Restoration ecology

Ecological restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed. It is distinct from conservation in that it attempts to retroactively repair already damaged ecosystems rather than take preventative measures. Ecological restoration can reverse biodiversity loss, combat climate change, and support local economies. The United Nations named 2021-2030 the Decade on Ecosystem Restoration.

Scientists estimate that the current species extinction rate, or the rate of the Holocene extinction, is 1,000 to 10,000 times higher than the normal, background rate. Habitat loss is a leading cause of species extinctions and ecosystem service decline.

Two methods have been identified to slow the rate of species extinction and ecosystem service decline: conservation of quality habitat and restoration of degraded habitat. The number and size of ecological restoration projects have increased exponentially in recent years.

Restoration goals reflect political choices, and restoration goals differ by place and culture.

Recently constructed wetland regeneration in Australia,
on a site previously used for agriculture

Definition

Restoration ecology is the academic study of the science of restoration, whereas ecological restoration is the implementation by practitioners. The Society for Ecological Restoration defines restoration as "the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed." Ecological restoration includes a wide diversity of methods including erosion control, reforestation, removal of non-native species and weeds, revegetation of disturbed areas, daylighting streams, the reintroduction of native species, and habitat and range improvement for targeted species. Many scholars and practitioners argue that ecological restoration must include local communities and stakeholders: they call this process the "social-ecological restoration".

Rehabilitation of a portion of Johnson Creek, to restore bioswale and flood control functions of the land
which had long been converted to pasture for cow grazing. The horizontal logs can float, but are
anchored by the posts. Just-planted trees will eventually stabilize the soil. The fallen trees with roots
jutting into the stream are intended to enhance wildlife habitat. The meandering of the stream is
enhanced here by a factor of about three times, perhaps to its original course.

Rationale

• There are many reasons to restore ecosystems. Some include :
• Restoring natural capital such as drinkable water or wildlife populations
• Helping human communities and the ecosystems upon which they depend adapt to the impacts of climate change (through ecosystem-based adaptation)
• Mitigating climate change (e.g. through carbon sequestration)
• Helping threatened or endangered species
• Aesthetic reasons 
• Moral reasons: human intervention has unnaturally destroyed many habitats, and there exists an innate obligation to restore these destroyed habitats
• Regulated use/harvest, particularly for subsistence
• Cultural importance to indigenous people
• The environmental health of nearby populations 

There exist considerable differences of opinion on how to set restoration goals and how to define their success. As Laura J. Martin writes, "Restoration targets are moral and political matters as well as logistical and scientific ones." Some restorationists urge active restoration (e.g. killing invasive animals) and others believe that protected areas should have the bare minimum of human interference, such as rewilding.

Sankey diagram for the evolution of keywords used in
publications about ecological restoration in Canada over time

Ecological restoration has generated controversy. Skeptics doubt that the benefits justify the economic investment or point to failed restoration projects and question the feasibility of restoration altogether. It can be difficult to set restoration goals because, as Anthony Bradshaw writes, "ecosystems are not static, but in a state of dynamic equilibrium." Some scientists argue that, though an ecosystem may not be returned to its original state, the functions of a "novel ecosystem" are still valuable.

Ecosystem restoration can mitigate climate change through activities such as afforestation. Afforestation involves replanting forests that remove carbon dioxide from the air. Forestry-based carbon offsetting is controversial and is sometimes critiqued as carbon colonialism. Another driver of restoration projects in the United States is the legal framework of the Clean Water Act, which often requires mitigation for damage inflicted on aquatic systems by development or other activities.

Forest restoration in action at the Buffelsdraai Landfill Site
Community Reforestation Project in South Africa

Theoretical foundations

Restoration ecology draws on a wide range of ecological concepts.

Disturbance

Disturbance is a change in environmental conditions that disrupt the functioning of an ecosystem. Disturbance can occur at a variety of spatial and temporal scales, and is a natural component of many communities. For example, many forest and grassland restorations implement fire as a natural disturbance regime. However the severity and scope of anthropogenic impact has grown in the last few centuries. Differentiating between human-caused and naturally occurring disturbances is important if we are to understand how to restore natural processes and minimize anthropogenic impacts on the ecosystems.

Succession

Ecological succession is the process by which a community changes over time, especially following a disturbance. In many instances, an ecosystem will change from a simple level of organization with a few dominant pioneer species to an increasingly complex community with many interdependent species. Restoration often consists of initiating, assisting, or accelerating ecological successional processes, depending on the severity of the disturbance. Following mild to moderate natural and anthropogenic disturbances, restoration in these systems involves hastening natural successional trajectories through careful management. However, in a system that has experienced a more severe disturbance (such as in urban ecosystems), restoration may require intensive efforts to recreate environmental conditions that favor natural successional processes.

