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AP Biology
Everything You Need to Know About Ecology
Essential knowledge 4.A.5: Communities are composed of populations of
organisms that interact in complex ways.
Topic: Population Dynamics and Ecosystem Components
Ecosystem Hierarchy
A biosphere is made of
ecosystems (Earth).
An ecosystem is made up of
various communities of living
(biotic) and nonliving items
(abiotic).
A community is made of many
populations.
A population is made of
individuals from a species.
The two forest communities above have the same species richness (four
species), but the relative abundance of the four species is different between
the two communities.
Communities with higher species diversity are often more resistant to
disruption by invasive exotic species than less diverse communities. This
may be because a more varied community, consisting of many species with
different ecological requirements, uses a greater diversity of the available
resources. This leaves fewer resources for an intruding newcomer with
specific needs.
Gray Squirrel 
A species is a group of
individuals that can breed and
have viable offspring.
Topic: Communities
The structure of a community is measured and described in terms of species
composition and species diversity. To measure species diversity of a
community, two factors are used: species richness and species abundance.
Species richness is a count of how many different species exist in a
community. Species abundance is how many of each individual is present of
the species in the community.
Question: What happens to all three populations with the introduction of
an invasive species (gray squirrel)?
Topic: Population Growth Types
Question:
A population is a group of individuals of the same species. Populations may
show different patterns of growth. The growth pattern depends partly on
the environmental conditions under which a population lives.
Exponential Growth
Under ideal conditions,
populations of most species can
grow at exponential rates.
Curve A in the figure to the right
represents exponential growth.
The population starts out
growing slowly. As population
size increases, the growth rate
also increases. The larger the
population becomes, the faster
it grows.
Logistic Growth
Most populations do not live under ideal conditions. Therefore, most do not
grow exponentially. Certainly, no population can keep growing
exponentially for very long. Many factors may limit (slow down) growth.
Often, the factors are density dependent (known as density-dependent
factors). These are factors that are influential when the population becomes
too large and crowded. For example, the population may start to run out of
food or be poisoned by its own wastes. As a result, population growth slows
and population size levels off. Curve B in the figure above represents this
pattern of growth, which is called logistic growth.
At what population size does growth start to slow in the logistic model of
growth? That depends on the population’s carrying capacity (see figure
above). The carrying capacity (K) is the largest population size that can be
supported in an area without harming the environment. Population growth
hits a ceiling at that size in the logistic growth model.
K-Selected and r-Selected Species
Species can be divided into two basic types when it comes to how their
populations grow.
•Species that live in stable environments are likely to be K-selected. Their
population growth is controlled by density-dependent factors. Population
size is generally at or near the carrying capacity. These species are
represented by curve B in the figure above. Examples include rhinos and
elephants.
Age structure diagrams show the various ages of a population and separate
male and female numbers of the population.
•Species that live in unstable environments are likely to be r-selected. Their
potential population growth is rapid (fast). For example, they have large
numbers of offspring. However, individuals are likely to die young. Thus,
population size is usually well below the carrying capacity. These species are
represented by the lower part of curve A in the figure above (r is the
population growth rate). Examples include roaches and rabbits.
Summary
•Under ideal conditions, populations can grow exponentially.
•The growth rate increases as the population gets larger.
•Most populations do not live under ideal conditions and grow logistically
instead.
•Density-dependent factors slow population growth as population size
nears the carrying capacity.
Question: Why does China’s age distribution diagram look this way?
Question: Draw a graph of a cockroach population. How would it grow?
Then, what would happen to the population with the introduction of a bird
that ate the cockroach?
Topic: Demographics
Demography is the statistical study of populations, especially human beings.
Demography encompasses the study of the size, structure, and distribution
of these populations, and spatial or temporal changes in them in response
to birth, migration, aging, and death.
Fecundity is the ability to produce an abundance of offspring or new
growth; it is a measure of fertility.
Question: What is the overall trend of this data? What do the lines above
the bar
represent?
level (Rule of 10). The other 90% is lost as heat into the environment. All
food webs and chains are dependent upon primary productivity (the
amount of photosynthesis done by the producers). This is measures in gross
primary production. If you account for how much a plant is
photosynthesizing and then respiring, this is called net primary production.
Changes in regional and global climates, and in levels of carbon dioxide, can
greatly influence primary production. For example, high levels of CO2 may
lead to increased rates of PP. NPP = GPP - R
Question: Beaked whales feed at various depths, but they defecate at the
ocean’s surface. Nitrogen-rich whale feces deposited in surface waters
supply nutrients for algae that are eaten by surface dwelling fish. Which of
the following best predicts what would happen if the whale population
decreased?
(A) There would be a reduction in surface nitrogen concentration, which
would cause an algal bloom.
(B) The surface fish populations would decline due to reduced populations
of algae.
(C) The remaining whales would accumulate mutations at a faster rate.
(D) The remaining whales would be forced to forage in the deepest parts of
the ocean.
