Download Ch. 4 Populations and communities

SGCEP SCIE 1121
Environmental Science
Spring 2012
Section 20531
Steve Thompson: [email protected]
http://www.bioinfo4u.net/
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Wednesday, January 25, 2012
Populations, communities,
and dynamics
Remember . . .
Population: a group of members of the
same species living in an area, and . . .
Community: populations of different
species living together in an area.
Let’s start with how populations
change in their number of
individuals.
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Population growth
Some real, real easy math . . .
Populations grow with births and immigration;
They decline with deaths and emigration:
Therefore . . .
(Births + Immigration) – (Deaths + Emigration)
= Change in population number (positive or negative)
Equilibrium means: (Births + Immigration) =
(Deaths + Emigration)
However, population growth is often not equal to
zero.
Population growth rate is the amount the
population has changed divided by the time it had to
make those changes.
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Exponential growth
Species can increase their population exponentially under
favorable conditions.
Exponential increase: 2, 4, 8, 16, 32, 64 . . .
The doubling time remains constant.
For example, if it takes t wo days to go from 8 to 16
individuals, it would still take t wo days to go from 1,000
to 2,000 individuals.
Such growth is called “explosive,” and . . .
graphs out as a J-shaped cur ve.
Population growth cur ves show how populations grow (or
shrink), and are used to find things like:
How fast a population could grow;
How many individuals there are now;
What the future population size could be; . . .
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Population growth cur ves
And some example
scenarios . . .
Disease can
do this too!
Exponential
growth i.e.
This one
is logistic.
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The equation’s not that bad.
The per capita rate of increase (or
decrease) of a population is called “r”.
G is the number of individuals added
per unit time. And . . .
N is the number of individuals at the
start of a given time inter val.
Therefore, . . .
G = rN
This is exponential growth.
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Outcomes of population increase
Exponential growth often results from unusual
disturbances, such as the . . .
Introduction of a foreign species, a suddenly
changed habitat, the elimination of a predator,
arrival in a new habitat, etc.
So the population continues to grow and then it
crashes and dies off. Versus, more commonly . . .
Logistic growth: some process slows growth so it
levels off near carrying capacity. This . . .
Results in an S-shaped curve, which . . .
Levels off at K (the carrying capacity).
As the population approaches K, growth slows.
The population remains steady and growth = 0
The maximum rate of population growth occurs
half way to K.
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Those processes are called
environmental resistance.
Abiotic and biotic factors cause mortality (death) —
Prevents unlimited population growth.
Environmental resistance: the biotic and abiotic
factors that may limit a population’s increase.
Biotic: predators, parasites, competitors, lack of
food.
Abiotic: unusual temperatures, moisture, light,
salinity, pH, lack of nutrients, fire.
Environmental resistance can also lower
reproduction, which slows population growth . . .
Loss of suitable habitat, pollution;
Changed migratory habits of animals.
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Reproductive strategies
The interplay of environmental resistance and
biotic potential (capability to reproduce) drives
the success of t wo reproductive strategies:
r-strategists (population growth rate-selected
species): produce lots of young, but leave their
sur vival to nature. They usually have . . .
Rapid reproduction, rapid movement, short
life spans, small bodies; and are . . .
Adapted to a rapidly changing environment.
They have “Boom-and-bust” populations.
Often called “weedy” or “opportunistic.”
An example is the housefly.
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Versus . . . . K-strategists
K-strategists (carrying capacity-selected
species) have a much lower reproductive
capacity. They . . .
Care for and protect their young;
Live in a stable environment already
populated by the species; Are . . .
Larger, longer lived, and well-adapted to
normal environmental fluctuations.
Their populations fluctuate around
carrying capacity. They are . . .
Also called equilibrial species.
For example, elephant, California condor.
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Life histories go right along.
Life history: the progression of changes in an
organism’s life. This includes . . .
Age at first reproduction, length of life, etc.
Can be visualized in a survivorship graph.
