 2001. Resource utilization and interspecific relations of sympatric bobcats and coyotes. Oikos 94: 236 249. We used scat analysis and radiotelemetry to characterize use of foods and habitats by sympatric bobcats and coyotes, and evaluated these in the context of spatial and temporal relationships to assess the potential for, and evidence of, interspecific competition. Bobcats and coyotes exhibited broad and overlapping diets. However, diets of the two predators differed in the relative contributions of small and large prey, with bobcats consuming relatively more rodent and lagomorph biomass and coyotes consuming relatively more ungulate biomass. Consumption among rodent prey species was highly correlated between bobcats and coyotes, indicating no evidence of prey partitioning within this group. Habitat selection by the two predators differed slightly at the landscape scale but not within home ranges. Bobcats and coyotes occupied small, overlapping home ranges, such that the likelihood of interspecific encounters (direct or indirect) was high. Bobcats displayed slight avoidance of overlapping coyote core areas during coyote reproductive seasons (winter and spring), when coyotes are typically most territorial (toward conspecifics), but displayed slight attraction during times of year when coyotes were not engaged in reproductive activities. Relative to coyotes, which were strongly nocturnal, diel activity patterns of bobcats were more diurnal and variable. However, activity patterns were not inversely correlated. Overall, these predators appeared to use resources independently and we found little evidence of negative interactions. Differences in resource use by bobcats and coyotes appeared to relate to fundamental niche differences as opposed to competition-related resource partitioning. J. C. C. Neale and B. N. Sacks, Dept of En ironmental Science, Policy and Management, Uni. of California, Berkeley, CA 94720, USA (present address: JCCN: Dept of En ironmental Toxicology, One Shields A enue, Uni. of California, Da is, CA 95616, USA [jcneale@ucda is.edu]; BNS: John Muir Inst. of the En ironment, One Shields A enue, Uni. of California, Da is, CA 95616, USA). Interspecific competition is thought to play an important role in structuring communities (Schoener 1982) and, according to theory, should be especially important in the top trophic level (Hairston et al. 1960, Oksanen et al. 1981). Empirically, the importance of competition among terrestrial carnivores has been difficult to assess due to the paucity of intensive field studies (Palomares and Caro 1999). Accordingly, we investigated resource utilization and interspecific interactions of two mesopredators with greatly overlapping geographic ranges (Nowak 1991), the coyote (Canis latrans) and the bobcat (Lynx rufus), the putative inferior competitor (Litvaitis 1992). Relationships between these two species have been difficult to assess. Because bobcats and coyotes have similar life requisites, they might be expected to compete where they co-occur. Evidence suggestive of competition between bobcats and coyotes includes declining bobcat populations in many parts of North America (Knowlton and Tzilkowski 1979) associated with range expansion of the coyote (Litvaitis and Harrison 1989, Parker 1995), an inverse relationship between popula- Accepted 19 March 2001 Copyright OIKOS 2001 ISSN 0030-1299 Printed in Ireland all rights reserved 236 OIKOS 94:2 (2001)

tion indices of bobcats and coyotes (Linhart and Robinson 1972), apparent increases in bobcat densities following population reduction of coyotes (Nunley 1977, Henke and Bryant 1999), and reports of coyote-caused bobcat mortality (Anderson 1986, Knick 1990). Other studies, however, reported positively related patterns of abundance of bobcats and coyotes (Schnell et al. 1985) or no relationship (Lovell et al. 1998, Main et al. 1999), suggesting a variable or ambiguous relationship. The major niche dimensions that competitors may use to partition life requisites in short supply are habitat and food (Schoener 1986). Contingency models of resource partitioning suggest that when resources are abundant, animals should be maximally specialized in use of food or habitat type (Schoener 1974a); morphological differences are particularly effective in allowing specialization by food size (Schoener 1974b). As food becomes limiting, breadth of food and habitat types used should increase. Unfortunately, theoretical predictions related to competition and resource utilization have largely ignored spatial relationships, which add another layer of complexity to, and interact with, interspecific dynamics. Assessing resource use at the population level and without respect to space or time can generate ambiguous findings with respect to competitive mechanisms, especially when only one niche axis is examined, as in many comparative studies of carnivore food habits. Spatially explicit and temporal patterns of resource overlap and separation can aid interpretation. We investigated use of food and habitat types as well as spatial and temporal relationships between sympatric bobcats and coyotes. Our study took place in northern California during July 1994 December 