The Ecological Impacts of Feral Swine

DAVID R. SYNATZSKE, Texas Parks and Wildlife Dept.

P.O. Box 115, Artesia Wells, TX 78001


Abstract: Feral swine, non-indigenous to North America, have the potential to significantly impact ecosystems. Whether these ecological impacts are viewed as negative or positive depends largely upon the perspective of the viewer. Significant impacts have been documented on vegetation, vertebrate and invertebrate fauna, soil properties, and water quality thru studies on feral hog food habits, rooting/wallowing activities and behavioral patterns. Review of the literature is presented documenting a variety of these impacts. Feral swine, like man, are a part of the ecosystem and strategies must be integrated into future management planning to control the potential impacts of this species.


Ecological impacts of feral swine – a very broad subject to try and address in a limited time period. The term ecology is one commonly encountered at this stage of man’s environmental awareness. Prior to any discussion of ecological impacts, one must first define “ecology”; although I’m sure we each have our own perception of it’s meaning. The word ecology, first coined in 1869 by German biologist Ernst Haeckel, is derived from the Greek oikos, meaning “house” or “place to live”. Odum (1971)states “Literally, ecology is the study of organisms “at home” and is concerned especially with the biology of groups of organisms and with functional processes on the lands, in the oceans, and in fresh waters. More in keeping with the modern emphasis on ecology is to define ecology as the study of the structure and function of nature, it being understood that mankind is a part of nature”. Therefore ecology is “environmental biology”.

Odum further states that “communities (all populations of organisms) and the non-living environment function together as an ecological system or ecosystem”; another term frequently used today. “Characteristics of ecosystems cannot be predicted from isolated populations, i.e. one must study the forest (the whole) as well as the trees (the parts).” Liebig (1840) recognized that all organisms have an ecological minimum and maximum with a range in between known as “limits of tolerance”, and that conditions approaching or exceeding these limits are “limiting factors”. Odum states, “the broad concept of limiting factors however; is not restricted to physical factors since biological interrelations are just as important in controlling the actual distribution and abundance of organisms in nature”. This brings us to “competition”- the interaction of organisms striving for the same thing, basically negative interactions. Interaction may also be positive, or of mutual benefit to different organisms.

Whether we consider ourselves as “ecologists” or “conservationists” brings different connotations to how we view the concerns of ecological impacts of feral swine. Odum states the goals or aims of conservation as “1) to insure the preservation of a quality environment that considers esthetic and recreational as well as product needs, and 2) to insure a continuous yield of useful plants, animals and materials by establishing a balanced cycle of harvest and renewal”. Therefore our view point dictates our interpretation of whether ecological impacts are positive or negative. Finally, Odum describes a phenomena known as “Ecological backlash (or ecological boomerang)-an unforeseen detrimental consequence of an environmental modification which cancels out the projected gain”. Since feral swine (or feral hogs) are not native to North America but are descendants of domesticated swine and/or European wild hogs that man, as an integral part of the ecosystem, introduced as early as the 16th century, we most probably are dealing with an “ecological backlash” in the case of feral swine.

The literature abounds with research that has been conducted on virtually all aspects of feral hog life history, behavioral patterns, food habits, and impacts on habitats and ecosystems. This paper will attempt to “hit some of the high spots” and invoke awareness that ecological impacts can be both positive and negative and that we must treat the “illness rather than the symptoms” and look at the “forest as well as the trees”.

Competitive Relationships

Potential for competition with indigenous wildlife presents a major concern for managers. Competition may be for food, water, or space. Food habit studies conducted on feral hogs yield information pertinent to the “potential” for competition with other species. Feral hogs are omnivorous, as indicated by numerous food habit studies conducted in various habitats. Kroll 1985, Springer 1977, Tate 1984, Thompson and Elisor 1973, Elisor and Harwell 1979, and Everitt and Alaniz 1980. Hog diets may include oak mast, soft mast, succulent grasses and forbs, fungi, roots and tubers, cacti, and animal matter. Being omnivorous, hogs have the ability to utilize different components of the habitat available as food resources and thus have advantages over other species with narrower tolerances as to diet. Springer (1977) noted potential competition for food in South Texas with white-tailed deer, turkey, and javelina but found, on the Aransas Refuge, that competition at present populations levels of feral hogs was a potential for only seasonally available fruits. Tate (1984) indicated potential competition for food with deer, turkey, bear, squirrel, chipmunks, skunks, raccoons, opossums, foxes, and bobcats. Thompson noted potential for significant competition for food with deer, turkey, squirrels, and waterfowl. Everitt and Alaniz (1980) indicated competition potential with sandhill cranes. Elisor (1973 and 1979) noted the potential for seasonally limited foods such as prickly pear fruits and mesquite beans to be competed for with white-tailed deer and javelina. He also noted that feral hogs and javelina were socially incompatible indicating spatial competition, with feral hogs fleeing from javelina upon confrontation. Kroll (1985) noted potential for competition to be highest in fall and winter with white-tailed deer. The potential obviously exists for competition for food with other species of wildlife not mentioned, however little is available in the literature to document such occurrence.

