Cascading effects of overexploitation on ecosystems

         
              All extractive systems in which the over harvested resource is one or more biological populations, can lead to pervasive trophic cascades and other unintended ecosystem-level consequences to non-target species. Most hunting, fishing, and collecting activities affect not only the primary target species, but also species that are taken accidentally or opportunistically. Furthermore, exploitation often causes physical damage to the environment, and has ramifications for other species through cascading interactions and changes in food webs. In addition, overexploitation may severely erode the ecological role of resource populations in natural communities. In other words, overexploited populations need not be entirely extirpated before they become ecologically extinct. In communities that are “half-empty” (Redford and Feinsinger 2001), populations may be reduced to sufficiently low numbers so that, although still present in the community, they no longer interact significantly with other species (Estes et al. 1989). Communities with reduced levels of species interactions may become pale shadows of their former selves. 

              Although difficult to measure, severe declines in large vertebrate populations may result in multi-trophic cascades that may profoundly alter the structure of marine ecosystems such as kelp forests, coral reefs and estuaries (Jackson et al. 2001), and analogous processes may occur in many terrestrial ecosystems. Plant reproduction in endemic island floras can be severely affected by population declines in flying foxes (pteropodid fruit bats) that serve as strong mutualists as pollinators and seed dispersers (Cox et al. 1991). In some Pacific archipelagos, several species may become functionally extinct, ceasing to effectively disperse large seeds long before becoming rare (McConkey and Drake 2006). A key agenda for future research will involve understanding the non-linearities between functional responses to the numeric abundance of strong interactors reduced by exploitation pressure and the quality of ecological services that depleted populations can perform. For example, what is the critical density of any given exploited population below which it can no longer fulfill its community-wide ecological role? 

                 In this section I concentrate on poorly known interaction cascades in tropical forest and marine environments, and discuss a few examples of how apparently innocuous extractive activities targeted to one or a few species can drastically affect the structure and functioning of these terrestrial and aquatic ecosystems.

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Years of growth and evolution

                        Although conservation biology has been an organized field only since the mid-1980s, it ispossible to identify and summarize at least several salient trends that have shaped it since.

Implementation and transformation

                 Conservation biologists now work in a much more elaborate field than existed at the time of its founding. Much of the early energy and debate in conservation biology focused on questions of the genetics and demographics of small populations, population and habitat viability, landscape fragmentation, reserve design, and management of natural areas and endangered species. These topics remain close to the core of conservation biology, but the field has grown around them. Conservation biologists now tend to work more flexibly, at varied scales and in varied ways. In recent years, for example, more attention has focused on landscape permeability and connectivity, the role of strongly interacting
species in top-down ecosystem regulation, and the impacts of global warming on biodiversity (Hudson 1991; Lovejoy and Peters 1994; Soulé and Terborgh 1999; Ripple and Beschta 2005; Pringle et al. 2007; Pringle 2008; see Chapters 5
and 8).


                   Innovative techniques and technologies (such as computer modeling and geographic information systems) have obviously played an important role in the growth of conservation biology. The most revolutionary changes, however, have involved the reconceptualizing of science’s role in conservation. The principles of conservation biology
have spawned creative applications among conservation visionaries, practitioners, planners, and policy-makers (Noss et al. 1997; Adams 2005). To safeguard biological diversity, larger-scale and longer-term thinking and planning had to take hold. It has done so under many rubrics, including: adaptation of the biosphere reserve concept (Batisse 1986); the development of gap analysis (Scott et al. 1993); the movement toward ecosystem management and adaptive management (Grumbine 1994b; Salafsky et al. 2001; Meffe et al. 2002); ecoregional planning and analogous efforts at other scales (Redford et al. 2003); and the establishment of marine protected areas and networks (Roberts et al. 2001). Even as conservation biologists have honed tools for designing protected area networks and managing protected areas more effectively, they have looked beyond reserve boundary lines to the matrix of surrounding lands (Knight and Landres 1998). Conservation biologists play increasingly important roles in defining the biodiversity values of aquatic ecosystems, private lands, and agroecosystems.


                   The result is much greater attention to private land conservation, more research and demonstration at the interface of agriculture and biodiversity conservation, and a growing watershed- and community-based conservation movement. Conservation biologists are now active across the entire landscape continuum, from wildlands to agricultural lands and from suburbs to cities, where conservation planning now meets urban design and green infrastructure mapping (e.g. Wang and Moskovits 2001; CNT and Openlands Project 2004).

