The effect is to limit or deny access to food supplies and areas of shelter. Potentially, it affects breeding habits as well. Multiplied over a larger area, this effect can drive species to extinction unless a reserve habitat area is created for them - the protected "island" in the theory.
This assumption draws a sharp line between the biodiversity in the human-influenced and natural habitats - and the theory has worked relatively well in certain areas. The theory also leads to dire predictions for planet Earth, with an inevitable clash between the resources required to feed and support an estimated 9 billion people by 2050 and the species that are displaced by human interaction.
However, a research team from Stanford University suggests in a recent study that the dividing line may not be so sharp, and the predictions may be too dire. Their study was published in the latest issue of Nature.
According to the team, the island theory doesn't always hold true because the island assumption itself doesn't always hold true. The island theory was developed using literal islands in lake or ocean environments, and does not take into account the alternate pathways available within land-based habitats. The researchers suggest that a more holistic approach to management of farmland and habitats can better balance the needs of both humans and wildlife, and keep up the levels of biodiversity.
To test these approaches, the group studied a variety of bats that are extremely sensitive to loss of a forest habitat. For contrast, the bats were studied in a mixed area of coffee plantations and forest located in Costa Rica and also in Panamanian islands located within a lake.
The theory performed as expected on the island population of bats, but it did a poor job of predicting the Costa Rican results. The island theory predicted that the coffee plantations could not support any bat species - but the actual reduction in species was minimized. Forested areas and reserves were able to support 23-28 bat species, while coffee plantations supported 18 species.
This finding should not imply that loss of any bat species, or any species at all, is trivial. However, it does suggest that biodiversity can be achieved in cooperation with development by using proper management techniques. Instead of isolating and dividing, we may be able to maintain biodiversity by integrating wildlife and development needs.
The authors noted that not only can the island theory predict excessive extinction, it can also fail to take into account novel species that do well in the altered environment. The overall message is that both humans and wildlife can benefit from a new theory that takes into account adaptability in land-based habitats.