Although animal matter has been found to be a component of the diet of feral hogs, most studies indicate such material makes up a relatively small percentage of the total diet. Everitt and Alaniz (1980) indicated animal matter to comprise 5% of feral hog diets in South Texas. The question as to whether animal matter presence is due to predation or is taken as carrion is largely unanswered. Tisdell (1982) documented hog predation on young lambs and Turman (1980) states “feral hogs are currently the number two predator (behind the coyote) on sheep and goats in Bandera, Real and Edwards Counties of Texas”, estimating that in 1988 some 1,400 sheep and goats were killed by hogs in Texas. Kroll (1985) documented the presence of lizards, mice, birds, juvenile pigs and deer remains in hogs on the Engling WMA and Tisdell (1982) documented cannibalism in hogs. Springer (1977) indicated snakes were taken every season of the year in South Texas and also noted leopard frogs and deer fawn remains in hogs sampled. Wood and Lynn (1977) indicated hogs were important predators of wild turkey nests while Henry (1967) indicated them to be haphazard nests predators and that hogs were not additive to nest predation, but only replaced that which would have occurred by other predators either driven off or preyed upon by feral hogs, especially snakes. Thompson found hogs present minor problems in predation of ground nesting birds, rabbits and freshwater turtles. Wood, et. al. (1992) found that in Georgia 80 percent of sea turtle nests were lost on Ossabow Island due to hog predation. Singer, et. al. (1982) indicated predation of the red-cheeked salamander, a threatened species.

Impacts on invertebrate life have also been documented. Kroll (1985) found earthworms to be highly preferred food items while South Texas studies indicate caterpillars may be substantial components of the diet at times (Taylor, unpublished). Schwartz (1988) indicated the habit of hogs stripping bark from trees to dig out overwintering insects. Bratton indicated hog rootings may be detrimental to a nationally endangered land snail. Whether invertebrates are taken purposely or incidental to rooting and feeding activities, impacts are apparent on invertebrate populations.

Effects on Vegetation

Impacts by feral hogs on vegetation can occur in “take” of vegetation as food, by indirect impacts in response to rooting and digging activities, and by changes in successional patterns, soil properties, and water infiltration rates.

Feral hogs are adaptable to a wide range of habitat types, being found in marsh, mountainous, forested, semi-arid brush habitats and gradients in-between. Feral hogs tend to use freshwater marsh, bog, and swamp habitats when available (Kroll 1985); although habitat preference tests in South Carolina indicated hogs did not use habitats out of proportion to their availability (Wood and Brenneman 1980). Tate (1984) indicated that 50 non-woody species are known to be eaten, uprooted or trampled in the Smoky Mountains. He noted that vegetation production of gray beech tended to be stimulated while hog activities were of detriment to reproduction and foliage height of some species. Bratton noted damage to unique wildflower areas. Singer et. al. (1982) stated that feral swine impacts could have “worldwide implications for forest ecosystems and for agricultural lands”. Andrvzejewski and Jezierski (1979) stated, “European boars are believed to enhance the growth of pines in poorer European soils”. Boar removal from native habitats is now suspected to have decreased the rate of nutrient cycling and upset the stability of affected European forests (Grodzinski 1975). It should be noted that European wild boar are a natural component of European ecosystems. Lucas noted that in Hawaii, with no native mammals other than bats, feral hogs have created problems where plants aren’t adapted to pressure from herbivores.

Feral hog rooting and digging activity is perhaps one of the first indicators of feral hog presence observed. These activities are in pursuit of food resources in the form of roots, tubers, and invertebrates and should not be confused with wallows, where hogs are seeking relief from heat and insect infestation. Hone (1988) found that environmental conditions such as altitudes, rocks, trees, etc. dictate level of disturbance by hogs. Rooting activities have been noted to have a wide range of impacts on the ecosystem. Springer (1977) noted an increase in numbers of plant species in rooted area compared to disturbed areas, with increased occurrence of early succession plants such as crotons, partridge pea, composites, crabgrass, sedges, and rushes. This might be considered positive in the overview of enhancing biodiversity. Tate (1984) viewed rooting as disruptive of vegetative communities and successional patterns, altering forest floor habitat, altering nutrient cycling, and detrimental to aesthetic values of national parks. Singer (1982, 1984) noted rooting activity mixed soil horizons, reduced ground vegetative cover and leaf litter, accelerated decomposition of organic matter, accelerated leaching of certain minerals and altered ecosystem nitrogen transformation processes. Nitrate concentrations were found higher in soil, soil water, and stream water in rooted areas and infiltration by rainfall was noted to increase. Belden, et. al. (1976) found that hogs may contribute to bacteria loads of streams near rootings and wallows. Lucas found that hogs destablized surface soils.

