The science of conservation biology was developed during the seventies and eighties, when society started to become aware of the importance of addressing the ongoing biodiversity crisis. The multidisciplinary basis of conservation biology has led to new subdisciplines, such as a theoretical branch and a subdiscipline to find solutions to practical problems. Conservation biology is considered a crisis science and has two specific goals: to investigate the human activity that affects diversity and to develop strategies to prevent species extinction.
Conservation biology is currently facing a major global issue: an unprecedented acceleration in the extinction rate of species. All studies have drawn similar conclusions: 20,000 to 30,000 species go extinct every year (or 50 to 80 species per day).
Species go extinct for many different reasons. While there may be biological causes that affect a species in a specific context (such as a disease outbreak or competition with other species), most extinctions are caused by human activity; some examples are overexploitation or the introduction of non-indigenous species. However, edge effects and habitat fragmentation are, by far, the main causes of species extinction.
What are edge effects?
Edge effects are the result of an abrupt transition between two significantly different natural habitats that are adjacent to each other in the same ecosystem. In essence, it is a break in continuity between two adjacent habitats, leading to changes in the environmental and biological conditions. Edge effects may appear, for example, as a consequence of the construction of a highway or of a forest fire. As mentioned previously, this is one of the main causes of species extinction.
Not all habitat edges present the same characteristics, as they may have significant differences, depending on the ecosystem and the conditions under which they have appeared. In this sense, there might be a smooth transition area between two adjacent habitats, which allows for a higher biological circulation (movement of species between habitats) and a lower flow of physical variables (such as the existence of a lower temperature gradient). On the other hand, more “abrupt” edges present less species exchange between habitats and a higher flow of physical variables, which is detrimental to species stability.
In any case, habitat edges are not the ideal environment for plant and animal species, as they present adverse effects, such as stronger insolation, the alteration of light cycles, noise, drastic temperature and humidity fluctuations, etc. Therefore, organisms will try to settle in one of the adjacent habitats, where conditions are more homogenous.
Habitat fragmentation and the occurrence of edge effects
Edge effects are usually linked to habitat fragmentation, destruction or degradation. When habitat fragmentation occurs, the perimeter of a habitat increases, creating new borders and increasing edge effects. Additionally, fragmentation breaks habitat continuity, reducing reproductive success, genetic exchange and, therefore, reducing genetic diversity in species.
Generally, when a habitat is fragmented, it breaks up into smaller areas. This is yet another disadvantage, as large areas have more species than when the same space is divided into several small habitats. This is because large habitats present homogeneous environments and, in small areas, inbreeding (the breeding of individuals or organisms that are closely related genetically) and genetic drift are more common. In fact, certain species cannot live in small habitats, such as migrating species, which require large territories and are often affected by edge effects.
Fragmentation is a common phenomenon around the world, but the extent of its impact varies. For example, equatorial habitats, which are already less densely populated per se, present a significant decline in the number of individuals and species when fragmentation occurs. Furthermore, these environments have higher endemism rates – more species with restricted distribution –, so their species will be more affected by any type of fragmentation or degradation of their habitat. Therefore, the impact of habitat fragmentation is greater in certain environments.
Other consequences of edge effects and fragmentation
The environmental and habitat alterations caused by edge effects and fragmentation favour the appearance of new species that are able to adapt to disruption. These species are usually invasive; they settle in a new area and naturalise successfully (meaning that they are able to reproduce and maintain their population), competing for resources with the other species. Invasive species are detrimental to the natural environment, as they can cause severe damage: the migration of indigenous species, hybridisation, etc.
New areas of research
Habitat degradation and loss, which are caused by fragmentation and edge effects, are behind 30% of all species extinctions. These processes especially affect sensitive organisms, such as amphibians or birds. Additionally, they lead to loss of genetic variability, which may ultimately make a species go extinct.
The viability of fragmented habitats and of the species that live in edge areas is currently being studied. An example of such research is the study of metapopulations: groups of populations that live in “patches” of fragmented habitats and that are connected through biological corridors and/or the migration of their individuals. Such research might allow conservation biology to put a halt mass extinction, while there is still time.
Translated by Carlos Heras