How Is an Ecosystem Structured and Sustained

An ecosystem is basically a conceptualization of a natural economy involving a chain of consumption and transformation into production of energy and nutrients. In this economy (Figure 3.1a), plants (primary producers) "consume" raw materials (solar energy, nutrients, CO2) and produce edible tissue; herbivores (primary consumers) eat plants to produce herbivore tissue (secondary production). Herbivore production, in turn, is consumed by predators (secondary consumers) leading to tertiary production. Ecologists refer to such consumer-producer interactions as trophic interactions. Agents engaging in a particular kind of trophic interaction belong to the same trophic level of the food chain. So, for example, agents engaging in herbivory belong to the herbivore trophic level, agents preying on herbivores belong to the carnivore trophic level, and so on.

The conversion of energy and materials from one trophic level to another in the chain must obey thermodynamic laws, which means that transformation of energy and materials from one form to another does not occur at 100% effi ciency (Odum 1997). Various indices are used to quantify efficiency, including nutrient (resource) use efficiency (the quantity produced per quantity of nutrient uptake), production efficiency (the quantity produced per quantity of resource assimilated), and ecosystem metabolism (rate of production after accounting for total energy respired). By themselves, these measures are useful indicators of efficiency, but they will not be appropriate indicators of sus-tainability without consideration of the kind of system being measured. That is, agents in a system can use resources efficiently even though the system might not be sustainable. Thus, the ecological sciences distinguish explicitly between efficiency and sustainability, whereas these concepts are often used interchangeably in the popular mindset.

Figure 3.1 Depiction of materials flow through consumption-production systems. Straight arrows indicate flow from producer to consumer; dashed arrows indicate loss due to metabolic processes and leaching; curved arrows indicate self-regulating negative feedback. (a) An open system in which materials flow up the trophic chain and dissipate out of the system. (b) A closed system due to materials recycling via decomposition and due to self-regulating negative feedback.

Figure 3.1 Depiction of materials flow through consumption-production systems. Straight arrows indicate flow from producer to consumer; dashed arrows indicate loss due to metabolic processes and leaching; curved arrows indicate self-regulating negative feedback. (a) An open system in which materials flow up the trophic chain and dissipate out of the system. (b) A closed system due to materials recycling via decomposition and due to self-regulating negative feedback.

Open vs. Closed Systems and Persistence

Figure 3.1a depicts an open system in the sense that energy and nutrients (resources) enter the system through producers and pass up the food chain to the top consumers. As resources move up the chain, some fraction dissipates out of the system through respiration (metabolic processes) or leaks out through leaching; hence transformation is inefficient. Such a system will only be stable (sustainable) if there is an inexhaustible supply of resources entering the bottom of the consumption chain. This condition is met for systems driven only by solar-derived energy which, for all practical purposes, is in near-infinite supply. The condition will not be met, however, for systems that are supported by nonrenewable energy and materials (e.g., fossil fuels, minerals, and nutrients) which occur in limited quantities (DeAngelis et al. 1989). The distinction between unlimited and limited resource supplies in an open system is critical, especially for promoting sustainable technologies.

In many cases, there is demand for technological innovations that would increase the transfer (or use) efficiency of energy and materials available in limited supply in order to promote sustainable resource use (e.g., improving fuel economy of automobiles that burn fossil fuels). While these actions increase the life span of the resource pool, they are not long-term solutions because the resource will eventually be depleted and the economy based on this resource will collapse. Moreover, given that all mineral and nutrient resources on Earth are in fixed supply (Gordon et al. 2006), sequentially transitioning economies from one limiting resource to the next will likewise not be sustainable over the long term because supplies of particular resources dwindle. Increasing use efficiency of any limited resource is, by itself, a stop-gap measure that buys society time to transition to alternative, sustainable approaches. This then begs the question: What are the requirements for sustainability in those systems that rely on limited resources?

An ecological system dependent upon limited resources will become sustainable when it becomes a closed system (DeAngelis et al. 1989). Through feedbacks, systems can become closed in two ways (Figure 3.1b).

First, all spent and unused materials that are in limited supply must be returned back to the pool of available resources that support future production. This feedback is known in the ecological sciences as elemental or materials cycling; in the vernacular, it is known as recycling. Recycling requires another trophic level—decomposers—to break production back down to its elemental components for reuse in new production. Decomposition is an important limiting step in materials cycling in that it is dependent both on the decomposers' capacity to break down materials and the speed with which materials are broken down. Decomposition is a critical limiting step in maintaining sustainabil-ity. Consequently, technological societies that ultimately depend on materials recycling to become sustainable must ensure that cradle-to-grave life cycle assessments are conducted whenever a new product is designed to ensure that the products are not only durable while in use, but also comparatively easily broken down into their elemental components when discarded. Ignoring decomposition at the design stage can lead to products that are costly or even not economically feasible to recycle. Discarded products end up in landfills. From a global perspective, this is an unsustainable practice because it leads to conversion of land to purposes that jeopardize some of the very ecosystems services (e.g., maintenance of clean water supplies, food production, gas regulation and environmental health) that sustain human livelihoods (Dodds 2008).

