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Range Expansion, Population Sizes, and Management of Wild Pigs in California

John D. Waithman, Richard A. Sweitzer, Dirk Van Vuren, John D. Drew, Amy J. Brinkhaus, Ian A. Gardner and Walter M. Boyce
The Journal of Wildlife Management
Vol. 63, No. 1 (Jan., 1999), pp. 298-308
Published by: Wiley on behalf of the Wildlife Society
DOI: 10.2307/3802513
Stable URL: http://www.jstor.org/stable/3802513
Page Count: 11
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Range Expansion, Population Sizes, and Management of Wild Pigs in California
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Abstract

The introduction and spread of nonnative organisms to new regions can disrupt ecosystems by causing declines or extinctions of native species. Widely introduced wild pigs (Sus scrofa) have contributed to declines and extinctions of numerous species on oceanic islands and can have pronounced negative ecological effects on mainland areas when population densities are high. Although range expansion by introduced wild pigs has ceased in many regions of the United States, it has increased significantly since the 1950s in California. Our analyses of data from annual hunter surveys and mapped locations of hunter-killed wild pigs shows that the distribution of wild pigs increased from around 10 coastal counties in the early 1960s to parts of 49 of California's 58 counties by 1996. An index to density based on locations of hunter-killed wild pigs plotted in a Geographic Information System (GIS) indicated that within the 79,550 km2 (25%) of the total land area of the state now occupied by wild pigs, populations are most abundant in the central and north-coast regions. By stratifying each county into 1 of 3 relative abundance classes and assigning density values based on research at multiple sites, we estimated there were around 133,106 (range = 106,485-159,727) wild pigs in California in 1996. The recent increase in the range of wild pigs in California was facilitated by a combination of multiple hunting-related introductions, deliberate releases of domestic pigs, and, possibly, increased forage availability associated with agricultural development. Natural range expansion also has occurred, however, and the dynamics of the spread of wild pigs in California appears typical of invasions observed among other organisms. Forage and water availability are important factors influencing the distribution and abundance of wild pigs in California, and predation may be important to an unknown extent. Up to 40% of California's wild pigs are removed from the population annually, which may control populations in some areas but not others. Wild pigs may be causing ecological damage in some coastal regions where population densities are very high, and detailed studies are needed in those areas to help mitigate potential problems with this increasingly widespread mammal in California.

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