Population Ecology
An Overview of Population Ecology
A population is a group of individuals of a single species that occupy the same
general area at the same time
Individuals in a population
rely on the same resources
are influenced by the same environmental factors
are likely to interact and breed with one another
Population ecology is concerned with changes in population size and the factors
that regulate populations over time
A population ecologist might describe a population in terms of its size (number
of individuals), age structure (proportion of individuals of different ages),
and/or density (number of individuals per unit area or volume)
Population ecologists also study population dynamics, the interactions
between biotic and abiotic factors that cause variation in population size
One important aspect of population dynamics is population growth
Population ecology also provides critical information for conservation and
restoration projects, is being used to develop sustainable fisheries throughout
the world, and is used to manage wildlife populations
Studying the population ecology of pests and pathogens provides insight into
controlling how they spread
Population Density
Population density is the number of individuals of a species per unit area or
volume of the habitat
Examples include the number of
largemouth bass per cubic kilometer (km3) of a lake
oak trees per square kilometer (km2) in a forest
nematodes per cubic meter (m3) in a forest’s soil
How do we measure population density?
In most cases, it is impractical or impossible to count all individuals in a
population
Instead, ecologists use a variety of sampling techniques to estimate
population density
Population densities may also be estimated by indicators such as number
of bird nests or rodent burrows rather than by actual counts of
organisms
Population Age Structure
The age structure of a population is the distribution of individuals in different
age-groups
The age structure of a population provides insight into the history of a
population’s survival, reproductive success, and how the population relates to
environmental factors
Life Tables
Life tables
track survivorship (the chance of an individual in a given population
surviving to various ages)
help to determine the most vulnerable stages of the life cycle
Survivorship Curves
Survivorship curves plot the number of individuals still alive at each age in
the maximum life span
By using a percentage scale instead of actual ages on the x-axis, we can
compare species with widely varying life spans
There are three idealized survivorship curves
Late loss (type I)
Constant loss (type II)
Early loss (type III)
Species that exhibit a Type I curve usually produce few offspring but provide
good parental care, increasing the likelihood that they will survive to maturity
Species that exhibit a Type II curve are intermediate, with survivorship
constant over the life span
Species that exhibit a Type III curve have low survivorship for the very young,
followed by a period when survivorship is high for those few individuals who
live to a certain age
Life History Traits as Evolutionary Adaptations
An organism’s life history is the set of traits that affect the organism’s
schedule of reproduction and survival
Key life history traits include the
age at first reproduction
frequency of reproduction
number of offspring
amount of parental care provided
Reproductive success is key to evolutionary success
The combination of life history traits represents evolutionary trade-offs that
balance the demands of reproduction and survival
Because selective pressures vary, life histories
are very diverse
Organisms with an opportunistic life history take immediate advantage of
favorable conditions and typically exhibit a type III survivorship curve
Organisms with an equilibrial life history
develop and reach sexual maturity slowly
produce few, well-cared-for offspring
are typically larger-bodied and longer-lived
typically exhibit a type I survivorship curve
Population Growth Models
Population size fluctuates as new individuals are born or move into an area
and others die or move out of an area
Population ecologists use idealized models to investigate how the size of a
particular population may change over time under different conditions
What factors determine the overall change in a population over time?
Growth rate (r) = Birth rate - death rate = population’s intrinsic rate of
increase
Immigration
Emigration
Exponential Growth
All populations have the potential to grow exponentially
Anytime birth rate > death rate, that population will grow exponentially
This is true no matter how slowly a population is growing
However, in order to continue indefinitely, there would have to be an
unlimited environment
The increasing speed of population growth produces a J-shaped curve that is
typical of exponential growth
The slope of the curve shows how rapidly the population is growing
Exponential population growth is common in certain situations, such as
following a disturbance, such as a fire, flood, hurricane, drought, or cold
snap, that may suddenly reduce the size of a population
Limiting factors
are environmental factors that restrict population growth
ultimately control the number of individuals that can occupy a habitat
Logistic Growth Model
The carrying capacity (K) is the maximum population size that a particular
environment can sustain
Logistic population growth occurs when the growth rate decreases as the
population size approaches carrying capacity
When the population is at carrying capacity, the growth rate is zero
Population-limiting factors
Density-dependent factors
The effect has the greater intensity with greater population density
Food, shelter, other resources
Density-independent factors
The effect has the same intensity regardless of the population density
Weather, fires, killing frosts, floods, etc.
