Environmental Science - 12e - Chapter 02.pdf - po ang - Januszek66

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Science, Matter,

and Energy

Carrying Out a Controlled

Scientific Experiment

CORE CASE STUDY

One way in which scientists learn about how nature works is to

conduct a controlled experiment. To begin, scientists isolate vari-

ables, or factors that can change within a system or situation

being studied. An experiment involving single-variable analysis is

designed to isolate and study the effects of one variable at a

time.

To do such an experiment, scientists set up two groups. One

is an experimental group in which a chosen variable is changed

in a known way and the other is a control group in which the

chosen variable is not changed. With proper experimental de-

sign, any difference between the two groups should result from

the variable that was changed in the experimental group.

In the 1960s, botanist Frank H. Bormann, forest ecologist

Gene Likens, and their colleagues began carrying out a classic

controlled experiment. The goal was to compare the loss of water

and nutrients from an uncut forest ecosystem (the control site )

with one that was stripped of its trees (the experimental site ).

They built V-shaped concrete dams across the creeks at

the bottoms of several forested valleys in the Hubbard Brook

Experimental Forest in New Hampshire (Figure 2-1). The dams

were anchored on impenetrable bedrock so all surface water

leaving each forested valley had to flow across a dam, where sci-

entists could measure its volume and dissolved nutrient content.

The first experiment measured the amounts of water and

dissolved plant nutrients that entered and left an undisturbed

forested area (the control site, Figure 2-1, left). These baseline

data showed that an undisturbed mature forest is very efficient

at storing water and retaining chemical nutrients in its soils.

The next experiment involved disturbing the system and

observing any changes that occurred. One winter, the investiga-

tors cut down all trees and shrubs in one valley (the experimen-

tal site), left them where they fell, and sprayed with herbicides

to prevent the regrowth of vegetation. Then they compared the

inflow and outflow of water and nutrients in this modified ex-

perimental site (Figure 2-1, right) with those in the control site

for 3 years.

With no plants to help absorb and retain water, runoff of

water in the deforested valley increased by 30–40%. As this ex-

cess water ran rapidly over the surface of the ground, it eroded

soil and carried dissolved nutrients out of the deforested site.

Overall, the loss of key nutrients from the experimental forest

was six to eight times that in the nearby undisturbed forest.

Figure 2-1 Controlled field

experiment to measure the effects

of deforestation on the loss of wa-

ter and soil nutrients from a forest.

V–notched dams were built into

the impenetrable bedrock at the

bottoms of several forested valleys

(left) so that all water and nutri-

ents flowing from each valley

could be collected and measured

for volume and mineral content.

Baseline data were collected

on the forested valley (left) that

acted as the control site. Then all

the trees in one valley (the experi-

mental site) were cut (right) and

the flows of water and soil nutri-

ents from this experimental valley

were measured for three years.

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Key Questions and Concepts

2-1 What is science?

CONCEPT 2-1 Scientists collect data and develop theories,

models, and laws about how nature works.

CONCEPT 2-4B Whenever energy is changed from one form to

another, we end up with lower quality or less usable energy than

we started with (second law of thermodynamics).

2-5 How can we use matter and energy more

sustainably?

CONCEPT 2-5A The processes of life must conform to the

law of conservation of matter and the two laws of thermo-

dynamics.

CONCEPT 2-5B We can live more sustainably by using and

wasting less matter and energy, recycling and reusing most matter

resources, and controlling human population growth.

2-2 What is matter?

CONCEPT 2-2 Matter consists of elements and compounds,

which are in turn made up of atoms, ions, or molecules.

2-3 How can matter change?

CONCEPT 2-3 When matter undergoes a physical or chemical

change, no atoms are created or destroyed (the law of conservation

of matter).

2-4 What is energy and how can it change its form?

CONCEPT 2-4A When energy is converted from one form to

another in a physical or chemical change, no energy is created or

destroyed (first law of thermodynamics).

Note: Supplements 1, 2, 6, 7, and 18 can be used with this chapter.

Science is an adventure of the human spirit.

It is essentially an artistic enterprise,

stimulated largely by curiosity,

served largely by disciplined imagination,

and based largely on faith in the reasonableness, order,

and beauty of the universe.

WARREN WEAVER

What Is Science?

2-1

CONCEPT 2-1 Scientists collect data and develop theories, models, and laws about how

nature works.

