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Ecological restoration aims to recreate, initiate, or accelerate the recovery of an ecosystem that has been disturbed. Disturbances are environmental changes that alter ecosystem structure and function. Common disturbances include logging, damming rivers, intense grazing, hurricanes, floods, and fires. Restoration activities may be designed to replicate a pre-disturbance ecosystem or to create a new ecosystem where it had not previously occurred. Restoration ecology is the scientific study of repairing disturbed ecosystems through human intervention.
Goals
Restoration projects differ in their objectives and their methods of achieving those goals. Many restoration projects aim to establish ecosystems composed of a native species; other projects attempt to restore, improve, or create particular ecosystem functions, such as pollination or erosion control. Some examples of different kinds of restoration include the following:
- Revegetation- the establishment of vegetation on sites where it has been previously lost, often with erosion control as the primary goal. For example, vegetated buffers are strips of vegetation that protect water quality in riparian ecosystems from urban or agricultural runoff.
- Habitat enhancement- the process of increasing the suitability of a site as habitat for some desired species.
- Remediation: improving an existing ecosystem or creating a new one with the aim of replacing another that has deteriorated or been destroyed.
- Mitigation: legally mandated remediation for loss of protected species or ecosystems.
At a given restoration site, it may be possible to establish a number of different communities. When choosing a target state for a restoration project however, restorationists generally select only one (or a small range) of possible community types. Often some sort of "natural" pre-disturbance condition, or reference state, is selected, along with its presumed properties (e.g., previous flooding or fire patterns). This is often represented by a nearby undisturbed reference site. Even with good working knowledge of an historical ecosystem’s species composition and functions, practitioners must still decide how far in the past is defined as "natural." For some ecosystems the reference state may be before any human disturbance, but more commonly the reference state is before agricultural or industrial intensification (such as pre-European contact in the US). However, sometimes an historical target may no longer be appropriate under current or projected climatic or biotic conditions. For example, future climates may not support certain species, and some species may have already gone extinct in an area. Under these circumstances practitioners may decide to create an ecosystem that did not exist historically at the project site, but which corresponds to current or projected future conditions. Sometimes, restoration efforts are designed to maintain a desirable human-derived state, such as montane meadows or Scottish moors.
History
The idea of restoring the land dates back centuries, but modern restoration ecology and its practice began in the early 1900s when people such as renowned conservationist Aldo Leopold began promoting the movement. It has since grown to include a wide variety of ecological restoration activities that range from large-scale projects (e.g., of the Everglades, Louisiana wetlands, or the Mau Forest in Kenya) to small-scale projects (e.g., tree planting). It is a defining characteristic of ecological restoration that many projects are locally initiated and implemented by community volunteers. Because restoration projects generally involve complex collaborations and negotiations among a diverse group of interested parties, social science is an integral part of restoration at all scales.
Ecological research on restoration has largely focused on community ecology and ecosystem ecology, with particular attention to plants. However, animal reintroduction, a common element of conservation biology, is also essentially restoration. Gaining momentum in the latter half of the twentieth century, restoration ecology is now established as a science and studied in many research institutions. International societies and journals, such as the Society for Ecological Restoration (est. 1988) and its journals Ecological Restoration (est. 1981) and Restoration Ecology (est. 1993), are dedicated to furthering knowledge of restoration science and practice. Starting in the 1990s, the number of books and journal articles on ecological restoration has risen exponentially. There has been a strong push to formalize the science and practice of restoration, linking it explicitly with ecological theories. In fact, ecological restoration can be used as a practical test of our ecological understanding. Conversely, failures in ecological restoration can reveal gaps in our understanding of ecology.
