Feral Pig (Sus scrofa) Soil Disturbance in Henry Coe State Park, California Halliday Dresser Abstract Exotic feral pigs (Sus scrofa) exist at increasing densities in California's oak woodlands, where their disturbance of the soil in the course of gathering roots and grubs constitutes a major change in disturbance regime and hence a management problem. In the summer and fall of 2003, I gathered data on feral pig soil disturbance and its associated habitat by walking a series of transects across Henry Coe State Park, a wilderness area in which a trapping program to control the feral pig population had recently begun. By comparing my data with data gathered in 1982 on the same transects, I determined: 1. That pig rooting has declined significantly between the two years of data-gathering; and 2. That this reduction was greater in the wetter of the two large watersheds of the study area. The implications for management are that, at least in this central California location, trapping feral pigs is an effective method of controlling their soil disturbance in the short term, and that it is evidently more effective where less precipitation is received. The ecological implication is that pigs may be concentrating in wetter areas, and thus estimates of pig population based on observation of those areas are likely to be faulty.Introduction Invasive species – organisms released into an environment to which they are not endemic, and in which their population is capable of expanding – are considered by many to be one of the most significant threats to global biodiversity. Invasive species may compete directly with native species whose numbers are often limited by predation. Invasives may also alter the environment, thereby transforming the habitat upon which native species depend (Ehrlich 1977). The feral pig, Sus scrofa, is a good example of an invasive organism, competing with native small mammals for acorns, grubs, and below-ground storage organs (Barrett 1987); reducing already-compromised oak recruitment by acorn predation; and producing a change in disturbance regime by their aggressive soil excavation in search of food. The feral pig common in the west is descended from domestic pigs escaped since 1769 and European wild boar introduced for sport hunting since 1923 (Barrett 1987). The majority of feral pigs’ active hours are spent rooting in the ground with their cartilaginous snouts for bulbs, worms and grubs, their main sources of protein (Krosniunas 1984, Van Vuren 1984). Soil disturbance is the most significant effect of feral pigs on their environment; pigs dig up areas of up to several hectares a day to a depth of 10 cm, turning over the soil and uprooting or burying all plant matter within the area. This "rototilling" impacts both plant communities and soil characteristics. Soil disturbance by feral pigs is regarded as a significant management problem in California as in many other places in the world. The question, then, for managers, is how to control the numbers and behavior of these animals, which travel hundreds of km daily and bear two litters of six to 12 piglets each year (Barrett 1987). They are omnivores and thus difficult to target specifically with poison. Additionally, no reasonable method of excluding them from vast areas of backcountry is available. Trapping and killing appears to be the only realistic recourse. Given their vagility and fecundity, however, there is no guarantee that this method will work either, certainly no guarantee that it will prove cost-effective. Such a trapping program has recently been instituted in Henry W. Coe State Park, an oak woodland wilderness representative of feral pig habitat in California. By recording data on a series of transects in Henry Coe State Park, and comparing them with similar data gathered in 1986, I endeavored to address several questions: Which habitat types at Coe State Park are most commonly disturbed by the pigs? What is the extent of this disturbance, by habitat type? What is the effectiveness of the Park’s trapping program? And what is thelonger-term trend in pig soil disturbance within the Park? Methods In the summer of 2003 I gathered data on feral pig (Sus scrofa) soil disturbance in Henry Coe State Park (Coe SP), an 80,000 acre area of oak and digger pine grassland in the Diablo Range of central California. The park consists of two large watersheds; there is a rainshadow effect, so the eastern watershed receives significantly less precipitation than the western watershed (fig. 1). In 2002 Coe SP instituted a trapping program to attempt to control pig densities, and thereby their soil disturbance. Figure 1. Henry W. Coe State ParkI walked 13 km of East/West transects due East/West across Henry Coe State Park, gathering data on successive 10-m square quadrats as I went, so as to ultimately survey a swath 10 m wide centered on each transect (Hone 1988; Anderson 1994). These transects duplicated, to within 50 m, those on which data, incorporated into my study, had been gathered in 1986 by Martha Schaub, a student in the lab of Reginald Barrett, UC Berkeley. I recorded habitat type (according to the California Wildlife Habitat Relationships System (CWHRS)) and area of soil disturbed by feral pigs for each quadrat. Soil disturbance was defined as an area of soil overturned to between 10 and 20 cm, with no new growth of grass or herbaceous groundcover showing, thus limiting my survey to areas disturbed within the previous year. Plots 10 m on a side were paced out or estimated visually; the percent of each quadrat's area disturbed was likewise estimated by eye. Using contingency tables, I compared: 1) The proportion of quadrats in which soil had been disturbed in 2003 with that in 1986, 2) the 2003 with the 1986 data in the Western watershed of the park, which receives more precipitation, and 3) the 2003 with the 1986 data in the Eastern watershed of the park, which receives less. Results I compared observed frequency of feral pig soil disturbance in 1982 and 2003 on ten transects. The results are presented below (Fig. 2). These data show a significant decline in soil rooting between 1982 and 2003 (Fisher’s Exact Statistic, p<0.0001; all contingency tables constructed and analyzed using JMP statistical software package). Individual transects all fall wholly within one or the other of Coe’s two watersheds: the Western, receiving an annual average of 66 cm of
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