1995, shortly after the end of a severe drought (1987 1992; Sacks 1998) when resources appeared superabundant (Neale 1996) and predator populations likely were approaching equilibrium levels. Densities of bobcats (Neale 1996) and coyotes (Sacks 1996) at our study site approximated the maxima reported for these species (Camenzind 1978, Hall and Newsom 1978, Jachowski 1981, Andelt 1985, Lembeck 1986), increasing the likelihood of chance encounters. To explore resource use in this context, we (1) characterized diets of the two predators through fecal analysis utilizing two approaches frequency of occurrence and estimated biomass consumption to compare use of foods, (2) investigated habitat selection at both the landscape and within-home range scales using radiotelemetry, (3) assessed spatial relationships on multiple scales, and (4) compared diel activity patterns. Methods Study area The study area was centered on the 21-km 2 Hopland Research and Extension Center (HREC; 39 00 N, 123 05 W) in Mendocino County, California, USA. This site was located in the Coast Range mountains in the Russian River drainage and had a primarily southwest aspect; topography was hilly to semi-rugged, with elevations ranging 150 915 m. Vegetation consisted of a mosaic of four principal types: chaparral, mixed evergreen-deciduous forest (forest), annual grassland (grassland) and woodland (Murphy and Heady 1983). Cover density was greatest in chaparral and forest and least in grassland. The climate was characterized by cool, wet winters and warm, dry summers. Numerous potential prey of coyotes and bobcats occurred on the site including two lagomorph species, 13 rodent species, blacktailed deer (Odocoileus hemionus), and various birds, reptiles, and invertebrates (Neale 1996). In addition to wild prey, between 900 and 2500 domestic sheep were regularly maintained on the site. Coyotes on the study area suffered high mortality due to routine removal to reduce sheep depredation (Sacks et al. 1999a). Bobcats generally were not associated with sheep kills (Neale et al. 1998) and none were removed during our study. Food habits Scats of bobcats and coyotes were collected bi-weekly from 21 transects (0.5 km in length) established throughout the study area (57% of scats), as well as opportunistically when found fresh (43%). Scats were assigned to predator species based primarily on size and shape (Murie 1954, Danner and Dodd 1982) as well as odor and other sign. Inspection of scats collected from known individuals from traps or during radiotracking confirmed our classification criteria. Approximately 10% of carnivore scats were of ambiguous origin and were discarded. Scats were oven-dried, placed in nylon bags, washed in an automatic clothes washer, and tumbledried, as described by Neale (1996). Hair, teeth, and bone were identified using reference skins, skulls, skeletons, hair keys, and photomicrographs (e.g., Mayer 1952, Glass 1973). Although frequency of occurrence (the percent of scats containing each food item) is commonly used to quantify diets of carnivores, this measure does not accurately reflect the proportional consumption of food items in carnivore diets. For example, frequency of occurrence tends to overestimate vegetative food items (Andelt and Andelt 1984), underestimate large mammalian prey (Weaver 1993), and can be biased with respect to detection of prey in scats due to differences among rodent prey in recovery rates of teeth and hair (Kelly 1991). Therefore, to more accurately assess food habits of these predators, we estimated biomass consumed. Fecal analysis was performed according to specifications of Kelly s (1991) residue-weight model, which OIKOS 94:2 (2001) 237

utilized correction factors derived from feeding trials to infer amount consumed from food remains. Depending on the scat contents, analysis involved separating and weighing hair, teeth, and bones, including count and weight of each tooth type present, as well as estimating percent volume for each food item in each scat (Kelly 1991). We used Program SCAT (Version 1.5.1) to estimate biomass consumption represented by scat contents and to calculate the proportion of total biomass represented by each food item. This program did not include a correction factor for reptiles or vegetation. However, based on visual estimates of percent volume, reptile remains did not contribute substantially to prey biomass for either predator. Occurrence of manzanita (Arctostaphylos spp.) berries was high in scats of coyotes (Neale 1996); we therefore calculated a correction factor for manzanita (1.85 g fresh berry consumed per 1 g residue) based on the number of berries represented per gram residue and the average weight of a fresh berry. Items found in scats but excluded from analyses included grass and occasional orchard fruits, as well as very rare occurrences of moles (Scapanus latimanus) and various carnivores. We compared bobcat and coyote food niches based on overall and seasonal diets, with seasonal periods designated as follows: summer (Jul. Sep.), fall (Oct. Dec.), winter (Jan. Mar.), and spring (Apr. Jun.). Rather than pool scats across seasons to estimate overall diets (which would have