Humans exert a powerful influence on biogeographic patterns

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The theory of island biogeography has provided surprising insight into the role human’s play in regional biogeographic patterns. As you saw in Key Concepts 53.3 and 53.4, Earth’s biomes are becoming smaller and more fragmented by local- and regional-scale forces such as agriculture, deforestation, and urbanization. They are being transformed into islandlike habitats—isolated patches of suitable habitat such as parks or forest fragments, surrounded by extensive areas of unsuitable habitat similar to the cattle pastures of Amazonia described at the opening of this chapter.

The fragmentation of the Amazon rainforest led Thomas Lovejoy and his colleagues to initiate, in 1979, one of the largest and longest-running ecological experiments ever conducted to investigate the effects of tropical forest fragmentation on species diversity (Investigating Life: The Largest Experiment on Earth). The Biological Dynamics of Forest Fragments Project (BDFFP) took advantage of an existing Brazilian law that required landowners who cut rainforest to leave half of it untouched. Lovejoy was able to survey the species diversity in different-sized fragments (either 1, 10, or 100 hectares [ha] in size surrounded by deforested land). Among a number of results, his group found that, after more than 30 years of isolation, even the largest deforested fragments surveyed (100 ha) had lost half of their bird species diversity. The researchers were also surprised by two additional findings. First, even minimal distances of 80 meters between fragments resulted in strong avoidance of the clearings, and thus isolation, by birds, insects, and tree-dwelling mammals that lived in the fragments. Second, the fragmentation exposed species within a fragment to a variety of potential hazards, including extreme heat, fires, hunting, predators, diseases, and invasive species. These so-called edge effects not only reduce the immigration rate from one fragment to another, thus contributing to a smaller species pool, but they effectively reduce the size of the fragment, thus increasing the extinction rate, by making the edge of the fragment much less hospitable habitat.

investigating life

The Largest Experiment on Earth

experiment

Original Paper: Ferraz, G. et al. 2003. Rates of species loss from Amazonian forest fragments. Proceedings of the National Academy of Sciences USA 100: 14069–14073.

Thomas Lovejoy and his colleagues asked what was the minimum area needed to maintain species diversity in the rainforest fragments created by logging near Manaus, Brazil. They conducted an experiment, starting in 1979, that took advantage of an existing Brazilian law that required landowners who cut rainforest to leave half of it untouched. In 2003, Ferraz and colleagues reported on one particular aspect of the experiment: the number of forest understory birds living in different-sized forest fragments surrounded by deforested land.

image

Animation 53.4 Edge Effects

www.Life11e.com/a53.4

work with the data

The data in the graph shown in the experiment can be used to calculate a scaling factor for the time it takes to lose half of the bird species within a fragment. The scaling factor shows that to increase the t50 (i.e., the time it takes fragments to lose half their species) by 10-fold, fragment area would need to increase 1,000-fold. Here you will use this scaling factor and other data provided in the graph to determine the area needed for the conservation of understory birds in the Amazon forests outside Manaus, Brazil.

QUESTIONS

Question 1

Graph the initial numbers of bird species by the size of the fragment area. Do these fragments follow the species–area relationship?

Yes, there is a positive relationship between number of bird species and size of the fragment area.

Question 2

Suppose you are consulting for the Brazilian government on the conservation of rainforests near Manaus. Assuming that the t50 for 1 hectare is 5 years, what is the minimum size that a rainforest fragment in the Manaus area needs to be to ensure that half of the bird species remain 50 years after the deforestation event that isolated them? How about a century after deforestation?

The scaling factor shows that to increase the t50 (i.e., the time it takes fragments to lose half their species) by 10-fold, the fragment area would need to increase 1,000-fold. Thus, if t50 for 1 hectare = 5 years, the fragment would need to be 1,000 hectares to ensure that half the species would remain after 50 years. The fragment would need to be 10,000 hectares to ensure that half the species would remain after 100 years.

Question 3

Suppose it takes 100 years for rainforests to fully recover after a deforestation event in the Manaus region. Given that the average size of a rainforest fragment in this area is no bigger than 1,000 hectares, will the number of species decline by half before the recovery of rainforest occurs around the largest fragments?

Yes, because the average area would need to be 10,000 hectares to ensure that half the species would remain after 100 years.

A similar work with the data exercise may be assigned in LaunchPad.