19 Oak Ecosystem Restoration on Santa Catalina Island, California

19 BROWSE IMPACTS OF INTRODUCED MULE DEER TO ISLAND SCRUB OAK HABITATS ON SANTA CATALINA ISLAND, CALIFORNIA Thad Manuwal and Rick Sweitzer Department of Biology, University of North Dakota Grand Forks, ND 58202 [email protected], phone 701-777-2164 [email protected], phone 701-777-4676 ABSTRACT: Islands commonly harbor unique species that are particularly susceptible to damage by introduced organisms. Historically, no large ungulates existed on Santa Catalina Island, but several were introduced including mule deer (Odocoileus hemionus). Previously, there were no data for assessing impacts of mule deer to endemic trees and shrubs on the island. Our objectives here are to investigate the impacts of introduced mule deer on island scrub oak (Quercus pacifica) habitats; more specifically, to (1) determine seasonal mule deer diets, (2) estimate browse use of two rare endemic trees/shrubs, (3) identify factors impinging on island scrub oak seedling survival, and (4) differentiate impacts of multiple introduced large ungulates on oak regeneration. Our results suggest deer select for forbs and grasses during the annual wet season and rely on woody browse during the annual dry season. Mule deer dramatically reduce available current annual growth twigs of rare island endemic shrubs. However, physical damage to oak seedlings by bison (Bos bison) and competition from non-native grasses appears more important for reduced seedling survival than browsing by mule deer. Further, island scrub oak seedling densities were higher in areas of the island where deer are present but other feral ungulates have been removed longest. KEYWORDS: Introduced ungulates, mule deer, Quercus pacifica, regeneration, Santa Catalina Island, seedlings INTRODUCTION The introduction and spread of nonnative species is an important conservation problem in general but especially on islands (Savidge 1987, Cree et al. 1995), which typically have higher proportions of endemic species and are more prone to invasion (Lodge 1993, Simberloff 1994). Further, insular endemic plant species often lack adequate chemical and structural defenses and are more susceptible to damage from introduced herbivores (Bowen and VanVuren 1997). Island ecosystem composition and function may be drastically altered by the introduction of large ungulates (Husheer et al. 2003). As an example, Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced to Haida Gwaii, Canada in the late 1800s. Once on the island, the black-tailed deer foraged preferentially on the seedlings and lower branches of western red cedar (Thuja plicata), which, as an island isolate population, had reduced chemical defenses against herbivores (Vourc’h et al. 2000). Over a 50 year time period, foraging by nonnative Sitka black-tailed deer on Haida Gwaii caused major reductions in overall plant cover (Stockton et al. 2005), songbird populations (55-70% reduction), and abundance and density of invertebrates compared to those same organisms on nearby islands without deer (Allombert et al. 2005a). Santa Catalina Island (hereafter Catalina Island), part of California’s Channel Islands (a group of islands off the coast of southern California), has no native large ungulates, and the largest native herbivore is the beechey ground squirrel (Spermophilus beecheyi nesioticus). However, the island has a long history of introduced ungulates including mule deer (Odocoileus hemionus). Catalina Island was occupied by feral goats (Capra hircus) before mule deer were introduced in the early 1930s (Coblentz 1977), whereas bison (Bos bison) and feral pigs (Sus scrofa) were introduced around the same time as mule deer (Sweitzer et al. 2005). By the 1980s conservationists were more aware of the myriad ecological Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA. 20 problems with introduced species, and in the 1990s the Santa Catalina Island Conservancy (hereafter the Conservancy), a non-profit organization which owns and manages 88% of Catalina Island, implemented an active restoration program to eradicate feral goats and feral pigs (Schuyler et al. 2002). By early 2005 feral goats were eradicated and nearly all feral pigs had been removed (Schuyler et al. 2002, Santa Catalina Island Conservancy unpublished records). Recent research on the ecological effects of the introduced bison population (Sweitzer et al. 2003, Constible et al. 2005) supported a decision by the Conservancy to reduce but not entirely remove the bison herd. Mule deer remain widespread and abundant at an unknown level, and very little is known of their foraging impacts and population ecology useful for