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Abstract. Human settlement and land use strongly influence native species in the Greater Yellowstone Ecosystem. This study reconstructs past interactions among ecosystem factors, native species, and human land use to provide a context for future management to sustain both ecological and human communities. Strong gradients in abiotic factors such as topography, climate, and soils appear to cause birds to be abundant and diverse only in localized "hot-spot" settings in the lowlands. Human land allocation and settlement appear to have also been influenced by these abiotic factors, with land use centered around these biodiversity hot spots. Outside of hot-spot habitats, forest management has reduced habitat quality by substituting clearcut logging for natural disturbances such as wildfire. Maintaining native species and human quality of life in the Greater Yellowstone Ecosystem will require coordinated management across public and private lands. Future management should focus on identifying, restoring, and protecting hot-spot habitats while using ecologically based forest management practices in uplands outside of hot spots.

The Greater Yellowstone area is one of the largest "intact" ecosystems remaining in the temperate zones of the world (Keiter and Boyce 1991). The vast nature reserves and wildernesses in the Greater Yellowstone Ecosystem (GYE) support a full complement of native birds and mammals, including predators such as grizzly bear and some of the last large herds of migratory ungulates in North America. The GYE offers a unique opportunity to study the role of abiotic factors (e.g., climate and soils) and disturbance (e.g., wildfire) in driving patterns of biodiversity in a "natural" system. At the same time, human development is proceeding rapidly on the private lands surrounding the nature reserves of this ecosystem. Some counties in the GYE have some of the highest human population growth rates in the nation. Thus far, there have been few efforts to quantify where in the GYE landscape humans are settling or the interactions between the human community and the ecosystem. Understanding past interactions between humans and ecosystems provides a context for developing ways to sustain both human and ecological communities in the future.

We are studying natural and human drivers of biodiversity in the northwest portion of the GYE (Fig. 8-1). The objectives of this work are to:

1. Determine the effects of abiotic factors (topography, climate, soils) and natural disturbance on patterns of vegetation and bird diversity.

2. Reconstruct patterns of human land allocation and land use (logging and rural residential development) across the study area from 1850 to present.

3. Evaluate and compare the range of variation in landscape patterns and bird habitats under natural drivers with the range of variation under human drivers.

4. Project vegetation and bird habitat patterns under alternative future management scenarios (present to 2195).

Fig. 8-2. Time periods examined for each of the study objectives.

The time periods of interest are 1700-1850 (prior to European human influence), 1850-1940 (initial European settlement), 1940-present (industrial logging and fire suppression), and present-2195 (Fig. 8-2). This paper focuses on objectives and portions of the study area for which analyses are most complete at this time.

Most of us think of vegetation composition and structure as strongly driving species diversity in ecosystems such as the Greater Yellowstone. While this is true, it is important not to overlook the importance of topography, climate, and soils to biodiversity (Hansen and Rotella, in press). Such "abiotic factors" are strongly expressed in the GYE, which is really a high mountainous plateau surrounded by lower plains (Fig.8-1). The differences in elevation from the center to the edge of the ecosystem strongly influence climate, with average annual temperatures substantially higher in valley bottoms than at higher elevations (Fig. 8-3). The soils in the valley bottoms are also much richer than those on the Yellowstone Plateau. These gradients in climate and soils influence patterns of vegetation. Lodgepole pine forests cover the poorer, volcanic soils at higher elevations. Douglas fir forests are found in better soils on lower slopes. Aspen, willow, and cottonwood communities occur on better soils on valley toe slopes and bottoms. These habitats are relatively rare, covering less than 3% of the study area. Environmental gradients also influence plant growth rates. In most years, primary productivity is very low at mid to high elevations on the volcanic soils and is high only in localized settings at lower elevations.

Fig. 8-3. Strong abiotic gradients exist in the GYE. This view of a portion of the current study area depicts: (a) average annual temperature, (b) topography (J. White and S. Running, University of Montana, unpublished data), (c) an index of primary productivity (derived from Normalized Difference Vegetation Index [NDVI] from satellite data; various studies have found correlations between NDVI and net primary productivity, though the key studies have not yet been done in the GYE), and (d) vegetation cover type (derived from Landsat Thematic Mapper imagery).

