Every time we clear land for roads or farms, build subdivisions, or excavate a mine, we break up natural ecosystems. Habitat fragmentation transforms continuous ecosystems into smaller, increasingly isolated patches of habitat, or habitat fragments. A patch of forest surrounded by a large area of land cleared for cattle pasture is a typical example. Habitat fragmentation affects the entire community of organisms and can reduce species richness because the populations of species dependent on the remaining patches of native ecosystem are generally reduced and their long-term survival threatened. Like islands, the smaller the fragment, the fewer species they can support.

In addition to the 90,000 km² of tropical forest that is cleared annually, some 20,000 km² are not entirely destroyed each year but rather damaged by logging, burning, and road building, resulting in habitat fragmentation (Figure 4.19). For the last three decades, the Biological Dynamics of Forest Fragments Project, a cooperative effort by Brazilian scientists and others from all over the world, has been investigating this phenomenon. Within a 1,000-km² study area, researchers established forest fragments of various sizes to observe and study. Each fragment was surrounded by land that had been cleared of forest to establish cattle pasture (Figure 4.20).The controls for this research are similar-sized forest plots completely surrounded by and connected to large expanses of forest (more than 200 kilometers in extent).

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The results of the study, which have been reported in hundreds of scientific papers, are clear: Compared with larger fragments, smaller forest fragments support fewer species of large mammals, primates, understory birds, dung beetles, ants, bees, termites, and butterflies. Most significant, forest fragments of all sizes support fewer species of forest animals compared with similar-sized areas embedded in continuous forest (Figure 4.21). This study and many others around the world have demonstrated repeatedly that habitat fragmentation results in the loss of biodiversity.

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When considering the size of an isolated fragment of forest, it’s important to remember that the newly created edge of that patch is different from the interior. Edges may be windier, drier, and hotter and might be subject to more insect pests. These edge effects often extend deep into the patch of forest. In the Amazon River Basin, 30 different edge effects have been identified, extending as far as 400 meters (1,300 feet) into forest fragments (Figure 4.22). While some species thrive where fragmentation produces edge effects, those that require the physical conditions of the interior of an ecosystem (e.g., higher humidity or lower sun exposure) do not. Consequently, as edge conditions proliferate with increasing fragmentation, these “interior species” decline in abundance and at some point disappear from the landscape.