r-Selected and K-Selected Species
r-selected species
These are opportunistic species
The pressures are usually density-independent
K-selected species
These are the equilibrium species
The pressures are usually density-dependent
Population Cycles
Sometimes, populations don’t follow the classic logistic growth patterns
Some populations of insects, birds, and mammals undergo dramatic
fluctuations in density with remarkable regularity
“Booms” characterized by rapid exponential growth are followed by
“busts,” during which the population falls back to a minimal level
Sometimes, two populations are dependent upon each other
The hare and lynx cycles may be caused by
winter food shortages that result from overgrazing
predator-prey interactions
a combination of food resource limitation and excessive
predation
Applications of Population Ecology
To a large extent, humans have converted Earth’s natural ecosystems to
ecosystems that produce goods and services for our own benefit
Population ecology is used to
increase populations of organisms we wish to harvest
decrease populations of pests
save populations of organisms threatened with extinction
Conservation of Endangered Species
The U.S. Endangered Species Act defines
an endangered species as one that is “in danger of extinction
throughout all or a significant portion of its range”
a threatened species as one that is likely to become endangered in
the foreseeable future
The challenge for conservationists is to determine the circumstances that
threaten a species with extinction and try to remedy the situation
The red-cockaded woodpecker requires longleaf pine forests, where it drills its
nest holes in mature, living pine trees
The numbers of red-cockaded woodpeckers declined as suitable habitats were
lost to logging, agriculture, and suppressing the fires that are a natural
occurrence in these ecosystems
Research revealed that breeding birds tend to abandon nests when vegetation
among the pines is thick and higher than ~4.5 m (15 feet)
Apparently, the birds require a clear flight path between their home
trees and the neighboring feeding grounds
Wildlife managers protected critical habitat and began a maintenance program
that included controlled fires to reduce forest undergrowth
As a result, populations of red-cockaded woodpeckers are beginning to
recover
Sustainable Resource Management
Sustainability can be a hard goal to achieve
According to the logistic growth model, the fastest growth rate occurs when
the population size is at roughly half the carrying capacity of the habitat
Theoretically, a resource manager should achieve the best results by
harvesting the population down to this level
However, the logistic model assumes that growth rate and carrying
capacity do not change over time
Fish, the only wild animals still hunted on a large scale, are particularly
vulnerable to over harvesting
In the northern Atlantic cod fishery, estimates of cod stocks were too high, and
the practice of discarding young cod (not of legal size) at sea caused a higher
mortality rate than was predicted
The fishery collapsed in 1992 and has not recovered
Sustainable catch rates can’t be estimated without knowing the essential life
history traits for the target species
In addition, knowledge of population ecology alone is not sufficient;
sustainable fisheries also require knowledge of community and ecosystem
characteristics
Invasive Species
Organisms that are introduced into non-native habitats can have a devastating
effect on the ecosystem
An invasive species
is a non-native species that has spread far beyond the original point of
introduction
causes environmental or economic damage by colonizing and dominating
suitable habitats
In the United States alone, there are hundreds of invasive species with an
estimated cost of $137 billion per year
Not every organism that is introduced to a new habitat is successful, and not
every species that survives in its new habitat becomes invasive
There is no single explanation for why any non-native species turns into a
damaging pest, but invasive species typically exhibit an opportunistic life
history pattern
Cheatgrass
is an invasive plant of the arid western United States that covers millions
of acres of rangeland that were formerly dominated by native grasses
and sagebrush
produces seeds earlier and in greater abundance than its competitors
matures in early summer, becoming extremely dry and flammable and
creating abundant fuel that is easily ignited by lightning or a stray spark
Cheatgrass fires are more intense and occur much more frequently than
the fires that native plants have evolved to tolerate
After a few fire cycles, the native plants are gone, robbing more
than 150 species of birds and mammals of the food and shelter they
derive from sagebrush
Global climate change is also hastening the transition of rangeland into
fields of cheatgrass
Studies have shown that cheatgrass responds to increased CO2
levels by growing faster and accumulating more tissue, which in
turn becomes more fuel for the fires that extend its domain
Burmese pythons
are another invasive species
were set loose in South Florida, either accidentally or deliberately
are now abundant in South Florida, eating many native species
Biological Control of Pests
Invasive species may benefit from the absence of pathogens and/or
predators
Biological control
is the intentional release of a natural enemy to attack a pest
population
is used to control insects, weeds, and other organisms that reduce
crop yield
Although biological control strategies have had many successes, the failures have
been monumental
