NATIONAL PARK SERVICE Mountain Goats in Olympic National Park: Biology and Management of an Introduced Species

Plants and herbivores are inescapably linked. The type and magnitude of the interactions between herbivores and plants depend on the food habits, behavior, and abundance of the herbivore; the growth form and physiology of the plants; and the dynamics of ecosystem processes where the herbivores and plants are both present (Harper 1977; Crawley 1983; Abrahamson 1989). Herbivore populations have direct and indirect effects on individual plants and indirect effects on plant communities and ecosystem processes (Floate 1981; Begon et al. 1986).

The direct influence of herbivores on individual plants is relatively straightforward. Plants either lose a portion of their biomass or sustain tissue injuries from being consumed or trampled (Caughley 1976b; Harper 1977; Floate 1981; Crawley 1983). Wallowing, especially by large herbivores, also may damage plant tissue; death of the plant often results when it is directly under the herbivore (Begon et al. 1986; Polley and Wallace 1986). Burrowing tends to cause root injuries, burial of plant parts, and presumably death if the plant is directly in the path of the larger burrowing animals (Platt 1975; del Moral 1984; Krueger 1986; Coffin and Lauenroth 1988; Whicker and Detling 1988).

Indirect effects of herbivores on individual plants, communities, and ecosystem processes are legion. Herbivores may alter plant growth form and reproductive output (Islam and Crawley 1983; Paige 1992) or cause individuals to produce tannins as secondary compounds to deter herbivory (Harper 1977; Rhodes 1985). Wallowing and burrowing expose bare mineral soil, which alters community composition in favor of ruderal plant species (Platt 1975; Pfitsch 1981; del Moral 1984; Polley and Wallace 1986; Whicker and Detling 1988). Relative abundance of plant species in the community may be altered because herbivores forage selectively (Ellison 1960; Anderson and Loucks 1979; McNaughton 1979; Day and Detling 1990; McInnes et al. 1992). A subtle but important effect of herbivores on ecosystem processes also results from redistribution of nitrogen in soils caused by urine and fecal deposition (Begon et al. 1986; Ruess and McNaughton 1987; Coffin and Lauenroth 1988; Day and Detling 1990; Frank and McNaughton 1992; Seagle et al. 1992). Deposition of nitrogen in feces may alter the rate nitrogen is cycled through the ecosystem, which in turn may affect the productivity of plant communities (Cargill and Jeffries 1984).

Our work demonstrated that introduced mountain goats have indirect and direct effects on the vegetation of the Olympic Mountains, a finding held in common with all other studies of ungulate grazing systems. We have no reason to believe that the general biotic effects of goats on the vegetation of the Olympics differs appreciably from mountain goat grazing systems where the animals are native. Nonetheless, changes in the park's vegetation have been substantial, and the status of rare plant populations is of concern.

Mountain goats modified the relative abundance of plants in subalpine plant communities of the Olympics; disturbance-oriented species such as Achillea millefolium increased with heavy grazing, and selected species such as Festuca idahoensis decreased. Mountain goats likely changed the competitive relations between these two species because a laboratory study demonstrated that Festuca was the stronger competitor except when clipped (del Moral 1985). Changes in plant cover of these two species were also visible in comparative photographs and were reported from the exclosure studies of Pfitsch and Bliss (1985) and Schreiner (1987).

Modifications of plant communities also occurred without necessarily changing competitive interactions among plant species; cover of selected and nonselected plant species changed at Mount Dana, but this did not change community dominants. Cover of dominant, strongly competitive species such as Carex spectabilis did not always respond to lowered goat density. This does not necessarily mean that C. spectabilis was unaffected by goats. Carex spectabilis is highly productive (Kuramoto and Bliss 1970; del Moral 1983a, 1984, 1985) and may have compensated for loss of grazed plant tissues. Graminoid species are well known for their compensatory response to grazing (McNaughton 1979; Begon et al. 1986).

Additionally, mountain goats influenced the vegetation of Olympic National Park by wallowing. Wallows disturbed soils and created mineral substrates for colonization by plants. Studies of Klahhane Ridge goat wallows (Pfitsch 1981; Reid 1983) and bison wallows elsewhere (Polley and Wallace 1986) showed that disturbance-oriented plants dominated wallow edges and that these disturbed sites differed from surrounding vegetation. We documented presence of wallows in all study areas. On Klahhane Ridge, we noted disturbance-oriented species invaded wallow edges as they probably did elsewhere. Such a result is to be expected because Olympic subalpine and alpine plant communities are particularly sensitive to soil disturbance (Bell and Bliss 1973; Schreiner 1974; Belsky and del Moral 1982; del Moral 1984). Some investigators believe that physical disturbances associated with herbivory may be more important than grazing (Harper 1977; Floate 1981) in changing the vegetation.

Incidentally consumed forage species may suffer intensive grazing at either high (Futayama and Wasserman 1980) or low (Houston 1982) herbivore densities. These forages may be eliminated because they exert no feedback control on herbivore population size. In a hypothetical one herbivore-one plant species system, the herbivore and plant must reach a dynamic equilibrium or the herbivore goes extinct (Caughley 1982). Further, in a one herbivore-two plant system (assuming that the two plant populations have different growth rates), "The extinction of one of the two plants is a direct consequence of its sharing the area with the other. . . . It goes extinct when sharing the area because the herbivore numbers and hence grazing pressure are maintained at a higher level than would be possible if the slower growing plant were the only food available" (Caughley 1982:311). Thus, rare plants may be at risk due to mountain goats for at least two reasons: (1) mountain goat population densities likely are not controlled by plant abundance on summer ranges (Houston and Stevens 1988), let alone rare plant abundance, and (2) goats are generalist herbivores with the capacity to consume most plant species, including rare plants.

We remain concerned about the long-term persistence of rare plants in mountain goat habitat. We note that the effects of goats on individual plant taxa may be severe or potentially severe, especially for taxa with very restricted distributions (i.e., those that are present in less than five subpopulations). Rare, endemic plants have been driven to extinction or near extinction elsewhere by introduced herbivores (e.g., goats in Hawaii and the Galapagos Islands; Loope et al. 1988).

Our studies represent only a small part of the total picture of mountain goat herbivory. Recent studies demonstrate that ungulates alter nitrogen cycles and soil formation processes (Day and Detling 1990; Frank and McNaughton 1992; McInnes et al. 1992). Moreover, vegetative changes wrought by one herbivore may influence the feeding behavior of other herbivores (Krueger 1986; Day and Detling 1990). Given the relations between site productivity and the degree of interspecific plant competition in Olympic subalpine vegetation (del Moral 1983a, 1985), mountain goats are likely to have a far greater influence on individual plant populations, communities, and ecosystem processes than we have demonstrated here.