Ecological research on restoration has largely focused on community ecology and ecosystem ecology, with particular attention to plants. However, animal reintroduction, a common element of conservation biology, is also essentially restoration. Gaining momentum in the latter half of the twentieth century, restoration ecology is now established as a science and studied in many research institutions. International societies and journals, such as the Society for Ecological Restoration (est. 1988) and its journals Ecological Restoration (est. 1981) and Restoration Ecology (est. 1993), are dedicated to furthering knowledge of restoration science and practice. Starting in the 1990s, the number of books and journal articles on ecological restoration has risen exponentially. There has been a strong push to formalize the science and practice of restoration, linking it explicitly with ecological theories. In fact, ecological restoration can be used as a practical test of our ecological understanding. Conversely, failures in ecological restoration can reveal gaps in our understanding of ecology.
Concepts Underpinning Restoration
Disturbance
Disturbance events can occur at many scales and different levels of severity, and some are natural parts of every ecosystem. Disturbance events can alter species composition, nutrient cycling, and soil properties. Natural disturbances include severe weather damage, fire, flooding, treefalls and even volcanic eruptions. Anthropogenic (human-caused) disturbances can alter or destroy natural habitat (like clearing land for agriculture) and/or ecological functions (like damming rivers for flood control). Humans can also change natural disturbance events and cycles (like suppression of wildfires and prevention of periodic flooding). The goal of a restoration project may be to initiate or speed the recovery of an ecosystem after disturbance. Restoration activities may also be designed to reestablish natural disturbance regimes.
Genetics
Restoration projects also typically include genetic considerations. Plants (or animals) from local sources are more likely to be well adapted to the target ecosystem. Therefore, using animals or plant materials (like seeds or cuttings) collected from local sources may increase the chance of successful establishment. Including a large number of individual plants or animals can help ensure genetic diversity in the restored populations. Genetic diversity is thought to be critical to maintaining the ability of populations to evolve and recover from disturbances.
Succession
Ecological succession is the process by which biological community composition- the number and proportion of different species in an ecosystem- recover over time following a disturbance event. Passive restoration means simply allowing natural succession to occur in an ecosystem after removing a source of disturbance. The recovery of the deciduous forests in the eastern United States after the abandonment of agriculture is a classic example of passive restoration. Active restoration involves accelerating the process or attempting to change the trajectory of succession. For example, mine tailings would take so long to recover passively that active restoration is usually appropriate.
Community Assembly Theory
Community assembly theory suggests that similar sites can develop different biological communities depending on order of arrival of different species. In the context of restoration, sites may not always recover toward a desired or anticipated group of species or ecosystem functions. Composition of seed mixes, planting order and year of planting may be important considerations for restoration practitioners, particularly when goals include the establishment of certain ecological communities or the prevention of invasion by weeds or pests.
Landscape Ecology
Restoration draws on several concepts from landscape ecology. Restored areas are often relatively small and isolated, which makes them especially sensitive to problems associated with habitat fragmentation. Habitat fragmentation occurs when continuous areas of habitat become disconnected by natural or human causes (for example, building roads through a forest). Fragmentation generally leads to small, isolated patches of hospitable habitat. Smaller habitats support fewer species and smaller populations, which are at greater risk of inbreeding and local extinction. The theory of island biogeography predicts that populations are more likely to persist in habitat patches that are large and/or well connected with populations in other hospitable habitats. This theory assumes that the matrix—the region between habitat patches—is uniform and inhospitable. The most common examples of this concept are oceanic islands, dots of terrestrial species’ habitat surrounded by uninhabitable water. More recently, the classic dichotomy of hospitable versus inhospitable habitat has been modified to include the existence a multiple types of habitat patches which are juxtaposed to form a patch mosaic. These different patches within the mosaic may be more or less hospitable for the species, communities and ecosystem functions targeted by restoration activities.
Fragmentation may also intensify negative edge effects — impacts of one habitat on an adjacent habitat — by increasing the amount of edge habitat and reducing the distances among edges. For instance, invasive weeds are more abundant along forest edges, so small forest fragments (which have more edge habitat) are more likely to be invaded. Restoration activities often seek to improve connectivity among habitat patches in fragmented landscapes by creating or restoring linkages. Examples of linkages commonly used to improve connectivity are corridors and stepping stones. Corridors are relatively narrow, linear strips of habitat between otherwise isolated habitat patches. Stepping stones are small unconnected patches of habitat that are close enough together to allow movement across the landscape.