resulted in biases toward seasons with larger scat samples), we averaged across seasonal values to generate overall values. We calculated measures of dietary breadth (B) and food niche overlap ( ) based on Pianka (1973):, B=1 p i 2 = (p i q i ) p i2 q i2, where p i is the proportion of food item i in the diet of predator p, and q i is the proportion of food item i in the diet of predator q. In this study, a maximum of 13 categories were available such that breadth ranged from 1 (only 1 food item taken) to 13 (all food items taken, in equal proportions). The index of overlap ranges from 0 (complete dissimilarity) to 1 (complete similarity). Overlap indices should be based on resource categories at least as fine as those perceived by the predators (Krebs 1989), thus we categorized prey to the species level whenever possible (Greene and Jaksíc 1983). For comparative purposes, we also calculated overall frequency of occurrence and relative frequency of occurrence (the latter values were expressed as percent of all occurrences, to allow direct comparison with biomass) as well as total and seasonal breadth measures and overlap indices based on frequency of occurrence. An additional 226 coyote scats were analyzed using only frequency of occurrence due to the extensive time requirements of biomass estimation. Radiotelemetry study Bobcats and coyotes were captured using No. 3, padded-jaw leghold traps (Woodstream Corp., Littitz, PA, USA) or homemade snares (coyotes only) with stops to prevent strangulation. Devices were set along roads, ridges, fences, and drainages throughout HREC. Bobcats were sedated with a mixture of ketamine hydrochloride and xylazine hydrochloride (10 mg ketamine+1.6 mg xylazine/kg body mass). Captured animals were radiocollared, weighed, measured, and examined for reproductive and general condition (Neale 1996, Sacks et al. 1999b). Animal care and handling procedures were approved by the Animal Care and Use Committee, Univ. of California, Berkeley (Protocol cr190-0496). We conducted radiotelemetry of resident predators throughout the day and night 5 7 d per week using stationary and mobile (hand-held) units as described previously (Neale et al. 1998, Sacks et al. 1999b), locating most individuals at least once per day. Average telemetry error was estimated at 146 m, with 95% of errors 356 m (Sacks 1996). We used program CALHOME (Kie et al. 1996) to calculate annual and seasonal 65% and 90% adaptive kernel (AK) isopleths and annual 95% minimum convex polygons (MCP). The AK ranges reflected intensity of use (Worton 1989); the 65% and 90% isopleths corresponded to core areas and home ranges, respectively (Sacks et al. 1999b). The MCPs were used to bound areas of availability (e.g., of habitat) because, in contrast to AKs, MCPs were relatively insensitive to dispersion of locations (i.e., clumped, uniform, or random). We determined annual home ranges for resident bobcats and coyotes with 100 radiolocations and monitored during 9 months of the study. Home range sizes of bobcats did not vary significantly by sex (Neale 1996) and thus were pooled for estimation. Male and female coyotes of a territorial pair used virtually identical areas; thus we chose one range (the female s) to represent the territory. Seasonal core areas and home ranges were calculated for territorial female coyotes (range=42 306 locations per season, X SD=126 69). Habitat selection We assessed habitat selection, i.e., use relative to availability, by bobcats and coyotes at two spatial scales. We used a vegetation map derived from LAND- SAT imagery (Fox et al. 1997) in conjunction with a geographic information system (GIS; ARCVIEW 3.0a, Environmental Systems Research Institute, Redlands, CA, USA) to classify habitats into the four major types 238 OIKOS 94:2 (2001)

on our study area: chaparral, forest, grassland, and woodland. To compare habitat selection of the two carnivores at the level of home range establishment (i.e., landscape), we calculated habitat composition of the annual 95% MCPs using the GIS. We used individual home ranges as the sample unit and calculated 95% Bonferroni confidence intervals on arcsine transformed mean proportions (Zar 1984: 239). Next, we tested for habitat selection within home ranges by comparing the proportion of each individual s radiolocations in each habitat type (observed) to the proportion of that habitat in the individual s MCP (expected). Previous studies comparing habitat use of bobcats and coyotes (e.g., Major and Sherburne 1987, Litvaitis and Harrison 1989) have used the radiolocation as the sample unit. Although this has been common practice in habitat selection analyses (Neu et al. 1974, Byers et al. 1984), the use of radiolocations instead of individuals as the sample unit amounts to pseudoreplication (Hurlbert 1984), which in this particular application (i.e., for territorial species) is especially problematic (Litvaitis et al. 1994). To avoid this problem, we calculated habitat selection indices for each individual separately, for each habitat type, as logtransformed ratios ( +1; Zar 1984: 238 239) of observed to expected proportions of radiolocations minus log(2) plus 1 (the last two terms made a selection