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Population Ecology
I.
Populations possess characteristic features.
A. Population ecology includes the study of the numbers of individuals of a particular species at a
given time and location, and the interactions of that population with other populations and the
environment.
B. Ecology includes both biotic and abiotic factors.
C. Density and dispersion are important features of populations.
1. Population size, expressed in terms of density, is most useful.
a) Density is the number of individuals of a species per unit space.
2. Dispersion describes the spacing of individuals.
a) Uniform dispersion occurs when individuals are more evenly distributed than would be
expected randomly.
(1) Uniform dispersion is often a result of agonistic behavior such as territoriality.
b) Random dispersion results from lack of interaction between individuals or a homogeneous
environment.
c) Clumped dispersion is the most common type of distribution and results from patchy
distribution of resources or by asexual reproduction.
(1) Clumped distribution may include social behavior in animals.
II.
Mathematical models describe population growth.
A. Population numbers change due to natality (birth rate) and mortality (death rate).
1. ∆N / ∆t = b – d
a) ∆N is the change in numbers in the population.
b) ∆t is the change in time.
c) b is the natality rate.
d) d is the mortality.
2. r = b – d
a) r is the growth rate of the population.
b) If r = 0, the population is stable; if r > 1, the population is increasing in size; if r < 1, the
population is declining.
3. These equations can be expressed as dN / dt = rN
B. Dispersal affects the growth rate in some populations.
1. Dispersal is the movement of individuals among populations.
2. Immigration (i) is the migration of individuals into the population.
3. Emigration (e) is the migration of individuals out of the population.
C. Each population has a characteristic intrinsic rate of increase.
1. The intrinsic rate of increase is the maximum rate of increase under ideal conditions for that
population.
2. Life history characteristics determine the biotic potential for a species.
3. Typically, larger animals have a greater biotic potential.
D. No population can increase exponentially indefinitely.
1. Exponential growth may exist for a period of time until limiting factors become important.
2. Environmental resistance prevents continued exponential growth.
a) As N increases, environmental resistance increases.
3. The carrying capacity is the largest population that can be sustained for an indefinite period of
time.
4. Growth regulated by environmental resistance shows an S-shaped growth curve.
5.
6.
This growth may be described by dN / dt = rN [(K – N) / K]; the logistic equation, where K is the
carrying capacity and N is the population size.
a) When N is small, the expression [(K – N) / K] has a value of almost 1 and minimally limits
growth.
b) When N is large, the expression [(K – N) / K] has a value that approaches 0 and greatly limits
growth.
Populations do not always approach and stabilize at K, but may overshoot K, followed by a
population crash.
III.
Factors influencing population size
A. Density-dependent factors regulate population size.
1. Density-dependent factors are those that increase in proportional effect with increasing
population size.
a) Predation and disease are examples of density-dependent factors.
2. Competition is an important density-dependent factor.
a) Intraspecific competition exists between members of the same species.
(1) In scramble competition, members of the population share the resource equally.
(2) In contest competition, some members of the population obtain a “lion’s share” of the
resource.
b) Interspecific competition exists between members of different species.
3. The effects of different density-dependent factors are difficult to assess in nature.
a) Many populations are regulated by multiple density-dependent factors.
B. Density-independent factors limit population size.
1. Environmental factors that affect population regardless of size are density-independent factors.
2. Many climatic factors are density-independent.
3. Most density-independent factors have some relationship to population density.
IV.
Life history traits
A. Based on life-history strategies, organisms may be classified as r-strategists and K-strategists.
B. r-strategists have a high r, small size, large number of offspring, and may live in unpredictable
habitats.
1. Many weeds or pest species are r-strategists.
2. Many r-strategists reproduce asexually.
C. K-strategists have a population size near K, are larger in size, have fewer offspring, and may care
for their young.
1. Many endangered species are K-strategists.
2. K-strategists typically live in stable environments.
D. Survivorship is related to r and K selection.
1. Survivorship curves may be classified as type I, type II, or type III.
a) Humans exhibit type I survivorship, which is typical of K-selected organisms. Type I curves
are characterized by high mortality late in life.
b) Some birds and lizards have a type II survivorship, characterized by constant mortality over
time.
c) Type III survivorship is characterized by high mortality early in life, as seen in r-selected
organisms.