Fragmentation

Habitat fragmentation describes spatial discontinuities in a biological system, where ecosystems are broken up into smaller parts through land-use changes (e.g. agriculture) and natural disturbance. This both reduces the size of the population and increases the degree of isolation. These smaller and isolated populations are more vulnerable to extinction. Fragmenting ecosystems decreases the quality of the habitat. The edge of a fragment has a different range of environmental conditions and therefore supports different species than the interior. Restorative projects can increase the effective size of a population by adding suitable habitat and decrease isolation by creating habitat corridors that link isolated fragments. Reversing the effects of fragmentation is an important component of restoration ecology. The composition of the surrounding landscape can also influence the effectiveness of restoration projects. For example, a restoration site that is closer to remaining vegetation will be more likely to be naturally regenerated through seed disperal than a site that is further away.

Ecosystem function

Ecosystem function describes the most basic and essential foundational processes of any natural systems, including nutrient cycles and energy fluxes. An understanding of the complexity of these ecosystem functions is necessary to address any ecological processes that may be degraded. Ecosystem functions are emergent properties of the system as a whole, thus monitoring and management are crucial for the long-term stability of ecosystems. A completely self-perpetuating and fully functional ecosystem is the ultimate goal of restorative efforts. We must understand what ecosystem properties influence others to restore desired functions and reach this goal.

Community assembly

Community assembly "is a framework that can unify virtually all of (community) ecology under a single conceptual umbrella". Community assembly theory attempts to explain the existence of environmentally similar sites with differing assemblages of species. It assumes that species have similar niche requirements, so that community formation is a product of random fluctuations from a common species pool. Essentially, if all species are fairly ecologically equivalent, then random variation in colonization, and migration and extinction rates between species, drive differences in species composition between sites with comparable environmental conditions.

Population genetics

Genetic diversity has shown to be as important as species diversity for restoring ecosystem processes. Hence ecological restorations are increasingly factoring genetic processes into management practices. Population genetic processes that are important to consider in restored populations include founder effects, inbreeding depression, outbreeding depression, genetic drift, and gene flow. Such processes can predict whether or not a species successfully establishes at a restoration site.

Applications

Leaf litter accumulation

Leaf litter accumulation plays an important role in the restoration process. Higher quantities of leaf litter hold higher humidity levels, a key factor for the establishment of plants. The process of accumulation depends on factors like wind and species composition of the forest. The leaf litter found in primary forests is more abundant, deeper, and holds more humidity than in secondary forests. These technical considerations are important to take into account when planning a restoration project.

Soil heterogeneity effects on community heterogeneity

Spatial heterogeneity of resources can influence plant community composition, diversity, and assembly trajectory. Baer et al. (2005) manipulated soil resource heterogeneity in a tallgrass prairie restoration project. They found increasing resource heterogeneity, which on its own was insufficient to ensure species diversity in situations where one species may dominate across the range of resource levels. Their findings were consistent with the theory regarding the role of ecological filters on community assembly. The establishment of a single species, best adapted to the physical and biological conditions can play an inordinately important role in determining the community structure.

Invasion and restoration

Restoration is used as a tool for reducing the spread of invasive plant species many ways. The first method views restoration primarily as a means to reduce the presence of invasive species and limit their spread. As this approach emphasizes the control of invaders, the restoration techniques can differ from typical restoration projects. The goal of such projects is not necessarily to restore an entire ecosystem or habitat. These projects frequently use lower diversity mixes of aggressive native species seeded at high density. They are not always actively managed following seeding. The target areas for this type of restoration are those which are heavily dominated by invasive species. The goals are to first remove the species and then in so doing, reduce the number of invasive seeds being spread to surrounding areas. An example of this is through the use of biological control agents (such as herbivorous insects) which suppress invasive weed species while restoration practitioners concurrently seed in native plant species that take advantage of the freed resources. These approaches have been shown to be effective in reducing weeds, although it is not always a sustainable solution long term without additional weed control, such as mowing, or re-seeding.

Restoration projects are also used as a way to better understand what makes an ecological community resistant to invasion. As restoration projects have a broad range of implementation strategies and methods used to control invasive species, they can be used by ecologists to test theories about invasion. Restoration projects have been used to understand how the diversity of the species introduced in the restoration affects invasion. We know that generally higher diversity prairies have lower levels of invasion. The incorporation of functional ecology has shown that more functionally diverse restorations have lower levels of invasion. Furthermore, studies have shown that using native species functionally similar to invasive species are better able to compete with invasive species. Restoration ecologists have also used a variety of strategies employed at different restoration sites to better understand the most successful management techniques to control invasion. To develop restoration ecology into a full science and to improve its practice requires generalizations about the processes governing the development of restored communities. While new experiments can be designed , one way forward is to use data from existing restoration studies to relate plant species performance to their ecological trait.

