Download Test Review 52-53

Chapter 52
An Introduction to Ecology and the Biosphere
Lecture Outline
Overview: The Scope of Ecology
Biologists record and transmit data on the annual 8,000-km migration of gray whales that calve in
Baja California, collecting information that has helped this population recover from the brink of
extinction.
o
A century ago, whaling had reduced this population to a few hundred individuals.
o
After 70 years of protection, more than 20,000 gray whales travel to the Arctic each year.
What environmental factors determine the geographic distribution of gray whales? How do variations
in their food supply affect the size of the gray whale population?

These questions are the subject of ecology, the scientific study of the interactions between
organisms and their environment.
o
These interactions occur on a hierarchy of scales from organismal to global.
Concept 52.1 Ecology integrates all areas of biological research and informs environmental decision
making.
Ecology’s roots are in discovery science.

Naturalists have long observed organisms in nature and recorded their observations.
o
Natural history remains a fundamental part of the science of ecology.
Modern ecology is also a rigorous experimental science.

Ecologists generate hypotheses, manipulate the environment, and systematically record their
observations.
○
For example, Paul Hanson and his colleagues at Oak Ridge National Laboratory created drought
and wet conditions on experimental plots of native Tennessee forest.
o
By comparing the growth and survival of trees in each plot, the researchers found that flowering
dogwoods, Cornus florida, were more likely to die in drought conditions than were members of
any other woody species examined.
Ecology and evolutionary biology are closely related sciences.
Organisms adapt to their environment over many generations through the process of natural
selection.
The differential survival and reproduction of individuals that lead to evolution occur in ecological time
(minutes to years).
o
For example, a severe drought affected the survival of Galápagos finches through the evolution
of larger beaks.
o
Finches with bigger beaks were better able to eat the large, hard seeds available during the
drought.
o
Smaller-beaked birds, which could eat only the smaller, softer seeds that were in short supply,
were less likely to survive.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-1
The link between ecology and evolution is evident in the world around us.

For example, a farmer applies a fungicide to protect a wheat crop from a fungus, thus increasing crop
yield.

After a few years, the farmer has to apply more and more fungicide to obtain the same protection.

The fungicide has altered the gene pool of the fungus—an evolutionary effect—by selecting for
individuals that are resistant to the chemical.

Eventually, the fungicide works so poorly that the farmer needs a different, more potent chemical to
control the fungus.
Ecological research ranges from the adaptations of individual organisms to the dynamics of the
biosphere.
Ecology can be divided into different areas of study.
1. Organismal ecology is concerned with the behavioral, physiological, and morphological
ways individuals interact with the environment.
2. A population is a group of individuals of the same species living in a particular
geographic area. Population ecology examines factors that affect population size and
composition.
3. A community consists of all the organisms of all the species that inhabit a particular
area. Community ecology examines the interactions between species and considers
how factors such as predation, competition, disease, and disturbance affect community
structure and organization.
4. An ecosystem consists of all the abiotic factors in addition to the entire community of
species that exist in a certain area. Ecosystem ecology studies energy flow and cycling of
chemicals among the various abiotic and biotic components.
Define
and list
from
most to
least
inclusive
A landscape or seascape consists of several different ecosystems linked by exchanges of energy,
materials, and organisms. Landscape ecology deals with arrays of ecosystems and their arrangement
in a geographic region.
o
Each landscape or seascape consists of a mosaic of different types of patches, an environmental
characteristic ecologists refer to as patchiness.
o
Landscape ecological research focuses on the factors that control exchanges of energy, materials,
and organisms among ecosystem patches.
The biosphere is the global ecosystem, the sum of all of the planet’s ecosystems. The biosphere
includes the entire portion of Earth inhabited by life, ranging from the atmosphere to a height of
several kilometers to the oceans and water bearing rocks to a depth of several kilometers.
Ecology provides a scientific context for evaluating environmental issues.
Because of ecology’s usefulness in conservation and environmental efforts, many people associate
ecology with environmentalism, advocacy for the protection of nature, conservation and preserving
living things

Ecologists distinguish between science and advocacy, however.
o
How society uses ecological knowledge does not depend solely on science.
The distinction between knowledge and advocacy is clear in the guiding principles of the Ecological
Society of America, a scientific organization that strives to “ensure the appropriate use of ecological
science in environmental decision making.”
In her 1962 book Silent Spring, Rachel Carson wrote: “The ‘control of nature’ is a phrase conceived in
arrogance, born of the Neanderthal age of biology and philosophy, when it was supposed that nature
exists for the convenience of man.”
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-2
o
Carson warned that the widespread use of pesticides such as DDT was causing population
declines in many organisms besides the insects targeted for control.
o
Her efforts led to a ban on DDT use in the United States and more stringent controls on the use
of other chemicals.
Concept 52.2 Interactions between organisms and the environment limit the distribution of species.
Ecologists have long recognized distinct global and regional patterns in the distribution of organisms.
Biogeography is the study of past and present distributions of individual species in the context of
evolutionary theory.
Ecologists ask a series of questions to determine what limits the geographic distribution of any
species.

Ecologists ask why species occur where they do, focusing on the factors that determine the
distribution of species.

Biotic factors include all the living organisms in the individual’s environment.

Abiotic factors are nonliving chemical and physical factors such as temperature, light, water, and
nutrients that influence the distribution and abundance of organisms.
○
For example, red kangaroos are abundant in areas of Australia that have sparse and variable
rainfall.
o
An abiotic factor—precipitation—may determine where red kangaroos live.
o
It is also possible that climate influences red kangaroo populations indirectly, through biotic
factors such as pathogens, parasites, predators, competitors, and food availability.
Species dispersal contributes to the distribution of organisms.
The movement of individuals away from centers of high population density or from their area of
origin is called dispersal.
o
Perhaps there are no kangaroos in North America due to barriers to their dispersal.
The dispersal of organisms is crucial to understanding geographic isolation in evolution and the broad
patterns of current geographic distribution of species.
○
For example, cattle egrets were found only in Africa and southwestern Europe in the early 1800s.
o
After these birds crossed the Atlantic Ocean and colonized northeastern South America, they
were able to spread throughout Central America, reaching North America by the 1950s.
o
Today, cattle egrets have breeding populations as far west as the Pacific Coast and as far north as
southern Canada.
One way to determine whether dispersal is a key factor limiting distribution is to observe the results
when humans have accidentally or intentionally transplanted a species to areas where it was
previously absent.

