Scientists collect evidence to learn how nature works. Science is a field of study focused on discovering how nature works and using that knowledge to describe what is likely to happen in nature. Science is based on the assumption that events in the natural world follow orderly cause-and-effect patterns. These patterns can be understood through observations (by use of our senses and with instruments that expand our senses), measurements, and experimentation . The scientific method is a research process in which scientists identify a problem for study, gather relevant data, propose a hypothesis, and modify the hypothesis that explains the data, gather data to test the hypothesis, and modify the hypothesis as needed. Within this process, scientists use many different methods to learn more about how nature works. 

Suppose a logging company plans to cut down all of the trees on the land behind your house. You are concerned and want to know about the possible harmful environmental effects of this action. 

One way to learn about such effects is to conduct a controlled experiment, just as environmental scientists do. They begin by identifying key variables, such as water loss and soil nutrient content that might change after the trees are cut down. Then they set up two groups. One is the experimental group, in which a chosen variable is changed in a known way. The other is the control group, in which the chosen variable is not changed. They then compared the results from the two groups. 

Botanist F. Herbert Bormann, forest ecologist Gene Likens, and colleagues carried out such a controlled experiment. Their goal was to compare the loss of water and soil nutrients from an area of uncut forest (the control site) with one that had been stripped of its trees (the experimental site). 

The scientists built V-shaped concrete dams across the creeks at the bottoms of each forest in the Hubbard Brook Experimental Forest in New Hampshire. The dams were designed so that all surface water leaving each forested valley had to flow across a dam, where they could measure its volume and dissolved nutrient content. 

Firths, the researchers measured the amounts of water and dissolved soil nutrients flowing from an undisturbed forested area in one of the valleys (the control site). These measurements showed that the undisturbed mature forest was efficient at storing water and retaining chemical nutrients in its soils. 

Next, they set up an experimental forest area in a nearby valley. They cut down all the trees and shrubs in that valley, left them where they fell, and sprayed the area with herbicides to prevent regrowth of vegetaton. Then, for 3 years, they compared the outflow of water and nutrients in this experimental site with data from the control site. 

The scientists found that, without plants to help absorb and retain water, the amount of water flowing out of the deforested valley increased 30-40. This excess water ran over the ground rapidly, eroded soil, and removed dissolved nutrients from the topsoil. Overall, the loss of key soil nutrients from the experimental forest was six to eight times that in the nearby uncut control forest. 

Scientists use logical reasoning and critical thinking skills to learn about nature. Thinking critically involves three steps: 

1. Be skeptical about everything you read or hear.

2. Evaluate evidence and hypotheses using inputs and opinions from a variety of reliable sources. 

3. Identify and evaluate your personal assumptions, biases, and beliefs and distinguish between facts and opinions before coming to a conclusion. 

Logic and critical thinking are important tools in science, but imagination, creativity, and intuition are also vital. According to Albert Einstein, "There is no completely logical way to a new scientific idea." 

We should never take a scientific theory lightly. It has been tested widely, is supported by extensive evidence, and is accepted tas being a useful explanation of some phenomenon by most scientists in a particluar field or related fields of study. So when you hear someone say, "Oh, that's just a theory," you will know that he or she does not have a clear understanding of what a scientific theory is and how it is an important result of science. 

Another important and reliable outcome of science is a scientific law or law of nature-a well-tested and widely accepted description of what we find always happening in the same way in nature. An example is the law of gravity. After making many thousands of observations and measurements of objects falling from different heights, scientists developed the following scientific law: all objects fall to the earth's surface at predictatable speeds. Scientific laws cannot be broken. 

Reliable science consists of data, hypotheses, models, theories, and laws that are accepted by most of the scientists who are considered experts in the field under study. 

Scientific results and hypotheses that are presented as reliable without having undergone peer review, or are discarded as a result of peer review or additional research, are considered to be unreliable science. 

Preliminary scientific results without adequate testing and peer review as tentative science. Some of these results and hypotheses will be validated and classified as reliable. Others may be discredited and classified as unreliable. This is how scientific knowledge advances. 

Environmental science and science in general have several limitations. First limitation, scientists cannot prove anything absolutely because there is always some degree of uncertainty in measurements, observations, models, and the resulting hypotheses and theories. Instead, scientists try to establish that a particular scientific theory has a very high probability or certainty (typically 90-95%) of being useful for understanding some aspect of the natural world. 

Scientists do no use the word proof in the same way as many nonscientists use it, because it can falsely imply "absolute proof." For example, most scientists would not say: "Science has proven that cigarette smoking causes lung cancer." Instead, they might say: "Overwhelming evidence from thousands of studies indicates that people who smoke regularly for many years have a greatly increased chance of developing lung cancer." 

A second limitation of science is that scientists are human and not always free of bias about their own results and hypotheses. However, the high standards for evidence and peer review uncover or greatly reduce personal bias and falsified results. 

A third limitation is that many systems in the natural world involve a huge number of variables with complex interactions. This makes it it too difficult, costly, and time consuming to test one variable at a time in controlled experiments like our Core Case Study for today's lesson. To deal with this, scientists develop mathematical models that can take into account the interactions of many variables and run the models on high-speed computers. In addition, science and engineering projects can fail and teach us lessons. 

A fourth limitation of science involves the use of statistical tools. For example, there is no way to measure accurately the number of metric tons of soil eroded annually worldwide. Instead, scientists use statistical sampling and mathematical methods to estimate such numbers. 

Despite these limitations, science is the most useful way that we have of learning about how nature works and projecting how it might behave in the future. 

For years, the story of Easter Island has been used in textbooks as an example of how humans can seriously degrade their own life-support system and as a warning about what we are doing to our life-support system. What happened on this small island in the South Pacific is a story about environmental degradation and the collapse of an ancient civilization of Polynesians living there. Over the years, many researchers have studied the island and its remains, including hundreds of huge statues.