53

key concepts

53.1

Ecology Is the Study of the Interrelationships among Organisms and the Environment

53.2

Global Climate Is a Fundamental Component of the Physical Environment

53.3

Topography, Vegetation, and Humans Modify the Physical Environment

53.4

Biogeography Is the Study of How Organisms Are Distributed on Earth

53.5

Geographic Area and Humans Affect Regional Species Diversity

The Largest Experiment on Earth

Most people know that there are many species on Earth, and that new species are being discovered every day, but not everyone knows that roughly half of all species worldwide reside in one place: the tropical rainforests of the Amazon Basin. The statistics are staggering: an estimated 390 billion trees representing 16,000 species grow in Amazonia, one in five bird species globally resides there, and one-fifth of all fresh water falls on its slopes and valleys. The Amazon Basin is by far the largest watershed on Earth, with thousands of tributaries leading to the Amazon River and then out to sea. To date, 2,200 freshwater fish species have been described, which is more than all the fish species in the Atlantic Ocean.

It is reasonable to assume, then, that when the Amazon Basin is threatened by human activities, so, too, is global biodiversity. By far, the main destructive force is deforestation, which began about 50 years ago. As roads pushed their way into Amazonia’s forests, more rainforest was logged and converted to agriculture and settlements. It is estimated that nearly 20 percent of the Amazon Basin has been clear-cut. Because soil fertility is generally poor in tropical rainforests, the cleared land is often useful only for a decade or less, resulting in abandonment and new cycles of deforestation and fragmentation.

How does habitat fragmentation affect the species living in the Amazon Basin? Fragmentation makes habitats smaller, and it isolates populations. Deforestation forces species to sustain themselves in a smaller area or move to more suitable areas. In the late 1970s, some ecologists asked a deceptively simple question about Amazon deforestation: What is the minimum area needed to maintain species diversity within a rainforest fragment? They wanted to know whether the species remaining in the forest fragments could maintain themselves there, whether the sizes of the forest fragments mattered to this maintenance, and whether species would venture from one fragment to another. These ecologists conducted a large and long-running experiment in the Amazon forests near Manaus, Brazil. As you will see in Key Concept 53.5, the size of the fragments and the ability of species to move across the deforested landscape are key to maintaining the extraordinary diversity left in the Amazon Basin.

How do geographic area and isolation affect the biogeography of life on Earth?