V.
Human populations
A. Thomas Malthus recognized that the human population cannot continue in exponential growth
indefinitely.
1.
2.
Human population growth has been marked by a large decrease in the death rate.
Human population size will increase to unknown proportions due to uncertainty of the
carrying capacity for our species.
B. Not all countries have the same growth rate.
1. Demographics describe population statistics.
2. The growth rate is higher in developing countries, which have a lower degree of
industrialization and prosperity than the developed countries.
a) Moderately developed countries include most countries of South America, as well as
Mexico and Turkey.
b) Less developed countries have the highest growth rates and include many African and
Southeast Asian countries.
3. The doubling time is the time needed for a population to double in size.
a) Doubling time is less than 20 years in some developing countries, and hundreds of years in
some developed countries.
4. Replacement-level fertility is the number of children a couple must have to replace themselves.
a) Replacement-level fertility is typically 2.1 children in developed countries and 2.7 children in
developing countries.
5. The total fertility rate is the average number of children born to a woman during her lifetime.
C.
The age structure of a country can be used to predict future population growth.
1. Age structure shows the percentages of the population at different ages.
2. A broader base to the age structure indicates a growing population; a narrower base indicates a
stable or shrinking population.
D. Environmental degradation is related to population growth and resource consumption.
1. Developed nations require more resources per capita and cause more environmental problems
than developing nations.
2. People overpopulation is a result of too many people living in a certain area; consumption
overpopulation is a result of people over-utilizing resources.
Community Ecology
VI.
E.
Community structure and functioning
In a community, populations interact in a variety of ways.
1. Within a community, no species exists independently of other species.
F. The niche is a species role in the community.
1. The niche describes all aspects of an organism’s existence.
a) The niche describes not only the habitat but much more.
b) The fundamental niche is the niche that an organism would occupy in the absence of
competition.
c) The realized niche is the niche that an organism occupies in the presence of competition.
2. Limiting resources restrict the ecological niche of a species.
a) Limiting factors as varied as soil composition, climatic extremes, or population interactions
may influence the niche.
b) Any environmental resource that is scarce or unfavorable is called a limiting resource.
3. Biotic and abiotic factors may influence a species’ ecological niche.
a) Barnacles of two different genera in the intertidal zone are an example of competition for
their own niches.
G. Competition is intraspecific or interspecific.
1. Intraspecific competition exists between members of the same species.
2.
3.
Interspecific competition exists between members of different species.
Competition between two species with overlapping niches may lead to competitive exclusion.
a) Competitive exclusion results in the exclusion of one species due to interspecific
competition.
b) Coexistence occurs when overlap between niches is reduced (resource partitioning).
4. Character displacement reduces interspecific competition.
a) Darwin’s finches exemplify character displacement.
b) Competing species evolve slightly different niches to avoid competition.
H. Natural selection shapes the body forms and behaviors of both predators and prey.
1. Predation is the consumption of one species, the prey, by another, the predator.
2. Pursuit and ambush are two predator strategies.
a) Predators that chase their prey are pursuit predators.
b) Some predators are camouflaged to avoid notice by their prey; others may attract their
prey, like the anglerfish.
3. Chemical protection is an effective plant defense against herbivores.
a) Plants use spines, tough leaves, and chemicals to detract herbivores.
b) Milkweeds produce chemicals that are poisonous to all but a few insects.
(1) Monarch caterpillars graze on milkweeds and accumulate chemicals in their bodies that
make them unpalatable to birds and other predators.
4. Animals possess a variety of defensive adaptations to avoid predators.
a) Animals may flee, hide, live in groups, or have mechanical defenses to avoid being preyed
upon.
b) Warning coloration results in avoidance by experienced predators.
c) Batesian mimicry is the resemblance of a harmless organism to a harmful or unpalatable
organism.
d) Müllerian mimicry is the similar morphology of a group of harmful or unpalatable
organisms.
(1) Monarch and viceroy butterflies are examples of Müllerian mimicry.
I. Symbiosis involves close associations between species.
1. Symbiosis is any intimate relationship or association between members of 2 or more species.
2. In mutualism, benefits are shared.
a) Nitrogen-fixing bacteria and legumes are mutualists.
b) Zooxanthellae and reef-building coral are mutualists.
c) Mycorrhizae facilitate mutualistic relationships between fungi and the roots of a wide
variety of plants.