These gradients in climate, soil, and plant productivity strongly influence where native organisms are found. Many bird species are most abundant at lower elevations in habitats where climate is more equitable and primary productivity is highest (Hansen et al., unpublished report). The energy fixed by primary producers cascades through each trophic level in the food chain. Thus, sites high in primary productivity have more energy to support herbivorous, insectivorous, and predatory birds than do sites low in primary productivity. We found that this relationship holds within forest types (e.g., in mature lodgepole pine; Fig. 8-4) as well as among forest types (cottonwood, aspen, and willow communities are higher in net primary productivity and bird abundance and richness than other stand types). Habitats high both in net primary productivity and structural complexity (e.g., number of canopy layers) are "hot spots" for bird abundance and species richness (Fig. 8-5). Notice that these hot spots cover only a small portion of the ecosystem and are mostly at lower elevations. Such settings, however, have also been choice locations for human settlement.

Fig. 8-4. Mean and variation in the abundance of two individual bird species, bird species richness, and total bird abundance across three elevation classes (high > 2500 m; medium 2300-2500 m; low < 2300 m) within mature and old-growth lodgepole pine forests. Elevation classes with nonoverlapping yellow lines differ significantly (P < 0.05). We assume that climate and/or net primary productivity are correlated with elevation, which may explain the results. We are currently analyzing more directly the relationship among bird community attributes, climate, and net primary productivity. Fig. 8-5. Bird species richness extrapolated from 1995 field data across a portion of the current study area. Increasing richness is indicated from white to red.

Fig. 8-6. Portions of Yellowstone National Park burned by wildfires in 1988.

Natural disturbances, especially wildfire, have also left their imprint on the Yellowstone ecosystem. More than 40% of Yellowstone National Park was burned by wildfire in 1988 (Christensen et al. 1989). Driven by the wind, the fire tended to burn in long narrow strips (Fig. 8-6). Elongated islands of forest survived the fire and act as refugia for forest-dwelling organisms. Large fires like this are typical in Yellowstone, recurring about every 200-300 years. Native organisms are well adapted to wildfire. Many plant species recolonized quickly after the fire, and several bird species are dependent upon the habitats created by fire (Fig. 8-7). Vegetation patterns have varied dramatically over time in this system because of wildfire, creating a dynamic mosaic of habitats and maintaining the full suite of native species (Fig. 8-8).

      

Land Allocation and Logging

Like birds, people prefer to occupy certain places in the landscape, and current land allocation reflects this tendency. Homesteaders tended to choose lands at low elevations, on productive soils, and near streams (Fig. 8-9), and private lands continue to occupy these settings. Nature reserves (e.g., Yellowstone National Park, Lee Metcalf Wilderness) were placed at the highest elevations in the less productive sites (Fig. 8-10). Extractive federal lands (e.g., national forests) surround these nature reserves.

Fig. 8-9. Remains of a homestead on a productive valley bottom near the Gallatin River.

Human land use tends to increase in intensity at lower elevations. Logging on the Targhee National Forest initially occurred at the lowest elevations and gradually worked up to the Yellowstone National Park boundary. These patterns of land allocation and land use suggest that human activities are concentrated in low-elevation, productive landscape settings that are also important hot spots for native species.

Some land-use practices such as clearcut logging produce vegetation patterns that differ dramatically from those created by wildfire. Compared to clearcutting, wildfire leaves many standing and downed dead trees, scattered live trees, and understory plants that hasten recovery after the fire. Wildfire patches are also more variable in size and shape than clearcuts and the remaining forest is less fragmented under wildfire than logging (Fig. 8-11). Because of these differences, wildfire tends to maintain ecological processes and native organisms much better than clearcut logging.

Such comparisons of natural disturbance and human activities are helpful for managing future landscapes. Some ecologists have suggested that we can best achieve ecological objectives by mimicking the patterns and processes that were typical in these landscapes prior to modern human influences. The underlying assumption of this approach is that ecological processes and native organisms persisted through the Holocene and should continue if future landscapes are maintained within the range of variation typical of presettlement times. This approach is relatively new and questions remain about its feasibility and effectiveness. For example, the presettlement range of variation in vegetation cover type for a fire-driven system like the GYE is very broad (e.g., Fig. 8-8). Simply maintaining modern landscapes somewhere within this wide range may not accomplish ecological objectives. Attention to changes in spatial patterning over time is also needed. Moreover, it is likely socially unacceptable to maintain large disturbances in modern landscapes.

