The review frames the problem against a backdrop of steady habitat decline. Human development has been changing, destroying and simplifying the planet’s vegetation cover, splitting and reducing the area of natural habitat and transforming the landscape into a mosaic of human settlements, agricultural land and isolated fragments of remaining forest. Climate change adds a second layer of pressure, because it is expected to shift the distribution of ecosystems and species, with forests especially affected. Forests act as sinks that absorb carbon from the atmosphere and store it in biomass and soils, but when they are cleared or degraded they become a major source of greenhouse gas emissions.

In 1990 the world had 4,128 million hectares of forest; by 2015 that area had fallen to 3,999 million hectares, a change from 31.6 percent of the global land area being forest to 30.6 percent. Africa and South America experienced the highest net annual loss of forests between 2010 and 2015, although the review notes that the loss compared with the previous five years had been mitigated. Beyond the outright loss of area, the remaining forests have been intensely fragmented, with continuous forest broken into smaller fragments of varying isolation by human activity, natural factors and, in particular, forest fires.

Fragmentation carries consequences for wildlife. When the area of natural habitat decreases, both the richness and abundance of species are reduced and the rate of local extinctions increases, the review reports. The fragmentation of temperate forests, for example, has affected the diversity and abundance of birds living in those ecosystems. Isolation can also reduce the exchange of individuals between populations in different fragments, a problem for species that require continuous forests across different altitudes. In forest management, adjacent or nearby protected wild areas are rarely placed under a common management plan that would make their protection more effective, which is why the review identifies the maintenance of land as reserves of natural resources and biodiversity as a key management objective and a prerequisite for sustainable forestry.

Biological corridors were proposed as a response to this problem in 1975, when Wilson and Willis first put the idea forward. Since then, corridors have become a widely discussed option for confronting the degradation of global biodiversity. The review explains that the creation of corridors is intended to serve multiple objectives, including biodiversity conservation and the use of ecosystems’ environmental services, allowing the management of natural resources to be integrated with conservation. Yet the authors also flag a persistent difficulty: the concept has been used interchangeably with the use of land and its management, which is precisely why they set out to distinguish the different approaches found in the scientific literature.

Three definitions stand out in the review. The first describes a corridor as a territorial extension of different sizes and shapes whose main function is to connect protected areas, allowing the migration and dispersal of wild flora and fauna. The second defines it as a geographical space that provides connectivity between landscapes, ecosystems and natural or modified habitats, ensuring the maintenance of biodiversity and of ecological and evolutionary processes, and whose presence is considered essential in mitigating the effects of fragmentation. The third is broader and more social: a priority territory composed of private farms and state protected areas that aims to improve the quality of life of communities inside and outside the corridors through education in conservation and the sustainable management of biodiversity. All three concepts are tied to the sustainable management of natural resources on public or private territories, with the shared aim of preventing vulnerable species from living in confined and inadequate spaces through integrated management that avoids biodiversity loss within a framework of economic, sustainable and social development.

The design logic behind corridors rests on a widely held assumption, one supported by a large body of literature the review cites: that fragmented territories, when united by a corridor, see a decrease in the extinction rate of the species inhabiting them, because corridors reduce levels of inbreeding, support gene flow among animals and plants, and reduce the vulnerability of the landscape. The review stresses, however, that the effectiveness of a corridor is not due to its design alone but mainly to its proper functioning, framed in plans and programs of different scope and scale across a wide territorial basis. In other words, merely protecting areas within a corridor’s frame will not solve the problem if the corridor ultimately does not fulfil its functions, including the sustainable use of its resources.

In several countries, the review observes, the delimitation of protected areas has been assumed to be the only strategy for conserving ecosystems and their biodiversity. Taking habitat fragmentation and adjacent critical connectivity into account, the authors argue that the objective of a biological corridor should be the preservation and increase of an area’s forest cover. Management plans implemented in such an area would allow planning of the sustainable use of the soil, provide new knowledge about resources, create more stable environmental conditions, improve the resilience of ecosystems to disturbances, promote the diversification of ecosystem services, and include adequate silviculture for woodlands. Characterizing habitats for different groups of fauna species, particularly endemic ones, is described as a primary task, and corridors should be designed and evaluated with long-term monitoring that combines the economic development of human populations with the preservation of nature.