3. Commensalism is taking without harming.
a) Epiphytes living on tropical trees benefit from the habitat of the host, but the host is not
harmed or benefited.
4. Parasitism is taking at another’s expense.
a) The parasite benefits, while the host is harmed.
b) A well-adapted parasite does not kill the host.
c) A parasite that causes the death of the host is a pathogen.
J.Keystone species affect the character of the community.
1. Keystone species have a great effect on other species in the community.
a) Keystone species are commonly top predators.
2. Dominant species influence a community as a result of their greater size or abundance.
a) Trees are the dominant species of forests because they change the local environment;
similarly, cordgrass in salt marshes, prairie grasses in prairies, and kelp in kelp beds.
b) Animals can also be dominant species, such as coral in coral reefs and cattle in overgrazed
rangelands.
VII.
K.
Community biodiversity
Species richness is the number of species within a community; species diversity is a measure of the
relative importance of each species.
L. Ecologists seek to explain why some communities have more species than others.
1. Isolated communities are typically less diverse than continental communities.
2. Species diversity typically increases in less stressful habitats.
3. High latitude communities have lower species diversity.
4. Ecotonal zones have high species diversity, known as the edge effect.
5. Geologically older habitats have higher species diversity.
M. Species richness probably causes community stability.
1. The traditional view stated that stability was a result of community complexity.
2. Newer mathematical models suggest that complexity does not lead to stability.
VIII. Community development
N. Succession is the process of community development over time, with one species being replaced by
another.
1. Primary succession occurs when a community develops in a “lifeless” environment.
a) During primary succession on bare rock, the rock is eventually transformed into soil.
b) Primary succession occurs on newly formed volcanic lava and recently glaciated rock.
2. Secondary succession occurs when a community develops where a previous community existed.
a) Secondary succession occurs where soil already exists, such as areas that have been
denuded or modified by fire or agriculture.
b) Abandoned farmland has been extensively studied.
Ecosystems and the Biosphere
IX.
Energy flow through ecosystems
O. Energy flow is linear as energy cannot be reused by organisms.
1. Solar energy is trapped by photosynthesis as chemical energy.
2. Chemical energy is then available to do work.
P. Energy flow describes who eats whom in ecosystems.
1. A food chain describes the sequential passage of energy.
2. A food web shows the complexity of interconnected food chains.
Q. Ecological pyramids illustrate how ecosystems work.
1. The primary producers comprise the first trophic level, the herbivores make up the second
trophic level, and the primary consumers the third.
2. An ecological pyramid illustrates the trophic levels.
a) A pyramid of numbers shows the number of organisms in each level.
(1) Typical pyramids of numbers show diminishing numbers in successive levels.
(2) Inverted pyramids of numbers are seen among hosts and parasites, decomposers, and
others.
b) A pyramid of biomass shows the biomass in each trophic level.
(1) An inverted pyramid of biomass may be seen in very productive aquatic ecosystems.
c) A pyramid of energy cannot be inverted due to loss of energy between trophic levels.
(1) This loss of energy illustrates the limitation on the number of trophic levels.
R. Ecosystems vary in productivity.
1. The gross primary productivity of an ecosystem expresses the total rate of photosynthesis.
2. Net primary productivity is the energy remaining after cellular respiration.
3.
4.
5.
6.
7.
8.
X.
Both GPP and NPP can be expressed in terms of kilocalories fixed per area per time, or in terms
of grams carbon fixed.
Rates of productivity are influenced by environmental factors.
Tropical rain forests are the most productive terrestrial ecosystems.
Wetlands, coral reefs, and estuaries are the most productive aquatic ecosystems.
The relationship of productivity to biological diversity is complex.
a) Ecosystems may be more diverse as productivity increases, but after a certain point,
diversity will decline with increasing productivity.
b) This may be important when considering nutrient-enriched environments, such as those
impacted by human application of fertilizers and enrichment by animal wastes.
Humans consume an increasingly greater percentage of global primary productivity.
Cycles of matter in ecosystems
S. Biogeochemical cycles are cycles of matter between the abiotic and the biotic components of the
environment.