For the transplant to be considered successful, the organisms must not only survive in the new area
but also reproduce there.
○
If the transplant is successful, then the potential range of the species is larger than its actual
range.
o
In other words, the species could live in areas where it currently does not.
Ecologists rarely conduct transplant experiments today because species introduced into new
geographic locations may disrupt the communities and ecosystems to which they are introduced.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-3
o
Instead, ecologists study the outcome when a species has been transplanted accidentally or for
another purpose.
Behavior and habitat selection contribute to the distribution of organisms.
Sometimes organisms do not occupy all of their potential range but select particular habitats.
Does behavior play a role in limiting the distribution in such cases?
Habitat selection is one of the least-understood ecological processes, but it appears to play an
important role in limiting the distribution of many species.
o
Female insects often deposit eggs only in response to a very narrow set of stimuli, which may
restrict distribution of the insect to certain host plants.
o
For example, the European corn borer can feed on a wide variety of plants but is found only on
corn. Egg-laying females are attracted by the odors of corn plants.
Biotic factors affect the distribution of organisms.
Negative interactions with other organisms in the form of predation, parasitism, or competition may
limit the ability of organisms to survive and reproduce.

Survival and reproduction may also be limited by the absence of species on which the transplanted
species depends, such as pollinators for flowering plants.

Predators and herbivores are examples of biotic factors that limit the distribution of species.
In certain marine ecosystems, there is an inverse relationship between the abundances of sea urchins
and seaweeds.
o
Sea urchins graze on seaweeds, preventing the establishment of large stands of seaweeds.
W. J. Fletcher tested the hypothesis that sea urchins are a biotic factor limiting seaweed distribution
by removing sea urchins from experimental plots.

He observed a dramatic increase in seaweed cover, showing that urchins limit seaweed distribution.
The presence or absence of food resources, parasites, diseases, and competing organisms can act as
biotic limitations on species distribution.
Abiotic factors affect the distribution of organisms.
The global distribution of organisms broadly reflects the influence of abiotic factors such as
temperature, water, and sunlight.
The environment is characterized by spatial heterogeneity and temporal heterogeneity.
Organisms can temporarily avoid stressful conditions through behaviors such as dormancy or
hibernation.
Environmental temperature is an important factor in the distribution of organisms because of its
effect on biological processes.
o
Very few organisms can maintain an active metabolism at very high or very low temperatures.
o
Cells may rupture if the water they contain freezes, and most proteins denature at temperatures
above 450C.
Some organisms have extraordinary adaptations that allow them to live outside the temperature
range habitable for most other living things, but more organisms function best within a specific range
of temperatures.
o
Endotherms may expend energy regulating their internal temperature at temperatures outside
that range.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-4
The variation in water availability among habitats is an important factor in species distribution.
o
Intertidal species may face desiccation as the tide recedes.
o
Terrestrial organisms face a nearly constant threat of desiccation and have adaptations that
allow them to obtain and conserve water.
o
Desert organisms, for example, have a variety of adaptations for acquiring and conserving water
in dry environments.
The salt concentration of the environment affects the water balance of organisms through osmosis.
o
Most aquatic organisms have a limited ability for osmoregulation and are restricted to either
freshwater or marine habitats.
o
High-salinity habitats such as salt flats typically contain few species.
Sunlight provides the energy that drives nearly all ecosystems.
o
Light intensity is not the most important factor limiting plant growth in most terrestrial
environments, although shading by a forest canopy creates intense competition for light in the
understory.
In aquatic environments, light intensity limits the distribution of photosynthetic organisms.
o
Every meter of water depth selectively absorbs 45% of red light and 2% of blue light passing
through it.
o
As a result, most photosynthesis in aquatic environments occurs near the surface.
Too much light can also limit the survival of organisms.
o
The atmosphere is thinner at higher elevations, absorbing less ultraviolet radiation. The sun’s
rays are more likely to damage DNA and proteins in alpine environments.
The physical structure, pH, and mineral composition of soils and rocks limit the distribution of plants
and, thus, of the animals that feed on them, contributing to the patchiness of terrestrial ecosystems.
o
The distribution of organisms at extreme acidic and basic conditions is limited.
o
pH can also have indirect effects by altering the solubility of nutrients and toxins.
In streams and rivers, substrate composition can affect water chemistry, thus influencing the
distribution of organisms.
In freshwater and marine environments, the structure of substrates limits the organisms that can
attach to or burrow in those habitats.
Four abiotic factors are the major components of climate.
Climate is the long-term, prevailing weather conditions in a particular area.

Four abiotic factors—temperature, water, sunlight, and wind—are the major components of climate.

Climatic factors, especially temperature and water, have a major influence on the distribution of
organisms.
Climate patterns can be described on two scales.

Macroclimate patterns are on global, regional, or local levels, and microclimate patterns are very
fine patterns, such as the conditions experienced by a community of organisms under a fallen log.
Global climate patterns are determined by sunlight and Earth’s movement in space.

The sun’s warming effect on the atmosphere, land, and water establishes the temperature variations,
cycles of air movement, and evaporation of water that are responsible for latitudinal variations in
climate.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-5
Bodies of water and topographic features such as mountain ranges create regional climatic variations,
while smaller features of the landscape affect local climates.

Seasonal variation may influence climate.
Ocean currents influence climate along the coast by heating or cooling overlying air masses, which
may pass over land.
o
Coastal regions are generally moister than inland areas at the same latitude.
o
In general, oceans and large lakes moderate the climate of nearby terrestrial environments.
In certain regions, cool, dry ocean breezes are warmed when they move over land, absorbing
moisture and creating a hot, rainless climate slightly inland.
o
This pattern draws a cool breeze from the water across the land.
o
At night, air over the ocean rises, drawing cooler air from the land back out over the water and
replacing it with warmer offshore air.
o
This Mediterranean climate pattern occurs inland from the Mediterranean Sea.
Mountains have a significant effect on the amount of sunlight reaching an area as well as on local
temperature and rainfall.
o
In the Northern Hemisphere, south-facing slopes receive more sunlight than north-facing slopes
and are therefore warmer and drier.
o
These environmental differences affect species distribution.
At any given latitude, air temperature declines 6°C with every 1,000-m increase in elevation.
o
This temperature change is equivalent to the change caused by an 880-km increase in latitude.
o
Biological communities on mountains are similar to those at lower elevations farther from the
equator.
As moist, warm air approaches a mountain, it rises and cools, releasing moisture on the windward
side of the peak.

On the leeward side of the mountain, cool, dry air descends, absorbing moisture and producing a
“rain shadow.”
o
Deserts are commonly on the leeward side of mountain ranges.