Corridors have been implemented progressively in recent decades, and Costa Rica stands out as the country with the most approved corridors. The review lists the Cusingos-Las Nubes corridor, also known as the Alexander Skutch corridor, along with Amistosa, El Quetzal Tres Colinas, Fuente de la Vida La Amistad, Río Cañas and Fila Anguciana, as well as the Mesoamerican Corridor shared with Mexico and other Central American countries. Other examples cited include the Monteverde corridor in Costa Rica, the Guadiamar Corridor in Doñana, Spain, the Geneva Biological Corridor in Switzerland, the Noé Corridor in Australia, and the mountain ecological corridor in Chile’s Metropolitan Region. In these cases, national policies consist principally, if not only, of interconnecting protected wild areas to safeguard groups of endemic or vulnerable species. Elsewhere, some studies design corridors by associating vegetation and wildlife habitat, using the composition, structure and environmental factors of the habitat, together with its potential conditions, to define the corridor’s design, since the requirements and effects of connectivity depend heavily on how each species uses different types of landscape. The underlying idea is that only large protected areas, properly interconnected, maintain genetic exchange and therefore conserve biodiversity in the long term.

On monitoring, the review finds that corridors established so far have been designed for simple connectivity of ecosystems, without identifying the habitat characteristics that would allow a management plan tailored to the requirements of particular species. The authors distinguish two types of corridor: those whose objectives are to preserve the richness of fauna and flora together with the development of local human populations, and those whose objectives are to preserve, restore or connect as many fragments as possible of original ecosystems that limit the effects of highly developed human activities. For both groups, environmental education is highly recommended. Beyond education, the review calls for indicators that help track the level of achievement of the objectives for which a corridor was designed. Investigations have proposed frameworks using principles, criteria and indicators, including actual and potential habitat use, biodiversity indices, and the growth rates of fauna and flora populations, but the systematization of results and data for the appropriate management of the ecosystems included in corridors remains low. The authors argue that delimiting the scale of items to be evaluated is important to ensure the effectiveness of corridors.

With respect to forest management, the review describes how adequate zoning of the areas involved in a corridor’s design can allow flexible forestry practices, so that the management of forests and the production of goods and services achieve different conditions of soil cover according to the habitats and species present, particularly those in a degree of vulnerability. It is important, the authors note, that these activities should not alter the reproductive cycles of the species, and the lack of conjunction between uses and activities is a subject that must be improved in forest management. Among the main gaps in the literature, the review identifies a lack of forest management through adaptive silviculture as an option to meet both conservation and development goals, since in protected areas interconnected by corridors, forest management can help accomplish ecological and socioeconomic objectives through flexible silvicultural programs adapted to legal and biophysical conditions. The review’s conclusions emphasize that although the biological corridor is a relatively modern concept with many definitions depending on objectives, scale and context, achieving corridor goals requires a correct diagnosis of the factors involved in the territory to be preserved, and that monitoring of results with adequate indicators is necessary so that the biodiversity conservation strategy is continuously fed back.

Subject of Research: A review of the concepts, management and monitoring of biological corridors for biodiversity conservation in forest ecosystems.

Article Title: What about biological corridors? A review on some problems of concepts and their management

Article References: Moreno, R., & Guerrero-Jimenez, C. J. (2019). What about biological corridors? A review on some problems of concepts and their management. BioRisk, 14, 15-24. https://doi.org/10.3897/biorisk.14.32682

Image Credits: AI Generated

DOI: 10.3897/biorisk.14.32682

Keywords: biological corridors, biodiversity conservation, forest fragmentation, habitat connectivity, sustainable forest management, adaptive silviculture, protected areas, Costa Rica, habitat loss, climate change, wildlife corridors, ecosystem monitoring