ABSTRACT
Introduction Land management decisions on public and private lands affect important ecosystem processes and functions with potentially far-reaching and long-term consequences. Humans are now managers of much of the habitable land of the earth. Over half of all land that is not tundra, ice, boreal, desert, or rock is devoted to agriculture (Tilman et al. 2001). Including managed forests, the majority of habitable land on earth is actively managed for human uses. The expected increase of two to three billion people over the next 50 years will further increase human land-use needs. In a recent book, biologist Simon Levin states: ‘the central environmental challenge of our time is embodied in the staggering loss, both recent and projected, of biological diversity…’ (Levin 1999, p.1). Though other factors, such as the introduction of exotic species, over-harvesting, pollution, and climate change, contribute to the loss of biodiversity, habitat loss is thought to be the primary reason for the loss of terrestrial biodiversity (Ehrlich and Ehrlich 1981; Wilson 1988). In fact, a common method biologists use to predict the number of species present in a given land area is through construction of a ‘species-area curve,’ which is based on empirical evidence linking the amount of habitat and the number of species found in the habitat (MacArthur and Wilson 1967). Several studies have found that the current rate of species extinction is several orders of magnitude above the ‘natural’ or background rate of extinction (National Research Council 1995; Pimm et al. 1995). Given land conversion trends, projections into the future are for even higher extinction rates (Wilson 1988). In the U.S., the Endangered Species Act has focused attention on the relationship between land use and species conservation. Under the Endangered Species Act, otherwise lawful land uses may be prohibited if they would result in harm to a species listed as endangered or threatened. Included in activities that cause harm are land uses that significantly modify habitat in ways that kill or injure listed species or interfere with essential activities such as breeding, feeding or sheltering. A number of recent high-profile endangered species cases have highlighted actual and potential restrictions on various land uses, including timber harvesting and urban development. These cases include timber harvest restrictions
to protect the spotted owl and marbelled murralet in the Pacific Northwest, potentially wide-ranging restrictions on urban and rural land use to protect salmon from Washington to California, restrictions on land development in Southern California to protect the California gnatcatcher, Stephens’ kangaroo rat and other species, and timber harvest restrictions in the Southeast to protect the red-cockaded woodpecker. As the Endangered Species Act examples demonstrate, society faces difficult choices over whether to allow habitat conversion for economic gain versus conserving habitat to protect biodiversity. In each of the cases mentioned above, there is a tradeoff between economic activity (e.g., timber harvest, housing development, etc.) and conserving the habitat of threatened and endangered species on certain lands. Because the conservation of biodiversity and the material wellbeing of the human population are both important goals, it is important to set conservation priorities intelligently and to minimize the reduction in other goals from pursuing conservation. In this chapter, our objective is to ensure that the maximum amount of biodiversity is conserved for any given level of cost. We illustrate our approach to this problem using land-value data, and taxonomic and geographical distribution data for breeding bird species in Oregon. In the next section, we present a general framework for cost-effective conservation decision-making that was first described by Solow et al. (1993). The framework requires specifying a biodiversity measure as an indicator of the relative worth of possible conservation outcomes, the probabilities of various outcomes occurring under a given set of management actions, the cost of these management actions and the conservation budget. We then discuss in more depth biodiversity measures and management actions. We apply our framework to a practical problem of selecting biological reserves under a budget constraint to maximize two measures of species diversity using taxonomic information, geographical distribution data of bird species, and land values in Oregon. The final section contains concluding comments. A Conservation Decision-Making Framework We begin by explaining the conservation decision-making framework of Solow et al. (1993). In this framework, the goal is to maximize expected biodiversity conserved under a budget constraint. There are three important components to this framework. First, what is the definition of biodiversity? In other words, what is the objective of conservation? For example, the definition of biodiversity could be the total number of species conserved or it could be a measure of the value of ecosystem services provided. In order to proceed with the analysis, however, there must be a clearly defined objective. In the applications that follow we will use two different biodiversity measures based on presence or absence of species: (1) the total number of species present (species richness), and (2) a measure of phylogenetic diversity of the conserved species. Second, how do various management actions affect biodiversity? A wide range of management actions can be considered in this framework. Management
actions could include such things as setting aside habitat as biological reserves, alternative pesticide application or tillage practices on agricultural land, or alternative timber harvest rotations in forests. Management actions could also include consideration of public policies such as zoning laws that restrict allowable land uses on certain parcels, or decisions on where to put infrastructure, such as roads or sewers, which will affect the pattern of future development activities. Since habitat decline is probably the single most important cause of biodiversity decline, land-use and land management decisions are particularly important to analyze. In the application to follow, we focus on the conservation strategy of setting aside land for biological reserves. Once it is decided what management actions to consider, there must be some way to assess the biological consequences of implementing those management actions. In practice, there may be limited ecological knowledge on which to base this assessment. At present, lack of ecological understanding is a key limiting factor in our ability to make intelligent choices regarding conservation. In the reserve-site selection problem we will consider the typical assumptions made are that species that are represented in those land areas selected in reserves will be conserved while those outside of any reserve area will be lost. Third, what are the costs of various management actions? At a very general level, these costs represent sacrifices in other goals that must be made in order to further conservation. In the application to follow we measure these costs in dollar terms. For example, if a public land management agency decides to prohibit timber harvesting or grazing on public land in order to protect habitat for certain species, the cost of this restriction would be the foregone income that could have been earned had timber harvesting or grazing been allowed. It is important to note that these costs are what economists would call opportunity costs in that they represent the costs of foregone opportunities to society that are required for conservation. Opportunity costs are not necessarily the same thing as the budgetary consequences for an agency or landowner. Prohibiting logging in order to protect an endangered species may not require any budgetary outlay but it does impose an opportunity cost in terms of lost income from timber operations. Formally, the conservation decision-making problem combining these three elements can be written as: (1)∑
∈Ss x sPsDMax )()( ,
subject to BxC ≤)( , where D(s) is the measure of the biological diversity outcome, S is the set of possible outcomes, Px(s) is the probability that outcome s will occur under conservation strategy x that describes what management actions will be taken, C(x) is the opportunity cost of implementing conservation strategy x, and B is the
conservation budget. By varying the budget, one can trace out the maximum achievable level of biodiversity conservation for various budget levels. In other words, following this procedure one can establish the cost curve for biodiversity conservation. Biodiversity Measures, Species Richness, and Phylogenetic Diversity Before turning to the application, we discuss in more detail biodiversity measures, specifically focusing on measures based on species presence and absence, and management actions, focusing on the reserve-site selection problem. What is referred to as ‘biodiversity’ can mean many different things. For example, one heavily referenced definition of biodiversity is the following:
Biodiversity is the variety of life and its processes. It includes the variety of living organisms, the genetic differences among them, the communities and ecosystems in which they occur, and the ecological and evolutionary processes that keep them functioning, and yet ever changing (Keystone Center 1991).
