Exam 3 Study Guide
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The goal of science is to investigate and understand the natural world , and to use those explanations to make useful predictions . In other words, science is an organized way of using evidence to learn about the natural world.
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A key assumption in science is that experimental results can be reproduced because nature behaves in a consistent manner. When one particular variable is manipulated in a given set of variables , the result should always be the same . Today’s researchers often publish a report of their own work in a scientific journal . Other scientists review the experimental procedure to make sure that the design was without flaws . They often repeat experiments to be sure that the results match those already obtained. o Page 14
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In science, the word theory applies to a well-tested explanation that unifies a broad range of observations . A theory enables scientists to make accurate predictions about new situations. o Page 16
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The word biology means the study of life (The Greek word bios means “ life ” and logy means “ study of ”) Biology is the science that seeks to understand the living world. A biologist is someone who uses scientific methods to study living things.
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No single characteristic is enough to describe a living thing. Living things share the following characteristics:
Living things are made up of units called cells . Living things reproduce . Living things are based on a universal genetic code. Living things grow and develop . Living things obtain and use materials and energy. Living things respond to their environment. Living things maintain a stable internal environment. Taken as a group living things change over time (evolution). o Page 20
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Some of the levels at which life can be studied include molecules , cells, organisms , populations of a single kind of organism, communities of different organisms in an area, and the biosphere . At all these levels, smaller living systems are found within larger systems.
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Levels of organization in an ECOSYSTEM are species, population , community, ecosystem , biome, and biosphere . o Page 63
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Ecology is the scientific study of interactions among organisms and between organisms and their environment or surroundings.
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Autotrophs use energy from the environment to fuel the assembly of simple inorganic compounds into complex organic molecules . These organic molecules combine and recombine to produce living tissue . Because they make their own food autotrophs are also called producers . Organisms that rely on other organisms for their energy and food supply are called heterotrophs . Heterotrophs are also called consumers .
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There are many different types of heterotrophs . Herbivores obtain energy by eating only plants . Carnivores including snakes dogs and owls eat animals . Humans, bears, crows and other omnivores eat both plants and animals. Detritivores such as mites, earthworms, snails and crabs feed on plants and animal remains and other dead matter.
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Energy flows through an ecosystem in one direction from the sun or inorganic compounds to autotrophs and then to various heterotrophs . The energy stored by producers can be passed through an ecosystem along a food chain , a series of steps in which organisms transfer energy by eating and being eaten. When the feeding relationships among the various organisms in an ecosystem form a network of complex interactions, ecologists

Exam 3 Study Guide Name_______________________________ describe these relationships as a food web . Each step in a food chain or food web is called a trophic level.
Producers make up the first trophic level. Consumers make up the second, third or higher trophic levels. Each consumer depends on the trophic level below it for energy. Only about 10 percent of the energy available within one trophic level is transferred to organisms at the next trophic level. o Notes
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The remaining 90% of energy that does not get transferred to the next trophic level is lost in the form of heat . o Page 74
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Energy and matter flow through the biosphere very differently. Unlike the one-way flow of energy, matter is recycled within and between ecosystems. Elements, chemical compounds and other forms of matter are passed from one organism to another and from one part of the biosphere to another through biogeochemical cycles. As the long word suggest, biogeochemical cycles connect biological , geological , and chemical aspects of the biosphere.
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All organisms require nitrogen to make amino acids , which in turn are used to build proteins .
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Although nitrogen gas is the most abundant form of nitrogen on Earth only certain types of bacteria can use this form directly. Such bacteria which live in the soil and on the roots of plants called legumes convert nitrogen gas into ammonia in a process known as nitrogen fixation . o Page 87
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Temperatures on Earth remain within a range suitable for life because the biosphere has a natural insulating blanket – the atmosphere . Carbon dioxide , methane, water vapor , and a few other atmospheric gases trap heat energy and maintain the Earth’s temperature range.
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The natural situation in which heat is retained by this layer of greenhouse gases is called the greenhouse effect. o Page 90
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Ecosystems are influenced by a combination of biological and physical factors. The biological influences on organisms within an ecosystem are called biotic factors. These include the entire living cast of characters with which an organism might interact .
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Physical and nonliving factors that shape an ecosystem are called abiotic factors. For example the climate of an area includes abiotic factors such as temperature , precipitation, and humidity .
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Together biotic and abiotic factors determine the survival and growth of an organism and the productivity of the ecosystem in which the organism lives. The area where an organism lives is called its habitat . A habitat includes both biotic and abiotic factors.
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If an organism’s habitat is its address, its niche is its occupation. A niche is the full range of physical and biological conditions in which an organism lives and the way in which the organism uses those conditions.
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A niche includes the type of food an organism eats, how it obtains this food, and which other species use the organism as food. o Page 92
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Community interactions such as competition , predation and various forms of symbiosis , can powerfully affect an ecosystem .
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Direct competition in nature often results in a winner and a loser – with the losing organism failing to survive . A fundamental rule in ecology, the competitive exclusion principle , states that no two species can occupy the same niche in the same habitat at the same time .
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An interaction in which one organism captures and feeds on another organism is called predation .
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Any relationship in which 2 species live closely together is called symbiosis .
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In mutualism both species benefit from the relationship.
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In commensalism on member of the association benefits and the other is neither helped nor harmed .