Adoption and integration

                  Since the emergence of conservation biology, the conceptual boundaries between it and other fields have become increasingly porous. Researchers and practitioners from other fields have come into conservation biology’s circle, adopting and applying its core concepts while contributing in turn to its further development. Botanists, ecosystem ecologists, marine biologists, and agricultural scientists (among other groups) were underrepresented in the field’s early years. The role of the social sciences in conservation biology has also expanded within the field (Mascia et al. 2003). Meanwhile, conservation biology’s concepts, approaches, and findings have filtered into other fields. This “permeation” (Noss 1999) is reflected in the number of biodiversity conservation-related articles appearing in the general science journals such as Science and Nature, and in more specialized ecological and resource management journals.


                  Since 1986 several new journals with related content have appeared, including Ecological Applications (1991), the Journal of Applied Ecology (1998), the on-line journal Conservation Ecology(1997) (now calledEcology and Society), Frontiers in Ecology and the Environment (2003), and Conservation Letters (2008). The influence of conservation biology is even more broadly evident in environmental design, planning, and decision-making. Conservation biologists are now routinely involved in land-use and urban planning, ecological design, landscape architecture, and agriculture (e.g. Soulé 1991; Nassauer 1997; Babbitt 1999; Jackson and Jackson 2002; Miller and Hobbs 2002; Imhoff and Carra 2003; Orr 2004). Conservation biology has spurred activity within such emerging areas of interest as conservation psychology (Saunders 2003) and conservation medicine (Grifo and Rosenthal 1997; Pokras et al. 1997; Tabor et al. 2001; Aguirre et al. 2002). Lidicker (1998) noted that “conservation needs conservation biologists for sure, but it also needs conservation sociologists, conservation political scientists, conservation chemists, conservation economists, conservation psychologists, and conservation humanitarians.” Conservation biology has helped to meet this need by catalyzing communication and action among colleagues across a wide spectrum of disciplines.

Marine and freshwater conservation biology

                    Conservation biology’s “permeation” has been especially notable with regard to aquatic ecosystems and marine environments. In response to long-standing concerns over “maximum sustained yield” fisheries management, protection of marine mammals, depletion of salmon stocks, degradation of coral reef systems, and other issues, marine conservation biology has emerged as a distinct focus area (Norse 1993; Boersma 1996; Bohnsack and Ault 1996; Safina 1998; Thorne-Miller 1998; Norse and Crowder 2005). The application of conservation biology in marine environments has been pursued by a number of non-governmental organizations, including SCB’s Marine Section, the Ocean Conservancy, the Marine Conservation Biology Institute, the Center for Marine Biodiversity and Conservation at the Scripps Institution of Oceanography, the Blue Ocean Institute, and the Pew Institute for Ocean Science. Interest in freshwater conservation biology has also increased as intensified human demands continue to affect water quality, quantity, distribution, and use. Conservationists have come to appreciate even more deeply the essential hydrological connections between groundwater, surface waters, and atmospheric waters, and the impact of human land use on the health and biological diversity of aquatic ecosystems (Leopold 1990; Baron et al. 2002; Glennon 2002; Hunt and Wilcox 2003; Postel and Richter 2003). Conservation biologists have become vital partners in interdisciplinary efforts, often at the watershed level, to steward freshwater as both an essential ecosystem component and a basic human need.

Building capacity

                At the time of its founding, conservation biology was little known beyond the core group of scientists and conservationists who had created it. Now the field is broadly accepted and well represented as a distinct body of interdisciplinary knowledge worldwide. Several textbooks appeared soon after conservation biology gained its footing (Primack 1993; Meffe and Carroll 1994; Hunter 1996). These are now into their second and third editions. Additional textbooks have been published in more specialized subject areas, including insect conservation biology (Samways 1994), conservation of plant biodiversity (Frankel et al. 1995), forest biodiversity (Hunter and Seymour 1999), conservation genetics (Frankham et al. 2002), marine conservation biology (Norse and Crowder 2005), and tropical conservation biology (Sodhi et al. 2007). Academic training programs in conservation biology have expanded and now exist around the world (Jacobson 1990; Jacobson et al. 1995; Rodríguez et al. 2005). The interdisciplinary skills of conservation biologists have found acceptance within universities, agencies, non-governmental organizations, and the private sector. Funders have likewise helped build conservation biology’s capacity through support for students, academicprograms, and basic research and field projects.


