             
 The LUHNA Book!
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Institute of Environmental Studies, P.O. Box AB, Route 44A Millbrook, NY 12545-0129; STAPickett@aol.com
NOTE: The following document is posted to provide a look at
new ideas and developing components of the LUHNA project. No reproduction
or other use of the materials posted here should be undertaken without
consent of, and discussion with, the author. Please note e-mail
address, above. Abstract When are land use and land cover useful ecological or environmental variables? In situations where the linkage between land cover and an ecological response is direct, or when a sound model links land cover or land use to ecological responses, they can serve as ecological variables. However, without such a mechanistic connection they must be considered to be promising exploratory factors. Land Use and Land Cover Land use is a description of how a parcel of land is employed by people, or the degree to which that land reflects natural versus human-generated cover. Land cover is a closely related concept, which more readily describes the natural and human surfaces or structures that blanket the Earth. Land cover is the more general variable, and land use is more specific. Land use describes how a parcel of surface is employed -- or not -- by people, and therefore requires more information than land cover. The two variables are not redundant, because a given land cover type may be used in more than one way. Land use and land cover are often characterized using remote sensing with subsequent confirmation by surveys on the ground or in private or public records. Because of the reliance on remote sensing, the land use/land cover categories that can be resolved are often rather crude. For instance, residential land cover can sometimes be broken down in to high and low densities, but many suburban developments may look, from high altitudes or using a few satellite bands, like a closed canopy forest. Hence land cover, and especially land use are not always easy parameters to measure. The covers of the Earth's surface are, in fact, complex, three dimensional bodies, rather than a simple layer that is draped over the geomorphology. Such three dimensionality, is however, what makes land use and land cover so important as ecological variables. Different cover types, due to their soil and substrate, their vegetation depth, height and layering, their hospitability, or hostility to the movement, shelter, and nourishment of different animals, their permeability to flows of water, nutrients, wind, and seeds, and their arrangement over space, all can influence biological diversity and the provision of ecosystem services. The three-dimensionality of land cover and land uses emphasizes that the spatial arrangement of land covers and land uses is significant to understanding their implications. Landscape comparisons may rest on knowing simply the mixture of land use and land cover types. What types are present in a landscape, and the relative proportions of the types may yield valuable information about the structure and function of a landscape. Comparison of contrasting landscapes using indices of land cover and land use richness, and the evenness of land cover types may be especially revealing. However, still more information can be gleaned from the actual map of land cover types and quantification of neighborhood, distances between similar or contrasting patch types, the nature and shape of boundaries, and the like. Direct vs. Indirect Effects of Land Use What does land use and land cover mean ecologically? In other words, can land use or land cover be used as ecological variables? The answer depends on the ecological arena and the research questions asked of that domain of study. If the ecological phenomena of interest respond directly to the land use/land cover matrix, then the variables that describe land use should be close determinants of the response. For example, birds may respond to the neighboring vegetation, or mule deer may respond to the viscosity of the boundary between forest and steppe. Even in the cases of "direct" response, there may be one or more mechanisms that an ecologist will have to interpose between the features of land use and the ecological response. In the case of the birds, a more detailed mechanism may be the avoidance of predators. In the case of the deer a similar kind of mechanism may come into play, although the scale of sensitivity may differ because the size and search range of the predators may differ. Both of these cases interpose a relatively simple ecological model between the land use and the ecological response. These cases can be considered direct, but there still needs to be an ecological model to translate between the abstractions of the map of land cover types and the behavioral or functional response of the ecological system. When traditional ecological variables are used, most ecologists who are familiar with the subject may carry the general model in their heads, and so land use/land cover can be readily perceived as a part of the roster of environmental variables. However, even such direct uses of land use and land cover have a relatively short history in ecology, dating in some cases to the introduction of island biogeography, and in other more complex and subtle cases, to the recognition of landscape ecology as a legitimate ecological specialty. There are other situations in which land use and land cover are not environmental variables. Again, when such a disconnect occurs between land use and ecology depends on the topic areas, the scales, and the question of interest. One kind of situation where land use is a poor ecological variable is one in which there is little direct connection between it and the ecological response. Human population density is one aspect of land cover that is often readily available. There may even be intriguing correlations between human population and some ecological