The changing angle of the sun over the course of a year affects local environments.
Belts of wet and dry air on either side of the equator shift with the changing angle of the sun,
producing marked wet and dry seasons at around 20° latitude, where many tropical deciduous
forests grow.
If the tilt of the Earth’s axis were increased, summers and winters in the United States would
likely become warmer and colder, respectively.


Seasonal changes in wind patterns produce variations in ocean currents, occasionally causing the
upwelling of nutrient-rich, cold water from deep ocean layers.
o
This nutrient-rich water stimulates the growth of surface-dwelling phytoplankton and the
organisms that feed on them.
Many features in the environment influence microclimates.
Forest trees moderate the microclimate beneath them.
o
Cleared areas experience greater temperature extremes than the forest interior.
Within a forest, low-lying ground is usually wetter than high ground and tends to be occupied by
different species of trees.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-6
A log or large stone shelters organisms, buffering them from temperature and moisture fluctuations.

Every environment on Earth is characterized by a mosaic of small-scale differences in abiotic factors
that influence the local distribution of organisms.
Long-term climate changes profoundly affect the biosphere.
One way to predict the possible effects of current climate changes is to consider the changes that
have occurred in temperate regions since the end of the last ice age.

Until about 16,000 years ago, continental glaciers covered much of North America and Eurasia.

As the climate warmed and the glaciers melted, the distribution of trees expanded northward.
o
A detailed record of these migrations is captured in fossil pollen in lake and pond sediments.
If researchers can determine the climatic limits of current geographic distributions for individual
species, they can predict how that species distribution will change with global warming.

A major question for tree species is whether seed dispersal rate is rapid enough to sustain the
migration of the species as climate changes.
1. An example to consider is the eastern hemlock, whose movement north was delayed
nearly 2,500 years at the end of the last ice age.

This delay in seed dispersal was partly attributable to the lack of “wings” on the seeds, which tend to
fall close to the parent tree.
The fossil record can inform predictions about the biological impact of current global warming trends
on the geographic range of the American beech, Fagus grandifolia.
Two different climate-change models are used to compare the current and predicted geographic
ranges of this tree.

These models predict that the northern limit of the beech’s range will move 700–900 km north over
the next century and its southern range will move northward even farther.

The beech will have to migrate 7–9 km per year to maintain its distribution in a warming climate.

However, since the ice age, the beech has migrated into its present range at a rate of only 0.2 km per
year.

Without human assistance, the beech will have a much smaller range and may become extinct.
Concept 52.3 Aquatic biomes are diverse and dynamic systems that cover most of Earth.
Varying combinations of biotic and abiotic factors determine the nature of Earth’s biomes, major
types of ecological associations that occupy broad geographic regions of land or water.
Ecologists distinguish between freshwater and marine biomes on the basis of physical and chemical
differences.

Marine biomes generally have salt concentrations that average 3%, whereas freshwater biomes have
salt concentrations of less than 0.1%.
Marine biomes cover approximately 75% of Earth’s surface and have an enormous effect on the
biosphere.
o
The evaporation of water from the oceans provides most of Earth’s rainfall.
o
Ocean temperatures have a major effect on world climate and wind patterns.
o
Photosynthesis by marine algae and photosynthetic bacteria produces a substantial proportion
of Earth’s oxygen. Respiration by these organisms consumes huge amounts of atmospheric
carbon dioxide.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-7
Freshwater biomes are closely linked to the soils and biotic components of the terrestrial biomes
through which they pass.

The pattern and speed of water flow and the surrounding climate are also important factors.
Most aquatic biomes are physically and chemically stratified.
Light is absorbed by the water and by photosynthetic organisms, so light intensity decreases rapidly
with depth.

There is sufficient light for photosynthesis in the upper photic zone.

Very little light penetrates to the lower aphotic zone.
The substrate at the bottom of an aquatic biome is the benthic zone.
o
The benthic zone is made up of sand and sediments and is occupied by communities of
organisms called benthos.
o
A major food source for benthos is dead organic material or detritus, which “rains” down from
the productive surface waters of the photic zone.
The most extensive part of the open ocean is the abyssal zone, regions where the water is 2,000–
6,000 m deep.
Sunlight warms surface waters, while deeper waters remain cold.

As a result, water temperature in lakes is stratified, especially in summer and winter.

In the ocean and most lakes, a narrow stratum of rapid temperature change called a thermocline
separates the more uniformly warm upper layer from more uniformly cold deeper waters.
Lakes tend to be particularly layered with respect to temperature, especially during summer and
winter.
o
Many temperate lakes undergo a semiannual turnover of oxygenated surface waters and
nutrient-rich bottom waters in spring and autumn because the density of water changes as
seasonal temperature changes
o
This turnover is essential to the survival and growth of shallow and deep-water organisms.
In aquatic biomes, community distribution is determined by the depth of the water, degree of light
penetration, distance from shore, and open water versus bottom.
o
In marine and deep lake communities, phytoplankton, zooplankton, and many fish species live in
the relatively shallow photic zone.
o
The aphotic zone contains little life, except for microorganisms and relatively sparse populations
of luminescent fishes and invertebrates.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-8
Chapter 53
Population Ecology
Lecture Outline
Overview: Counting Sheep

Ecologists have been studying a population of sheep on the Scottish island of Hirta for more
than 50 years.
o These Soay sheep are the closest living relatives of domesticated sheep.
o In 1932, the population of sheep was established on Hirta Island, where food is plentiful
and predators are absent.
The population of Soay sheep has fluctuated dramatically, sometimes changing by more
than 50% from year to year.
Why do populations of some species fluctuate greatly while others do not?
Population ecology is the study of populations in relation to their environment, including
biotic and abiotic influences on population density, distribution, size, and age structure.
Concept 53.1 Dynamic biological processes influence population density, dispersion,
and demographics.
A population is a group of individuals of a single species that live in the same general area.
Members of a population rely on the same resources, are influenced by similar
environmental factors, and have a high likelihood of interacting and breeding with one
another.
Populations can evolve through natural selection acting on heritable variations among
individuals and changing the frequencies of various traits over time.