1. The carbon, nitrogen, phosphorus, and water cycles are central to life on Earth.
2. The carbon, nitrogen, and water cycles have atmospheric components and cycle on a global
scale.
3. Phosphorus has no atmospheric component and cycles on a local scale.
T. Carbon dioxide is the pivotal molecule of the carbon cycle.
1. Carbon is present as carbon dioxide in the atmosphere; as carbon dioxide, carbonate, and
bicarbonate in aquatic habitats; and in rocks such as limestone.
2. Carbon is fixed by photosynthesis, which incorporates carbon into organic molecules such as
glucose.
3. Organic molecules may be used in cellular respiration or may be passed to consumers.
4. Organic carbon may be stored in the wood of trees or stored even longer in fossil fuels.
a) Combustion of fossil fuels releases the carbon to the atmosphere.
5. The organic carbon of marine organisms may end up in limestone.
a) Erosion returns this carbon to the water.
6. Human activities have disturbed the global carbon budget.
a) Since the beginning of the Industrial Revolution, humans have contributed an increasing
amount of carbon dioxide to the atmosphere by burning fossil fuels.
U. Bacteria are essential to the nitrogen cycle.
1. Nitrogen gas in the atmosphere is very abundant but is such a stable molecule that bacteria are
needed to break it apart.
2. Nitrogen fixation is the conversion of gaseous nitrogen to ammonia.
a) Most nitrogen fixation is accomplished by nitrogen-fixing bacteria.
b) Nitrogenase, which functions only in the absence of oxygen, is present in some
cyanobacteria, some soil bacteria, and Rhizobium, which lives in root nodules of legumes.
3. Nitrification is the conversion of ammonia to nitrate.
a) Nitrifying bacteria convert the nitrogen compounds to produce metabolic energy.
b) Nitrosomonas and Nitrococcus are soil bacteria that convert ammonia to nitrite.
c) Nitrobacter, another soil bacterium, converts nitrite to nitrate.
d) Assimilation is the uptake of nitrate or ammonia by plant roots.
(1) Animals assimilate nitrogen by consuming plants or other animals.
e) Ammonification is the conversion of organic nitrogen compounds into ammonia.
(1) Nitrogen-containing wastes are decomposed by ammonifying bacteria, which produce
ammonia.
(2) These bacteria are recyclers of organic nitrogen.
f)
Denitrification is the conversion of nitrate to gaseous nitrogen, which represents a net loss
of available nitrogen in the system.
(1) Denitrifying bacteria are anaerobes.
g) Human interaction with the nitrogen cycle can cause water pollution.
(1) Production of nitrogenous fertilizers can enrich bodies of water when agricultural areas
are eroded.
(2) Nitrogen oxides are produced in automobile exhaust.
(3) Nitrogen oxides are also an ingredient of photochemical smog.
V. The phosphorus cycle lacks a gaseous component.
1. Phosphorus cycles from the land to sediments and back to the land.
2. Erosion releases phosphate to the soil where it may be assimilated by plant roots.
3. Phosphate is returned to the soil by decomposers.
4. Phosphate may be deposited in oceanic sediments where it remains unavailable for millions of
years.
5. Humans affect the natural cycling of phosphorus.
a) Fertilizers, runoff containing animal wastes, and sewage introduce phosphorus into aquatic
ecosystems.
b) Phosphorus loss from ecosystems is accelerated by clear-cutting.
W. Water moves among the ocean, land, and atmosphere in the hydrological cycle.
1. Water may evaporate from land and enter the atmosphere, flow to the ocean, or enter the
groundwater.
2. Water in the atmosphere is ultimately precipitated on the land or ocean.
XI.
XII.
Ecosystem regulation from the bottom up and the top down.
X. Bottom-up processes are based on food webs; they regulate ecosystem function by nutrient cycling
and other aspects of the abiotic environment.
1. Bottom-up processes predominate in certain aquatic ecosystems where nitrogen or phosphorus
is limiting.
Y. Top-down processes regulate ecosystem function by trophic interactions, particularly from the
highest trophic level.
1. Top-down regulation appears to predominate in ecosystems with few trophic levels and low
species richness.