Second, systems will become closed when there is self-regulating or negative feedback in the consumer trophic levels. In ecological systems, self-regulating feedback is achieved when members of a trophic level compete for the limited resource. That is, the inefficient capacity to take up and convert resources coupled with demands on resources by the members of the trophic level (competition) limits the growth and ultimate size of the trophic level in a classic Malthusian sense. Technologically advanced societies have developed the capacity to produce in greater quantities and to convert production more efficiently. However, this has often been accomplished with little or no regard to maintain the self-regulating feedback in the system. Consequently, these improvements overcome the self-limiting feedback over the short term but cause long-term problems because of unsustainable positive feedback. For example, production of food was limited globally by the abundance of elemental nitrogen, which was derived largely from nitrogen-fixing plants and natural, physical nitrifying processes. The invention of the Haber-Bosch process for industrial nitrogen synthesis greatly increased humankind's capacity to produce food and thus avert hunger. One could argue, however, that it also played a role in increasing the human population growth rate. In turn, rising human population size, and the associated increase demand for food, requires increasingly more land to be converted for agricultural production, which then increases the demand for water supplies and leads to overfertilization and ultimately pollution (Tilman et al. 2001; MEA 2005). The solution here is to limit consumption and conversion of production (e.g., food) into secondary production (e.g., children) in an effort to reestablish the negative feedback. Reestablishing negative feedback by restraining consumption and population growth is perhaps one of the most politically sensitive yet crucial policy issues facing global sustainability today (Dodds 2008; Speth 2008).

Ecological theory (Loreau 1995) has shown that production and consumption process can be fine-tuned to maximize the flow rate of energy and materials through resource-limited, closed systems (i.e., to maximize sustainable economic activity). This can only be achieved if the total supply (stock) of a limiting resource within the entire system exceeds at least some threshold. This makes intuitive sense. When resources stocks are low, producers cannot persist at steady state. Theory, however, shows that above this threshold, but below twice the threshold level, consumers and producers can persist, but consumers will reduce materials or energy flow. It is only for resource supplies above twice the threshold that consumers are able to maximize flow rates. There are, however, upper limits. If consumption rates are so high that most of the material is bound up for long time periods in the consumer trophic level, rather than in the decomposition process, then production will diminish or even halt. In other words, decomposition, not consumption, must be the rate-limiting step to foster the maximization of flow rates in a sustainable (persistent) system.

Measuring Persistence

An index of system persistence requires the measurement of rates of flow and loss of materials and energy through the production-consumption chain, the rate of decomposition. From an ecological perspective, this is typically accomplished by developing a classic materials or energy budget for the entire production-consumption, decomposition-elemental pool chain. The most persistent closed system will be one for which the net budget equals zero; that is, for which materials recycled back into the system meet the materials demands for production. Within a persistent system, it is also possible to develop criteria to maximize the flow rates among trophic groups, in essence, a form of efficiency. This requires quantifying the threshold levels of resources stocks needed to make a particular production economy efficient and then to identify, in relation to that threshold, the rate of production and consumption that maximizes materials and energy flows. Finally, the index must account for the strengths of self-regulating feedback and how this may change with changes in production levels and transfer efficiency.

Network Complexity and Reliability

Persistence alone is a good indicator of sustainability when conceptualizing stocks and flows through chains of production, consumption, and decomposition in which there is a single agent within each trophic level. Such a conceptualization, however, oversimplifies real ecological systems in which there is a diversity of producer and consumer agents (species) that are linked together in highly interconnected networks (Levin 1998, 1999). A major concern in ecology is to understand how consumer and producer diversity is related to system stability (Hooper et al. 2005). This endeavor began with the question (MacArthur 1955): Why is it that in some ecosystems the abundances of most species changed little, whereas in other systems the species abundances fluctuated wildly? Inasmuch as species contribute toward ecosystem functioning and provisioning of services (Hooper et al. 2005), then by extension the question could be rephrased: Why are ecosystem functions and services fairly steady in some systems but fluctuate widely in other systems? These issues speak to another measure of sustainability known as reliability (Naeem and Li 1997), where systems with steady levels of functions and services are considered to be more reliable (sustainable) than systems with high fluctuations in functions and services. That is, while systems may be sustainable by one measure (persistence), they may still differ in their sustainability according to another measure (reliability).

MacArthur (1955) initially posited that reliability-type stability arose from the degree of interconnectedness among species or agents in a system. Consider, for example, two systems with identical numbers of species in different trophic levels (Figure 3.2). These systems differ only in the extent to which species are connected to each other via consumer-resource links and hence the number of pathways along which energy and materials can flow. In the simplest system comprising two parallel and independent production-consumption chains (Figure 3.2a), the effects of fluctuations in producer species 1 (P1) would reverberate up the trophic chain and cause fluctuations of flow to both the primary consumer species (C11) and the secondary consumer species (C12) that form part of that trophic chain. In a somewhat more complex system (Figure 3.2b), Cu is linked also to the second producer species (P2) in

Sustainability of Ecosystem Services and Functions (b) (c)

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