Rehabilitating degraded lands
The rehabilitation of degraded lands and the restoration of ecosystems have become increasingly important elements of resource conservation throughout the world. Rehabilitation, which aims to revive important ecological services on degraded lands, is becoming particularly important in mountainous regions, arid lands, and irrigated crop lands. In mountainous areas, the loss of forest and other vegetative cover has often increased soil erosion. Arid lands have suffered declining soil fertility and increased erosion as a result of agriculture and overgrazing. Resource productivity has declined on an estimated 80 percent of rangelands and 60 percent of croplands in arid regions of developing countries, and irrigated croplands have been degraded by salinization, waterlogging, and alkalinization.
Restoring degraded lands
More ambitiously, restoration attempts to bring lands modified by human use back to their natural state. Because determining the “pre-disturbance” state of most ecosystems is difficult and because ecosystems continually change, complete restoration is rarely a realistic goal. But the approximate re-creation of natural communities is becoming central to efforts to maintain biodiversity and restore important ecological services.
Physical and biological interventions
Both restoration and rehabilitation make use of physical and biological interventions. Physical means include drainage systems in water-logged lands and check dams or contour plowing to slow erosion rates, while biological interventions include growing grasses to slow erosion, nitrogen-fixing trees to increase the nutrient content of soils, drought-adapted trees for hillside reforestation, and so forth. Rehabilitation often makes use of exotic species, since the primary goal is to restore critical ecological services rather than the natural community. In contrast, restoration attempts to restore the natural complement of species.
Ecosystem restoration does not always requires interventions, however. Left to natural processes, many ecosystems will return to something like their pre-disturbance condition if populations of the original species still exist nearby. How long natural recovery takes depends upon the type of ecosystem and the type of disturbance. In Brazil’s caatinga forest, natural recovery of slash-and-burn agricultural sites requires more than a century, but sites cleared by bulldozer may take 1,000 or more years to recover. Similarly, sites in tropical lowland wet and dry forest require an estimated 1,000 and 150 years, respectively, to recover from timber harvest.
If an ecosystem has been physically transformed or if pre-disturbance species cannot disperse to the site, natural processes alone won’t restore it. Numerous lakes throughout the world and the prairies of central North America have been fundamentally changed by introduced species and can’t return to their natural states unless the exotic species are removed. Elsewhere, soil erosion, salinization, or the loss of mycorrhizal mutualists has changed ecological systems so radically that native species can’t become established without such interventions as seeding, planting, inoculation of soils with mycorrhizal fungi, and weed, fire, or predator control.
But even where communities and ecosystems might naturally revive after disturbance, restoration technologies can speed recovery. In recent years, for instance, the recovery (and creation) of numerous coastal salt marshes has been significantly accelerated by planting common salt marsh species.
Restoration ecology falls in an “economic and psychological no-man’s land, appealing to neither agricultural interests nor to environmentalists concerned with the preservation of pristine natural areas.” Yet, restoration ecology can enhance in situ conservation. Where existing protected areas are too small to maintain certain species, ecological restoration can be used to enlarge areas of natural habitat or to establish conservation corridors between reserves.
Restoring Guanacaste National Park Ecosystem
The planned 800-km2 Guanacaste National Park in Costa Rica will be created largely through ecological restoration. The park is located in a region formerly covered by tropical dry forest–the most threatened of all major lowland tropical forest habitats. Some 700 square kilometers is now being restored from pasture and forest fragments. This restoration will help save many of the dry forest species from extinction, boost the local economy, and help re-establish the cultural ties of the people to their land. The restoration will rely primarily on natural processes because remaining remnants of forest can provide seeds to the restored site; however, to speed the process, fires will be suppressed and some cattle may be allowed to graze initially since reducing grass height enhances tree seedling survival.
In Guanacaste Park, the remaining fragments of natural communities are big enough that the region’s native species are still present. But in many restoration programs, in situ and ex situ conservation efforts must be closely coordinated. Populations of endangered species are increasingly being maintained in captivity only until suitable wild habitat has been restored. As part of the recovery program for the red wolf in the United States, an effort was made in the mid-1970s to capture all of the remaining wild wolves. Captive breeding populations were then established while scientists looked for suitable habitat for the wolves in their natural range. Since 1986, the wolves have gradually been returned to one of these habitats.
Restoration and Rehabilitation Tools and Priorities
Just as restoration helps conserve biodiversity, rehabilitation and restoration programs requires the very species that biodiversity conservation seeks to maintain. For instance, rehabilitation projects are benefiting from the more widespread use of species that can grow under stressful environmental conditions, and both restoration and rehabilitation draw upon species maintained in protected areas, zoos, botanic gardens, and seed banks to establish populations.
This current focus on stressful environmental conditions is not misplaced. As the threat of climate change to biodiversity grows, ecological restoration and rehabilitation will become increasingly important. Along with habitat fragmentation, rapid climate change will prevent many species from migrating to more suitable climates. Increasingly, restoration techniques will be used to introduce species to new ranges and to accelerate the re-establishment of such communities.