Exam 3 Study Guide Name_______________________________
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In parasitism one organism lives on or inside another organism and harms it. o Page 94
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Ecosystems are constantly changing in response to natural and human disturbances . As an ecosystem changes, older inhabitants gradually die out and new organisms move in, causing further changes in the community . This series of predictable changes that occurs in a community over time is called ecological succession .
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On land, succession that occurs on surfaces where no soil exists is called primary succession.
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When a disturbance of some kind changes an existing community without removing the soil, then secondary succession can follow. o Page 119
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Population density is the number of individual’s per unit area.
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Three factors can affect population size: the number of births , the number of deaths and the number of individuals that enter or leave a population.
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Immigration the movement of individuals into an area.
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Emigration the movement of individuals out of an area can cause a population to decrease in size.
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Exponential growth occurs when the individuals in a population reproduce at a constant rate.
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Under ideal conditions with unlimited resources, a population will grow exponentially .
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As resources become less available, the growth of a population slows or stops.
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Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth.
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The largest number of individuals that a given environment can support is the carrying capacity.
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In the context of populations, a limiting factor is a factor that causes population growth to decrease.
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A limiting factor that depends on population size is called a density dependent limiting factor.
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Density dependent limiting factors include competition, predation, parasitism, and disease.
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Densityindependent limiting factors affect all populations in similar ways, regardless of the population size.
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Unusual weather , natural disasters , seasonal cycles , and certain human activities – such as damming rivers and clear- cutting forests – are all examples of density-independent limiting factors. o Page 142
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The green revolution greatly increased the worlds food supply.
Environmental goods and services may be classified as either renewable or nonrenewable .
Renewable resources can regenerate if they are alive or can be replenished by biogeochemical cycles if they are nonliving.
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A nonrenewable resource is one that cannot be replenished by natural processes. The fossil fuels coal, oil and natural gas are nonrenewable resources.
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Sustainable development is a way of using natural resources without depleting them and of providing for human needs without causing long term environmental harm.
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Plowing the land removes the roots that hold soil in place. This increases the rate of soil erosion – wearing away of surface soil by water and wind.
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In certain parts of the world with dry climates, a combination of farming, overgrazing and drought has turned once productive areas into deserts . This process is called desertification .
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Loss of forests of deforestation has several effects. Deforestation can led to severe erosion as soil is exposed to heavy rains. Erosion can wash away nutrients in the topsoil.
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There are a variety of sustainable development strategies for forest management. In some forests mature trees can be harvested selectively to promote the growth of younger trees and preserve the forest ecosystem. In

Exam 3 Study Guide Name_______________________________ areas where forests have already been cut foresters today often plant, manage, harvest, and replant tree farms .
Tree farms can now be planted and harvested efficiently making them fully renewable resources. o Page 148
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If you live in a large city, you have probably seen smog , a mixture of sulfur and nitrogen that occurs as a graybrown haze in the atmosphere. Smog is primarily due to automobile exhaust and industrial emissions.
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Acid Rain can kill plants by damaging their leaves and changing the chemistry of soils and standing water ecosystems.
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Another word for variety is diversity . Therefore, biological diversity or biodiversity is the sum total of the genetically based variety of all organisms in the biosphere.
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Human activity can reduce biodiversity by altering habitats , hunting species to extinction , introducing toxic
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Many forms of pollution can threaten biodiversity but one of the most serious problems occurs when toxic compounds accumulate in the tissues of organisms. The history of DDT , one of the first widely used pesticides explains the situation well. At first, DDT seemed to be a perfect pesticide . When DDT was sprayed it drained into rivers and streams at low concentrations that seemed harmless. But DDT has two properties that make it hazardous . DDT is nonbiodegradable , and when it is picked up by organisms they do not eliminate it from their bodies . When an aquatic plants picks up DDT from the water the DDT is stored in the plants tissues . When herbivores eat those plants they too store the DDT. In this process called biological magnification , concentrations of a harmful substance increase in organisms at higher trophic levels in a food chain or food web.
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Invasive species increase their populations because their now habitat lacks the parasites and predators that control their population “back home.” o Page 157
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Between 20-50 kilometers above the Earth’s surface the atmosphere contains a relatively high concentration of ozone gas called the ozone layer . It absorbs a good deal of ultra violet (UV) radiation from sunlight before it reaches Earth’s surface. Overexposure to UV radiation is the cause of sunburn . It can also cause cancer , damage eyes, and decrease an organisms resistance to disease.
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Chlorofluorocarbons or CFCs could damage the ozone layer, causing ozone depletion. o Page 176
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The nucleus contains nearly all the cell’s DNA and with it the coded instructions for making proteins and other important molecules .
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Plant and animal cells are eukaryotic cells , which have a nucleus .
Prokaryotic cells do not have a nucleus.
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An organ system is a group of organs that work together to perform a specific function. o NOTES
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Natural gas drilling, deforestation, and construction and all example of Habitat fragmentation.
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Invasive species in Pennsylvania are Rats, Asian Lady Beetle, and Stinkbug.
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A species that is confined to a specific habitat is an endemic species.
ESSAY’S TO STUDY
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What causes acid rain? What is the effect of acid rain on natural habitats?
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What are introduced species? How are they a threat to biodiversity?