Three fundamental characteristics of individuals in any population are density, dispersion,
and demographics.
Every population has a specific size and specific geographic boundaries.
The density of a population is the number of individuals per unit area or xvolume.
The dispersion of a population is the pattern of spacing among individuals within the
geographic boundaries.
Measuring the density of populations is a difficult task.
Ecologists can count individuals (which is very accurate but very difficult to do), but they
usually estimate population numbers.
o It is almost always impractical to count all the individuals in a population.
Ecologists use a variety of sampling techniques to estimate densities and total population
sizes.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-9
o
o
For example, ecologists might count the number of individuals in randomly located
plots, calculate the average density in the samples, and extrapolate to estimate the
population size in the entire area.
 Such estimates are accurate when there are many sample plots and a homogeneous
habitat.
In other cases, instead of counting individuals, population ecologists estimate density
from an index of population size, such as the number of nests, burrows, tracks, calls, or
fecal droppings.
A sampling technique that researchers commonly use to estimate wildlife populations is the
mark-recapture method.
N = m·n/x
N = estimation population
m = number of marked and released individuals from first capture
x = number of recaptured marked individuals in the second capture



n = total number of individuals captured in the second capture
Individuals are trapped and captured, marked with a tag, recorded, and then released.
After a period of time has elapsed, traps are set again and individuals are captured and
identified.
The second capture yields both marked and unmarked individuals.
From counts of these individuals, researchers estimate the total number of individuals in the
population.ion
o The mark-recapture method assumes that each marked individual has the same
probability of being trapped as each unmarked individual.
o This may not be a safe assumption because trapped individuals may be more or less
likely to be trapped a second time.
Population density results from the dynamic interplay between processes that add
individuals to a population and processes that remove individuals from it.
o Additions to a population occur through birth (including all forms of reproduction) and
immigration (the influx of new individuals from other areas).
o The factors that remove individuals from a population are death (mortality) and
emigration (the movement of individuals out of a population).
o Immigration and emigration may represent biologically significant exchanges between
populations.



Within a population’s geographic range, local densities may vary substantially.
Variations in local density are important population characteristics, providing insight into
the environmental and social interactions of individuals within a population.
Some habitat patches are more suitable than others.
Social interactions between members of a population may maintain patterns of spacing.
Dispersion is clumped when individuals aggregate in patches.
o Plants and fungi are often clumped where soil conditions favor germination and growth.
o Animals may clump in favorable microenvironments (such as salamanders under a fallen
log) or to facilitate mating interactions.
o Group living may increase the effectiveness of certain predators, such as a wolf pack.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-10
Dispersion is uniform when individuals are evenly spaced.
o For example, some plants secrete chemicals that inhibit the germination and growth of
nearby competitors.
o
Animals often exhibit uniform dispersion as a result of territoriality, the defense of a
bounded space against encroachment by others.
o When members of a population are uniformly distributed, it
suggests that the members of the population are competing for
access to a resource
In random dispersion, the position of each individual is independent of the others, and
spacing is unpredictable.
o Random dispersion occurs in the absence of strong attraction or repulsion among
individuals in a population, or when key physical or chemical factors are relatively
homogeneously distributed.
o For example, plants may grow where windblown seeds land.
o Random patterns are uncommon in nature.
Demography is the study of the vital statistics of populations and how they change over
time.
o Of particular interest are birth rates and how they vary among individuals (specifically
females) and death rates.
A life table is an age-specific summary of the survival pattern of a population.
The best way to construct a life table is to follow the fate of a cohort, a group of individuals of
the same age, from birth throughout their lifetimes until all are dead.
 Population ecologists follow the fate of the same aged cohorts to determine the birth
and death rate of each group in the population.
.
 To build a life table, researchers need to determine the number of individuals that die in
each age group and calculate the proportion of the cohort surviving from one age to the
next.
A graphic way of representing the data in a life table is a survivorship curve, a plot of the
numbers or proportion of individuals in a cohort of 1,000 individuals still alive at each age.
Natural populations exhibit several patterns of survivorship.
A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle
life, and drops steeply as death rates increase among older age groups.
o Humans and many other large mammals exhibit Type I survivorship curves.
The Type II curve is intermediate, with constant mortality over an organism’s life span.
o Many species of rodent, various invertebrates, and some annual plants show Type II
survivorship curves.
A Type III curve drops sharply at the start, reflecting very high death rates early in life, but
flattens out as death rates decline for the few individuals that survive to a critical age.
○
Type III survivorship curves are associated with organisms that produce large numbers
of offspring but provide little or no parental care.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-11
o
Examples are many fishes, long-lived plants, and marine invertebrates.
Many species fall somewhere between these basic survivorship curves or show more
complex curves.
o Some invertebrates, such as crabs, show a “stair-stepped” curve, with increased
mortality during molts.
o In birds, mortality is often high among chicks (Type II) but is fairly constant among adults
(Type III).
In populations without immigration or emigration, survivorship and reproductive rates are
the two key factors determining changes in population size.
Demographers who study sexually reproducing populations usually ignore males and focus
on females because only females give birth to offspring.
A reproductive table is an age-specific summary of the reproductive rates in a population,
constructed by measuring the reproductive outputs of cohorts from birth until death.


For sexual species, a reproductive table tallies the number of female offspring produced by
each age group.
The reproductive output for sexual species is the product of the proportion of females of a
given age that are breeding and the number of female offspring of those breeding females.
Reproductive tables vary greatly from species to species.
o Squirrels have a litter of two to six young once a year for less than a decade, for
example, whereas oak trees drop thousands of acorns each year for tens or hundreds of
years.
Concept 53.2 Life history traits are products of natural selection.
Natural selection favors traits that improve an organism’s chances of survival and
reproductive success.
In every species, there are trade-offs between survival and traits such as frequency of
reproduction, number of offspring produced, and investment in parental care.
The traits that affect an organism’s schedule of reproduction and survival make up its life
history.
Life histories are highly diverse, but they exhibit patterns in their variability.
Life histories involve three basic variables: when reproduction begins, how often the
organism reproduces, and how many offspring are produced during each reproductive
episode.
Life history traits are evolutionary outcomes reflected in the development, physiology, and
behavior of an organism.
Some organisms, such as the Pacific salmon, exhibit what is known as big-bang
reproduction, in which a salmon returns to freshwater streams to spawn and then die.
o This pattern, known as semelparity, also occurs in plants such as the agave or century
plant.
o Agaves grow in arid climates with unpredictable rainfall and poor soils.
o Agaves grow for years, accumulating nutrients in their tissues, until there is an unusually
wet year. They then send up a large flowering stalk, produce seeds, and die.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-12
o
This life history is an adaptation to the harsh desert conditions in which the agave lives.
By contrast, some organisms produce only a few offspring during repeated reproductive
episodes.
o This pattern, known as iteroparity, is typical of lizards that produce a few large eggs
annually for several years.
What factors contribute to the evolution of semelparity versus iteroparity?
o In other words, how much does an individual gain in reproductive success through one
pattern versus the other?
A current hypothesis answers this question based on the survival rate of the offspring and
the likelihood the adult will survive to reproduce again.
When the survival rate of offspring is low, as in highly variable or unpredictable
environments, big-bang reproduction (semelparity) is favored.
o Adults are also less likely to survive in such environments.
Repeated reproduction (iteroparity) is favored in dependable environments, where adults
are more likely to survive to breed again and competition for resources is intense.
o In such environments, a few, well-provisioned offspring have a better chance of
surviving to reproductive age.
Oak trees and sea urchins are intermediate between the two extremes. Both live a long
time, but repeatedly produce large numbers of offspring.
Limited resources mandate trade-offs between investment in reproduction and survival.