In California, soil erosion has been found to increase in shallow soils over bedrock subjected to rooting activity. Lacki and Lancia (1983) found an increase in gaseous CO2 in soil air due to evaluated decomposition of organic matter. Singer (1982) also observed that vertebrate species depending largely on leaf litter for habitat were in some cases eliminated in rooted areas; however, higher densities of mice and chipmunks were noted in extremely rooted areas. An 80 percent reduction of macroinvertebrates in soil was observed in some areas. Howe, et. al. (1979) noted siltation or contamination of streams from nearby rootings and/or wallows which could be detrimental to native brook trout. Thompson noted that hog activity in marsh/ wetland areas provided positive and negative values; “hog activity can destroy plants of little or no value to other wildlife while providing soil disturbances that stimulate the growth of more valuable plants”. Bratton noted 1.5 to 4 times increase in vegetative cover upon exclusion of feral hogs in the Great Smokey Mountains. In Tennessee, potential negatives of hogs included setting back or speeding up plant succession, consumption of natural seed crops to point of impeding reproduction, limiting species composition and quantity of vegetation, encouraging erosion and physical damage to trees. Positive impacts were decreases in abundance of forest insect pests and increased quality of seed beds. Everitt and Alaniz (1980) noted that some shift in plant succession in response to rooting is beneficial to some wildlife. Spatz and Mueller-Dossbois (1975) found that in Hawaii, pig digging greatly enlarged the component of introduced species in communities with a former high percentage of native species, particularly in grasses. However, they also noted that despite the replacement of decadent native grasses by introduced grasses, when some introduced grasses come into competition with native grasses the natives may eventually become dominant over the exotic. Of note here is the decadent condition of native grasses.

It appears that the perceived impact of soil disturbance by hogs is relative to the management goals of the observer. While, hog disturbance can be viewed as highly detrimental to “native communities” it might be viewed as somewhat positive in terms of changing succession and perhaps increasing biodiversity through the creation of different seral stages.

Disease Reservoirs

Considerable research has been done on parasite loads and disease potential of feral hogs. Feral hogs have been found to be potential reservoirs for domestic livestock diseases, among which are hog chlorea, brucellosis, trichinosis, foot and mouth disease, African swine fever, and pseudorabies. Hogs have been found to test positive for leptospirosis and areas of heavy hog use have been found to harbor higher concentrations of fecal coliform bacteria. The potential of transmission of disease to other wildlife, livestock, and man does exist, as the documentation of the loss of 30 head of cattle in Dimmit Co., Texas in 1989 due to transmission of pseudorabies by feral hogs (Turman, ADC, USDA) would indicate.

Man-Feral Swine Relationships

Potential impact implications of feral hogs on man are an important consideration. A survey of landowners by Springer (1977) indicated that feral hogs 1) were considered as pests, 2) were implicated as predators on sheep, calves, goats, turkey, quail, small mammals, deer and exotics, and 3) damaged facilities, roads, waterings, fences, fields and tame pastures. However, 30 percent of respondents indicated they realized income from feral hog hunting. The National Park Service views feral hogs as detrimental to native communities, aesthetically unpleasing to the public with possibilities of hogs creating road hazards and confrontations with the public. In 1977 President Carter signed a proclamation directing National Parks to rid themselves of exotic species. Stu Coleman, chief of the Smokies Resource Management Division stated “as far as the NPS is concerned, wild hogs are the scourge of the earth. They don’t belong here and we don’t want them” (in Schwartz, 1988). To serve this goal, Great Smoky Mountains National Park was spending $200,000 annually to remove hogs in the late 1980’s. Likewise, environmental groups have voiced concerns over hog impacts on “natural area”.

Despite the negatives associated with feral hogs, hunters exhibit a tenacious attraction to hog hunting and many farmers and ranchers encourage presence of feral hogs due to the income derived from hunting. Although “control” efforts are ongoing by agencies such as Animal Damage Control/U.S. Dept. of Agriculture, in many cases control efforts are limited by the landowner, i.e. “take some but don’t take all of the hogs”. It’s common practice for landowners in South Texas to catch and release hogs, pen hogs to be fattened for human consumption, take some hogs thru harvest yet “leave the sow with young for seed”, and even stock feral hogs. We, as mankind, have developed an aesthetic value for hogs as illustrated by wild hog cook offs, and hog races, and an economic value in the form of marketing hunting rights for feral hogs and even marketing the hogs themselves as exotic table fare to restaurants.

Conclusions

In reviewing past literature on feral swine it becomes apparant that it’s hard to find much complimentary or positive about the species! Like it or not, feral hogs have become, due to man’s influence, a part of our ecosystem. We have created an “ecological backlash” by 1) introducing hogs, a non-native species to North America, 2) magnifying the potential impacts of expansion of feral hog range thru creation of habitat suitable for hogs out of previously poor habitat and 3) developing a desire to control but a reluctance, because of aesthetic and economic values, to adequately control feral hog numbers. Feral swine, like coyotes and white-tailed deer, are highly adaptable animals. To control impacts we must endeavor to develop management strategies that realistically access both potential negative and positive implications of feral hogs as they integrate with our management policies.

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