Abiotic factors in ecosystems
Z. The sun warms the Earth.
1. Solar energy powers biogeochemical cycles.
a) All solar energy is ultimately radiated back into space.
AA. Temperature changes with latitude.
1. The sun’s rays strike the equatorial regions vertically, which results in warmer temperatures.
BB. Temperature changes with season.
1. The tilting of the Earth’s axis causes the amount of solar radiation to vary during the year.
CC. The atmosphere contains several gases essential to organisms.
1. The atmosphere protects the Earth’s surface from radiation.
2. The sun drives global atmospheric circulation.
a) Differential warming of the Earth results in atmospheric circulation.
(1) Heated air at the equator rises, flows away from the equator, cools, and sinks.
3. The atmosphere exhibits complex horizontal movements.
a) Horizontal movements are called winds.
(1) The Coriolis effect is the deflection of winds by the Earth’s rotation.
(2) Winds are deflected to the right in the Northern Hemisphere and to the left in the
Southern Hemisphere.
DD. The global ocean covers most of Earth’s surface.
1. Four interconnected oceans may be described: the Pacific, Atlantic, Indian, and Arctic Oceans.
a) The Pacific Ocean contains more than half of the Earth’s water.
2.
Surface ocean currents are driven by winds.
a) Ocean currents are produced by prevailing winds.
b) Circular ocean currents are gyres.
c) The continents affect oceanic circulation.
(1) The Southern Hemisphere has a circumpolar flow of water.
(2) The current along the eastern United States is northward; the current along the
western United States is southward.
3. The ocean interacts with the atmosphere.
a) The El Niño-Southern Oscillation (ENSO) is a periodic warming of the ocean in the tropical
East Pacific that alters climatic patterns in many far distant areas.
EE. Climate profoundly affects organisms.
1. Weather is the atmospheric condition at a given place and time.
a) Climate is the average weather.
2. Air and water movements and surface features affect precipitation patterns.
a) Precipitation occurs when air reaches its saturation point.
b) Precipitation is highest on the windward coast of continents.
c) Temperate continental interiors are drier.
3. Some mountains cause rain shadows.
a) As air rises over mountains, it cools, and precipitation occurs.
b) The leeward side of mountains has little precipitation.
4. Microclimates are local variations in climate.
a) Microclimates may be created by differences in terrain, or by the organisms themselves.
FF. Fires are a common disturbance in some areas.
1. Fires free nutrients tied up in dry organic material.
2. Fires remove plant cover and promote germination of many seeds.
3. Fires are a natural event in the African savanna, the California chaparral, the North American
grasslands, and the Ponderosa pine forests.
4. Controlled burning is used to burn plant litter before it reaches dangerous levels.
Conservation Biology
XIII.
The biodiversity crisis
GG. Biological extinction is the eventual fate of all species, but humans are greatly increasing the
extinction rate.
1. Environmental sustainability is the ability to meet human needs (wants?) without compromising
the ability of future generations to meet their needs.
2. Extinction is the death of all members of a species.
3. Biodiversity is the variations among organisms.
a) Species richness is the number of species.
b) Genetic diversity is the genetic variety within a species.
c) Ecosystem diversity is the variety of ecosystems.
d) Minimum viable population is the smallest population that has a high chance of enduring
into the future; small population numbers can lead to lack of genetic diversity and thus
extinction due to inbreeding.
4.
According to the United States Endangered Species Act, a species is declared endangered when
it is in danger of becoming extinct.
5. A threatened species is one whose population has become greatly diminished and will probably
become endangered.
HH. Human activities contribute to declining biological diversity.
1. Fragmentation of habitat leads to species decline.
2. Pollution greatly affects populations.
3. Biotic pollution (introduction of exotics) harms native species due to competition, predation, or
interbreeding; invasive species are those foreign species that cause economic or environmental
harm.
a) Island populations are particularly sensitive to introductions.
b) Another example is the zebra mussel, introduced into the American Great Lakes.
4. Some species have become endangered or extinct by deliberate human action to eradicate
them.
a) Prairie dogs, wolves, mountain lions, and grizzly bears are examples.
5. Unregulated hunting, or overhunting, has led to the decline or extinction of many birds and large
mammals.
6. Commercial harvesting removes live animals and plants from the wild for research, zoos, and
the pet trade.