Science is a Search for Order

in Nature

Have you ever seen an area in a forest where all the

trees were cut down? If so, you might wonder about the

effects of cutting down all those trees. You might won-

der how it affected the animals and people living in that

area and how it affected the land itself. That is ex actly

what scientists Bormann and Likens ( Core Case

Study ) thought about when they designed their

experiment.

Such curiosity is what motivates scientists. Science

is an endeavor to discover how nature works and to

use that knowledge to make predictions about what

is likely to happen in nature. It is based on the assump-

tion that events in the natural world follow orderly

cause and effect patterns that can be understood

through careful observation, measurements, experi-

mentation, and modeling. Figure 2-2 summarizes the

scientific process.

There is nothing mysterious about this process. You

use it all the time in making decisions. Here is an ex-

ample of applying the scientific process to an everyday

situation:

Observation: You switch on your flashlight and

nothing happens.

Question: Why didn’t the light come on?

Hypothesis: Maybe the batteries are dead.

Test the hypothesis: Put in new batteries and switch

on the flashlight.

Result: Flashlight still does not work.

Links:

refers to the Core Case Study.

refers to the book’s sustainability theme.

indicates links to key concepts in earlier chapters.

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ter and soil nutrients from cutover forests as a

problem worth studying.

Identify a problem

•

Find out what is known about the problem.

Bormann and Likens searched the scientific litera-

ture to find out what was known about the reten-

tion and loss of water and soil nutrients in forests.

Find out what is known

about the problem

(literature search)

•

Ask a question to be investigated. The scien-

tists asked: “How does clearing forested land affect

its ability to store water and retain soil nutrients?

Ask a question to be

investigated

•

Design an experiment to answer the question

and collect data. To collect data, or information

needed to answer their questions, scientists often

conduct experiments, or procedures carried out

under controlled conditions to gather information

and test ideas. Bormann and Likens collected

and analyzed data on the water and soil nutrients

flowing from a patch of an undisturbed forest (Fig-

ure 2-1, left) and from a nearby patch of forest

which they had cleared of trees for their experi-

ment (Figure 2-1, right).

Perform an experiment

to answer the question

and collect data

Scientific law

Well-accepted

pattern in data

Analyze data

(check for patterns)

Propose an hypothesis

to explain data

•

Propose an hypothesis to explain the data.

Scientists suggest a scientific hypothesis, or pos-

sible explanation of what they observe in nature.

The data collected by Bormann and Likens showed

a decrease in the ability of a cleared forest to store

water and retain soil nutrients such as nitrogen.

They came up with the following hypothesis, or

tentative explanation for their data: When a forest

is cleared, it retains less water and loses large

quantities of its soil nutrients when water from

rain and melting snow flows across its exposed

soil.

Use hypothesis to make

testable predictions

Perform an experiment

to test predictions

hypothesis

Revise

hypothesis

Make testable

predictions

•

Make testable predictions. Scientists use an

hypothesis to make testable predictions about what

should happen if the hypothesis is valid. They

often do this by making “If ...then” predictions.

Bormann and Likens predicted that if their original

hypothesis was valid for nitrogen, then a cleared

forest should also lose other soil nutrients such as

phosphorus.

Test

predictions

Scientific theory

Well-tested and

widely accepted

hypothesis

Figure 2-2 What scientists do. The essence of science is this process

for testing ideas about how nature works. Scientists do not necessar-

ily follow the order of steps described here. For example, sometimes

a scientist might start by formulating an hypothesis to answer the

initial questions and then run experiments to test the hypothesis.

•

Test the predictions with further experi-

ments, models, or observations. To test their

prediction, Bormann and Likens repeated their

controlled experiment and measured the phos-

phorus content of the soil. Another way to test

predictions is to develop a model, an approximate

representation or simulation of a system being

studied. Since Bormann and Likens performed

their experiments, scientists have developed in-

creasingly sophisticated mathematical and com-

puter models of how a forest system works. Data

from Bormann and Likens’s research and that of

other scientists can be fed into such models and

used to predict the loss of phosphorus and other

types of soil nutrients. These predictions can be

compared with the actual measured losses to test

the validity of such models.

New hypothesis: Maybe the bulb is burned out.

Experiment: Replace bulb with a new bulb, and

switch on flashlight.

Result: Flashlight works.

Conclusion: Second hypothesis is verified.