Successional trajectories

Progress along a desired successional pathway may be difficult if multiple stable states exist. Looking over 40 years of wetland restoration data, Klötzli and Gootjans (2001) argue that unexpected and undesired vegetation assemblies "may indicate that environmental conditions are not suitable for target communities". Succession may move in unpredicted directions, but constricting environmental conditions within a narrow range may rein in the possible successional trajectories and increase the likelihood of the desired outcome.

Sourcing land for restoration

A study quantified climate change mitigation potentials of 'high-income' nations shifting diets - away from meat-consumption - and restoration of the spared land. They find that the hypothetical dietary change "could reduce annual agricultural production emissions of high-income nations' diets by 61% while sequestering as much as 98.3 (55.6-143.7) GtCO2 equivalent, equal to approximately 14 years of current global agricultural emissions until natural vegetation matures", outcomes they call 'double climate dividend'.

Sourcing material for restoration

For most restoration projects it is generally recommended to source material from local populations, to increase the chance of restoration success and minimize the effects of maladaptation. However the definition of local can vary based on species, habitat and region. US Forest Service recently developed provisional seed zones based on a combination of minimum winter temperature zones, aridity, and the Level III ecoregions. Rather than putting strict distance recommendations, other guidelines recommend sourcing seeds to match similar environmental conditions that the species is exposed to, either now, or under projected climate change. For example, sourcing for Castilleja levisecta found that farther source populations that matched similar environmental variables were better suited for the restoration project than closer source populations. Similarly, a suite of new methods are surveying gene-environment interactions in order to identify the optimum source populations based on genetic adaptation to environmental conditions.

Ecosystem restoration for the superb parrot on an abandoned railway line in Australia

Challenges

Some view ecosystem restoration as impractical, partially because restorations often fall short of their goals. Hilderbrand et al. point out that many times uncertainty (about ecosystem functions, species relationships, and such) is not addressed, and that the time-scales set out for 'complete' restoration are unreasonably short, while other critical markers for full-scale restoration are either ignored or abridged due to feasibility concerns. In other instances an ecosystem may be so degraded that abandonment (allowing a severely degraded ecosystem to recover on its own) may be the wisest option. Local communities sometimes object to restorations that include the introduction of large predators or plants that require disturbance regimes such as regular fires, citing threat to human habitation in the area. High economic costs can also be perceived as a negative impact of the restoration process.

Public opinion is very important in the feasibility of a restoration; if the public believes that the costs of restoration outweigh the benefits they will not support it.

Many failures have occurred in past restoration projects, many times because clear goals were not set out as the aim of the restoration, or an incomplete understanding of the underlying ecological framework lead to insufficient measures. This may be because, as Peter Alpert says, "people may not [always] know how to manage natural systems effectively". Furthermore, many assumptions are made about myths of restoration such as carbon copy, where a restoration plan, which worked in one area, is applied to another with the same results expected, but not realized.

Restored prairie at the West Eugene Wetlands in Eugene, Oregon

Science-practice gap

One of the struggles for both fields is a divide between restoration ecology and ecological restoration in practice. Many restoration practitioners as well as scientists feel that science is not being adequately incorporated into ecological restoration projects. In a 2009 survey of practitioners and scientists, the "science-practice gap" was listed as the second most commonly cited reason limiting the growth of both science and practice of restoration.

There are a variety of theories about the cause of this gap. However, it has been well established that one of the main issues is that the questions studied by restoration ecologists are frequently not found useful or easily applicable by land managers. For instance, many publications in restoration ecology characterize the scope of a problem in-depth, without providing concrete solutions. Additionally many restoration ecology studies are carried out under controlled conditions and frequently at scales much smaller than actual restorations. Whether or not these patterns hold true in an applied context is often unknown. There is evidence that these small-scale experiments inflate type II error rates and differ from ecological patterns in actual restorations. One approach to addressing this gap has been the development of International Principles & Standards for the Practice of Ecological Restoration by the Society for Ecological restoration (see below) - however this approach is contended, with scientists active in the field suggesting that this is restrictive, and instead principles and guidelines offer flexibility.

There is further complication in that restoration ecologists who want to collect large-scale data on restoration projects can face enormous hurdles in obtaining the data. Managers vary in how much data they collect, and how many records they keep. Some agencies keep only a handful of physical copies of data that make it difficult for the researcher to access. Many restoration projects are limited by time and money, so data collection and record-keeping are not always feasible. However, this limits the ability of scientists to analyze restoration projects and give recommendations based on empirical data.