II. Where is the problem of declining biological diversity greatest?
1. Declines in diversity are greatest in the United States in California, Hawaii, Nevada, Alabama,
Utah, and Florida.
2. Worldwide, tropical rain forests are adversely affected by human populations.
JJ. Conservation biology addresses the issue of declining biological diversity.
1. In situ conservation includes establishment of preserves to protect wildlife in its natural and
original habitat.
2. In situ conservation is the best way to preserve biological diversity.
a) In situ conservation preserves natural ecosystems.
3. Restoring damaged or destroyed habitats is the goal of restoration ecology.
4. Ex situ conservation includes protection of species in zoos and seed banks.
5. Ex situ conservation is used in an attempt to save species on the brink of extinction.
a) Zoos, aquaria, and botanical gardens are examples of ex situ conservation.
b) Artificial insemination and host mothering may be used to increase the number of offspring.
XIV. Deforestation
KK. After deforestation, soils erode, causing increased turbidity of rivers and lakes; this may lead to
desertification in dry areas.
1. Deforestation leads to loss of biological diversity.
2. Globally, loss of forests interferes with the hydrologic cycle by reducing sylvan (originating from
trees) precipitation.
3. Deforestation is involved in global warming.
LL. Where and why are forests disappearing?
1. Significant loss of forests in temperate regions has occurred during the past 1,000 years.
a) Deforestation in the tropics has accelerated during the last few decades.
b) Deforestation in tropical areas is caused primarily by subsistence agriculture, commercial
logging, and cattle ranching.
2. Subsistence agriculture is the most important cause of deforestation.
a) Subsistence agriculture is the production of food to feed the family.
b) Slash-and-burn agriculture produces high crop yields initially, followed by rapid decline as
soil fertility decreases.
3.
4.
c) This type of agriculture is only sustainable with low population density.
Commercial loggers remove vast tracts of tropical rain forests.
a) Commercial logging accounts for over 20% of tropical deforestation.
b) Logging is removing forests at a rate much greater than is sustainable.
Cattle ranching also causes deforestation.
a) Cattle ranching in South and Central America supplies developed countries with inexpensive
beef.
XV.
XVI. Global warming
MM.
Global warming is a long-term increase in average temperatures.
1. The past two decades have been the warmest in the 20 th century.
2. During the 20th century, Earth’s average surface temperature increased 0.6 degrees C.
NN. Greenhouse gases probably cause global warming.
1. Carbon dioxide, methane, surface ozone, nitrous oxide, and chlorofluorocarbons are increasing
in concentration in the atmosphere.
2. Combustion of fossil fuels, burning of rain forests, leaking of old refrigerants, and
decomposition all contribute these gases.
3. These gases all retain atmospheric heat and contribute to the greenhouse effect.
OO. What are the probable effects of global warming?
1. With global warming, the sea level is expected to rise.
a) Thawing of the glaciers and icecaps will cause the sea level to rise.
b) Countries most vulnerable to this change are those that inhabit river deltas.
2. With global warming, precipitation patterns may change.
a) Some areas may experience desertification; others may have more frequent flooding.
3. With global warming, the ranges of organisms will change.
a) r-selected organisms are likely to prosper during this change.
4. Global warming will probably affect agriculture.
a) Global warming will change productivity and increase droughts and floods in different areas.
XVII. Declining stratospheric ozone
PP. Ozone is a natural component of the stratosphere.
1. Ozone shields the surface of Earth from UV radiation.
2. Ozone depletion appeared over Antarctica most demonstrably in 1985.
QQ. Certain chemicals destroy stratospheric ozone.
1. CFCs are broken down by UV in the stratosphere and react with ozone, forming molecular
oxygen.
2. CFCs are not used up in this reaction and are able to break down many thousands of ozone
molecules.
RR. Ozone depletion harms living organisms.
1. Exposure to UV is linked to disorders in humans, including cataracts, skin cancer, and weakened
immune systems.
2. Exposure to increasing UV is linked to declines in phytoplankton productivity.
XVIII. Connections among environmental problems
SS. Problems such as deforestation, global warming, and ozone depletion are interconnected, and all
will result in extinction of species.
TT. Consumption overpopulation refers to the excessive consumption by the developed countries, and
has a global impact.