index of 1 correspond to an observed-to-expected ratio of 1). We calculated 95% Bonferroni confidence intervals of selection indices for each habitat during wet (winter spring) and dry (summer fall) seasons. Spatial relationships We investigated overlap of adjacent bobcat and coyote annual 90% AK home ranges. We used the proportion of a bobcat home range overlapped by a neighboring coyote home range as a measure of interspecific spatial overlap (Bradley and Fagre 1988). Quantification of home range overlap is useful as a descriptor and may indicate the likelihood of encounters. However, with the possible exception of adjacent but non-overlapping home ranges with shared boundaries (indicative of strong interspecific territoriality), this metric is too coarse to indicate anything about avoidance (or attraction) behavior. Previous studies (Major and Sherburne 1987, Litvaitis and Harrison 1989) have attempted (and failed) to detect avoidance or attraction between coyotes and bobcats using a modification of the nearest neighbor analysis (Clark and Evans 1954, Keenan 1981) whereby separation distances of paired simultaneous locations of bobcats and coyotes with adjoining or overlapping home ranges are compared to randomly paired locations of the same two individuals. We used this technique (using locations separated by 1 h) in preliminary analyses and similarly found no avoidance. However, if the distance at which a bobcat would alter its course due to the presence of a coyote is short (e.g., 150 m) relative to radiotelemetry error (which seems likely, especially in dense vegetation or rugged topography), this technique would be extremely insensitive. Further, the distance between two simultaneous locations that are widely separated in time (up to 4 h; Litvaitis and Harrison 1989) is unlikely to differ in any meaningful way from that between randomly paired locations. Therefore, to determine whether bobcats avoided coyotes, we compared use versus availability of coyote core areas by spatially overlapping bobcats. We chose the core area of coyotes because this was the most intensively used portion of the home range, and therefore where territorial behavior would be expected to be most apparent. [We also conducted the analyses using coyote home ranges; results were similar and therefore, to avoid duplication, were not presented here.] Availability of coyote core areas was calculated for each bobcat, corresponding to the proportion of a bobcat s annual 95% MCP overlapping core areas of resident female coyotes. Because coyotes shifted their core areas seasonally, these were calculated separately for each season. Use was indicated by proportions of radiotelemetry locations falling inside coyote cores. Ratios of observed (use) to expected (available) proportions of locations within coyote core areas were used to calculate selection indices as described above (under Habitat selection ). Selection indices were calculated for bobcats with 20 locations in a season. Canids tend to be most territorial towards conspecifics during breeding (winter) and pup-rearing (spring) (Jaeger et al. 1996); it is unknown whether interspecific territoriality between coyotes and bobcats also reflects similar seasonality. Therefore, we tested for seasonal differences in bobcat selection indices for coyote core areas using analysis of variance. Acti ity To evaluate overlap between bobcats and coyotes in time, we examined diel activity patterns during wet and dry seasons. Activity (active, inactive, or ambiguous) was assessed at the time of radiolocation, and was inferred from amplitude fluctuation and bearing shift (Andelt 1985, Major and Sherburne 1987) or pulse-rate variation for a subset of animals (5 bobcats, 1 coyote) that wore motion-sensitive transmitters (Telonics, Inc., Mesa, AZ, USA). Environmental conditions such as high winds can influence amplitude fluctuation. Therefore, we only evaluated activity for locations in which we were confident the animal was active or inactive (55% of locations; Sacks 1996). We calculated proportions of these locations coded as active during eight 3-h diel periods. Only animals with 20 activity-coded locations in a diel period were included in calculations of diel period averages. OIKOS 94:2 (2001) 239

Table 1. Proportional biomass (percent of the estimated total biomass of prey consumed represented by each food item) and frequency of occurrence (percent of scats containing each food item) of food items a in bobcat and coyote scats, Jul. 1994 Dec. 1995. For direct comparison with proportional biomass, the relative frequency of occurrence of food items (number of occurrences of a food item/total number of occurrences of all items in sample, expressed as percent) is given in parentheses. Values for composite groups are left-justified for clarity. Bobcat Coyote biomass (%) freq. of occ. (%) biomass (%) freq. of occ. (%) n=226 scats n=226 scats, n=311 scats n=537 scats, 416 occurrences 865 occurrences Ungulate a 18.1 16.7 (9.1) 59.1 41.2 (25.6) Deer 12.6 11.0 (6.0) 32.4 23.4 (14.5) Sheep 4.9 4.8 (2.6) 23.7 18.7 (11.6) Lagomorph b 23.3 27.8 (15.1) 6.0 9.0 (5.6) Rodent a 57.2 75.95 (41.3) 30.0 42.3 (26.3) Squirrel c 4.7 11.6 (6.3) 