Food security and nature degradation

A range of activities in the name of "nature restoration", such as monoculture tree plantations, "degrade nature-destroying biodiversity, increasing pollution, and removing land from food production".

Consideration as a substitute for steep emission reductions

Climate benefits from nature restoration are "dwarfed by the scale of ongoing fossil fuel emissions". It risks "over-relying on land for mitigation at the expense of phasing out fossil fuels". Despite these issues, nature restoration is receiving increasing attention, with a study concluding that "Land restoration is an important option for tackling climate change but cannot compensate for delays in reducing fossil fuel emissions" as it's "unlikely to be done quickly enough to notably reduce the global peak temperatures expected in the next few decades".

For instance, researchers have compared reforestation and prevention of (mainly tropical) deforestation in specific:

This section is an excerpt from Reforestation § Comparison to forest protection.

Researchers have found that, in terms of environmental services, it is better to avoid deforestation than to allow for deforestation to subsequently reforest, as the former leads to irreversible effects in terms of biodiversity loss and soil degradation. Furthermore, the probability that legacy carbon will be released from soil is higher in younger boreal forest. Global greenhouse gas emissions caused by damage to tropical rainforests may have been substantially underestimated until around 2019. Additionally, the effects of af- or reforestation will be farther in the future than keeping existing forests intact. It takes much longer - several decades - for the benefits for global warming to manifest to the same carbon sequestration benefits from mature trees in tropical forests and hence from limiting deforestation. Therefore, scientists consider "the protection and recovery of carbon-rich and long-lived ecosystems, especially natural forests" to be "the major climate solution".

Contrasting restoration ecology and conservation biology

Both restoration ecologists and conservation biologists agree that protecting and restoring habitat is important for protecting biodiversity. However, conservation biology is primarily rooted in population biology. Because of that, it is generally organized at the population genetic level and assesses specific species populations (i.e. endangered species). Restoration ecology is organized at the community level, which focuses on broader groups within ecosystems.

In addition, conservation biology often concentrates on vertebrate and invertebrate animals because of their salience and popularity, whereas restoration ecology concentrates on plants. Restoration ecology focuses on plants because restoration projects typically begin by establishing plant communities. Ecological restoration, despite being focused on plants, may also have "umbrella species" for individual ecosystems and restoration projects.[95] For example, the Monarch butterfly is an umbrella species for conserving and restoring milkweed plant habitat, because Monarch butterflies require milkweed plants to reproduce. Finally, restoration ecology has a stronger focus on soils, soil structure, fungi, and microorganisms because soils provide the foundation of functional terrestrial ecosystems.

International Principles & Standards for the Practice of Ecological Restoration

The Society for Ecological Restoration (SER) released the second edition of the International Standards for the Practice of Ecological Restoration on September 27, 2019, in Cape Town, South Africa, at SER's 8th World Conference on Ecological Restoration.[98]  The publication provides updated and expanded guidance on the practice of ecological restoration, clarifies the breadth of ecological restoration and allied environmental repair activities, and includes ideas and input from a diverse international group of restoration scientists and practitioners.

The second edition builds on the first edition of the Standards, which was released December 12, 2016, at the Convention on Biological Diversity's 13th Conference of the Parties in Cancun, Mexico. The development of these Standards has been broadly consultative. The first edition was circulated to dozens of practitioners and experts for feedback and review. After release of the first edition, SER held workshops and listening sessions, sought feedback from key international partners and stakeholders, opened a survey to members, affiliates and supporters, and considered and responded to published critiques.

The International Principles and Standards for the Practice of Ecological Restoration:

Present a robust framework to guide restoration projects toward achieving intended goals.

Address restoration challenges including: effective design and implementation, accounting for complex ecosystem dynamics (especially in the context of climate change), and navigating trade-offs associated with land management priorities and decisions.

Highlight the role of ecological restoration in connecting social, community, productivity, and sustainability goals.

Recommend performance measures for restorative activities for industries, communities, and governments to consider.

Enhance the list of practices and actions that guide practitioners in planning, implementation, and monitoring activities, including: appropriate approaches to site assessment and identification of reference ecosystems, different restoration approaches including natural regeneration, and the role of ecological restoration in global restoration initiatives.

Include an expanded glossary of restoration terminology.

Provide a technical appendix on sourcing of seeds and other propagules for restoration.

History

Indigenous peoples, land managers, stewards, and laypeople have been practicing ecological restoration or ecological management for thousands of years. Restoration ecology emerged as a separate field in ecology in the late twentieth century. The term was coined by John Aber and William Jordan III when they were at the University of Wisconsin-Madison.