Biodiversity: Aspen stands have the lead, but will nonnative species take over?

Abstract

We investigated vascular plant and butterfly diversity in Rocky Mountain National Park. We identified 188 vascular plant species unique to the aspen vegetation type. The slope of the mean species-area curve for the aspen vegetation type was the steepest of the 10 types sampled, thus, an increase in aspen area could have much greater positive impacts on plant species richness than other vegetation types. Aspen plots contained the greatest number of butterfly species and ranked highest with all diversity indices tested. However, aspen plots were the most heavily invaded by nonnative plant species, which could have negative effects on native plant and butterfly species diversity. The importance of aspen (Populus tremuloides Michaux) for maintaining biodiversity in western landscapes is well introduced in DeByle et al. (1985), where aspen stands are noted for their own genetic diversity, as well as providing habitat for insects, birds, and mammals. Stohlgren et al. (1997a,b) found a disproportionately high number of vascular plant species in aspen stands in relation to their coverage in the Beaver Meadows area (750 ha) of Rocky Mountain National Park, Colorado. There, aspen covered only 1.2% of the landscape, but it contained 45% of the plant species sampled. Resource managers in Rocky Mountain National Park (the Park) are concerned that elk (Cervus elaphus Nelsoni) may be harming vegetation in portions of the Park (Berry et al. 1997). Localized studies have reported little or no aspen regeneration in elk winter range (Baker et al. 1997; Olmstead 1997), while a more extensive study did find successful regeneration at landscape scales in areas of low elk use (Suzuki et al. 1999). Similar concern and controversy over regeneration exists in the Greater Yellowstone Ecosystem (Barnett and Stohlgren 2000; Bartos et al. 1991; Gruell and Loope 1974; Krebill 1972; Romme et al. 1995; Weinstein 1979). Another potential harmful effect on aspen stands and the diversity that they support is invasion by nonnative plant species. Work in other species-rich habitat types has found that hotspots of native plant diversity are being invaded by nonnative plant species (Stohlgren et al. 1998b, 1999a,b,c). These invasions may have long-term, negative consequences for native diversity, especially in vegetation types such as aspen that are small, scattered, and rare on the landscape in parts of their range. Our objective was to assess a variety of vegetation types’ contributions to plant and butterfly species richness. We used species-log(area) curves (Gleason 1925; Rejmanek and Ejvind 1992; Shmida 1984) to compare the relative contributions to vascular plant species richness made by 10 different vegetation types in the Park. Species-area curves allow comparisons across vegetation types, and even other studies, because the slopes of the curves can be calculated and compared without the difficulties posed by other diversity indices that often require abundance data (for evenness) and vary greatly depending on study design (Ludwig and Reynolds 1988). In addition, species-area models allow one to estimate the number of species expected in an area larger than the area