                  Despite such growth, most conservation biologists would likely agree that the capacity does not nearly meet the need, given the urgent problems in biodiversity conservation. Even the existing support is highly vulnerable to budget cutbacks, changing priorities, and political pressures.

Internationalization

                Conservation biology has greatly expanded its international reach (Meffe 2002; Meffe 2003). The scientific roots of biodiversity conservation are obviously not limited to one nation or continent. Although the international conservation movement dates back more than a century, the history of the science from an international perspective has been inadequately studied (Blandin 2004). This has occasionally led to healthy debate over the origins and development of conservation biology. Such debates, however, have not hindered the trend toward greater international collaboration and representation within the field (e.g. Medellín 1998).This growth is reflected in the expanding institutional and membership base of the Society for Conservation Biology.


                 The need to reach across national boundaries was recognized by the founders of the SCB. From its initial issue Conservation Biology included Spanish translations of article abstracts. The Society has diversified its editorial board, recognized the accomplishments of leading conservation biologists from around the world, and regularly convened its meetings outside the USA. A significant move toward greater international participation in the SCB came when, in 2000, the SCB began to develop its regional sections.

Seeking a policy voice

                 Conservation biology has long sought to define an appropriate and effective role for itself in shaping public policy (Grumbine 1994a). Most who call themselves conservation biologists feel obligated to be advocates for biodiversity (Odenbaugh 2003). How that obligation ought to be fulfilled has been a source of continuing debate within the field. Some scientists are wary of playing an active advocacy or policy role, lest their objectivity be called into question. Conversely, biodiversity advocates have responded to the effect that “if you don’t use your science to shape policy, we will.” Conservation biology’s inherent mix of science and ethics all but invited such debate. Far from avoiding controversy, Conservation Biology’s founding editor David Ehrenfeld built dialogue on conservation issues and policy into the journal at the outset.


                 Conservation Biology has regularly published letters and editorials on the question of values, advocacy, and the role of science in shaping policy. Conservation biologists have not achieved final resolution on thematter. Perhaps in the end it is irresolvable, a matter of personal judgment involving a mixture of scientific confidence levels, uncertainty, and individual conscience and responsibility. “Responsibility” is the key word, as all parties to the debate seemto agree that advocacy, to be responsible, must rest on a foundation of
solid science andmust be undertakenwith honesty
and integrity (Noss 1999).

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Historical foundations of conservation

                            Since conservation biology’s emergence, commentary on (and in) the field has rightly emphasized its departure from prior conservation science and practice. However, the main “thread” of the field the description, explanation, appreciation, protection, and perpetuation of biological diversity can be traced much further back through the historical tapestry of the biological sciences and the conservation movement (Mayr 1982;McIntosh 1985; Grumbine 1996; Quammen 1996). That thread weaves through related themes and concepts in conservation, including wilderness protection, sustained yield, wildlife protection and management, the diversity-stability hypothesis, ecological restoration, sustainability, and ecosystem health. By focusing on the thread itself, conservation biology brought the theme of biological diversity to the fore. In so doing, conservation biology has reconnected conservation to deep sources in Western natural history and science, and to cultural traditions of respect for the natural world both within and beyond the Western experience.

                 Long before environmentalism began to reshape “conservation in the old sense” in the 1960s prior even to the Progressive Era conservation movement of the early 1900s the foundations of conservation biology were being laid over the course of biology’s epic advances over the last four centuries. The “discovery of diversity” (to use Ernst Mayr’s phrase) was the driving force behind the growth of biological thought. “Hardly any aspect of life is more characteristic than its almost unlimited diversity,” wrote Mayr (1982:133). “Indeed, there is hardly any biological process or phenomenon where diversity is not involved.” This “discovery” unfolded as colonialism, the Industrial Revolution, human population growth, expansion of capitalist and collectivist economies, and developing trade networks transformed human social, economic, political, and ecological relationships ever more quickly and profoundly (e.g. Crosby 1986; Grove 1995; Diamond 1997). Technological change accelerated humanity’s capacity to reshape the world to meet human needs and desires. In so doing, it amplified tensions along basic philosophical fault lines: mechanistic/organic; utilitarian/reverential; imperialist/arcadian; reductionism/ holism (Thomas et al. 1956; Worster 1985). As recognition of human environmental impacts grew, an array of 19th century philosophers, scientists, naturalists, theologians, artists, writers, and poets began to regard the naturalworld within an expanded sphere of moral concern (Nash 1989).

