Thresholds, novel ecosystems, and the sanctity of history
Submitted by Brandon Bestelmeyer on Tue, 08/19/2014 - 02:58
How is an ecosystem supposed to be? The answers determine how millions of dollars are spent and how ecosystems are transformed, with effects lasting centuries. Conflict over this question used to be between industry and environmentalists. Now ecologists are doing battle with one another too.
The standard for ecosystem management is historical or “reference” condition. Ecologists argue about how long ago we should look to, what historical ecosystems looked like, and what processes shaped them, but history captures much of what we value in nature. History set the evolutionary stage for the Earth’s biological diversity, unspoiled by human domination (but perhaps aided by human activities, such as fire use). Historical soil forming processes produced the deep, dark soils of the Great Plains (although humans produced them in the Amazon). Ecologists bicker about the details, but history is at the core of every land management framework—land potential, wilderness, ecosystem health, historical range of variation, and resilience. When we allow ecosystems to deviate from historical conditions, we risk losing the diversity of life and our precious soil. In fact, we don’t even know what we are losing, because we know so little about the machinery of evolution and ecosystem function. So historical conditions must be preserved and restored at even great cost. History is, in a word, sacred.
Proponents of “novel ecosystems”, then, are heretics. These scientists propose that preservation or restoration of historical conditions is sometimes impractical (1). The core of their argument is that ecosystems can cross ecological thresholds beyond which recovery to historical conditions is limited. Some ecosystems can cross thresholds and be restored with effort. Woody plants can be removed, allowing grass to recover and the fire cycle to be restored (2). There is another kind of threshold, however. When soil is stripped away by erosion after careless grazing or earthmovers seeking minerals; when a suite of invasive plants pervade a landscape, then historical conditions cannot be entirely recreated (3-5). Add to that climate change, which will prevent recovery of historical species, and we have difficult decisions to make (6).
The problem is that we often choose to ignore the difficult cases. We abandon ecosystems because they are “irreversibly degraded”. Even if we believe they are not a total loss, they are placed at the bottom category of triage—treatment is indefinitely delayed. “Novel ecosystems” proponents offer us another option (1). They suggest that we consider accepting these cases for what they are and managing them for whatever ecosystem services can be supported by the new ecosystem. Although the ecosystem differs from its historical state, it can have value as a wild ecosystem. We adapt. Or is the word—surrender (7)?
Some conservation and invasive species biologists are attacking the concept of novel ecosystem on two grounds (8). First is that there are no ecological thresholds that would ultimately prevent restoration to historical conditions with enough effort. Second, a focus on ecosystem services is a challenge to the inherent value of biodiversity and complexity of undomesticated historical ecosystems (9). “For those who care about global extinctions or about preserving historical ecosystems, [novel ecosystems] are bad news”(10).
This critique denies fundamental realities about ecosystem change and management. While it is true that not all ecosystems that have been characterized as “crossing a threshold” are unrestorable, there is clear evidence that highly persistent or irreversible transitions have occurred in certain cases, often involving invasive species and soil degradation. In contrast to the assertion that “no proof of ecological thresholds that would prevent restoration has ever been demonstrated” (8), I would take the opposite position that there is no evidence that restoration to the historical reference can occur even with intensive efforts in such cases (3, 5, 11). I would also assert that limited restoration potential puts an even greater premium on preserving historical ecosystems. We acknowledge that restoration cannot fully mitigate additional losses of intact ecosystems (11).
The critique also takes an unnecessarily polar view of biodiversity vs. human well-being and fails to acknowledge land use. Within wildland and low intensity land uses such as rangeland, historical conditions will continue to be prized whether or not novel systems are considered. This is because options for most ecosystem services, including wildness and soil fertility, are maximized in historical ecosystems. A critical problem, however, is the loss of wildlands and rangelands to more intensive cropland and urban land uses. Conversion of rangelands to cropland is accelerating in several parts of the world, including novel ecosystems such as some retired croplands in the US (Conservation Reserve Program lands) or degraded forests in Argentina (12, 13). Further, energy development may be increasingly focused on “degraded” ecosystems (14). A decision to manage land as a novel ecosystem is a decision to retain wildland or rangeland land uses for the benefit of biodiversity conservation.
It might reconcile the debate to consider novel ecosystems not as a type of ecosystem but as a land use. It means that that we will value and maintain an area as wildland in spite of its limitations. We manage it to maintain biodiversity, ecosystem complexity, and low intensity uses as well as it can. To condemn such areas as degraded, to wait in vain for restoration to occur, is to invite conversion to other land uses. Conservation biologists should not let the perfect, or the sacred, become the enemy of conservation.
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2. Twidwell D, Fuhlendorf SD, Taylor CA, & Rogers WE (2013) Refining thresholds in coupled fire-vegetation models to improve management of encroaching woody plants in grasslands. J Appl Ecol 50(3):603-613.
3. Peters DPC, et al. (2006) Disentangling complex landscapes: New insights into arid and semiarid system dynamics. Bioscience 56(6):491-501.
4. Chambers J, et al. (2014) Resilience to stress and disturbance, and resistance to Bromus tectorum L. invasion in cold desert shrublands of western North America. Ecosystems 17(2):360-375.
5. Herrick JE, Havstad KM, & Rango A (2006) Remediation research in the Jornada Basin: Past and future. . Structure and function of a Chihuahuan Desert ecosystem: The Jornada Basin LTER., eds Havstad KM, Schlesinger WH, & Huenneke LF (Oxford University Press, New York, NY), pp 278–304.
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7. Perring M, Audet P, & Lamb D (2014) Novel ecosystems in ecological restoration and rehabilitation: Innovative planning or lowering the bar? Ecological Processes 3(1):8.
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10. Marris E (2009) Ragamuffin earth. Nature 460(7254):450-453.
11. Coffman JM, Bestelmeyer BT, Kelly JF, Wright TF, & Schooley RL (2014) Restoration practices have positive effects on breeding bird species of concern in the Chihuahuan Desert. Restor Ecol 22(3):336-344.
12. Wright CK & Wimberly MC (2013) Recent land use change in the Western Corn Belt threatens grasslands and wetlands. P Natl Acad Sci USA 110(10):4134-4139.
13. Zak MR, Cabido M, Caceres D, & Diaz S (2008) What drives accelerated land cover change in central Argentina? Synergistic consequences of climatic, socioeconomic, and technological factors. Environ Manage 42(2):181-189.
14. Stoms DM, Dashiell SL, & Davis FW (2013) Siting solar energy development to minimize biological impacts. Renewable Energy 57(0):289-298.