Life histories represent an evolutionary resolution of conflicting demands.
Organisms may make trade-offs between survival and reproduction when resources are
limited.
o For example, red deer females have a higher mortality rate in winters that follow
summers in which they reproduce.
Selective pressures also influence the trade-off between number and size of offspring.
o Plants and animals whose young are subject to high mortality rates often produce large
numbers of relatively small offspring.
o Plants that colonize disturbed environments usually produce many small seeds, only a
few of which reach a suitable habitat.
o Smaller seed size may increase the chance of seedling becoming established by enabling
seeds to be carried longer distances to a broader range of habitats.
o Animals that suffer high predation rates, like quail and rabbits, sardines, and mice, tend
to produce many offspring.
In other organisms, extra investment on the part of the parent greatly increases the
offspring’s chances of survival.
o Walnuts trees and coconut palms both have large seeds with a large store of energy and
nutrients to help the seedlings become established.
o In animals, parental care does not always end after incubation or gestation.
o Primates provide an extended period of parental care to only a few offspring.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-13
Concept 53.3 The exponential model describes population growth in an idealized,
unlimited environment.
All populations have a tremendous capacity for growth.
Unlimited population increase does not occur indefinitely for any species, however, either in
the laboratory or in nature.
The study of population growth in an idealized, unlimited environment reveals the capacity
of species for increase and the conditions in which that capacity may be expressed.
Imagine a hypothetical population living in an ideal, unlimited environment.

For simplicity’s sake, we ignore immigration and emigration and define a change in
population size during a fixed time interval using the following verbal equation:
Change in population size
=
Births during
−
Deaths during
during time interval
time interval
time interval
Using mathematical notation, we can express this relationship more concisely.
o If N represents population size and t represents time, then N is the change in
population size and t is the time interval.
o We can rewrite the verbal equation as
N/t = B − D
where B is the number of births during the time interval and D is the number of
deaths.
We can convert this simple model into one in which births and deaths are expressed as the
average number of births and deaths per individual during the specified time period.
The per capita birth rate is the number of offspring produced per unit time by an average
member of the population.
o For example, if there are 34 births per year in a population of 1,000 individuals, the
annual per capita birth rate is 34/1000, or 0.034.
If we know the annual per capita birth rate (expressed as b), we can use the formula B = bN
to calculate the expected number of births per unit time in a population of any size.
Similarly, knowing the per capita death rate (symbolized by d) allows us to calculate the
expected number of deaths per unit time in a population of any size using the formula D =
dN.
o If d = 0.016 per year, we would expect 16 deaths per year in a population of 1,000
individuals.
Now we can revise the population growth equation, using per capita birth and death
rates:
N/t = bN − dN

Population ecologists are most interested in the differences between the per capita birth
rate and the per capita death rate.
This difference is the per capita rate of increase, or r, which equals b − d.
The value of r indicates whether a population is growing (r > 0) or declining (r < 0).
If r = 0, then there is zero population growth (ZPG).
o Births and deaths still occur, but they balance exactly.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-14
Using the per capita rate of increase, we rewrite the equation for the change in population
size as
N/t = rN
Ecologists use differential calculus to express population growth instantaneously, as the
growth rate at a particular instant in time:
dN/dt = rinstN
where rinst is the instantaneous per capita rate of increase.
Population growth under ideal conditions is called exponential population growth.
o Under these conditions, we may assume the maximum growth rate for the population
(rmax), called the intrinsic rate of increase.
The equation for exponential population growth is
dN/dt = rmaxN
The size of a population that is growing exponentially increases at a constant rate, resulting
in a J-shaped growth curve when the population size is plotted over time.
o Although the maximum rate of increase is constant, the population accumulates more
new individuals per unit of time when it is large.
o As a result, the curve gets steeper over time.
EX PROBLEM USING dN/dt = rmaxN
A population of ground squirrels has an annual per capita birth
rate of 0.10 and an annual per capita death rate of 0.02. Estimate
the number of individuals added to (or lost from) a population of
1,000 individuals in one year.
Ans: rmax = .10 - .02 = .08
N =1000
.08 x 1000 = 80 INDIVIDUALS added
o
A population with a high intrinsic rate of increase grows faster than one with a lower rate of
increase.
The larger a population (the higher the value of N) is the faster its rate of growth will be.
o EX: A small population of white-footed mice has the same
intrinsic rate of increase (r) as a large population. If everything
else is equal, the large population will add more individuals
per unit time
o
J-shaped curves are characteristic of populations that are introduced into a new or unfilled
environment or whose numbers have been drastically reduced by a catastrophic event and
are rebounding.
Concept 53.4 The logistic growth model describes how a population grows more
slowly as it nears its carrying capacity.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-15
Typically, resources are limited and, as population density increases, each individual has
access to an increasingly smaller share of available resources.

Ultimately, there is a limit to the number of individuals that can occupy a habitat.
Ecologists define carrying capacity (K) as the maximum stable population size that a
particular environment can support.
o Carrying capacity is not fixed but varies over space and time with the abundance of
limiting resources.
o A population’s carrying capacity may change as environmental conditions change.
Energy limitations often determine the carrying capacity, although other factors, such as
shelters, refuges from predators, nutrient availability, water, and suitable nesting sites, can
all be limiting.
If individuals cannot obtain sufficient resources to reproduce, then the per capita birth rate
b declines.

