In addition to such technical tools, scientists use a variety of conceptual tools to help them answer questions about nature. The method that underlies most scientific research is the hypothesis-prediction (H-P) approach. The H-P approach allows scientists to modify their conclusions as new information becomes available. The method has five steps:
1. Making observations
2. Asking questions
3. Forming hypotheses, which are tentative answers to the questions
4. Making predictions based on the hypotheses
5. Testing the predictions by making additional observations or conducting experiments
If the results of the testing support the hypothesis, it is subjected to additional predictions and tests. If they continue to support it, confidence in its correctness increases, and the hypothesis comes to be considered a theory. If the results do not support the hypothesis, it is abandoned or modified in accordance with the new information. Then new predictions are made, and more tests are conducted.
Hypotheses are tested in two major ways Tests of hypotheses are varied, but most are of two types: controlled experiments and the comparative method. When possible, scientists use controlled experiments to test predictions from hypotheses. That is what Pieter Johnson was doing when he hatched frog eggs in the laboratory. He predicted that if his hypothesis—that the parasite Ribeiroia caused deformities in frogs—was correct, then frogs raised with the parasite would develop deformities and frogs raised in the absence of the parasite would not. The advantage of controlled experiments is that all factors other than the one hypothesized to be causing the effect can be kept constant; that is, any other factors that might influence the outcome (such as water temperature and pH in Pieter's experiment) are controlled. The most powerful experiments are those that have the ability to demonstrate that the hypothesis or the predictions made from it are wrong.
But many hypotheses cannot be tested with controlled experiments. Such hypotheses are tested by making predictions about patterns that should exist in nature if the hypothesis is correct. Data are then gathered to determine whether those patterns in fact do exist. This approach is called the comparative method. It is the primary approach of scientists in some fields, such as astronomy, in which experiments are rarely possible. Biologists regularly use the comparative method.
A single piece of supporting evidence rarely leads to widespread acceptance of a hypothesis. Similarly, a single contrary result rarely leads to abandonment of a hypothesis. Results that do not support the hypothesis can be obtained for many reasons, only one of which is that the hypothesis is wrong. For example, incorrect predictions can be made from a correct hypothesis. Poor experimental design, or the use of an inappropriate organism, can also lead to erroneous results.
We will now show how the H-P method was used by other researchers to investigate the larger question that concerned Pieter Johnson: Why are amphibian populations declining dramatically in many places on Earth?
step 1: making observations. Amphibian populations, like populations of most organisms, fluctuate over time. Before we decide that the current declines are different from "normal" population fluctuations, we first need to establish that they are unusual. To assess whether the current declines are unusual, an international group of scientists has been gathering worldwide data on amphibian populations. The group's data show that amphibian populations are declining seriously in some parts of the world, especially western North America, Central America, and northeastern Australia, but not others, such as the Amazon Basin. Their data also show that population declines are greater in mountains than in adjacent lowlands. These scientists also discovered that no data on population trends in amphibians are available from Africa or Asia.
step 2: asking questions. Two questions were suggested by these observations: Why are amphibian declines greater at high elevations? Why are amphibians declining in some regions, but not in others?
steps 3 and 4: formulating hypotheses and making predictions. To develop hypotheses about the first question, scientists first identified the environmental factors that change with elevation. Temperatures drop and rainfall increases with elevation worldwide, and in temperate regions, summer levels of ultraviolet-B (UV-B) radiation increase about 18 percent per 1,000 meters of elevation gain. One hypothesis is that declines in the populations of some amphibian species are due to global increases in UV-B radiation resulting from reductions in atmospheric ozone concentrations. If increased levels of UV-B are adversely affecting amphibian populations, we predict that experimentally reducing UV-B over ponds where amphibian eggs are incubating and larvae are developing should improve their survival.
Figure 1.9 describes one of many experiments in which the UV-B hypothesis has been tested. Some other experiments have yielded similar results, while others have shown no effects of UV-B exposure, or have shown a negative effect of UV-B exposure only when it is associated with low pH.
Several hypotheses have also been proposed to account for regional differences in amphibian population declines, including the adverse effects of habitat alteration by humans. Two obvious forms of human habitat alteration are air pollution from areas of urban and industrial growth, and the airborne pesticides used in agriculture.
A straightforward prediction from the habitat alteration hypothesis is that amphibian declines should be more noticeable in areas exposed to higher amounts of human-generated air
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