3.2 3.9 (2.5) Woodrat 18.2 26.3 (14.3) 10.0 10.3 (6.4) Pocket gopher 7.7 11.2 (6.1) 3.0 8.2 (5.1) Kangaroo rat 3.6 5.5 (3.0) 1.1 2.0 (1.2) Chipmunk 0.2 1.2 (0.6) 0.1 0.6 (0.4) Vole 14.0 25.4 (13.8) 7.7 15.4 (9.6) Mice d 5.8 20 (10.9) 3.3 8.6 (5.3) Bird 1.1 11.1 (6.0) 0.6 7.8 (4.8) Insect 0.2 8.5 (4.6) 0.4 11.7 (7.3) Reptile n/a 17.2 (9.3) n/a 7.8 (4.8) Manzanita 0.0 2.4 (1.3) 3.9 33.7 (20.9) a Ungulate and rodent categories include unidentified large and small mammals, respectively. b Syl ilagus bachmani and Lepus californicus. c Sciurus griseus and Spermophilus beecheyi. d Peromyscus sp. and Reithrodontomys megalotis. Results Food habits Most of the biomass consumed by both predators was represented by three mammalian taxa (ungulates, rodents, and lagomorphs; Table 1). The two predators differed in their relative consumption of large and small prey, with small mammals dominating the bobcat diet and ungulates dominating the coyote diet (Fig. 1). The slope of the regression line in Fig. 1 indicated that for coyotes the proportion of total biomass composed by rodents was 54% that of bobcats. Within the rodent category, however, relative ranking of species was quite similar for bobcats and coyotes (revealed by the close fit of points about the trend line for rodents). Based on biomass, overall niche breadth was 6.71 for bobcats and 4.91 for coyotes; overall niche overlap was 0.63. Based on frequency of occurrence, overall niche breadth was 8.84 for bobcats and 8.35 for coyotes; overall niche overlap was 0.62. Prey selection by bobcats and coyotes changed over time, but was not generally linked to season per se (i.e., showing similarity between successive summers or falls). Ungulate consumption by bobcats, although much lower, roughly paralleled that of coyotes over time (Fig. 2A). Bobcats displayed a decreasing trend in rodent consumption and an increasing trend in lagomorph consumption, whereas no consistent trends were evident for coyote consumption of these prey (Fig. 2B, C). Seasonality of manzanita berries in coyote scats was apparent in frequency of occurrence data, which showed manzanita berries frequently consumed by coyotes during summer and fall seasons; in contrast, manzanita berries composed only a small proportion of the biomass consumed in all seasons (Fig. 3). Bobcat seasonal niche breadth, based on biomass, paralleled bob- Fig. 1. Consumption of ungulate, rodent, and lagomorph prey by coyotes versus bobcats, expressed as proportion of the estimated total fresh biomass of prey consumed, Hopland Research and Extension Center, July 1994 December 1995. Filled circles represent rodent species, open circles represent deer and sheep. Rodent regression line intercept was constrained to the origin. 240 OIKOS 94:2 (2001)

cat-coyote niche overlap and both decreased during the latter part of the study; no pattern was evident in coyote niche breadth (Fig. 4A). Seasonal breadth and overlap measures based on frequency of occurrence showed no obvious trends (Fig. 4B). Radiotelemetry study We captured and monitored 13 coyotes and 11 bobcats. Bobcats were categorized by sex and estimated age class at capture, and weights were calculated for adult females (X SE=5.03 0.07 kg, n=3), subadult males (5.78 0.38 kg, n=5), and adult males (7.70+0.36 kg, n=3). Average weights ( SE) of coyotes were 9.77 ( 0.34) kg for females and 11.36 ( 0.63) kg for males. We obtained 2770 radiolocations of bobcats and 4498 radiolocations of coyotes. Of the radiocollared Fig. 2. Proportion of prey biomass consumed by bobcats and coyotes consisting of (A) ungulate, (B) rodent, and (C) lagomorph prey during six seasons, Hopland Research and Extension Center, July 1994 December 1995. Seasonal sample sizes for bobcat and coyote scats, respectively: 34, 52 (summer 1994); 20, 52 (fall 1994); 76, 53 (winter 1995); 49, 51 (spring 1995); 27, 51 (summer 1995); and 20, 52 (fall 1995). bobcats, there was one death (cause undetermined) during the study period. All bobcats appeared to be in good condition at initial capture, subsequent captures, and during visual observations. Habitat selection Habitat composition of the 95% MCP was similar for bobcats and coyotes, although coyote home ranges tended to include more grassland and less forest than did bobcat home ranges (Fig. 5). Habitat selection Fig. 3. Seasonal use of manzanita berries by coyotes, as measured by occurrence in scats versus biomass consumed, Hopland Research and Extension Center, July 1994 December 1995. OIKOS 94:2 (2001) 241

