When and where to actively restore ecosystems?
Research highlights
► We propose a framework for when land managers should actively restore ecosystems. ► The rate of natural regeneration, project goals, and available resources should be considered. ► Evaluating these factors should result in a more efficient use of restoration resources.
Introduction
Conservation efforts have traditionally focused on protecting areas where the land cover has not been heavily altered by humans and such efforts must remain a priority. Given the extent of land use and land cover change by humans on a global scale, conservation efforts have increasingly focused on natural recovery and active restoration of degraded ecosystems in order to restore both ecosystem services and biodiversity (Aronson et al., 2007, Chazdon, 2008b, Rey Benayas et al., 2008). These restoration efforts range from removing human disturbances (e.g. fire, grazing, water removal from rivers) in order to allow for natural or unassisted recovery (“passive restoration” sensu DellaSala et al., 2003, Rey Benayas et al., 2008) to humans actively intervening in an effort to accelerate and influence the successional trajectory of recovery (“active restoration”).
Given that natural recovery in many ecosystems can take decades, there is often considerable social pressure to intervene to accelerate this process, particularly in urban settings where degraded areas are highly visible. Active restoration projects are becoming increasingly common, often at a cost of considerable time and labor. In these projects, land managers intervene in a range of ways to facilitate recovery, including restoring predisturbance topography (in terrestrial and wetland systems) or river channel patterns; reintroducing propagules of plants or animals; and actively manipulating disturbance regimes such as fire and flooding (Perrow and Davy, 2002, Van Andel and Aronson, 2006).
There is considerable debate, however, regarding whether active restoration is always necessary (Prach and Hobbs, 2008, Clewell and McDonald, 2009, Rey Benayas et al., 2009), given the numerous examples of ecosystems recovering over a period of decades without human intervention (Jones and Schmitz, 2009). For example, the vast majority of forest in the eastern United States was logged approximately a century ago and is now second growth forest, which has recovered the structure and much of the predisturbance species composition of the original forest (Duffy and Meier, 1992, McLachlan and Bazely, 2001). Similar patterns of forest recovery have been documented in some tropical ecosystems (Guariguata et al., 1997, Finegan and Delgado, 2000, Letcher and Chazdon, 2009), although rates of tropical forest recovery are highly variable (Holl, 2007, Chazdon, 2008a) for several reasons that are discussed in more detail later. One of the most dramatic examples of extensive forest recovery is Puerto Rico, where by the end of the 1930s forest covered less than 10% of the island. The subsequent decline of the agriculture sector has facilitated natural forest regeneration, and by the year 2000, forest cover increased to more than 40% (Helmer, 2004, Páres-Ramos et al., 2008). Forest biomass and species richness in these secondary forests is similar to mature forests after 30–40 years of recovery (Aide et al., 2000).
In some cases restoration efforts may actually slow ecosystem recovery or have a strong influence on the direction of the successional trajectory. For instance, Alnus acuminata (Andean alder) is often planted with the goal of accelerating forest recovery in the Andes, but after 30 years of regeneration these areas have lower alpha and beta diversity in comparison with secondary forests of the same age (Murcia, 1997). Similarly, mechanically planting trees to restore the density and diversity of tropical dry forest trees in central Brazil causes damage to naturally resprouting species, resulting in no net gain in tree establishment (Sampaio et al., 2007).
These examples beg the question of when and where humans should intervene to actively restore ecosystems vs. simply allowing the ecosystems to passively recovery. Here we provide a conceptual framework for deciding whether and to what extent humans should intervene to facilitate forest recovery. These decisions are critically important given the limited resources available for restoration. We focus the majority of our examples on tropical forests, in keeping with the special issue, although the framework applies broadly to a range of ecosystems.
Tropical forests provide an excellent ecosystem in which to explore these ideas since over half the tropical moist forest cover worldwide has been reduced to less than 50% tree cover (Asner et al., 2009), which has contributed to extensive loss of biodiversity and greater than 12% of global carbon dioxide emissions (van der Werf et al., 2009). Although deforestation has been the dominant process in tropical forest, during the last 20 years there has been a substantial increase in tropical secondary forest due to primarily passive restoration (i.e. natural regeneration), but also to active restoration (Lamb et al., 2005, Wright and Muller-Landau, 2006, Chazdon, 2008b).
Section snippets
Conceptual framework
The decision of which restoration strategy should be employed in a degraded system depends on the natural rate of recovery and the desired endpoint for the ecosystem (Fig. 1). The rate of recovery is affected by the intrinsic ecosystem resilience (defined as the degree and pace with which an ecosystem recovers the initial structure and function following disturbance, sensu Westman, 1978), the level of human degradation, and the characteristics of the landscape around the focal area (Fig. 1).
Applying the framework to restoration decisions
In order to apply our framework to decision-making we recommend that managers begin by clearly identifying the goals of the restoration project, assessing the resources available, and answering the three questions we discuss below. This process will certainly be iterative, as the goals may need to be modified to fit the available resources.
Conclusions
Our framework does not provide thresholds where there is a clear yes or no answer to land managers about when and how to intervene to facilitate recovery. While this vagueness is frustrating, we argue that yes/no answers do not reflect the ecological and social realities of most degraded sites. We advocate that if managers consider the questions we have outlined both during the planning process, as well as over time, monitoring results and using this information to implement an adaptive
Acknowledgements
We thank the Holl lab group, Martha Bonilla, and an anonymous reviewer for their comments on previous versions of this manuscript.
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