If individuals cannot find and consume enough energy to maintain themselves, or if disease
or parasitism increases with density, then the per capita death rate d increases.
A decrease in b or an increase in d results in a lower per capita rate of increase r.
Ecologists can modify the mathematical model to incorporate changes in the growth rate as
the population size nears the carrying capacity.
In the logistic population growth model, the per capita rate of increase declines as the
carrying capacity is reached.
Mathematically, we start with the equation for exponential growth and add an expression
that reduces the per capita rate of increase as N increases.
If the maximum sustainable population size (carrying capacity) is K, then K − N is the number
of additional individuals the environment can accommodate and (K − N)/K is the fraction of
K that is still available for population growth.
By multiplying the intrinsic rate of increase rmax by (K − N)/K, we modify the growth rate of
the population as N increases:
dN/dt = rmaxN[(K − N)/K]
When N is small compared to K, the term (K − N)/K is large and the per capita rate of
increase is close to the intrinsic rate of increase.

When N is large and resources are limiting, N approaches K., d (death rate) increases. The
term (K − N)/K is small and so is the rate of population growth.

According to the logistic growth equation dN/dt = rmaxN(K — N)/k, population growth is
zero when N is equal to K
Population growth is greatest when the population is approximately half of the carrying
capacity.
o At this population size, there are many reproducing individuals, and the per capita rate
of increase remains relatively high.
r
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-16

New individuals are added to the population most rapidly at intermediate population sizes,
when there is not only a substantial breeding population but also lots of available space and
other resources in the population.
Why does the population growth rate slow as N approaches K? Population growth slows
as the birth rate b decreases or the death rate d increases.
How well does the logistic growth model fit the growth of real populations?

The growth of laboratory populations of some organisms fits an S-shaped curve fairly well.
These populations are grown in a constant environment without predators or competitors.
1 Some of the basic assumptions built into the logistic growth model do not apply to all
populations, however.
1 The logistic growth model assumes that populations adjust instantaneously and approach
the carrying capacity smoothly.
o In most natural populations, there is a lag time before the negative effects of increasing
population are realized.
o Populations may overshoot their carrying capacity before settling down to a relatively
stable density.
o
Some populations fluctuate greatly, making it difficult to define the carrying capacity.
The logistic growth model assumes that, regardless of the population density, each
individual added to the population has the same negative effect on the population growth
rate.
o Some populations show an Allee effect, in which individuals may have a more difficult
time surviving or reproducing if the population is too small.
o Animals may not be able to find mates in the breeding season at small population sizes.
o A plant may be protected in a clump of individuals but vulnerable to excessive wind if it
stands alone.
The logistic population growth model provides a basis from which ecologists can consider
how real populations grow and how to construct more complex models.
o The model is useful in conservation biology for estimating how rapidly a particular
population might increase after it has been reduced to a small size, or for estimating
sustainable harvest rates for fish or wildlife populations.
○
Conservation biologists can use the model to estimate the lower critical size below
which populations of certain species may become extinct.
The logistic growth model predicts different per capita growth rates for populations of low
or high density relative to the carrying capacity of the environment.
o At high densities, each individual has few resources available, and the population grows
slowly.
o At low densities, per capita resources are abundant, and the population can grow
rapidly.

Different life history features are favored under low and high population densities.
At high population density, selection favors adaptations that enable organisms to survive
and reproduce using few resources.
o Competitive ability and efficient use of resources should be favored in populations that
are at or near their carrying capacity.
o These are traits associated with iteroparity.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-17

At low population density, adaptations that promote rapid reproduction, such as the
production of numerous, small offspring, should be favored.
o These are traits associated with semelparity.
Ecologists have attempted to connect these differences in favored traits at different
population densities with the logistic model of population growth.
Selection for life history traits that are sensitive to population density is known as Kselection, or density-dependent selection.
○
○
○
o
K-selection tends to maximize population size and operates in populations living at a
density near K.
Wolves are an example of population that thrives as an iteroparous, K- selection
population.
Selection for life history traits that maximize reproductive success at low densities is
known as r-selection, or density-independent selection.
r-selection tends to maximize r, the per capita rate of increase, and occurs in
environments in which population densities fluctuate well below K or when individuals
face little competition.
 Such conditions are often found in disturbed habitats.
Concept 53.5 Many factors that regulate population growth are density-dependent.
Why do all populations eventually strop growing?
o What environmental factors stop a population from growing indefinitely?
o Why do some populations show radical fluctuations in size over time, while others
remain relatively stable?
Population regulation is an area of ecology with many practical applications.
o A farmer may want to reduce the abundance of an agricultural pest.
o A conservation ecologist may need to know what environmental factors create a
favorable feeding or breeding habitat for an endangered species.
The first step in answering these questions is to examine the effects of increased population
density on rates of birth, death, immigration, and emigration.
A birth rate or death rate that does not change with population density is densityindependent.
o For example, mortality of dune fescue grass is due to physical factors that kill a similar
proportion of a local population regardless of density.
A death rate that increases or a birth rate that falls as population density rises is said to be
density-dependent.
o Reproduction by dune fescue declines as the population density increases, due to
greater seed herbivory.
In dune fescue, the factors that regulate birth rate are density-dependent, whereas the
death rate is regulated by density-independent factors.
Negative feedback prevents unlimited population growth.
Density-dependent regulation provides negative feedback on population growth, operating
through mechanisms that help to reduce birth rates and increase death rates.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-18
In crowded populations, increasing population density increases
competition for nutrients and other resources, thus reducing
reproductive output.
On Hirta Island, the effects of increasing density reduce the birth rate most strongly for
the youngest sheep, one-year-old juveniles.
o
o
In animal populations, territoriality may limit density.
o In this case, space is the resource for which individuals compete.
o Oceanic birds often nest on rocky islands to avoid predators. Beyond a certain
population density, additional birds cannot find a suitable nesting spot and do not
reproduce.
o Songbird males defend territories commensurate with the size
from which they can derive adequate resources for themselves,
their mate, and their chicks.
o
The presence of nonbreeding individuals in a population is an indication that
territoriality is restricting population growth.
Population density can also influence the health and thus the survival of organisms.
o As crowding increases, the transmission rate of a disease may increase.
o If the transmission rate of a disease depends on a certain level of crowding in a
population, then the disease’s impact may be density-dependent.
o Tuberculosis, caused by bacteria that spread through the air when an infected person
coughs or sneezes, affects a higher percentage of people living in high-density cities
than in rural areas.
Predation may be an important cause of density-dependent mortality for a prey species if a
predator encounters and captures more food as the population density of the prey
increases.
o As a prey population builds up, predators may feed preferentially on that species,
consuming a higher percentage of individuals.
o For example, trout may concentrate for a few days on a particular species of insect that
is emerging from its aquatic larval stage, and then switch prey as another insect species
becomes more abundant.
The accumulation of toxic wastes can contribute to density-dependent regulation of
population size.
o In laboratory cultures of microorganisms, metabolic by-products accumulate as
populations grow, poisoning the organisms within this limited artificial environment.
o For example, as a yeast population increases, the yeast accumulate alcohol during
fermentation.
o Yeast can withstand an alcohol concentration of only approximately 13% before they
begin to die.
For some animal species, intrinsic (physiological) rather than extrinsic (environmental)
factors appear to regulate population size.
o White-footed mice individuals become more aggressive as population size increases,
even when food and shelter are provided in abundance.
o Eventually the mouse population ceases to grow.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-19
o
These behavioral changes may be due to hormonal changes caused by stress, which
delay sexual maturation and depress the immune system.
All populations for which data are available show some fluctuation in numbers.
The study of population dynamics focuses on the complex interactions between biotic and
abiotic factors that cause variations in population size.
Populations of large mammals, such as deer and moose, were once thought to remain
relatively stable over time, but long-term studies have challenged that view.
The numbers of Soay sheep on Hirta Island may rise or fall by more than 50% from one year
to the next.
What causes the size of this population to change so dramatically? The most important
factor appears to be the weather.
o Harsh weather, particularly cold, wet winters, weakens the sheep and decreases food
availability, thus reducing the size of the population.
o When sheep numbers are high, other factors, such as an increase in the density of
parasites, also cause the population to shrink.
o Conversely, when sheep numbers are low and the weather is mild, food is readily
available and the population grows quickly.