Type I survivorship: low mortality in early life.
Most live the bulk of their life span (e.g., humans).
Type III sur vivorship: many offspring that die young.
Few live to the end of their life (e.g. oysters,
dandelions).
Type II survivorship: intermediate survivorship
pattern (e.g. squirrels, many birds, coral).
K-strategists usually have a Type I pattern; rstrategists show Type III. All are extremes.
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Survivorship cur ves
More Kselected
like
More rselected
like
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Predictable pattern in species
There is a predictable pattern to the way human
activities affect species.
r-strategists become pests when humans change
an area. For example . . .
Houseflies, dandelions, cockroaches increase.
K-strategists become rarer or extinct with
change. For example . . .
Eagles, bears, and oaks decline.
An exception: rare opportunistic species (rselected) so separated from new habitat . . .
That they cannot succeed, despite high biotic
potential.
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Why does a population grow?
A population’s size depends on the interplay
bet ween its biotic potential and the environmental
resistance it encounters.
A population’s biotic potential remains constant.
But . . .
Environmental resistance changes.
Population balance is a dynamic balance
Additions (births, immigration) and subtractions
(death, emigration) occur continually.
The population may fluctuate widely or very
little.
A population is at equilibrium when populations
restore their numbers and the ecosystem’s capacity
is not exceeded.
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Limits on populations
The carrying capacity may not explain
what limits a population.
Population density: number of individuals
per unit area.
The higher the density, the more likely a
factor (e.g., crowding or disease) affects
the population.
Some factors (e.g., a tornado) keep a
population from increasing, but have
nothing to do with the density of the
organisms.
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Density dependence and
independence
Density-dependent factor: increases with
increased population density. Includes . . .
Predation, disease, food shortage, etc.
Logistic growth occurs when populations
become more crowded (approach carrying
capacity).
Density-independent factor: one whose effects
are independent of the density of the population.
Spring freeze, flood, earthquake, fire, etc.
Is not involved in maintaining population
equilibrium in the logistic growth.
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Regulating a population
Only density-dependent factors can regulate
a population (keep it in equilibrium).
Top-down regulation: control of a
population (species) by predation.
Bottom-up regulation: control of a
population occurs as a result of scarcity of
a resource (food).
Factors controlling a population determine
the effects on an ecosystem of adding or
removing a species.
Removal of a species can affect species
that don’t directly interact with it.
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Critical number
Critical number: the minimum population base
allowing the sur vival and recovery of a
population. For instance, . . .
A pack of wolves, flock of birds, school of
fish, etc.
The group is necessary to provide protection
and support to all members.
If a population falls below this number:
Surviving members become more vulnerable;
Breeding fails; and . . .
Extinction is almost inevitable.
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Community interactions
Relationships bet ween species may be positive
(helpful), negative (harmful), or neutral for each
species.
Predation: one member benefits, the other is
harmed;
Includes parasitism, herbivory.
Competition: both species are harmed. Either . . .
Interspecific competition: bet ween different
species; or . . .
Intraspecific competition: bet ween the same
species.
Mutualism: both species benefit.
Commensalism: One species benefits, the other is
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Major types
of
interactions
bet ween
species
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Predation
Predator: the organism that does the
feeding.
Prey: the organism that is fed upon.
Predator-prey interaction: carnivores (meat
eaters) eat herbivores (plant eaters).
Herbivores feed on plants.
Parasites feed on hosts (a type of predation).
Parasite: an organism (plant or animal)
that feeds on its “prey,” usually without
killing it.
Host: the organism that is being fed upon.
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Parasitic organisms . . .
Include tapeworms, disease-causing bacteria,
viruses, fungi.
Parasites affect host populations in a densitydependent way.
Increased population density makes it easier
for parasites and their vectors (carriers of the
parasite) to find new hosts.
Pathogens: bacteria and viruses that cause
disease.
No real ecological difference from other
parasites. They are just . . .