Here is a more formal outline of steps scientists of-

ten take in trying to understand nature, although not

always in the order listed:

•

Identify a problem. Bormann and Likens

( Core Case Study ) identified the loss of wa-

CONCEPT 2-1

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• Accept or reject the hypothesis. If their new

data do not support their hypotheses, scientists

come up with other testable explanations. This

process continues until there is general agreement

among scientists in the field being studied that a

particular hypothesis is the best explanation of the

data. After Bormann and Likens confirmed that

the soil in a cleared forest also loses phosphorus,

they measured losses of other soil nutrients,

which also supported their hypothesis. A well-

tested and widely accepted scientific hypothesis or

a group of related hypotheses is called a scientific

theory. Thus, Bormann and Likens and their col-

leagues developed a theory that trees and other

plants hold soil in place and help it to retain water

and nutrients needed by the plants for their

growth.

Three important features of any scientific process

are skepticism, peer review of results by other scientists,

and reproducibility. Scientists tend to be highly skeptical

of new data and hypotheses until they can be verified.

Peer review happens when scientists report details of

the methods they used, the results of their experiments

and models, and the reasoning behind their hypotheses

for other scientists working in the same field (their

peers) to examine and criticize. Ideally, other scientists

repeat and analyze the work to see if the data can be

reproduced and whether the proposed hypothesis is

reasonable and useful.

For example, the results of the forest experim ents

by Bormann and Likens ( Core Case Study ) were

submitted to other soil and forest experts for

their review before a respected scientific journal would

publish their results. Other scientists have repeated the

measurements of soil content in undisturbed and

cleared forests of the same type and also for different

types of forests. Their results have also been subjected

to peer review. In addition, computer models of forest

systems have been used to evaluate this problem, with

the results subjected to peer review. Scientific knowl-

edge advances because scientists continually question

measurements, make new measurements, and try to

come up with new and better hypotheses (Science

Focus, at right).

Another important outcome of science is a scien-

tific, or natural, law: a well-tested and widely ac-

cepted description of what we find happening over and

over in the same way in nature. An example is the law

of gravity, based on countless observations and meas-

urements of objects falling from different heights. Ac-

cording to this law, all objects fall to the earth’s surface

at predictable speeds.

A good way to summarize the most important out-

comes of science is to say that scientists collect data and

develop theories, models, and laws that describe and

explain how nature works ( Concept 2-1 ). Scientists use

reasoning and critical thinking skills (pp. 2–4). But the

best scientists also use intuition, imagination, and cre-

ativity in asking important questions, developing hy-

potheses, and designing ways to test them. Scientist

Warren Weaver’s quotation found at the opening of

this chapter reflects this aspect of science.

The Results of Science Can Be

Tentative, Reliable, or Unreliable

A fundamental part of science is testing. Scientists insist

on testing their hypotheses, models, methods, and re-

sults over and over again to establish the reliability of

these scientific tools, and the resulting conclusions.

Media news reports often focus on disputes among

scientists over the validity of scientific data, hypothe-

ses, models, methods, or results. This reveals differ-

ences in the reliability of various scientific tools and

results. Simply put, some science is more reliable than

other science, depending on how carefully it has been

done and on how thoroughly the hypotheses, models,

methods, and results have been tested.

Sometimes, preliminary results that capture news

headlines are controversial because they have not been

widely tested and accepted by peer review. They are

not yet considered reliable, and can be thought of as

tentative science or frontier science. Some of these

results will be validated and classified as reliable and

some will be discredited and classified as unreliable. At

the frontier stage, it is normal for scientists to disagree

about the meaning and accuracy of data and the valid-

ity of hypotheses and results. This is how scientific

knowledge advances.

By contrast, reliable science consists of data, hy-

potheses, theories, and laws that are widely accepted by

scientists who are considered experts in the field. The

results of reliable science are based on the self-correct-

ing process of testing, open peer review, reproducibility,

and debate. New evidence and better hypotheses (Sci-

ence Focus, at right) may discredit or alter tried and ac-

cepted views. But unless that happens, those views are

considered to be the results of reliable science.

Scientific hypotheses and results that are presented

as reliable without having undergone the rigors of peer

Scientific Theories and Laws

Are the Most Important Results

of Science

If an overwhelming body of observations and measure-

ments supports a scientific hypothesis, it becomes a sci-

entific theory. Scientific theories are not to be taken lightly.

They have been tested widely, are supported by exten-

sive evidence, and are accepted by most scientists in a

particular field or related fields of study.

CHAPTER 2

Science, Matter, and Energy

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SCIENCE FOCUS

Easter Island: Some Revisions in a Popular Environmental Story

or years, the story of Easter Island

has been used in textbooks as an ex-

ample of how humans can seriously degrade

their own life-support system. It concerns a

civilization that once thrived and then disap-

peared from a small, isolated island in the

great expanse of the South Pacific, located

about 3,600 kilometers (2,200 miles) off the

coast of Chile.