Fragmentation may also intensify negative edge effects — impacts of one habitat on an adjacent habitat — by increasing the amount of edge habitat and reducing the distances among edges. For instance, invasive weeds are more abundant along forest edges, so small forest fragments (which have more edge habitat) are more likely to be invaded. Restoration activities often seek to improve connectivity among habitat patches in fragmented landscapes by creating or restoring linkages. Examples of linkages commonly used to improve connectivity are corridors and stepping stones. Corridors are relatively narrow, linear strips of habitat between otherwise isolated habitat patches. Stepping stones are small unconnected patches of habitat that are close enough together to allow movement across the landscape.
Application
Applied restoration is a multi-step process, which may include some or all of these stages:
- Assessing the site: A thorough appraisal of the current conditions at the restoration site is essential for determining what kind of actions will be necessary. In this step, the causes of ecosystem disturbance and methods for stopping or reversing them are identified.
- Formulating project goals: To determine targets for the restored community, practitioners may visit reference sites (similar, nearby environments in natural condition) and/or consult historical sources that detail the pre-disturbance community. Goals may also include considerations of what species will be best suited to present or future climate conditions.
- Removing sources of disturbance: Before restoration can be successful, forces of disturbance may need to be removed. Examples include cessation of mining or farming or causes of erosion, restricting livestock from riparian areas, removing toxic materials from soil or sediments, and eradicating invasive exotic species.
- Restoring processes/disturbance cycles: Sometimes restoring important ecological processes such as natural flood or fire regimes is enough to restore ecosystem integrity. In these cases, native plants and animals that have evolved to tolerate or require natural disturbance regimes may come back on their own without direct action by practitioners.
- Rehabilitating substrates: This can include any activity aimed at repairing altered soil texture or chemistry, or restoring hydrological regimes or water quality.
- Restoring vegetation: In many cases, restoration activities involve direct revegetation of a site. Usually, native species suited to local environmental conditions are chosen for planting. Seeds or cuttings are generally collected from a variety of sources within a local region in order to ensure genetic diversity. Vegetation can be planted as seeds, or seedlings.
- Monitoring and maintenance: Monitoring the restoration site over time is critical to determining whether goals are being met, and can inform future management decisions. Observations made at the site may indicate that further action, such as periodic weed removal, is necessary in ensuring the long-term success of the project. Ideally restoration projects would eventually achieve a self-sustaining ecosystem without the need for future human intervention.
Virtually all the worlds' ecosystem types have been the subject of restoration efforts, but particular attention has been paid to ecosystems most impacted by human activities, such as wetlands, grasslands/rangelands, riparian areas, and tropical forests.
Broader Considerations
In a world with a rapidly changing climate, restorationists plan for an uncertain future. One new and controversial approach to dealing with climate change is assisted migration: the idea of establishing a species in a place where it does not presently occur and has not occurred in the recent past, but where the climate is predicted to be suitable for that species in the future. If the climate is currently changing faster than many plants and animals can move (through dispersal or migration), and many hospitable habitat patches are now isolated, then it may be necessary to actively move species to new habitats.
Strategies to avert future biodiversity loss are likely to include many of the techniques of ecological restoration, but its practice is not without controversy. One contentious issue is the process of mitigation, in which destruction of protected populations or habitats is allowed if there are offsetting mitigation plantings. Even mitigations that fulfill legal requirements often fail to fully compensate for the lost populations or communities. Some fear that restoration provides an excuse for activities that are destructive of biodiversity. Restoration activities should instead be viewed as complementary to, not a substitute for, efforts for the conservation of biodiversity.