For example, beetles were brought in to combat St. John’s wort, a perennial
(long-lived) European weed that invaded the western United States and had
overgrown millions of acres of rangeland and pasture, leaving few edible
plants for grazing livestock
Researchers imported leaf beetles from the plant’s native region that feed
exclusively on St. John’s wort
The shiny, pea-sized insects reduced the weed to less than 5 percent of
its former abundance, restoring the land’s value to ranchers
One cautionary tale comes from the introduction of mongooses to control
rats
Sugar cane planters imported the small Indian mongoose to deal with the
problem
Mongooses were introduced to dozens of natural habitats, including all of the
largest Caribbean and Hawaiian islands—and became invasive themselves
On island after island, populations of reptiles, amphibians, and groundnesting birds have declined or vanished as mongoose populations have
grown and spread
Pest Management
Agricultural operations create their own highly managed ecosystems that
have genetically similar individuals (a monoculture), are planted in close
proximity to each other, and are a banquet for plant-eating animals and
pathogenic bacteria, viruses, and fungi
Pesticides may
result in populations that are not affected by a pesticide as a result of
natural selection
kill both the pest and their natural predators
inadvertently kill pollinators that are essential for both agricultural
and natural ecosystems
Integrated Pest Management
Integrated pest management (IPM) uses a combination of biological,
chemical, and cultural methods for sustainable control of agricultural pests
Integrated pest management (IPM) advocates
tolerating a low level of pests instead of total eradication
lowering the habitat’s carrying capacity for the pest population by
using pest-resistant varieties of crops, mixed-species plantings, and
crop rotation to deprive the pest of a dependable food source
Biological controls can also be used when appropriate
Human Population Growth
There are about 3 humans added to the world’s population every second
Every 20 minutes, there are 3,600 new humans added to the world
Every 20 minutes, there is one species of plant or animal that becomes
extinct
The human population is expected to continue increasing for at least the next
several decades
But the number of people added to the population each year has been
declining since the 1980s
The History of Human Population Growth
Economic development in Europe and the United States led to advances in
nutrition, sanitation and, later, medical care
At first, the death rate decreased while the birth rate remained the
same so population growth began to pick up by the beginning of the
1900s
By the mid-1900s, improvements in nutrition, sanitation, and health
care had spread to the developing world, spurring growth at a
breakneck pace as birth rates far outstripped death rates
As the world population skyrocketed from 2 billion in 1927 to 3 billion in
1960, some scientists became alarmed, but the overall growth rate peaked in
1962
In developed nations, advanced medical care continued to improve
survivorship, but effective contraceptives held down the birth rate
As a result, the overall growth rate of the world’s population began a
downward trend as the difference between birth rate and death rate
decreased
Age Structures of Human Populations
Age structures help predict a population’s future growth
Every day, ~350,000 people are born and ~150,000 die
1.3 billion people live in absolute poverty
2 billion people lack basic health care or clean drinking water
>2 billion people have no sanitation services
Each year, 14 million people, mostly children die from hunger or hunger-related
problems
What is Earth’s actual carrying capacity for humans?
No one really knows
Will the planet be able to handle all of those people, and if so, how?
Our Ecological Footprint
World food production must increase dramatically to accommodate all the
people expected to live on our planet in the coming decades and improve the
diets of those who are currently malnourished or undernourished
Agricultural lands are already under pressure
Overgrazing by growing herds of livestock is turning vast areas of grassland
into desert
Water use has risen 6x over the past 70 years
Changes in precipitation patterns due to global warming are already causing
food shortages in some regions of the world
Because so much open space will be needed to support the expanding human
population, many other species are expected to become extinct
An ecological footprint is an estimate of the amount of land and water required to
provide the resources an individual or a nation consumes, including food, fuel, and
housing, and the ability to absorb the waste generated,
especially carbon emissions
Comparing our demand for resources with Earth’s capacity to renew these
resources, or biocapacity, gives us a broad view of the sustainability of
human activities
According to the World Wildlife Fund, in 2008
(the most recent year for
which data are available), the average ecological footprint for the world’s
population was roughly 1.5 times the planet’s biocapacity per person
Humans are depleting the Earth’s resources
Affluent nations such as the United States and Australia consume a
disproportionate amount of resources
The problem is not just overpopulation, but overconsumption
The world’s richest countries, with 15% of the global population,
account for 36% of humanity’s total footprint
If the world lived like we do in the United States, we would require the
resources of more than four planet Earths