Highly specialized parasites.
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Regulation of prey
Predators can regulate herbivore numbers (top-down
control). For example . . .
Moose crossed to Isle Royale, in Lake Superior.
Several years later, wolves also reached the island.
Without wolves, the moose population grew rapidly.
Low environmental resistance for the moose.
The wolf population also increased and preyed on
moose.
Low environmental resistance for the wolves.
Fewer moose (high environmental resistance)
resulted in fewer wolves (high environmental
resistance).
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Predator-prey relationships
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Other factors affect populations
Wolf predation wasn’t the only factor affecting the
moose. Other things mattered . . .
Deep snow limited access to food;
A tick infestation caused significant mortality.
A sharp decline in moose popuations kept wolf
populations low because there were . . .
Not enough calves to catch.
Wolves can’t catch a mature moose in good
condition.
Predator-prey relationships involve top-down (on the
prey) and bottom-up (on the predator) regulation.
Parasites weaken hosts, making them more
vulnerable to predation.
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Overgrazing
If herbivores eat plants faster than they can
grow, plants are depleted and animals
eventually suffer.
Reindeer were brought to St. Matthew Island.
At first, they were healthy and wellnourished.
They ate up the lichens.
A few years later, they were malnourished.
With little food and harsh weather, almost
the entire herd starved.
No population can escape the ultimate
limitations set by environmental resistance.
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Plant-herbivore interactions
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Predator removal
Eliminating predators upsets plantherbivore relationships.
White-tailed deer in the U.S. were originally
controlled by wolves, mountain lion, bears.
But these were killed because they were
thought to threaten livestock and
humans.
Deer populations can get so large they
overgraze the area.
Humans control numbers through
hunting.
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Keystone species
Removal of one species can create a cascade of
effects . . .
Impacting far more than just the other species
they interact with. For example . . .
Sea stars eat mussels in rocky intertidal zones.
Removing sea stars allows mussels to crowd
out all other species, reducing diversity.
Keystone species: play a crucial role in
maintaining ecosystem biotic structure. They . . .
Moderate other species that would take over;
thereby allowing other, less-competitive
species to flourish.
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Competition is . . .
Interactions where both species
are harmed.
They compete for a scarce
resource.
Species that compete have
overlapping niches.
Over time, there is pressure to
reduce the overlap.
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Competition can be . . .
Intraspecific competition: competition bet ween
members of the same species. This . . .
Occurs over resources. And is all about . . .
Territory: an area defended by an individual or
group.
Is vigorously defended.
Most defense is intimidation—serious fights
are rare.
Organisms fight to protect an area for
nesting, establishing a harem, or food
resources.
Lack of territories: a density-dependent
limitation on a population.
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Territoriality as an advantage
It protects a population from the possibility of
everyone getting some resources, but nobody
getting enough to survive.
Without territoriality, when resources are
scarce . . .
Every encounter could end in a potentially
lethal fight. Plus . . .
All members would get only a part of what
they need and could die.
Competition lowers fitness and production of
offspring. However, . . .
Territoriality lowers the direct effects of
competition.
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Not everyone gets a territory.
Individuals unable to claim a territory . . .
Are often young and may obtain a
territory later. They . . .
May disperse, opening up new habitats
to the species. Or they . . .
May die.
Territoriality is most likely in Kstrategists. It is . . .
An adaptation that helps organisms
disperse and stabilizes populations.
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Intraspecific competition’s impact
Short-term impact on a population: it leads to
density-dependent regulation of a population . . .
Through territoriality, and . . .
Through self-thinning: crowded organisms
(e.g., trees) become less numerous as they get
bigger.
Long-term impact on a population: it leads to
long-term changes as the species adapts to its
environment:
Those better able to compete, sur vive, and
reproduce. And . . .
Their superior traits are passed on to
successive generations.
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Interspecific competition
Competitive exclusion principle: species
cannot survive competition if they compete
directly in many respects. Especially . . .