                For example, Alfred RusselWallace (1863) warned against the “extinction of the numerous forms of life which the progress of cultivation invariably entails” and urged his scientific colleagues to assume the responsibility for stewardship that came with knowledge of diversity. The first edition of George Perkins Marsh’s Man and Nature appeared the following year. In his second chapter, “Transfer, Modification, and Extirpation of Vegetable and of Animal Species,” Marsh examined the effect of humans on biotic diversity. Marsh described human beings as a “new geographical force” and surveyed human impacts on “minute organisms,” plants, insects, fish, “aquatic animals,” reptiles, birds, and “quadrupeds.” “All nature,” he wrote, “is linked together by invisible bonds, and every organic creature, however low, however feeble, however dependent, is necessary to the well-being of some other among the myriad forms of life with which the Creator has peopled the earth.” He concluded his chapter with the hope that people might“learn to put a wiser estimate on the works of
creation” (Marsh 1864). Through the veil of 19th century language, modern conservation biologists may recognize Marsh, Wallace, and others as common intellectual ancestors. Marsh’s landmark volume appeared just as the post-Civil War era of rampant resource exploitation commenced in the United States.A generation later, Marsh’s book undergirded the Progressive Era reforms that gave conservation in the United States its modern meaning and turned it into a national movement. That movement rode Theodore Roosevelt’s presidency into public consciousness and across the American landscape.

                Conservationists in the Progressive Era were famously split along utilitarian-preservationist lines. The utilitarian Resource Conservation Ethic, realized within new federal conservation agencies, was committed to the efficient, scientifically informed management of natural resources, to provide “the greatest good to the greatest number for the longest time” (Pinchot 1910:48). By contrast, the Romantic Transcendental Preservation Ethic, overshadowed but persistent through the Progressive Era, celebrated the aesthetic and spiritual value of contact with wild nature, and inspired campaigns for the protection of parklands, refuges, forests, and “wild life.” Callicott (1990) notes that both ethical camps were “essentially human-centered or ‘anthropocentric’. . . (and) regarded human beings or human interests as the only legitimate ends and nonhuman natural entities and nature as a whole as means.”Moreover, the science upon which both relied had not yet experienced its 20th century revolutions. Ecology had not yet united the scientific understanding of the abiotic, plant, and animal components of living systems. Evolutionary biology had not yet synthesized knowledge of genetics, population biology, and evolutionary biology. Geology, paleontology, and biogeography were just beginning to provide a coherent narrative of the temporal dynamics and spatial distribution of life on Earth.

                Although explicitly informed by the natural sciences, conservation in the Progressive Era was primarily economic in its orientation, reductionist in its tendencies, and selective in its application. New concepts from ecology and evolutionary biology began to filter into conservation and the resource management disciplines during the early 20th century. “Proto-conservation biologists” from this period include Henry C. Cowles, whose pioneering studies of plant succession and the flora of the Indiana Dunes led him into active advocacy for their protection (Engel 1983);
Victor Shelford, who prodded his fellow ecologists to become active in establishing biologically representative nature reserves (Croker 1991); Arthur Tansley, who similarly advocated establishment of nature reserves in Britain, and who in 1935 contributed the concept of the “ecosystem” to science (McIntosh 1985; Golley 1993); Charles Elton, whose text Animal Ecology (1927) provided the foundations for a more dynamic ecology through his definition of food chains, food webs, trophic levels, the niche, and other basic concepts; Joseph Grinnell, Paul Errington, Olaus Murie, and other field biologists who challenged prevailing notions on the ecological role and value of predators (Dunlap 1988); and biologists who sought to place national park management in the USA on a sound ecological footing (Sellars 1997; Shafer 2001). Importantly, the crisis of the Dust Bowl in North America invited similar ecological critiques of agricultural practices during the 1930s (Worster 1979; Beeman and Pritchard 2001). By the late 1930s an array of conservation concerns soil erosion, watershed degradation, urban pollution, deforestation, depletion of fisheries and wildlife populations brought academic ecologists and resource managers closer together and generated a new awareness of conservation’ ecological foundations, in particular the significance of biological diversity. In 1939 Aldo Leopold summarized the point in a speech to a symbolically appropriate joint meeting of the Ecological Society of America and the Society of American Foresters:

