Ecosystem function

This week we’ve discussed the processes that move energy and materials in and out of ecosystems and between their components. Together, these determine an ecosystem’s function. The function of a particular ecosystem emerges from the collective activities of the specific plants, animals, and microbes that inhabit it, and their effect on physical and chemical conditions. A wetland, for example, produces certain kinds of plant biomass, stores carbon, and cycles certain nutrients—all differently than a tropical forest does. These functions emerge from that particular system. Ecosystem functions are varied and operate on vastly different scales, from a fallen tree decomposing in the woods to extensive forests regulating weather and climate.

Ecosystem resilience and the link to biodiversity

As we’ve seen in Gorongosa, species abundance fluctuates in response to disturbances, whether natural or caused by human activities. Some species and ecosystems are more resistant to disturbance, and experience less change, while those that experience change but can recover quickly are considered more resilient. For example, the Hudson River experienced major changes in species composition following its invasion by zebra mussels. While it was not resistant to this disturbance in the short term, as you’ll see in the next essay, it has exhibited considerable resilience over time.

Figure 1. Resilience of coral reef ecosystems. Three decades ago, scientists generally believed that coral reefs were very stable ecosystems, meaning that a coral reef was likely to remain a coral reef for thousands of years despite impacts such as hurricanes, fishing, and degraded water quality. Coral reefs seemed to have a natural, built-in resilience that enabled them to recover and persist after these impacts. In recent years, however, it has become evident that this resilience can become overwhelmed, so that seaweeds take over and the coral reef ecosystem is lost. At the root of this phenomenon is intense competition for space on the reef among corals, seaweeds, and sometimes other invertebrate groups. Certain species will have an advantage over others depending on the surrounding conditions, including the presence of certain herbivores (like parrotfishes) and water quality. Images by James Lui, from Brumbaugh 2014, reproduced with permission.

Back in the 1950s, ecologist Robert MacArthur proposed that diversity and connections among species can play a key role in how much variation an ecosystem experiences, its resistance or its resilience, and hence in maintaining the stability of ecosystems. Known as the Diversity-Stability Hypothesis, this has since been investigated experimentally and widely confirmed. One study showed that the plant biomass of a set of plots varied less from one year in plots with greater plant diversity than in those with less diversity. Another study showed that more diverse grasslands within Yellowstone National Park were more resistant, and experienced less changes in plant species composition, in times of drought. Greater diversity also helps ecosystems withstand biological invasions. The implications are clear: maintaining an ecosystem’s diversity protects it against the effects of disturbance, and buffers fluctuations in population numbers or biomass. Take a moment to consider the drastic reduction of diversity within Gorongosa National Park, and what ecosystem functions might have changed as a result.

How exactly does diversity support ecosystem functions? Some have argued that it is the diversity itself, the richness of species, that provides a sort of insurance to large fluctuations in composition and function. More recent evidence suggests that it is not just species diversity that drives this relationship, but also the nature of trophic linkages between species—the structure of the food webs—all of which are aspects of biodiversity (see Box 1). In other words, it’s the diversity of linkages among the species, not the sheer number of species, that helps stabilize ecosystems. The most stable systems have a mix of strong and weak trophic linkages, because many weak linkages can counterbalance changes to strong ones.

Figure 2. Wolf spider, damselbug, and aphid. Ecologists studying the effect of food webs on community and ecosystem stability designed an experiment with a specialist predator (wolfspiders, Pardosa mackenziana, shown above, and P. wyuta,), an omnivore predator (the damselbug Nabis alternatus), and their herbivorous prey (the aphid Macrosiphum valeriani, similar to the one pictured above). Wolfspider, ©Jacy Lucier/CC BY-NC-SA 2.0; damselbug, ©Pat Cassidy; ©aphid, Christophe Quintin/CC BY-NC 2.0

When biodiversity declines, for example if species diversity is reduced, we would expect the diversity of trophic linkages to decline as well, with linkages potentially becoming less diverse and stronger. This may make ecosystems less resilient to disturbances, and more vulnerable to destabilizing changes or even collapse. When an ecosystem experiences a major change, such that it does not bounce back to its previous state, it is considered to have crossed a threshold, or gone past a tipping point. Thresholds and tipping points are difficult to determine and probably vary across ecosystems (Figure 3). Given these uncertainties, we should err on the side of protecting the diversity of ecosystems that sustain human livelihoods and economies. In the words of American environmentalist Aldo Leopold: “To keep every cog and wheel is the first precaution of intelligent tinkering.”

