Download Chapter 48: Populations and Communities

Chapter 48: Populations and
Communities
Section 1: Population Growth
Population Growth
• In order to study relationships between
organisms, ecologists need to know how
groups of organisms change over time
• Ecologists study populations
• A group of organisms that all belong to the
same species and that live in a given area
Exponential Growth: A Baby Boom
• Almost any organism provided with ideal
conditions for growth and reproduction will
experience a rapid increase in its population
• The larger the population gets, the faster it
grows
• If nothing stops the population from growing,
it will continue to expand faster and faster
• Exponential growth curve
Exponential Growth Curve
Exponential Growth: A Baby Boom
• Exponential growth does not continue in
natural populations for long
• Most offspring of plants and animals do not
survive long enough to reproduce
• WHY?
Logistic Growth: A Step Closer to
Reality
• The population growth history of a particular
species is a bit more complicated than simple
exponential growth
• Most populations go through a number of
growth phases
• Can be shown on a logistic growth curve
Logistic Growth: A Step Closer to
Reality
• Suppose a few animals are introduced into a
new environment
• At first their numbers will begin to grow
slowly
• Soon, however, the population will begin to
grow very rapidly
• Few animals are dying and a great many are
being produced
Logistic Growth: A Step Closer to
Reality
• Exponential growth does not continue for long
• A population grows more when more
organisms are produced in a given period of
time than die during the same period
• Birthrate is greater than deathrate
• Population growth may slow down because
either the birthrate decreases or because the
deathrate increases or both
Logistic Growth: A Step Closer to
Reality
• When the birthrate and deathrate are the same,
population growth will stop
• During a steady state, the average growth rate
is zero
• Rises and falls of populations average out
around a certain population size
Logistic Growth: A Step Closer to
Reality
• If you were to draw a horizontal line through
the middle of the steady state region, that line
will tell you how big the population is in
steady state
• Carrying capacity
• Once a population reaches the carrying
capacity of its environment, certain factors
keep the population from growing any further
• Lack of food, overcrowding, and
competition among the individuals in the
population
Chapter 48: Populations and
Communities
Section 2: Factors That Control
Population Growth
Factors That Control Population
Growth
• The growth of individuals can be controlled by
limiting factors
• Both plant and animal populations can be
controlled by several factors
• No single species has ever threatened to
overpopulate the entire planet
Density-Dependent Limiting Factor
• When factors that control population size operate
more strongly on large populations than on small
ones, they are called density-dependent limiting
factors
• Usually operate only when a population is large
and crowded
• Do not affect small, widely scattered
populations much
• Density-dependent limiting factors
include competition, predation,
parasitism, and crowding
Competition
• When populations become crowded, both plants and
animals compete, or struggle, with one another for
food, water, space, sunlight, and other essentials of life
• Competition between members of different yet similar
species is a major force behind evolutionary change
• No two organisms can occupy the same niche in
the same place at the same time
• When two species compete, both find themselves
under pressure from natural selection to change in
ways that decrease their competition
• Ties ecology and evolution together
Predation
• Just about every species serves as food for some
other species
• Predators and prey coexist over long periods of time
• Have become accustomed to each other’s
strengths and weaknesses
• Prey have evolved defenses against predators
• Poisonous chemicals, shells, camouflage
• Predators have evolved counter defenses
• Stronger teeth and jaws, powerful digestive
enzymes, extra keen eyesight
Predation
• Typically, at some point the prey population
grows so large that prey are numerous and easy
to find
• With such a large and available food supply to
feast upon, there may soon be almost as many
predators as prey
• This situation cannot last because each
predator needs many prey to satisfy its
energy needs
• Predator-prey relationships are important in
controlling natural populations
Parasitism
• Parasites live off their hosts, weakening them
and causing disease
• Like predators, parasites work most effectively
if hosts are present in large numbers
• Parasitism works as a density-dependent
limiting factor on population growth
Crowding and Stress
• Most animals have a built-in behavioral need
for a certain amount of space
• Room to hunt
• Nesting
• Territory
• The number of suitable territories regulates
population size in a density-dependent manner
• Certain species fight among themselves if they
are overcrowded
• Creates stress
Density-Independent Limiting
Factors
• Many species show boom-and-bust growth
curves
• Populations grow exponentially for some
time and then suddenly crash
• Insects that feed on plant buds and leaves
can be washed out by a rainstorm
• They may also be harmed by long hot
periods of dry weather
• Frosts, too, can cause sudden drops in
insect populations
Density-Independent Limiting
Factors
–For these species, storms, cold
weather, dry weather, or other natural
occurrences can nearly wipe out the
population
• Happen regardless of how large or
small the population is
• Density independent limiting
factors
Human Population Growth
• Human populations tend to increase in size with time
• For a long period of time the human population grew
slowly
• Then, about 500 years ago, the world’s human
population grew exponentially
• Today, population growth in the US and parts of
Europe has slowed down
• Most of the world’s people do not live in these
countries
• Instead, they live in China, India, and part of Africa
and Latin America – places where populations are
still growing very rapidly
Chapter 48: Populations and
Communities
Section 3: Interactions Within and
Between Communities
Interactions Within and Between
Communities
• After populations, the next larger biological
units are communities
• A community consists of all the populations
of organisms living in a given area
• Populations in communities interact with one
another in many ways
Interactions Within and Between
Communities
• Example:
• Plant species compete for water, nutrients,
and sunlight. At the same time, some plants
have evolved defenses against herbivores.
Herbivores compete with one another for
food and space. These herbivores may have
evolved counter defenses against the plant
species. While this is going on, carnivores
are hunting the herbivores.
Symbiosis
• There are several relationships that play an
important role in nature
• Symbiosis
• “Living together”
–Parasitism
–Commensalism
–Mutualism
Parasitism
• One species benefits and the other is harmed
• Example:
• Tapeworm feeding off of nutrients in the
digestive tract of an organism
• http://media2.foxnews.com/112008/worm_tum
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Commensalism
• One member benefits and the other is not harmed
• Example:
• Shrimp live within the stinging tentacles of sea
anemones
• The shrimp are not affected by the anemone’s
poison
• As a result, the shrimp are protected from
predators that cannot tolerate the anemone’s
stings
• Anemones are not harmed by shrimp living on
them, but they are not helped either
Mutualism
• Two species live together in such a way that
both species benefit
• Example:
• Clownfish benefit from living within the
stinging tentacles of the sea anemone in the
same way shrimp do
• However, clownfish also help the anemones
by chasing away several species of
anemone-eating fish
Interactions Among Ecosystems
• Not only do populations and communities
interact, ecosystems also interact with one
another in many ways
• Nearly every ecosystem is connected, either
directly or indirectly, with other ecosystems