management. Catalina Island harbors six species of plants found only on this island and 15 that are endemic to the Channel Islands. All of these plants are at risk from foraging and other activities of introduced ungulates. The island scrub oak (Quercus pacifica) is an especially important endemic tree and a dominant component of the island scrub oak woodland/chaparral habitat type or community of the Channel Islands (Junak et al. 1995). Oaks and their associated woodland habitats are of great ecological importance throughout the world and especially in California. Oak woodland communities in California are especially diverse with more than 1400 species of flowering plants, over 300 species of vertebrates, and thousands of invertebrate species (Pavlik et al. 1991, Tyler et al. 2006). Several vertebrate species depend on oak woodland habitats on Catalina Island, including the endemic island fox (Urocyon littoralis catalinae; Moore and Collins 1995) and the orange crowned warbler (Vermivora celata sordida, Sillett pers. comm.). However, recent observations of island scrub oak have indicated low regeneration rates (Stratton 2001). This phenomenon is not specific to Catalina Island; a relative lack of regeneration among several species of oaks has been reported in mainland California (White 1966, Griffin 1971), however see Tyler et al. (2006). Reported causes for limited regeneration include physical and/or foraging damage by deer, feral pigs, and cattle (White 1966, Borchert et al. 1989, Sweitzer and Van Vuren 2002), predation by rodents (Borchert et al. 1989), water stress (Matzner et al. 2003), and competition with non-native grasses (Gordon and Rice 2000). Because of the known impacts of nonnative ungulates on insular plants, there is concern that mule deer are damaging oak woodlands and endemic trees and shrubs on Catalina Island by browsing. A major focus of our research was investigating the impacts of introduced ungulates (deer and bison) on oak woodland habitats and oak seedling recruitment. Our primary objectives were to: (1) determine seasonal mule deer diets, (2) estimate browse use of two rare endemic trees/shrubs, (3) identify factors impinging on island scrub oak seedling survival, and (4) differentiate impacts of several introduced large ungulates on oak regeneration. STUDY AREA Catalina Island is a 194-km2 island located 40 km south of coastal Los Angeles, in Los Angeles County, California. Elevation on the island ranges from sea level to 640 m, with a topography dominated by a northwest-southeast mountain range containing a series of lateral canyons (Schuyler et al. 2002). The climate is Mediterranean with relatively mild temperatures throughout the year and a long term mean annual precipitation of 290 ± (S.D.) 155 mm, mostly occurring between November and April (Schoenherr et al. 1999). There are four common habitat types on the island: (1) coastal sage scrub, characterized by coastal sage (Artemisia californica) and prickly pear cactus (Opuntia littoralis); (2) grassland, dominated by exotic annual grasses and forbs, such as bromes (Bromus spp.) and storksbill (Erodium spp.), and interspersed with native bunch grasses (Nasella spp.); (3) island chaparral represented by evergreen and drought-resistant shrubs and low trees such as island scrub oak and lemonade berry (Rhus integrifolia); and (4) riparian habitats, limited to a few permanent or ephemeral streams in relatively deep canyons and marshy wetland areas, represented by cottonwood (Populus trichocarpa), willow (Salix spp.) various sedges and rushes, and mule fat (Baccharis pilularis) (Knapp 2002). Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA. 21 Three cross-island fences were established on Catalina Island in the 1990s to facilitate the eradication of feral pigs and goats (Schuyler et al. 2002), effectively dividing the island into four zones (Figure 1). These two feral ungulates were then eradicated in each zone sequentially (zone 1 – 1998, zone 2 – 2000, zone 3 – 2003, zone 4 – all goats and most pigs by January 2005). Because feral pigs are known to negatively impact tree seedling survival by rooting (Sweitzer and Van Vuren 2002) and feral goats seriously damage trees and shrubs by browsing (Coblentz 1977), we anticipated the presence of greater densities of oak seedlings in zone 1 compared to zones 2 and 3. Further, because bison on Catalina Island rarely ventured across the isthmus into zone 1 historically, and were prevented from crossing into zone 1 by fence after the early 1990s (Sweitzer et al. 2005), any possible bison-related impacts to tree seedlings would be focused in zones 2 and

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