Comparative Method

Hypothesis: Airborne pesticides from agricultural fields and urban air pollutants are contributing to the decline of amphibian populations.

PREDICTION If pesticides and urban air pollutants are factors in amphibian population declines, populations close to and downwind from agricultural and urban areas should have decreased more strikingly than populations upwind and farther away from those sources of air pollutants.

Census (count) and then compare persistence of populations of species of amphibians at suitable habitat sites that lie upwind and downwind of major agricultural and urban areas.

Populations of some species, as illustrated here by Rana aurora, persist in areas upwind of or remote from sources of urban and agricultural pollutants, but this amphibian is largely absent from areas close to or downwind of air pollution sources. (Distributions of three other species of Rana were similar to that of R. aurora.)

• Rana aurora present

• Rana aurora absent

—»- Average wind direction | Agriculture Urban area

Populations of some species, as illustrated here by Rana aurora, persist in areas upwind of or remote from sources of urban and agricultural pollutants, but this amphibian is largely absent from areas close to or downwind of air pollution sources. (Distributions of three other species of Rana were similar to that of R. aurora.)

• Rana aurora present

• Rana aurora absent

—»- Average wind direction | Agriculture Urban area

Upwind San Francisco

Greater Los Angeles

Plasma Membrane Images

San Diego

Upwind San Francisco

Greater Los Angeles

San Diego

Conclusion: Airborne agricultural pesticides and urban air pollutants are contributing to declines in populations of some amphibian species.

Figure 1.10 Using the Comparative Method to Test a Hypothesis The effects of human-generated airborne pollutants on amphibian populations can be assessed by determining whether species persist in, or are absent from, suitable habitats that lie upwind or downwind from sources of airborne pollutants.

phibians respond in exactly the same way to changes in the environment. In their responses to environmental changes, amphibians are like most living things. They live in complex and changing environments, and they interact with many other species.

Simple explanations that account for everything should not be expected or trusted. Its complexities make biology a difficult science, but they also make it exciting and challenging.

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