Essential knowledge 4.A.6: Interactions among living systems and with their
environment result in the movement of matter and energy.
Topic: Energy Flows; Food Chains and Food Webs
Primary Producer  Primary Consumer  Secondary Consumer  Tertiary Consumer
Energy flows in an ecosystem. As energy passes through each trophic level,
energy is lost. Approximately 10% is efficiently harnessed at the next trophic
Question:
Topic: Nutrient Cycles
Nutrients
(matter) is
recycled in an
ecosystem. These
biogeochemical
cycles can takes
years to cycle
through the
system.
Nitrogen Cycle Video:
http://studyjams.scholastic.com/studyjams/jams/science/ecosystems/nitro
gen-cycle.htm
Essential knowledge 4.B.3: Interactions between and within populations
influence patterns of species distribution and abundance.
Topic: Population Interactions
Interactions between populations affect the distributions and abundance of
populations. Competition, parasitism, predation, mutualism, and
commensalism can affect population dynamics. And relationships among
interacting populations can be characterized by positive and negative
effects, and can be modeled mathematically (predator/prey,
epidemiological models, invasive species). Many complex symbiotic
relationships exist in an ecosystem, and feedback control systems play a
role in the functioning of these ecosystems.
A keystone species is a species on which other species in an ecosystem
largely depend, such that if it were removed the ecosystem would change
drastically. An example of such is a sea otter.’
Symbioses (Relationships) to Know
-Mutualism: Both species benefit from a relationship together. Ex: oxpecker
and rhino.
-Obligate mutualism: organism cannot survive without the other.
-Commensalism: One species benefits from the relationship, but the other
does not and is not harmed. Ex: cow egrets and cows
-Parasitism: One species is harmed (the host), while the other species
benefits. Ex: tapeworm and humans.
Carbon Cycle Video:
http://studyjams.scholastic.com/studyjams/jams/science/ecosystems/carbo
n-cycle.htm
Question:
Essential knowledge 4.B.4: Distribution of local and global ecosystems
changes over time.
Topic: Human Impact and the Ecosystem
Human impact accelerates change at local and global levels. Logging (cutting
down trees), slash and burn agriculture, urbanization, mono-cropping,
infrastructure development (dams, transmission lines, roads), urban runoff
(from fertilizers, pesticides, and trash) and global climate change threaten
ecosystems and life on Earth. Most often, we reduce the number of species
in the area, thus decreasing biodiversity.
An introduced species/exotic/non-native can exploit a new niche free of
predators or competitors, thus exploiting new resources. An example is the
Cane Toad of Australia. And, the introduction of new diseases can devastate
native species. Such examples include Dutch elm disease, potato blight, or
small pox (Native Americans).
Also, geological and meteorological events impact ecosystem distribution.
• El Nino: During the winter in No. America, warmer water leads to
increased rain leads. This causes animals to change migration routes and
can cause species loss with flooding.
• Continental drift: Caused significant change in presence of species due to
movement of the plates.
• Meteor impact on dinosaurs: Caused a mass extinction of the land
dinosaurs.
Topic: Ecological Succession
Ecological succession is the gradual process by which ecosystems change
and develop over time. Nothing remains the same and habitats are
constantly changing. There are two main types of succession, primary and
secondary.
Primary succession is the series of community changes which occur on an
entirely new habitat which has never been colonized before. For example, a
newly quarried rock face or sand dunes.
Secondary succession is the series of community changes which take place
on a previously colonized, but disturbed or damaged habitat. For example,
after felling trees in a woodland, land clearance or a fire.
For example, not all individuals in a population in a disease outbreak are
equally affected; some may not show symptoms, some may have mild
symptoms, or some may be naturally immune and resistant to the disease.
Essential knowledge 4.C.4: The diversity of species within an ecosystem
may influence the stability of the ecosystem.
Natural and artificial ecosystems with fewer component parts and with little
diversity among the parts are often less resilient to changes in the
environment.
Keystone species, producers, and essential abiotic and biotic factors
contribute to maintaining the diversity of an ecosystem. The effects of
keystone species on the ecosystem are disproportionate relative to their
abundance in the ecosystem, and when they are removed from the
ecosystem, the ecosystem often collapses.
Practice FRQs
Essential knowledge 4.C.3: The level of variation in a population affects
population dynamics.
Topic: Variation in a Population
Population ability to respond to changes in the environment is affected by
genetic diversity. Species and populations with little genetic diversity are at
risk for extinction. A great example is the California condor. The California
condor has only about 200 individuals left in its population. Condors were
hunted for their feathers and also became sick when they ingested lead
bullets from carrion (dead animals that were hunted). They also have
intrinsic characteristics that make it more difficult to rebound their
population. For example, they have low birth rates and require a large
territory to live in.
Because so few individuals are left, the genetic diversity of the population is
low. Genetic diversity is great because it ensures the survival of the species.