COMPETITIVE INTERACTIONS BETWEEN COYOTES AND SAN JOAQUIN KIT FOXES BRIAN L. CYPHER AND KENNETH A. SPENCER Enterprise Advisory Services, Inc., P.O. Box 178, Tupman, CA 93276 Present address of KAS: Colorado State University, Center for Ecological Management of Military Lands, Fort Hunter Liggett, CA 93928 Downloaded from https://academic.oup.com/jmammal/article/79/1/204/841958 by guest on 30 September 2021 Competitive interactions between coyotes (Canis latrans) and federally endangered San Joaquin kit foxes (Vuipes macrotis mutica) were investigated at the Naval Petroleum Re­ serves in California (NPRC) during 1984-1995. Coyotes and kit foxes used similar food items, indicating the potential for exploitative competition. Leporids were the primary prey for coyotes in all years, but small rodents were the primary prey for kit foxes in most years, although leporids were primary prey in other years. Coyotes were the main cause of mortality to kit foxes at NPRC, indicating that interference competition may be occur­ ring. Population trends of kit foxes appeared to be strongly i!1.fluenced by food availability, but competition from coyotes also may have affected population dynamics of kit foxes. Mechanisms employed by kit foxes, such as resource partitioning, greater dietary breadth, and year-round den use, may facilitate coexistence with coyotes. However, use of anthro­ pogenic food sources by coyotes may intensify competitive interactions during periods of low prey availability. Key words: Vulpes macrotis mutica, kit fox, Canis tatrans, coyote, endangered species, competition, California The San Joaquin kit fox (Vuipes macrotis tition. Coyotes also have been identified as mutica) is a federally endangered and Cal­ an important source of kit fox mortality ifornia tlueatened species occurring in the (Cypher and Scrivner, 1992; Disney and southern San Joaquin Valley and adjacent Spiegel, 1992; Ralls and White, 1995), and Salinas, Valley in California. Primary this mortality potentially constitutes inter­ threats to the continued existence of the kit ference competition. The objectives of this fox are loss, degradation, and fragmentation investigation were to detennine the extent of habitat due to agricultural, industrial, and of both exploitative and interference com­ urban development (United States Fish and petition between San Joaquin kit foxes and Wildlife Service, 1983). Pesticide use, coyotes, and to assess the effect of any predator control programs, and illegal competitive interactions on kit fox abun­ shooting and trapping may constitute sec­ dance. ondary threats. Competition from other predators, particularly coyotes (Canis la­ MATERIALS AND METHODS trans), also has been identified as a poten­ Study area.-Our investigation was conduct­ tial tlueat to kit faxes (Ralls and White, ed at the Naval Petroleum Reserves in California 1995). However, the extent and significance (NPRC) located ca. 42 Ian southwest of Bakers­ of competitive interactions between kit fox­ field, Kern Co. The NPRC is an area of active es and coyotes have not been determined. petroleum production and is comprised of the Coyotes apparently consume many of the 19,186-ha Naval Petroleum Reserve No. 1 same food items-used by kit foxes (Cypher (NPR-I) and the adjacent 12,173-ha Naval Pe M et aI., 1994; White et aI., 1995), and this troleum Reserve No.2 (NPR-2). Physiographi­ potentially constitutes exploitative compe- cally, NPR-l and NPR-2 each encompass anti- Journal of Mammalogy, 79(1):204-214, 1998 204 February 1998 CYPHER AND SPENCER-COYOTE-KIT FOX COMPETITION 205 clinal ridges that are highly dissected by steep pare total and annual food-item use between kit draws and dry stream channels. Alluvial plains faxes and coyotes. So that data from coyotes and and flat valley lands surround the ridges. Ele­ foxes would be more comparable, food-item use vation ranges from 88 m to 473 m (Woodring et by kit foxes was determined using feces col­ al., 1932). Climate is characterized by hot dry lected on NPR-l during 1985-1988 and feces summers and cool wet winters with frequent fog. from both NPR-l and NPR-2 during 1989- Temperatures in summer often exceed 38°C and 1990. For this analysis, food items were cate­ seldom go below O°C in winter. Annual precip­ gorized as leporid, rodent, insect, or other. An­ Downloaded from https://academic.oup.com/jmammal/article/79/1/204/841958 by guest on 30 September 2021 itation averages ca. 12 cm and occurs primarily nual and total dietary diversity were detennined as rain falling between November and April for both species by calculating a Shannon di­ (National Oceanic and Atmospheric Administra­ versity index (Brower and Zar, 1984). For each tion, 1992). year, a Student's t-test was used to test if diver­ Vegetation at NPRC is classified as either sity indices differed between kit foxes and coy­ Lower Sonoran Grassland (Twisselman, 1967), otes (Hutcheson, 1970). Hom's similarity index Valley Grassland (Heady, 1977), or Valley Salt­ (Hom, 1966) was used to estimate dietary over­ bush Scrub (Holland, 1986). Dominant shrubs lap between species. include desert saltbush (Atriplex polycarpa), Leporids were identified as an important food cheesebush (Hymenoclea salsola), and bladder­ item at NPRC for both kit foxes (Scrivner et aI., pod (lsomeris arborea). Herbaceous cover is 1987) and coyotes (Cypher et aI., 1994). Abun­ dominated by red brome (Bromus madritensis) dance of leporids was assessed annually at and red-stemmed filaree (Erodium cicutarium). NPRC beginning in 1984 by conducting line­ Food-item use.