Scientists used anthropological evidence

and scientific measurements to estimate the

ages of certain artifacts found on Easter

Island (also called Rapa Nui). They hypothe-

sized that about 2,900 years ago, Polynesians

used double-hulled, seagoing canoes to colo-

nize the island. The settlers probably found a

paradise with fertile soil that supported dense

and diverse forests and lush grasses. Accord-

ing to this hypothesis, the islanders thrived,

and their population increased to as many as

15,000 people.

Measurements made by scientists indi-

cated that over time the Polynesians began

living unsustainably by using the island’s for-

est and soil resources faster than they could

be renewed. When they had used up the

large trees, the islanders could no longer

build their traditional seagoing canoes for

fishing in deeper offshore waters, and no one

could escape the island by boat.

Without the once-great forests to absorb

and slowly release water, springs and streams

dried up, exposed soils were eroded, crop

yields plummeted, and famine struck. There

was no firewood for cooking or keeping

warm. According to the original hypothesis,

the population and the civilization collapsed

as rival clans fought one another for dwin-

dling food supplies and the island’s popula-

tion dropped sharply. By the late 1870s, only

about 100 native islanders were left.

But in 2006, anthropologist Terry L. Hunt

evaluated the accuracy of past measurements

and other evidence and carried out new

measurements to estimate the ages of various

artifacts. He used these data to formulate an

alternative hypothesis describing the human

tragedy on Easter Island.

Hunt came to several conclusions. First,

the Polynesians arrived on the island about

800 years ago, not 2,900 years ago. Second,

their population size probably never exceeded

3,000, contrary to the earlier estimate of up to

15,000. Third, the Polynesians did use the is-

land’s trees and other vegetation in an unsus-

tainable manner, and by 1722 visitors reported

that most of the island’s trees were gone.

But one question not answered by the

earlier hypothesis was, why did the trees

never grow back? Recent evidence and Hunt’s

new hypothesis suggest that rats (which came

along with the original settlers either as stow-

aways or as a source of protein for their long

ocean voyage) played a key role in the island’s

permanent deforestation. Over the years, the

rats multiplied rapidly into the millions and

devoured the seeds that would have regener-

ated the forests.

Another of Hunt’s conclusions was that

after 1722, the population of Polynesians on

the island dropped to about 100, mostly from

contact with European visitors and invaders.

These newcomers introduced fatal diseases,

killed off some of the islanders, and took large

numbers of them away to be sold as slaves.

This story is an excellent example of how

science works. The gathering of new scientific

data and reevaluation of older data led to a

revised hypothesis that challenged our think-

ing about the decline of civilization on Easter

Island. The tragedy may not be as clear an ex-

ample of ecological collapse caused mostly by

humans as was once thought. However, there

is evidence that other earlier civilizations did

suffer ecological collapse largely from unsus-

tainable use of soil, water, and other re-

sources, as described in Supplement 6 on

p. S31.

Critical Thinking

Does the new doubt about the original Easter

Island hypothesis mean that we should not be

concerned about our apparent and growing

unsustainable use of essential natural capital

on the island in space we call the earth?

Explain.

review, or that have been discarded as a result of peer

review, are considered to be unreliable science. Here

are some critical thinking questions you can use to un-

cover unreliable science:

•

• Are the conclusions of the research widely accepted

by other experts in this field?

If “yes” is the answer to each of these questions,

then the results can be classified as reliable science.

Otherwise, the results may represent tentative science

that needs further testing and evaluation, or they can

be classified as unreliable science. See Supplement 17 on

pp. S73–S80 on How to Analyze a Scientific Paper.

Was the experiment well designed? Did it invol ve

enough testing? Did it involve a control

group? ( Core Case Study ).

•

Have the data supporting the proposed hypotheses

been verified? Have the results been reproduced by

other scientists?

•

Do the conclusions and hypotheses follow logically

from the data?

Science and Environmental Science

Have Some Limitations

Before we continue our study of environmental sci-

ence, we need to recognize some of its limitations, as

well as those of science in general. First, scientists can

disprove things but cannot prove anything absolutely

because there is always some degree of uncertainty in

scientific measurements, observations, and models.

•

Are there no better scientific explanations?

•

Are the investigators unbiased in their interpre-

tations of the results? Are they free of a hidden

agenda? Were they funded by an unbiased source?

•

Have the conclusions been verified by impartial

peer review?

CONCEPT 2-1

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