When habitats are destroyed or made uninhabitable, the organisms living there are lost. But habitat loss affects even those remaining habitats that are not destroyed. As habitat fragments become smaller and more isolated, the fragments become more disturbed by "edge effects," in which an ecological community is changed by physical and biological factors coming from an adjacent community.
In 1979, researchers initiated a study on habitat patches in the rainforest north of the city of Manaus, Brazil, which highlighted the importance of edge effects. The rainforest in the study area was continuous at the time, but was scheduled to be logged. One of the key findings of the research was that the smaller fragments were disturbed to a greater extent by surrounding influences than were the larger fragments.
Biologists counted the species in the future fragments while the areas were still part of the continuous forest. Soon after the surrounding forest was cut and converted to pasture, species began to disappear from isolated fragments.
The first species to be eliminated were monkeys that travel over large areas. Army ants and the birds that follow army ant swarms also soon disappeared. Small fragments cannot maintain populations of species that require large areas, and they can support only small populations of many of the species that can survive in small fragments.
In addition, in a phenomenon known as the edge effect, physical and biological factors outside the fragment influence the life at the edge of the fragments. In this example of a relatively large fragment, roughly 30% of the fragment is influenced by edge effects.
Close to the edges of forest fragments, the areas have stronger winds, higher temperatures, lower humidity, and higher light levels than farther inside the forest. Additionally, species from surrounding habitats often colonize the edges of fragments to compete with or prey on the species living there.
The smaller the fragment of habitat, the greater the proportion of the fragment that is influenced by the surrounding environment. Tree species adapted to the dark, humid forest interior die at the edge of the fragments, opening the way for the growth of pioneer tree species. Smaller fragments typically show the greatest change in forest composition.
The Manaus, Brazil, research project is important because researchers were able to assess which species lived in the forested areas before the surrounding trees were logged. Soon after the surrounding forest was converted to pasture, the researchers could assess that some of the original species were beginning to disappear from isolated forest fragments.
On a positive note, some of the pastures that surrounded the experimental forest fragments in Brazil have been abandoned, and young forests are now growing in them. Within 7–9 years of abandonment, army ants and some of the birds that follow them recolonized forest fragments that were connected to larger forest fragments by young forests. Other species of birds that forage in the forest canopy also reestablished themselves. The young forest is not a suitable permanent habitat for most of these species, but they can disperse through it to find good habitat.