within home ranges was very similar for bobcats and coyotes in all seasons (Fig. 6). Grassland tended to be used less than expected (based on availability) by both carnivores, but this was significant only for coyotes in winter spring 1995. Forest was marginally selected for by coyotes during winter spring 1995 as was woodland for bobcats. Spatial relationships Annual home range size of bobcats averaged ( SE) 8.7 1.3 km 2 (90% AK) and 9.8 1.5 km 2 (95% MCP). Coyote annual home ranges averaged 4.4 0.9 km 2 (90% AK) and 5.2 1.1 km 2 (95% MCP). An average of 16% (range=2 46%) of a bobcat s home range was overlapped by each neighboring coyote home range, and most bobcat home ranges were overlapped by 2 (radiocollared) coyote territories. Coyote territories were mutually exclusive. Fig. 5. Habitat composition of home ranges of bobcats (n= 11) and coyote pairs (n=4), expressed as proportions of the 95% minimum convex polygon consisting of each of four major habitat types, Hopland Research and Extension Center, July 1994 December 1995. Error bars correspond to 95% Bonferroni confidence intervals. Bobcat use of coyote core areas, as indicated by selection indices, differed seasonally (F=3.40, df= 5,24, P=0.018). In particular, bobcats avoided coyote core areas most during winter and spring (Fig. 7). Although sample sizes were too small (n=3 large males) to include bobcat body size as a factor in statistical analyses, only small bobcats appeared to avoid coyote cores during winter, whereas all bobcats appeared to avoid coyote core areas during spring (Fig. 8). Acti ity Relative to coyotes, diel activity of bobcats was variable, both within and among seasons (Fig. 9). Whereas coyotes were more consistently nocturnal, bobcats were relatively less active during 0300 0559 h in both seasons for which data were collected for this diel period. Coyotes were generally least active during 1200 1759 h. Diel activity patterns of bobcats and coyotes were not negatively correlated in any season (r=0.11, P= 0.11, summer fall 1994; r=0.06, P=0.90, winter spring 1995; r=0.26, P=0.26, summer fall 1995). Fig. 4. Seasonal estimates of dietary breadth for bobcats and coyotes, and bobcat coyote food niche overlap, based on (A) biomass consumption and (B) frequency of occurrence of food items, Hopland Research and Extension Center, July 1994 December 1995. Discussion Food habits Food use represents a primary mode of resource partitioning between two ecologically similar species. At HREC, overall food niche overlap of bobcats and coyotes was moderate when compared with values from other carnivore studies based on the same index (Jedrzejewska and Jedrzejewski 1998). Dietary differences were primarily associated with prey size. Whereas small 242 OIKOS 94:2 (2001)

Fig. 6. Habitat selection indices for bobcats (open circles) and coyotes (closed circles) during summer fall 1994 (A, B), winter spring 1995 (C, D), and summer fall 1995 (E, F), Hopland Research and Extension Center, July 1994 December 1995. Values 1=positive selection, 1=negative selection, 1=no selection. Sample sizes were 7, 8, and 5 bobcats, and 4, 4, and 3 coyotes in chronological order. Error bars correspond to 95% Bonferroni confidence intervals. mammals were the principal prey of bobcats, ungulates comprised the bulk of prey biomass for coyotes. In contrast to bobcats, which tend to be solitary (Bailey 1974, Major and Sherburne 1987), coyotes often hunt in pairs or groups, which likely facilitates predation on ungulates (Gese et al. 1988, Gese and Grothe 1995, Sacks et al. 1999b). Body size is often an important constraint on prey size selection as well (Gittleman 1985). Whereas bobcats tend to be slightly smaller than coyotes on average throughout their ranges (Young 1958, Nowak 1991), bobcats on our study area were substantially smaller than coyotes (roughly half the mass on average), which also may have hindered their ability to kill ungulates. Another difference in the diets of bobcats and coyotes in this study was in consumption of vegetation. Because this difference is characteristic of canids and felids generally (Nowak 1991), it clearly reflected differences in fundamental niches of bobcats and coyotes. Interestingly, although most studies of bobcat and coyote food habits have concluded that fruit in the coyote diet but not bobcat diet was a major seasonal niche difference (e.g., Small 1971, Toweill and Anthony 1984, Witmer and DeCalesta 1986, Major and Sherburne 1987, Litvaitis and Harrison 1989, DiBello et al. 1990), the omnivorous status of the coyote contributed little to differences between bobcat and coyote diets in this study. Although manzanita occurred at a high frequency in coyote scats seasonally, it contributed little biomass. Thus, estimates of coyote niche breadth and bobcat coyote overlap based on frequency of occurrence were biased by undue influence of manzanita berries. Food niches of bobcats and coyotes became less similar over the course of our study, based on prey biomass. This pattern, as well as the decreasing trend in bobcat dietary breadth, may have resulted from the partial replacement of rodent species by lagomorphs OIKOS 94:2 (2001) 243

