Feral Pigs and Oak Woodland Vegetation

Oaks ‘n’ Folks – Volume 16, Issue 2 – August 2000

In the late Pleistocene, California was home to a diverse suite of large vertebrates, including digging and rooting animals like the peccary and grizzly bear. These two species, along with many others, are now extinct but their role in the community has been partially replaced by other herbivores, including domestic livestock. Native to the Old World, feral pigs are common in California.Rooting by these pigs can result in an important disturbance for grasslands and woodlands, with a potential impact on the native understory flora and on oak regeneration.

As a first step toward testing the impacts of pigs on oak woodland in the Inner South Coast Ranges, we designed a quantitative field survey to answer the following questions: Where do pigs occur? How is pig activity related to vegetation?


The study was conducted at the Ventana Ranch in San Benito County, California, east of King City in the Inner South Coast Ranges. The vegetation is a mixture of blue oak woodland, grassland, and chaparral, with rainfall generally under 750mm/year.

We randomly located 41 oak woodland sample plots in the fall and winter of 1998-9. The plots, each a 10 meter long transect, were marked with permanent stakes at each end. Along the transect, a total of 20 one-square-meter quadrats were placed on alternate sides of the tape. The slope, aspect, and general description of the plot were noted, and the variables listed in Table 1 were recorded for each of the 820 total quadrats. Occurrence of rooting was used to measure pig activity. We screened the data for statistical significance using a simple procedure based on a 95% confidence interval for proportions. The confidence interval, which is based on the binomial distribution, varies with the number of samples in each category, so the rarer categories have a wider confidence band.

Table 1. Variables Measured on each of the 820 Quadrats.

Oak canopy presence/absence and number of trees with canopy over the quadrat
Acorn class a description of the number of acorns or caps on each oak canopy above the quadrat

0=no acorns
1=one or two acorns visible after great searching
2=many acorns easily spotted
3=abundant acorns

Shrub cover shrub species and cover, measured as the length of measuring tape intercepted by each species
Rooting class a description of the degree of rooting evidence

0=no evidence
1=rooted patches with faded edges and plant growth
2=patches with distinct edges, loose soil, and no regrowth
3=fresh patches with moist soil

% bare ground amount of bare ground along the quadrat
# oak seedlings total number of oak seedlings
cow sign presence/absence and type if present
perennials number and species, when identifiable, of perennial herbs


Of the 820 quadrats, 504 (61%) contained oak canopy and 556 (68%) showed some evidence of pig rooting. Thirty-eight (5%) of the quadrats showed evidence of recent rooting (within a few days of sampling). Rooting was significantly more likely under oak canopy than in open grassland (Figure 1), although pigs still rooted in more than half of the open quadrats. Although pigs are known to eat acorns, overall rooting and recent rooting were not related to acorn abundance (Figure 2) nor to oak seedlings (Figure 3).

Oak seedlings were fairly common in the woodlands, but seedling presence was not related to slope, aspect, or rooting activity. We found a total of 105 oak seedlings in 71 quadrats; 64 (90%) of those quadrats were under canopy. Seedlings were significantly more likely under acorn class 3 than classes 0 or 1 (Figure 4) and also were more likely under denser canopy.

Figure 1. Is rooting related to oak canopy cover?
Relationship between pig rooting and presence of oak canopy cover. The proportions (approximately 95% confidence interval for the proportion is in parentheses) of quadrats with oak cover and rooting = 0.75 (0.7-0.8) and of quadrats withour oak cover and with rooting = 0.56 (0.5-0.6) are significantly different.
Figure 2. Is rooting related to acorn class?
Relationship between acorn class and presence or absence of rooting. The presence of rooting in a quadrat is not significantly related to acorn class.
Figure 3. Is rooting related to seedlings?
Relationship between rooting and presence or absence of oak seedlings. There is no significant relationship between presence of seedlings in a quadrat and evidence of rooting.
Figure 4. Location of Oak Seedlings
Oak seedlings were significantly more likely in quadrats under the abundant acorn class 3 than under classes 0 and 1. Proportions of quadrats with seedlings by acorn class (approximately 95% confidence interval for the proportion is in parentheses) are class 0=0.08 (0.04-0.12); class 1=0.07 (0.03-0.11); class 2=0.15 (0.10-0.20); class 3 =0.32 (0.15-0.65)


We found that pig activity was common on the ranch and positively related to oak canopy cover, although pig activity was also frequent in grasslands. Pig rooting, both overall and recent, was not related to measures of acorn abundance. Pigs apparently are attracted to rooting sites for factors other than acorn abundance. Oak seedlings were more common under high acorn producing trees and occurred in numbers probably sufficient for stand regeneration. Pig rooting was neither related to acorn abundance or to presence of oak seedlings. Although rooting created bare ground, oak seedlings and other native perennial plants were found in rooted areas. Further investigations of relationships between pig activity and community structure will require experimental manipulations.

We would like to thank Phil Berry for allowing access to the Ventana Ranch and providing financial and logistical support.


1 Edwards, S.W. 1996. A Rancholabrean-Age, Latest-Pleistocene bestiary for California Botanists. Four Seasons 10(2):5-34.

2 Barrett, R.H. 1978. The feral hog on the Dye Creek Ranch, California. Hilgardia 46:282-355.

3 Kotanen, P.M. 1997. Effects of experimental soil disturbance on revegetation by natives and exotics in coastal California. Journal of Applied Ecology 34:631-644.

edited by Adina Merenlender and Emily Heaton