A long-term population study has found that the moose population on Isle Royale in Lake
Superior also fluctuates over time.
Predation plays an important role in regulating the moose population.
o Moose from the mainland colonized the island in around 1900. Wolves followed 50
years later.
o In recent years, both populations have been isolated from immigration and emigration.
o The moose population experienced two major increases and collapses during the last 45
years.
o The first collapse coincided with a peak in the wolf population from 1975 to 1980.
o The second collapse in around 1995 coincided with harsh winter weather, which
increased the energy needs of the animals and made it harder for the moose to find
food under the deep snow.
Some populations undergo regular boom-and-bust cycles.
Some populations show cyclic population changes.
o Small herbivorous mammals such as voles and lemmings tend to have 3- to 4-year
cycles.
o Ruffled grouse and ptarmigan have 9- to 11-year cycles.



A striking example of population cycles is the 10-year cycles of lynx and snowshoe hare in
northern Canada and Alaska.
Lynx are specialist predators of snowshoe hares, so it is not surprising that lynx numbers
vary with the numbers of hares.
A Scientific study of the population cycles of the snowshoe hare and the lynx shows many
biotic and abiotic factors contribute the cycling of each species population
Why do hare numbers rise and fall in 10-year cycles? There are three main hypotheses.
1. The cycles may be caused by food shortages during the winter.
2. The cycles may be due to predator-prey interactions.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-20
3. The cycles may vary with changes in sunspot activity.
If hare population cycles are due to winter food shortages, as the first hypothesis suggests,
they should stop if extra food is added to a field population.
Researchers conducted such an experiment over 20 years.
o They found that hare population densities increased about threefold in areas that had
extra food, so the carrying capacity of a habitat for hares can clearly be increased by
adding food.
o The populations of lynx and hares continued to cycle, however, so food supplies alone
do not cause the hare cycles.
o The first hypothesis can be rejected.
To test the second hypothesis, field ecologists placed radio collars on hares to find them as
they die and determine the cause of death.
o Ninety percent of dead hares were killed by predators; none appear to have died of
starvation.
o These data support the second hypothesis.
Ecologists excluded predators from one area and excluded predators and added food to
another area.
o They found that the hare cycle is driven largely by predation but that food availability
also plays an important role, especially in winter.
o Perhaps better-fed hares are more likely to escape from predators.
To test the third hypothesis, ecologists compared the timing of hare cycles and sunspot
activity.
○
When sunspot activity is low, slightly less ozone is produced in the atmosphere, and
slightly more ultraviolet radiation reaches Earth’s surface.
○
In response, plants produce more chemicals that act as sunscreens and fewer chemicals
that deter herbivores, thus increasing the quality of the hares’ food.
The researchers found that periods of low sunspot activity were followed by peaks in
the hare population.
o
All of these experiments suggest that both predation and sunspot activity regulate hare
numbers and that food availability plays a less important role.

The availability of prey is the major factor influencing population changes for predators such
as lynx, great-horned owls, and weasels, which depend heavily on a single prey species.
When prey become scarce, predators often turn on one another.
o Coyotes kill both foxes and lynx, and great-horned owls kill smaller birds of prey as well
as weasels, thus accelerating the collapse of the predator populations.
Long-term experimental studies continue to help unravel the complex causes of these
population cycles.
Local populations can be linked to form a metapopulation.
Immigration and emigration can also influence populations, particularly when a group of
populations is linked together to form a metapopulation.
In a sense, local populations in a metapopulation occupy discrete patches of suitable habitat
within a sea of unsuitable habitat.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-21
o
o
o
The patches vary in size, quality, and isolation from other patches, influencing how
individuals move among the populations.
Patches with many individuals can supply more emigrants to other patches.
If one population becomes extinct, the patch it occupied can be recolonized by
immigrants from another population.
The metapopulation concept helps ecologists understand population dynamics and gene
flow in patchy habitats, providing a framework for the conservation of species living in a
network of habitat fragments and reserves.
Concept 53.6 The human population is no longer growing exponentially, but it is still
increasing rapidly.
The concepts of population dynamics can be applied to the specific case of the human
population.
It is unlikely that any other population of large animals has ever sustained so much
population growth for so long.


The human population increased relatively slowly until about 1650, when approximately
500 million people inhabited Earth.
Since then, human population numbers have doubled three times.
The global population is now more than 6.6 billion people and is increasing by about 75
million each year, or 200,000 people each day.
Population ecologists predict a population of 7.8–10.8 billion people on Earth by 2050.
Population ecologists predicted a population of about 7.0 billion people on Earth by 2010.
Although the global population is still growing, the rate of growth began to slow during the
1960s.
o The rate of increase in the global population peaked at 2.2% in 1962.
o By 2005, the rate of increase had declined to 1.15%.
o Current models project a decline in the overall growth rate to just over 0.4% by 2050.