An alternative approach is to use an understanding of the interactions between ecosystems and human land use to design landscapes to accomplish management objectives. For example, knowledge of environmental gradients, natural disturbance, and human activities described here could be used to tailor management strategies to sustain both ecological and human communities (e.g., Hansen and Rotella, in press). In the case of the GYE, the challenge is to integrate management of natural disturbance and land use across public and private lands so as to maintain suitable habitats for native species across the full elevational gradient.

Rural Residential Development

Fig. 8-12. The Gallatin Valley near Bozeman, Montana, looking south towards Yellowstone National Park.

Land use tends to shift from timber management to grazing, agriculture, rural residential, and urban as we move down in elevation from the Yellowstone Plateau to the Gallatin Valley (Fig. 8-12). A wave of "urban refugees" is emigrating to the Northern Rockies from across the United States, fueling subdivision of seminatural lands (Fig. 8-13, 8-14).

This human development may be influencing native biodiversity even more than previously expected. Our initial analyses suggest that grazing, agriculture, and rural residential development tend to focus on the locations and habitats that are high in net primary productivity and that may be hot spots for native species (Fig. 8-15). About 25% of the bird species we sampled were strongly associated with hot-spot habitats. Hot spots may also be very important to species that also use other habitats. Reproduction and survival may be especially high in hot-spot habitats, allowing these areas to serve as population source areas, producing abundant offspring that disperse widely and are critical for maintaining the viability of many plant and animal populations across the region. Our initial studies indicate, however, that birds in hot spots near human activity have low reproductive rates due to nest predators that are abundant in human landscapes (e.g., raccoons). Thus, human activities in and around biodiversity hot spots may reduce habitat quality and population viability for native species.

Implications for Ecosystem Management

These results suggest that human settlement and intense land use are centered on just the locations in the landscape that are most important to native species. Gradients in topography, climate, and soils result in hot spots for native species in localized settings in the lowlands. Many of the new immigrants to the GYE are attracted to these same localities, concentrating private lands, human settlement, and intense land use in the productive lowlands. Given this overlap, human impacts on biodiversity and ecological processes may be greater than would be predicted based on human population density alone. Knowledge of the relationships between abiotic factors, natural disturbance, biodiversity, and human land use is critical for deriving management strategies to sustain both native species and human communities.

Our results suggest that it is important to identify biodiversity hot spots in the GYE and to design conservation plans to protect them from land development. Conservation easements and land acquisition may be effective means of preserving hot-spot habitats that are now on private lands. It is also important to identify the environmental factors that cause variation in species abundance and richness so that management plans can be designed to maintain and improve the conditions required by species of concern. Further mapping of abiotic factors, natural disturbance, native species, and human land use across this ecosystem is needed to identify previously unknown hot spots. Beyond preserving hot spots, active restoration will be required in some locations. Restoration may be accomplished by reintroducing natural disturbances such as fire or flooding to restart vegetation succession, using silvicultural strategies to restore the structural complexity of vegetation, and restricting livestock densities and home placement near some biodiversity hot spots.

Outside of hot spots, management should strive to maintain the quality of habitats for native species. Timber harvest regimes should be designed to create the structural complexity and landscape patterns typical of natural disturbances such as wildfire. Rather than simply mimicking presettlement disturbances, the challenge is to manage the spatial scale, location, and frequency of disturbance so as to best maintain the full suite of habitats for native species.

Accomplishing these management objectives will require a new level of cooperation among government agencies, private land owners, and local governments. Clearly, the GYE is a complex and tightly linked ecosystem. Understanding and managing the strong linkages between biodiversity and socioeconomic forces here can help to maintain the current high quality of life for humans in this region.

Funding was provided by the U.S. Department of the Interior National Biological Service, the National Aeronautics and Space Administration, the U.S. Forest Service, the National Fish and Wildlife Foundation, the Montana Fish, Wildlife and Parks Department, Montana State University, the U.S. Fish and Wildlife Service, and the National Science Foundation. We thank Tom Sisk, Maury Nyquist, Susan Stitt, and Ralph Root, and three anonymous reviewers for comments on the presentation and/or for technical support.

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Past, present, and future landscapes in the Henry's Fork Watershed, Idaho: modeling forest plan management alternatives. Progress report for the Targhee National Forest. Biology Department, Montana State University, Bozeman, Mont.

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