(single taxon) in the bobcat diet. The tight relationship between bobcat and coyote consumption within rodent species (Fig. 1) suggested that these predators perceived rodents as a group, which they differentiated from lagomorphs and ungulates by size. When we calculated the seasonal niche breadth indices using lumped prey categories (i.e., rodent, lagomorph, ungulate), bobcat breadth did not decrease over time. Bobcats may have shifted from rodents to lagomorphs as a prey-switching response to changing relative abundances of these prey (Murdoch 1969), although we did not quantify prey abundance. of grassland by both predators was consistent with previous findings for bobcats (May 1981, Koehler and Hornocker 1991), but was uncharacteristic of coyotes, which tend to selectively use such open habitats (e.g., Major and Sherburne 1987). Negative selection of grassland by coyotes in this study was likely related primarily to avoidance of humans (Sacks 1996). Sheep intensively grazed much of the grassland habitat in our study and density of grassland rodents may have been relatively low in this habitat as a result (Hayward et al. 1997), further reducing its attractiveness to coyotes (and bobcats). Habitat selection Habitat segregation may occur independently of interspecific interactions. For example, felids often rely on dense understory cover to facilitate their stalk and ambush style of predation, in contrast to the open pursuit typical of canids (Kleiman and Eisenberg 1973); thus cover and open habitat types might be expected to be selected by bobcats and coyotes, respectively. In this study there was a slight tendency for bobcat home ranges to include more forest and less grassland than coyote home ranges. Further, bobcat home ranges tended to be located at higher elevations, spread along the primary mountain ridge, suggesting some degree of selection for features associated with these areas, such as steep hillsides and rugged, rocky terrain. Within their home ranges bobcats and coyotes displayed nearly identical habitat selection, suggesting that differences at the landscape scale were not related to interspecific interactions. The slight negative selection Fig. 7. Selection indices of observed versus expected use of overlapping coyote core areas by bobcats during six seasons, Hopland Research and Extension Center, July 1994 December 1995. Values 1=positive selection, 1=negative selection, 1=no selection. Sample sizes were 4, 5, 8, 7, 4, and 2 bobcats in chronological order. Error bars correspond to 95% Bonferroni confidence intervals. Spatial relationships The potential for chance encounters between bobcats and coyotes, and hence interference, on the study area was high due to high densities, small home ranges, and high interspecific home range overlap. Whereas exploitation competition, by definition, requires that a resource (usually food) be limiting, behavior associated with interference competition may be present regardless of current resource levels. For example, agonistic behavior and interspecific killing could be cued by detection of a particular species rather than being caused proximately by resource limitation, although such a response could be the evolutionary consequence of past resource competition. Bobcats displayed some avoidance of the most intensively used parts of coyote home ranges during winter and spring, which coincided with coyote breeding and pup-rearing, when coyotes tend to be most territorial toward conspecifics. Core avoidance during winter (coyote breeding) was only apparent for small bobcats, whereas large and small bobcats avoided coyote cores during spring (pup-rearing), when these areas corresponded to den sites, which were probably actively defended. These findings were consistent with other studies indicating that small bobcats (females and young males) are more vulnerable to agonistic interactions with coyotes (Anderson 1986, Litvaitis 1992). We suspect that the observed spatial avoidance of coyote cores by bobcats in this study reflected indirect (e.g., avoiding coyote scent) as opposed to direct (i.e., being chased) interactions. We found no evidence that bobcats were injured or killed by coyotes. Only one radiocollared bobcat died during our study and, although we recovered the carcass too long after death to determine the cause, we found no evidence of attack by another carnivore (e.g., tooth punctures in the skeleton). In addition to the bobcats captured in our study, many others were captured before and after the study during attempts to capture coyotes for other research (e.g., Sacks et al. 1999a, b); none of these bobcats had injuries indicative of coyote attacks. 244 OIKOS 94:2 (2001)

Fig. 8. Space use by female and small male bobcats (A, B) and large male bobcats (C, D) with respect to coyote territories during winter (A, C) and spring (B, D) 1995, Hopland Research and Extension Center, January 1995 June 1995. Seasonal bobcat locations (dots) and annual 95% MCPs (lines; female ranges have dashed line) are overlaid on seasonal coyote territories (shaded polygons; darker shading in core areas). Activity Partitioning of resources in time is rare relative to partitioning by food or habitat types (Schoener 1974a). Temporal segregation among carnivores is usually not an effective means of partitioning resources per se, because shared resources generally are not renewed within a diel period, and thus exploitation competition is not alleviated (Jaksíc et al. 1981). However, temporal spacing may reflect a response to agonistic interactions as with spatial segregation. Toweill (1986) reported different patterns of activity of bobcats and coyotes which he speculated helped to minimize interspecific contact. At HREC, although bobcats displayed some avoidance of coyotes in space, we found no evidence of a corresponding avoidance in time. Activity patterns of bobcats and coyotes were not negatively correlated and although the two predators displayed different diel patterns of activity, these differences did not coincide with seasons when spatial avoidance was most pronounced. In comparison, transient coyotes, which displayed strong spatial avoidance of residents at our study site (Sacks et al. 1999b), displayed diel activity patterns opposite those of resident coyotes (Sacks 1996). Nocturnal activity of resident coyotes was probably due primarily to high human exploitation. During the year previous to this study, no removal of coyotes OIKOS 94:2 (2001) 245