Human population growth has departed from true exponential growth, which assumes a
constant rate.
The declines are the result of fundamental changes in population dynamics due to diseases
such as AIDS and voluntary population control.
Population dynamics vary widely from region to region.
To maintain population stability, a regional human population can exist in one of two
configurations:
Zero population growth = High birth rates − High death rates
Zero population growth = Low birth rates − Low death rates
The movement from the first toward the second state is called the demographic transition.

After 1950, mortality rates have declined rapidly in most developing countries.
Birth rates have declined in a more variable manner.
o The fall in birth rate has been most dramatic in China, where the total fertility rate
(number of children per woman per lifetime) fell from an average of 5.9 children in 1970
to 1.7 children in 2004.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-22
o
o
In some countries of Africa, the transition to lower birth rates has also been rapid,
although birth rates remain high in most of sub-Saharan Africa.
In India, birth rates have fallen more slowly.
In the developed nations, populations are near equilibrium (growth rate of about 0.1% per
year), with reproductive rates near the replacement level (total fertility rate of 2.1
children per woman).
*The life history in industrialized nations is generalize k-selected.
In many developed nations, the reproductive rates are in fact below the replacement level.
o These populations will eventually decline without immigration and a rise in the birth
rate.
o The population is already declining in many eastern and central European countries.
Most population growth is concentrated in developing countries, where 80% of the world’s
people live.
A unique feature of human population growth is our ability to control it with family planning
and voluntary contraception.
o Reduced family size is the key to the demographic transition.
o Delayed marriage and reproduction help to decrease population growth rates and move
a society toward zero population growth.
World leaders continue to disagree about how much support should be provided for global
family planning efforts.
One important demographic variable is a country’s age structure.
Age structure, the relative number of individuals of each age, is illustrated as a pyramid
showing the percentage of the population at each age.



Age structures differ greatly from nation to nation.
Age structure diagrams can predict a population’s growth trends and future social
conditions.
EX: When considering several human populations of equal size and net reproductive rate, but
different in age structure. The population that is likely to grow the most during the next 30 years is
the one with the greatest fraction of people in the lowest age range

Infant mortality, the number of infant deaths per 1,000 live births, and life expectancy at
birth, the predicted average length of life at birth, vary widely among different human
populations.
o These differences reflect the quality of life faced by children at birth and influence the
reproductive choices parents make.
Although global life expectancy has been increasing since about 1950, more recently it has
dropped in a number of regions, including countries of the former Soviet Union and in subSaharan Africa.
o In these regions, the combination of social upheaval, decaying infrastructure, and
infectious diseases such as AIDS and tuberculosis is reducing life expectancy.
Estimating Earth’s carrying capacity for humans is a complex problem.
Predictions of the size of the human population vary from 7.8 to 10.8 billion people by 2050.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-23
o
o
How many humans can the biosphere support?
Will the world be overpopulated in 2050? Is it already overpopulated?
For more than three centuries, scientists have attempted to estimate the carrying capacity
of Earth for humans.
o Estimates have ranged from fewer than 1 billion to more than 1 trillion, with an average
of 10–15 billion.
Carrying capacity is difficult to estimate, and scientists have used different methods to
obtain their answers.
o Some use curves like those produced by the logistic growth equation to predict the
future maximum human population size.
o In models of sigmoidal (logistic) population growth curve,
population growth rate slows dramatically as N approaches K.
o
o
o
Others generalize from the existing “maximum” population density and multiply by the
area of habitable land.
Other estimates are based on a single limiting factor, usually food, and consider many
variables, including the amount of available farmland, the average yield of crops, the
prevalent diet (vegetarian or meat-based), and the number of calories needed per
person per day.
In calculating the carrying capacity of humans, scientists must consider multiple constraints.
Humans need food, water, fuel, building materials, and other resources, such as clothing
and transportation.
The concept of an ecological footprint summarizes the aggregate land and water area
appropriated by each person, city, or nation to produce all the resources it consumes and to
absorb all the waste it generates.
o If the area of all the ecologically productive land on Earth is divided by the global human
population, the result is about 2 hectares (ha) per person.
o Reserving some land for parks and conservation means reducing this allotment to 1.7 ha
per person—the benchmark for comparing actual ecological footprints.
o Anyone who consumes resources that require more than 1.7 ha to produce is using an
unsustainable share of Earth’s resources.
o A typical ecological footprint for a person in the United States is about 10 ha.


Ecologists sometimes calculate ecological footprints using other currencies besides land
area.
For instance, the amount of photosynthesis that occurs on Earth is finite, constrained by the
land and sea area and by the sun’s radiation.
Scientists determined the extent to which people around the world consume seven types of
photosynthetic products: plant foods, wood for building and fuel, paper, fiber, meat, milk,
and eggs (the last three based on estimates of how much plant material goes into their
production).
o Areas with high population densities, such as China and India, have high consumption
rates.
o Areas with much lower population densities but higher per capita consumption, such as
parts of the United States and Europe, have equally high consumption rates.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-24
The combination of population density and resource use per person determines our global
ecological footprint.
Food may be the main factor that will determine Earth’s ultimate carrying capacity for the
human population.
o Malnutrition and famine result mainly from unequal distribution of food rather than
inadequate production.
o So far, technological improvements in agriculture have enabled food supplies to keep up
with global population growth.
Perhaps we will eventually be limited by suitable space, like the gannets on oceanic islands.
o As our population grows, conflict over space will intensify, and agricultural land will be
developed for housing.
o There seem to be few limits, however, on how closely humans can be crowded together,
as long as adequate food and water are provided and space is available to dispose of
their waste.
Humans could also run out of nonrenewable resources, such as certain metals and fossil
fuels.
o The demands of many populations have already far exceeded the local and even
regional supplies of one renewable resource—water.
o More than 1 billion people do not have access to sufficient water to meet their basic
sanitation needs.
Perhaps the human population will eventually be limited by the capacity of the environment
to absorb its wastes.
o In such cases, Earth’s current human occupants could lower the planet’s long-term
carrying capacity for future generations.
Some optimists have suggested that because of our ability to develop technology, human
population growth has no practical limits.
o Technology has undoubtedly increased Earth’s carrying capacity for humans, but no
population can continue to grow indefinitely.

There is no single, agreed-on carrying capacity for humans on Earth.
How many people our planet can sustain depends on their quality of life and the distribution
of wealth across people and nations.
Humans can decide whether zero population growth will be attained through social changes
based on human choices or through increased mortality due to resource limitations,
plagues, war, and environmental degradation.
Lecture Outline for Campbell/Reece Biology, 8th Edition, © Pearson Education, Inc.
53-25