Figure 3. Hypothetical relationships between biodiversity loss and ecosystem function. This graph shows possible, hypothetical relationships between biodiversity loss and ecosystem function. While most evidence points towards a positive association between stability and diversity, ecologists are not sure of the shape of this relationship, and way in which ecosystem function may respond to decreases in diversity. Does stability drop proportionally to drops in diversity (curve B) or sharply (curve A) with small reductions in diversity? Or is it resistant to decreases in diversity until loss reaches a high threshold (curve C)? The shape of this relationship is likely to be different for different ecosystems and under different conditions, and has important implications for understanding and managing the consequences of biodiversity loss. ©Nadav Gazit/AMNH

Ecosystem services

Regardless of who we are and where we live, functioning ecosystems generate services from which every human being benefits. Ecosystem services are the components of nature that humans consume, enjoy, or that contribute directly or indirectly to human well-being.

There are various ways to classify ecosystem services. In 2000 the United Nations inaugurated the Millennium Ecosystem Assessment (MEA), an ongoing assessment of the world’s ecosystems and the services they provide. The MEA grouped ecosystem services into four broad categories:

Provisioning services: goods that directly benefit people, such as fruits and vegetables, and drinking water. Other types include timber, fisheries, fuelwood, medicinal plants, natural building materials, fibers, and other forest products.

Regulating services: processes that moderate natural phenomena, including water regulation for flood control, soil stabilization, removal of pollution from the air and water, disease control, pollination, and regulation of climate through the storage of carbon. These services help to make ecosystems sustainable, functional, and resilient.

Cultural services: a non-material benefit that contributes to people’s intellectual, cultural, and social development. This includes the role of ecosystems in people’s history and education, and in their spiritual, religious, artistic and recreational lives.

Supporting services: Supporting services have indirect or very long-term impacts on people, but underlie other ecosystem services, particularly provisioning services. Examples include the formation of soils, water and nutrient cycles, and photosynthesis (i.e., oxygen production).

Figure 4. Linkages between ecosystem services and human well-being. This connection circle is a model of our understanding of the linkages between categories of ecosystem services and common components of human well-being, with the strength of the relationship indicated by the thickness of the arrow. Adapted from Millennium Ecosystem Assessment (2005) by Nadav Gazit/AMNH

See Figure 4 for a model of the connection between these services and human well-being. Most ecosystems provide a combination of these kinds of ecosystem services. Boxes 2 through 4 provide some examples.

Figure 5. Hudson River ecosystem services. The Hudson River and its watershed provide an array of ecosystem services, including provisioning drinking water (top left), regulating the flow of nutrients through wetlands (bottom left), supporting wildlife (top right) and providing opportunities for recreation (bottom right). Reservoir, ©s58y/CC BY 2.0; wetland, © rvc845/CC BY-ND 2.0; hiking, ©KMcFaddenH/CC BY-ND 2.0; bald eagle, ©Jordan Confino/CC BY-NC-ND 2.0

Accounting for ecosystem services

Regardless of how we define and classify ecosystem services, rapid population growth and increasing demands for food, fresh water, timber, fiber, and fuel have presented unprecedented threats. Human well-being and economic development have greatly improved over the last two centuries, but at a cost: many ecosystem services have degraded, and Earth’s biodiversity has suffered substantial and largely irreversible losses. The MEA estimates that more than 60 percent of the world’s ecosystems are being used unsustainably.

