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

Plant-herbivore grazing systems have been the subject of increased study during the last 2 decades. Herbivores have been shown to affect numerous attributes of vegetation including plant morphology, species composition, species and plant abundances, net primary productivity, and the genetic makeup of plant populations (e.g., McNaughton 1983; Pastor et al. 1988; Frank and McNaughton 1992). The scale of plant-herbivore interactions ranges from effects on individual plants to ecosystems (Harper 1977; McNaughton 1979, 1983; Crawley 1983). Effects of herbivores have been documented in long-established natural ecosystems (Coppock et al. 1983) and where wild ungulates have been recently introduced (Caughley 1970a; Leader-Williams et al. 1987).

Characteristically, ungulates and vegetation are linked by strong feedback loops between the dynamics of the plants and the dynamics of the animals (Caughley 1970b, 1976b). Ungulate populations may be limited by available food resources; consequently, available per capita resources decrease (i.e., palatable plants) as their populations increase. Natural mortality increases and natality decreases because of insufficient food. The plants feedback to cause a new, lower level of the animal population. Plants and ungulate populations fluctuate with decreasing amplitude over time as dynamic equilibrium is achieved and profound changes usually occur in the vegetation. Caughley (1970a) described this process for another introduced mountain ungulate, the Himalayan thar (Hemitragus jemlahicus), in New Zealand.

Mountain goats are generalist herbivores strongly associated with cliffs and rock outcrops (see Chapter 6; Chadwick 1983). Diets seem to be dictated more by the available plant resource than by a preference for any particular species or growth form. Previous studies of the interactions between mountain goats and their summer range in the Olympic Mountains (Table 23) demonstrated that goats changed native ecosystems. Physical site characteristics—such as rock outcrops used for escape terrain and late-melting snow for thermoregulation—were the most important factors in habitat selection by goats; plant community composition was relatively unimportant (Stevens 1979; Pfitsch and Bliss 1985). As a consequence, mountain goats utilized all nine subalpine and alpine plant communities in one study area (Pfitsch and Bliss 1985). Plant community effects included reduced moss and lichen cover, increased soil disturbance and exposure of mineral soil from wallowing and trampling, an increase in ruderal (disturbance-oriented) species (Pfitsch 1981; Pike 1981; Pfitsch et al. 1983; Reid 1983; Pfitsch and Bliss 1985), and a decrease in palatable species. In addition, three Olympic Peninsula and one Olympic Peninsula and Vancouver Island endemic plant taxa were consumed by goats (Pike 1981; Kaye 1989; see Chapters 6 and 12).

Table 23. Studies of the interactions among introduced mountain goats (Oreamnos americanus), vegetation, and soils in Olympic National Park, 1976-1986.

Previous studies were conducted primarily on Klahhane Ridge from 1979 to 1982. We began investigations in 1981 to extend understanding of the interactions among mountain goats, vegetation, and soils in time and space. We also examined relations between plant communities and other herbivores on the 43,500 ha of land free of glacial ice above 1,520 m (i.e., goat summer range) and tested two general null hypotheses:

1. Reducing mountain goat densities will not result in changes to the relative abundance of plant species.

2. Mountain goats pose no threat to the long-term persistence of rare plants.

Independent study methods used during the investigations included the following:

1. extensive surveys of vegetation composition and herbivore use in areas of high, medium, and low goat density (see Chapter 9);

2. permanent plots to quantify plant community responses to intentional reductions in goat density (see Chapter 10);

3. a qualitative assessment of changes documented by repeat photographs to examine the effects of climate, human use, natural disturbance, and mountain goats on vegetation and soils over a 70-year period (see Chapter 11); and

4. measures of the general distribution, abundance, and trends in rare plants and a rare endemic (Olympic Mountain milkvetch [Astragalus australis var. olympicus]) to evaluate effects of mountain goats on the long-term persistence of these taxa (see Chapter 12).