was attempted and coyote activity patterns were less nocturnal (Sacks 1996). Bobcats suffered no exploitation and therefore did not share this pressure for nocturnality. Diurnal activity seems to be generally more typical of unexploited populations of bobcats (Kitchings and Story 1978) and coyotes (Gipson and Sealander 1972, Andelt 1985, Kitchen et al. 2000). Bobcat coyote niche relationships Taken together, our findings indicate that bobcats and coyotes used food and habitat resources independently of each other. The avoidance of coyote cores by bobcats during some seasons provided limited evidence of negative relations. However, the lack of evidence of physical harm or of temporal avoidance suggested that such behavior was not especially important, despite generally high densities of the two predators. It seems likely that prey size differences and/or abundance of food alleviated competition between these two carnivores. Other studies of bobcats and coyotes conducted in environments with mild, relatively stable climates had similar results (Witmer and DeCalesta 1986, Bradley and Fagre 1988). In these studies, as in ours, densities of both predators were high, interspecific home range overlap was extensive, and bobcats and coyotes used foods and habitats similarly. These authors did not report any evidence of interspecific avoidance in space or time, agonistic interactions, or coyote-caused bobcat injury or mortality. Fig. 9. Average diel activity of (A) bobcats and (B) coyotes, during summer fall 1994 (n=11 bobcats, 9 coyotes), winter spring 1995 (n=8, 6), and summer fall 1995 (n=5, 3), Hopland Research and Extension Center, July 1994 December 1995. Standard error bars and connecting lines are not shown in summer fall 1995 due to small sample sizes and missing data. 246 OIKOS 94:2 (2001)

In contrast, studies in harsh environments characterized by long, severe winters, reported that bobcats and coyotes occurred at relatively low densities and occupied large home ranges (Toweill 1986, Major and Sherburne 1987, Litvaitis and Harrison 1989). Bobcats and coyotes displayed moderate to extensive home range overlap but because of large use areas, chance interspecific encounters were likely rare even in overlap areas. High overlap in one niche dimension (e.g., food use) was often related to low overlap in another (e.g., habitat use; Toweill 1986), indicating niche complementarity. Agonistic interactions included at least one bobcat killed by coyotes in the Cascade Mountains of Oregon (Toweill 1986) and one bobcat killed in a trap by coyotes in eastern Maine (Litvaitis and Harrison 1989). Major and Sherburne (1987) working in western Maine reported no evidence of agonistic behavior, but believed that exploitation competition for deer may have been important, and presented evidence that bobcats were in generally worse condition during winter than coyotes. This may have reflected climatic stress rather than interspecific competition with coyotes, as the study area occurred in the northernmost boundary of the bobcat range. Thus, the importance of interspecific competition between bobcats and coyotes seems generally low but perhaps greater in highly seasonal climates characteristic of northern/northeastern North America and mountainous regions of the western U.S. as compared to mild, stable climates of the southern and west-coastal regions. Although prey biomass tends to be lower in northerly regions, low prey biomass per se does not imply prey scarcity because predator biomass on average tends to correspond to average prey biomass (e.g., Knowlton and Gese 1995). However, in northern North America resources regularly become limiting due to harsh winters and, in boreal regions, multiannual cycles in abundance of primary prey provide additional periods of prey scarcity (and abundance; Keith and Windberg 1978, O Donoghue et al. 1998). In contrast, Mediterranean or semi-arid climates of western and southern North America tend to be more stable and predator and prey populations are likely to be closer to equilibrium more of the time, such that prey are rarely very scarce (or abundant) relative to predator abundance. Acknowledgements We thank M. M. Jaeger and HREC for supporting this study. K. Blejwas, J. Dayton, J. Meisler, J. Poor, Jr., and T. Weller provided valuable field assistance. S. Ardley, J. Theade, E. Voight, and volunteers with the University Research Expedition Program helped with scat collection. M. E. Jaeger, K. Finn and C. Wu assisted with scat analysis. G. Trehey and C. Brooks provided GIS expertise and satellite images. 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