Saving What Remains


April 1, 2012

As forests are set aside as reserves, usually in the regions of the highest diversity, the question of reserve size comes into play. Obviously as much land as possible should be protected to some degree, but whether to keep a single large reservoir or several small reserves has been a controversial issue in conservation biology over the past two decades. Bitter fighting between the two camps in the SLOSS debate (single large or several small) has resulted in squandered time, money, resources, and credibility, and has divided groups that should be united in saving the planet's environment. A single large reserve is advantageous because it possesses larger populations of each species and a more stable environment. On the other hand, a single large reserve is subject to devastation by a single catastrophic event like a fire, flood, or disease. Breaking the reserve into separate pieces reduces the risk of complete population loss by a single event, but diminishes the size of the species populations and puts them at a higher risk of extinction. In addition, if the reserve is too small it may experience system decay resulting in the loss of many species. Small reserves are particularly affected by the invasion of alien species. Studies have shown that domestic mammals will venture up to three miles (5 km) into the rainforest, not only introducing disease and alien plant seeds, but also eating eggs, destroying nests, and crushing seedlings. Finally many species require a certain threshold-population size or range to persist.

Large reserves protect a larger area (Species—Area Math), including varied habitats, like eco-tones, forest edges, interior clearings, swamps, and ridges, which mean more niches, and hence greater diversity. It is important to preserve such zones, which both provide for and produce biodiversity.

Several studies have demonstrated the effects from a reduction in reserve size, including two famous projects in Latin America: Barro Colorado Island, Panama, and Thomas Lovejoy's experiment in the Brazilian Amazon (Are Forest Fragments Worth Saving?).

Barro Colorado Island was once a forested hilltop amid a rich tropical rainforest. When the Panama Canal was constructed, the Chagres river was dammed and the valley was flooded, leaving the hilltop an island of six square miles of forest. Barro Colorado Island was declared a biological reserve in 1923 and since has been the center of intensive research (since 1946 the island has been a research site run by the Smithsonian Institution). Over the last seven decades, researchers have recorded profound changes in the animal population. Large predators like the jaguar, puma, and harpy eagle were the first to go. Without large predators, mammals like pacas, agoutis, peccaries, and coatimundis populations skyrocketed to levels 2-10 times their normal concentration. However, by 1970, 45 birds species had disappeared due to the increased omnivore population, the loss of niches like meadows and forest edges, and the loss of area. Today toll has climbed to 65 bird species lost since the island's formation. Similar results have been recorded on islands created by hydroelectric projects in Thailand (Chiew Larn Hydroelectric Reservoir) and Venezuela (a reservoir created by the Guri dam).

To avoid further conflicts and help mitigate the problems with reserve size, some biologists have suggested a compromise solution, which is to create a series of small reserves connected by corridors of forest. This set-up would allow migration between the sections, but help protect against a mass die-off caused by a single event. Corridors are especially important should global warming occur, since species must be able to migrate as the climate changes. But more research is required to find the optimal reserve size and layout for sustaining the most biodiversity.

Sign warning that a tree is spiked with metal to discourage logging in West Kalimantan, Indonesian Borneo. Click image for more pictures of West Kalimantan. Photo by Rhett A. Butler

Review questions:

  • How does reserve size or area impact levels of biodiversity?

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Continued / Next: Valuing the Rainforest