Despite their critical importance, many ecosystem services are either undervalued or assigned no financial value at all. This makes it harder for sustainable practices, such as organic farming or sustainable forestry, to compete with practices like intensive agriculture that offer higher short-term economic benefits. And provisioning services are much easier to quantify than other types of services. One remedy is to provide financial incentives for sustainable practices, based on the importance of the ecosystem service or how much it would cost to replace it. (See Boxes 2 and 3 for examples of assessing the value of pollination and of clean water.) An indirect way to measure the value of an ecosystem is to poll various stakeholders in order to estimate how much maintaining a service would be worth to them. For example, how much would you pay for a view of a mountain? How much might people be willing to pay to prevent the mountaintop from being destroyed by strip mining?

Markets for ecosystem services like biodiversity and carbon sequestration are beginning to emerge around the world, both voluntary and legally enforced. Collectively known as Payments for Ecosystem Services (PES), these programs offer incentives for landowners or stewards to restore or maintain ecosystems and one or more of the services they provide.

Limitations of an Ecosystem Services Approach

Although monetizing ecosystem services can promote conservation, putting a price tag on nature is controversial and problematic. Critics pose four main questions:

Is it ethical, and is it wise? Society has traditionally designated certain things as worthy of protection or ineligible for commerce for moral reasons rather than economic ones. Selling body organs is generally prohibited on moral grounds, for example, and a similar argument can be made when it comes to the natural world. Since there are no substitutes for clean air and water, resources to which all humans should have access, how can we justify buying and selling them? What about aspects of an ecosystem, such as a rare species of spider or a pollinator of a native plant, which are not easily converted into a monetary value yet have an intrinsic right to exist? It also may not be wise to let the fate of resources depend upon markets, which assign value on purely monetary terms, fluctuate according to supply and demand, and are susceptible to speculation. A forest that reduces pests for surrounding coffee farms may lose its market value if the coffee farms are converted to pasture, for example, or an endangered species may become more valuable as its numbers diminish, creating incentives for poachers. Market forces cannot correct many of the problems they create.

Is it accurate? Economic values vary substantially depending on the characteristics of the ecosystem service being assessed and who is conducting the assessment, and values are often underestimated. A natural forest and a palm oil plantation may have similar carbon sequestration capacities, but their capacity to sustain biodiversity, support diverse livelihoods, and provide aesthetic pleasure differ widely. All too often, the economically quantifiable value alone is taken into consideration. It’s also hard to estimate the value people would place on a resource. In one study, most people were unwilling to pay to protect endangered birds, yet said they would be upset if a bird went extinct.

Is it working for people? Monetizing ecosystem services leads to environmental degradation, which can in turn harm human populations, especially many of the world’s poorest people who depend heavily on natural resources. How can we protect ecosystems so that human communities also benefit? When markets and private interests drive decision-making, particular stakeholders may benefit but society overall, especially poor communities, may not. Reducing a service to a single figure or metric also fails to capture the many ways in which people may value it, which devalues the service. A tropical forest, for example, may be valued by the government solely for its timber, while indigenous populations value its spiritual significance and other products it provides, such as food, fuelwood, or medicines.

Is it working for biodiversity? Many initiatives are tracking whether these new approaches that monetize ecosystem services are conserving biodiversity. But because many are relatively new and long-term evaluation is costly, substantial evidence linking monetization to improved environmental protection is still lacking. Preliminary analyses of multiple case studies have shown that biodiversity benefits from monetization approaches are rare.

Despite these limitations, the monetization of ecosystem services approach is important in order for our economic system to recognize and incorporate the true value of ecosystems. It can be valuable when applied in careful ways and together with other strategies that mitigate its shortcomings.

Thinking back to Gorongosa, the decline in wildlife likely affected cultural services, as well as provisioning and regulating services. By recognizing these multiple types of services, we may be able to better understand the connections between the ecological and human dimensions of the whole ecosystem, and in turn, support both social and ecosystem resilience.

REFERENCES

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