In simple habitats with species needing the
same resources.
But species do occupy the same area without
becoming extinct. How is this?
It’s because abiotic and biotic conditions in
an environment vary in space and time.
Adaptations of species to specific conditions
allows it to thrive and overcome its
competitors in one location or time, but not in
another.
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Environments are heterogeneous.
Variable environments support species with
different niches.
Competitors in the same habitat usually have
different niches. For instance . . .
Woodpeckers eat insects; but other birds
may be specialized to eat seeds.
Space can also be a resource.
Resource partitioning: the division of a
resource and specialization in different parts
of it.
With more intense competition, resources
are even further divided.
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For example:
resource
partitioning
in warblers
— each bird
has a
preferred
feeding
zone.
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Mutualism
An arrangement bet ween t wo species where
both benefit.
Examples of mutualism:
Pollinators (e.g., bees) receive nutrition
while plants receive pollination;
Fungi on roots (mycelium): the fungus gets
nutrition, the plant gets easier intake of
soil nutrients.
Lichens are made of a fungus and an algae.
The anemone fish protects the anemone
from predation by the butterfly fish, and
the anemone protects the fish.
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Commensalism
Commensalism: one species benefits;
the other is unaffected.
More rare than others. E.g. . . .
Buffalo stir up insects that cattle
egrets eat. Or . . .
Orchids live on trees but do not
harm or feed off of them.
Amensalism: one species is harmed;
the other is unaffected. This is . . .
Usually accomplished by natural
chemical compounds. E.g. . . .
Black walnut trees produce a
chemical that kills other plants.
Symbiosis: t wo species live close to
each other. This is huge in nature!
Can be beneficial (mutualism) or
harmful (parasitism).
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Evolution as a force for change
Predation and competition keep populations
under control:
Individuals with qualities that lower impacts
of negative interactions sur vive.
Predators and prey become adapted to each
other.
Intraspecific competition leads to improved
adaptations to the environment.
Interspecific competition promotes adaptations
in competitors.
They specialize in exploiting a resource.
Resource partitioning allows resource
sharing.
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Selective pressures
Most young plants and animals do not sur vive.
Selective pressures: environmental resistance
factors affect which individuals sur vive and
reproduce. For instance . . .
Predators, parasites, drought, lack of food.
Animals with traits that protect them or allow
them to escape can sur vive and reproduce.
Predators function as a selective pressure:
favoring sur vival of traits that enable prey
to escape predation.
Food is a selective pressure: predators with
keen eyesight or swift speed sur vive.
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Humans have introduced
thousands of species from
foreign ecosystems, some
accidentally, others
deliberately, changing
community and population
relationships, usually for
the worse. This causes . . .
Economic losses in the
U.S. of $138 billion/year!
For example: rabbits were
introduced into Australia
for sport shooting;
without natural enemies,
the population exploded,
devastating the
environment.
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Introduced
species
Kudzu, a vine introduced to
control erosion and for
cattle fodder, has invaded
and climbed over forests.
Introduced species often
become serious pests, e.g. . .
Japanese beetles, fire ants, ‘love’ bugs, gypsy
moths;
Chestnut blight, spotted knapweed, purple
loosestrife, cogongrass, wisteria, hydrilla;
Goats and pigs on islands;
Domestic and feral cats;
Zebra and quagga mussels;
Ctenophore jellyfish;
A lack of competition allows the invasive species
to displace native species.
The problem is increasing due to expanding world
trade and travel.
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Implications for human management
Ecologists study populations and communities:
To increase our understanding of the world;
To better manage natural resources by
protecting declining species and controlling
introduced pest species;
To study our impacts on populations.
Keystone species are such an integral part of the
ecosystem.
Removing them can cause the ecosystem to
collapse.
For example, removing beavers (by overhunting
and habitat destruction) hurt wetland
meadow species.
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Next time . . .
Looking into ecosystems
more carefully.
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