The emergence of ecology has placed the economic biologist in a peculiar dilemma: with one hand he points out the accumulated findings of his search for utility, or lack of utility, in this or that species; with the other he lifts the veil from a biota so complex, so conditioned by interwoven cooperations and competitions, that no man can say where utility begins or ends. No species can be ‘rated’ without the tongue in the cheek; the old categories of ‘useful’ and ‘harmful’ have validity only as conditioned
by time, place, and circumstance. The only sure conclusion is that the biota as a whole is useful, and (the) biota includes not only plants and animals, but soils and waters as well (Leopold 1991:266–67).

                 With appreciation of “the biota as a whole” came greater appreciation of the functioning of ecological communities and systems (Golley 1993). For Leopold and others, this translated into a redefinition of conservation’s aims: away from the narrow goal of sustaining outputs of discrete commodities, and toward the more complex goal of sustaining what we now call ecosystem health and resilience. As conservation’s aims were thus being redefined, its ethical foundations were being reconsidered. The accumulation of revolutionary biological insights, combined with a generation’s experience of fragmented policy, short-term economics, and environmental decline, yielded Leopold’s assertion of an Evolutionary-Ecological Land Ethic (Callicott 1990). A land ethic, Leopold wrote, “enlarges the boundaries of the community to include soils, waters, plants, and animals, or collectively: the land”; it “changes the role of Homo sapiens from conqueror of the land-community to plain member and citizen of it” (Leopold 1949:204). These ethical concepts only slowly gained ground in forestry, fisheries management, wildlife management, and other resource management disciplines; indeed, they are contentious still. In the years following World War II, as consumer demands increased and technologies evolved, resource development pressures grew. Resource managers responded by expanding their efforts to increase the yields of their particular commodities.

                 Meanwhile, the pace of scientific change accelerated in disciplines across the biological spectrum, from microbiology, genetics, systematics, and population biology to ecology, limnology, marine biology, and biogeography (Mayr 1982). As these advances accrued, maintaining healthy connections between the basic sciences and their application in resource management fields proved challenging. It fell to a diverse cohort of scientific researchers, interpreters, and advocates to enter the public policy fray (including such notable figures as Rachel Carson, Jacques-Yves Cousteau, Ray Dasmann, G. Evelyn Hutchinson, Julian Huxley, Eugene and Howard Odum, and Sir Peter Scott). Many of these had worldwide influence through their writings and students, their collaborations, and their ecological concepts and methodologies. Working from within traditional disciplines, government agencies, and academic seats, they stood at the complicated intersection of conservation science, policy, and practice a place that would come to define conservation biology.

              More pragmatically, new federal legislation in the USA and a growing body of international agreements expanded the role and responsibilities of biologists in conservation. In the USA the National Environmental Policy Act (1970) required analysis of environmental impacts in federal decision making. The Endangered Species Act (1973) called for an unprecedented degree of scientific involvement in the identification, protection, and recovery of threatened species (see Chapter 12). Other laws that broadened the role of biologists in conservation and environmental protection include the Marine Mammal Protection Act (1972), the Clean Water Act (1972), the Forest and Rangeland Renewable Resources Planning Act (1974), the National Forest Management Act (1976), and the Federal Land Policy Management Act (1976). At the international level, the responsibilities of biologists were also expanding in response to the adoption of bilateral treaties and multilateral agreements, including the UNESCO (United Nations Educational, Scientific and Cultural Organization) Man and the Biosphere Programme (1970), the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) (1975), and the Convention on Wetlands of International Importance (the “Ramsar Convention”) (1975). In 1966 the International Union for the Conservation of Nature (IUCN) published it first “red list” inventories of threatened species. In short, the need for rigorous science input into conservation decision-making was increasing, even as the science of conservation was changing. This state of affairs challenged the traditional orientation of resource managers and research biologists alike.

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