There is also some apprehension with the idea that we know enough to create functioning ecosystems. This unease stems from the fact that restoration is inherently uncertain at every step, from the planning (what really existed there before or how do we balance multiple objectives with conflicting requirements?), to the implementation (what is the best way to control weeds or how do we really restore flooding?), to the continued management (when can we judge a project to be truly successful?). Despite this uncertainty, ecological restoration is a rapidly growing field that represents a foundational change in our relationship to the natural world.
Strategies to avert future biodiversity loss are likely to include many of the techniques of ecological restoration, but its practice is not without controversy. One contentious issue is the process of mitigation, in which destruction of protected populations or habitats is allowed if there are offsetting mitigation plantings. Even mitigations that fulfill legal requirements often fail to fully compensate for the lost populations or communities. Some fear that restoration provides an excuse for activities that are destructive of biodiversity. Restoration activities should instead be viewed as complementary to, not a substitute for, efforts for the conservation of biodiversity.
There is also some apprehension with the idea that we know enough to create functioning ecosystems. This unease stems from the fact that restoration is inherently uncertain at every step, from the planning (what really existed there before or how do we balance multiple objectives with conflicting requirements?), to the implementation (what is the best way to control weeds or how do we really restore flooding?), to the continued management (when can we judge a project to be truly successful?). Despite this uncertainty, ecological restoration is a rapidly growing field that represents a foundational change in our relationship to the natural world.
References and Recommended Reading
Bell, S. S., Fonesca, M. S. et al. Linking restoration and landscape ecology. Restoration Ecology 5, 318–323 (1997).
Bradshaw, A. D. Restoration: the acid test for ecology. In Restoration Ecology: A Synthetic Approach to Ecological Research. eds. Jordan, W. R., Gilpin, M. E. et al. (Cambridge, UK: Cambridge University Press, 1987): 23–29.
Falk, D. A., Palmer, M. A. et al. Foundations of Restoration Ecology. Washington, DC: Island Press, 2006.
Hobbs, R. J. & Harris, J. A. Restoration ecology: Repairing the Earth's ecosystems in the new millennium. Restoration Ecology 9, 239–246 (2001).
Hobbs, R. J. & Norton, D. A. Towards a conceptual framework for restoration ecology. Restoration Ecology 4, 324–337 (1996).
Hobbs, R. J., Arico, S. et al. Novel ecosystems: theoretical and management aspects of the new ecological world order. Global Ecology and Biogeography 15, 1–7 (2006).
Holl, K. D., Loik, M. E. et al. Tropical montane forest restoration in Costa Rica: overcoming barriers to dispersal and establishment. Restoration Ecology 8, 339–349 (2000).
Lamb, D. Large-scale ecological restoration of degraded tropical forest lands: the potential role of timber plantations. Restoration Ecology 6, 271–279 (1998).
McKay, J. K., Christian, C. E. et al. "How local is local?": a review of practical and conceptual issues in the genetics of restoration. Restoration Ecology 13, 432–440 (2005).
Michener, W. K. Quantitatively evaluating restoration experiments: research design, statistical analysis, and data management considerations. Restoration Ecology 5, 93–110 (1997).
Montalvo, A. M., Williams, S. L. et al. Restoration biology: a population biology perspective. Restoration Ecology 5, 277–290 (1997).
Osborne, L. L. & Kovacic, D. A. Riparian vegetated buffer strips in water-quality restoration and stream management. Freshwater Biology 29, 243–258 (1993).
Palmer, M. A., Bernhardt. E. S. et al. Standards for ecologically successful river restoration. Journal of Applied Ecology 42, 208–217 (2005).
Temperton, V. M., Hobbs, R. J. Assembly Rules and Restoration Ecology. Washington, DC: Island Press, 2004.
Van Andel, J. and Aronson J. Restoration Ecology. Malden, MA: Blackwell Publishing, 2006.
Young, T. P. Restoration ecology and conservation biology. Biological Conservation 92, 73–83 (2000).
Young, T. P., Petersen, D. A. et al. The ecology of restoration: historical links, emerging issues, and unexplored realms. Ecology Letters 8, 662–673 (2005).