-Use of food items by coyo­ transect surveys (Harris, 1986). Surveys were tes and kit foxes was determined to assess ex­ conducted in summer (June) along 60 l.6-km ploitative competition. From 1985 to 1990, a transects established throughout NPRC. Annual coyote-reduction program was conducted at abundance of leporids was indexed by calculat­ NPRC in an effort to reduce predation on kit ing the number observed per km of transect. To faxes. Reduction was conducted on NPR-l from determine if abundance of leporids influenced 1985 to 1988 and on both NPR-l and NPR-2 their use by kit foxes and coyotes, annual fre­ from 1989 to 1990 (Cypher and Scrivner, 1992). quencies of occurrence of leporids in coyote This program resulted in the killing of 591 coy­ stomachs and kit fox feces was compared to an­ otes, which were taken in all seasons. Stomachs nual leporid indices using correlation analysis. were collected from 322 carcasses and frozen Frequencies of occurrence were arcsine-trans­ for analysis. Food habits of coyotes were deter­ formed (Zar, 1984) prior to analysis. mined by analysis of stomach contents (Cypher Kit fox mortality.-Sources and rates of mor­ et aI., 1994). tality of kit faxes were determined annually by Food habits of kit faxes were determined by monitoring fates of radiocollared kit faxes. Most analysis of feces. Feces were collected from fox­ adult faxes were collared (models L2B5, 300, es live-trapped during annual efforts to assess 95, and 80; Telonics, Inc., Mesa, AZ) during abundance, mortality rates, and reproductive live-trapping sessions conducted in summer success. Samples were collected in all seasons (July-August) and winter (Novetnber-January) except late winter-early spring (15 January-l to assess abundance. During these sessions, one May) when fox trapping was suspended to avoid wire-mesh box trap (37.5 by 37.5 by 105 cm) disturbing animals during parturition and early was placed in each quarter-section of NPR-2 and pup-rearing. Feces were oven-dried ~24 h and a 117-km2 study area on NPR-l, and set for 4 sent to a diagnostic laboratory (Global Ecosys­ consecutive nights. Traps were covered with tem Managers, Baton Rouge, LA) for analysis. tarps to provide protection from inclement Although highly disgestible items may have weather, baited with either canned mackerel or been more difficult to detect in feces compared pieces of road-killed leporid (not used after to stomachs, foods commonly consumed by kit 1981), and checked early the next morning. Cap­ foxes contain materials of low digestibility (e.g., tured foxes were weighed, aged (juvenile or hair, bone, exoskeleton) that were easily detect­ adult based on body mass and patterns of tooth ed in feces. eruption). ear-tagged, and inspected for injuries. Contingency-table analysis was used to com- All faxes were released at the capture site. Ju- 206 JOURNAL OF MAMMALOGY Vol. 79, No.1 venile foxes were collared (models 95, 80, and marks) were recorded. Annual indices of abun­ 2A; Telanies) during live-trapping conducted in dance were calculated by determining the pro­ spring (March-June) to assess reproductive suc­ portion of operable stations visited and multi­ cess of kit foxes. FOf this effort, traps were set plying by 1,000 (Harris, 1987; Roughton and at dens occupied by radiocollared adult females Sweeny, 1982). and pup-rearing dens identified by the presence Relationships between indices of abundance of prey remains, trampled vegetation, and sma11- of coyotes and kit foxes and between indices of sized tracks and feces (Morrell, 1972). coyote abundance and mortality rates of both Downloaded from https://academic.oup.com/jmammal/article/79/1/204/841958 by guest on 30 September 2021 Radiocollared kit foxes were monitored diur­ adult and juvenile foxes were examined using nally several times per week. If a fox was mo­ correlation analysis. The relationship between tionless for 4 hours, the pulse rate emitted by mortality rates of kit foxes and the proportion of the radiocollar would increase (about double) mortalities caused by predators also was exam­ producing a mortality signal. Foxes with trans­ ined using correlation analysis. This relationship mitters emitting a mortality signal were recov­ was expected to be nonsignificant if mortality ered and necropsied to determine cause of death. attributable to coyotes was compensatory and Mortality from predators was identified by the positive if this source of mortality was additive.