processes, such as number of native species. However, the functional relationships in such a case are likely to be indirect and multiple. Native biodiversity can be affected by reduction in habitat area, which accounts for an estimated 80% of species extinctions, for example. Alternatively, or in addition, loss of native species can be due to pollution, deliberate human extirpation or harvest, interaction with domestic predators, subsidy of wild predators, introduction of new feral predators or exotic diseases, among other causes. While these factors clearly relate to the nature and configuration of land use and land cover types, they represent an array of more direct ecological interactions that are affected by or allowed by the land use land cover setting. In order for an ecologist to understand both the patterns of species richness relative to land cover and use, and the mechanisms behind those patterns, an explicitly ecological model of the factors involved, their interactions and the outcomes of these interactions must be interposed between the land use variables and the ecological responses. Most attempts to use land use or land cover as ecological variables may fall into the indirect category. This is because those sorts of variables have not fallen within the domain of ecology for very long. Other variables such as elevation, climate, geology, and the structure of habitats and communities, are familiar to ecologists, and there are may formal and informal models for linking those often coarse scale variables with processes of ecological interest. For instance, climate variables are integrated through models of energy balance, photosynthesis, and the like. But there are few ecological models to relate land use and land cover to important ecological functions. This is a serious lapse since human influence is now so pervasive in the biosphere. Indeed it is spreading in many places. Therefore modern ecology seems to demand a better understanding of land use and land cover variables along with increased knowledge of traditional and emerging areas of ecology. An example of the link between ecology and land use, appears in the attempt to understand the response of ecosystems to abstract gradients of the degree of urbanization. Like climate or elevation, urbanization is a complex ecological gradient that encompasses many factors. Unlike natural gradients, for which ecologists have used direct measurement, multivariate statistics and ordination, reciprocal and common garden transplants, simulation and mechanistic models and the like, ecologists have not had practice in building the models that translate from a complex and general variable to detailed mechanistic ecosystem responses. However, such progress is possible and desirable. One exploratory attempt to assess the human role in forest function has been made in a project on the New York City Metropolitan region. The metropolitan area encompasses some of the densest and most highly developed land in the U.S., as well as open, thinly populated farming and forested landscapes. In other words it provides a wide range of conditions in which to discover the relationships between human population, development and land use patterns. Sample areas were established in several closed canopy forests in the urbanized Bronx, suburbanized areas of Westchester County, New York, and rural Litchfield County, Connecticut. All sites show the same bedrock and soil types. The forests differed in key ecosystem parameters, such as litter decay and nitrogen mineralization rates. What human factors might best explain the ecosystem functions? Several readily available variables were assembled for cells of 4x4 km centered on the forest patches. Of the variables examined, traffic volume explained the greatest proportion of the variation among forest processes. Population density, and road density also explained large amounts of the difference. However the fact that traffic volume can be more directly linked to the input of nitrogen in the nearby atmosphere suggests that it is not only the statistically best but also the ecologically most relevant variable. Clearly other variables describing human impact and potential mechanisms need to be examined and brought to bear via models so that the mechanisms can be compared for their efficacy. The plausible relationships discovered by the coarse scale statistical relationships, and the proposed mechanisms can be tested by field transplants, common garden or laboratory experiments. Land Use and a New Ecological Synthesis The results from the New York urban to rural gradient show the way that land use and land cover can enrich ecology. Because ecologists in the United States have not focused on relationships and functions involving humans, there are likely many patterns to be discovered. Not all such patterns will make the transition from intriguing, to plausible, to mechanistically defensible. But the search can profitably build from a broad base. A second advantage of examining land use and land cover from an ecological perspective is an expansion of the linkage between ecology and other sciences. Humans, and the rich variety of behaviors, structures, institutions, and processes they generate likely affect much of ecology. However the interaction of ecologists with social scientists, historians, economists, and geographers, among others who study humans and their institutions, is in its infancy. Maturation of such interaction may generate new questions about the human role in ecological systems.
With the integration of human and ecological studies, land use and land
cover can be used most intelligently to develop conceptual models and
empirical chains of causation that make them effective ecological variables.
New mechanisms and modes of interaction may emerge from the integrated study
of humans and ecological systems. Land use and land cover are the
appropriate entry points into the new arena.
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