The idea of sustainability has been the underpinning of formal thinking in the field of ecology since Aldo Leopold's (1953) path-defining work on how humankind should interact with the natural world in order to conserve Nature's services for perpetuity. There is now formal and widespread recognition that ecologists can and must offer a leading intellectual role in encouraging thinking about the very meaning of sustainability, the scientific insights needed to advance it rigorously, and how one goes about devising clear and measurable criteria to judge success (Lubchenco et al. 1991; Daily 1997; Levin et al. 1998; NRC 1999; Gunderson 2000; Myerson et al. 2005; Palmer et al. 2005).

Ecologists agree that the goal of sustainability must involve the reconciliation of human society's needs within environmental limits over the long term (Lubchenco et al. 1991; NRC 1999; Palmer et al. 2005). In this context, sus-tainability is fundamentally predicated on sound stewardship of the Earth's vital ecosystem services.

Ecosystem services fall into two broad categories: material goods and functions (Myers 1996; de Groot et al. 2002). Material goods subsume contributions with easily measured economic value such as new and improved foods, plant-based pharmaceuticals, raw materials for industry, and biomass for energy production. Material goods have values that are set by supply and demand pricing because they can be traded in markets (de Groot et al. 2002). Functions, on the other hand, typically do not have a marketable value because they cannot be easily sold. Functions (e.g., production, consumption, decomposition) provide a range of services that contribute toward human well-being by sustaining components of ecosystems on which major economies depend. These services include, for example, regulation of water quality, regulation of greenhouse gases, disturbance regulation (including flood and erosion control and resistance to invasive species), recycling of organic wastes and mineral elements, soil formation for agriculture, pollination (Myers 1996; Daily et al. 1997; de Groot et al. 2002), or reducing production costs (Schmitz 2007). The importance of sustaining ecosystem functions for human livelihoods is often overlooked even though, ironically, their value may rival or exceed the value of material goods traditionally managed by natural resource sectors such as forestry, fisheries, and agriculture (Costanza et al. 1997; Daily 1997).

Since ecological functions derive from biotic species that comprise ecosystems, one would accordingly expect that the level of those functions is related to the level of biotic diversity (biodiversity) within ecosystems. This is indeed supported by scientific evidence (Hooper et al. 2005).

A key challenge in moving society's actions toward sustainability, then, is understanding how energy and material stocks are bound up in and flow through species in ecosystems and the trade-offs humankind faces in terms of their use versus their conservation. To demonstrate such trade-offs, let us look at the following examples:

First, a major portion of grassland ecosystems in the western U.S. has been appropriated for cattle grazing and cereal crop production. Historically, grasslands harbored large predators (wolves, Canis lupus) that can prey on cattle and thus jeopardize the cattle industry. Consequently, these large predators have been systematically extirpated from much of their historical range (Leopold 1953; Schmitz 2007). The attendant consequence of this action has been a long-term increase in the densities of native herbivores such as elk (Cervus elaphus) and moose (Alces alces). High abundances of these herbivores lead to devastating impacts on riparian habitat due to overbrowsing (Beschta and Ripple 2006; Schmitz 2007. This, in turn, leads to declines in water flow and quality (Beschta and Ripple 2006) of the very water that is used to irrigate crops. Failure to maintain biodiversity and its particular function—in this case predator species and their capacity to regulate native herbivore abundances— can lead to a serious decline in an important ecosystem service for agricultural production.

Second, humans have appropriated natural ecosystems for the production of truck crops. Humans rely further on insects, bees in particular, to pollinate crops before they bear fruit (Kremen et al. 2002). Farmers have relied on this function for millennia and have cultivated extensively the European honey bee (Apis mellifera) to provide this function (Kremen et al. 2002). The European honey bee is, however, in serious decline due to diseases and poisoning from insecticides, and farming is thus in jeopardy (Kremen et al. 2002). One solution has been to enlist the diversity of native pollinators as a substitute. However, encouraging native pollinator species diversity requires maintaining their habitats in close proximity to crop fields to ensure successful pollination (Kremen et al. 2002). This means that farmers need to reduce the extent of agricultural land and create a portfolio of land use that trades habitat conservation for biodiversity against crop production. Failure to maintain biodiversity and its particular function—in this case insect species diversity and pollination—can diminish the capacity to produce important crops.

These examples illustrate the complex interdependencies and feedbacks between humans and nature in the provisioning and use of ecosystems services. Understanding how these interdependencies and feedbacks impact sustainability requires careful consideration of the way we define a system and, more importantly, the criteria for sustainability. My goal in this chapter is to relate concepts about sustainability of ecosystem services that ecologists have wrestled with to spur thinking about how we might develop indicators and measurement criteria for sustainable human dominated systems.

The value in relating ecology principles of sustainability is that ecosystems are, perhaps, the archetypal complex system (Levin 1998). They contain many different agents that interact directly and indirectly in highly interconnected and interdependent networks (Levin 1998). Moreover, higher-scale system properties, such as trophic structure, nutrient stocks and flows, and productivity, emerge from lower-scale interactions and selection among the agents (Levin 1998). Ecosystems are also considered complex adaptive systems in that there is a perpetual feedback loop in which higher-scale properties modify lower-scale interactions which then produce new emergent properties (Levin 1998). Thus, ecosystems represent powerful working metaphors for research programs aimed at identifying the level of functional complexity needed to consider modern issues of sustainability (Myerson et al. 2005).

Formal research on ecological sustainability began largely under a different guise: ecological stability (MacArthur 1955; Holling 1973; DeAngelis et al. 1989; McCann 2000; Ives and Carpenter 2007). In the ecological sciences, stability is defined and quantified in a myriad of ways, depending on the goal of research and management (McCann 2000; Ives and Carpenter 2007). The definitions relevant to sustainability of human-dominated systems can, however, be grouped into three broad categories:

1. Persistence: a system is considered highly sustainable if it can maintain functioning over the long term.

2. Reliability: a system is highly sustainable if there is little variation or fluctuation in the levels of functions.

3. Resilience: a system is highly sustainable if it can buffer disturbances or adapt quickly in response to them.

In this chapter, I present an overview of how these concepts are applied to understand sustainability of ecosystems. Fundamentally, applicability depends largely on the conceptualization of how an ecosystem is structured and how its functions are sustained.

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