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V. Food Chains and Food Webs
A. food chain: series of organisms that transfer food
between the trophic levels of an ecosystem
1. all food chains begin with producers
2. no ecosystem is simple enough to show with just
one food chain
B. food web: network of food chains representing the
feeding relationships among the organisms of an
ecosystem
1. includes all the food chains of an ecosystem
2. changes in one population of a food web send
ripples through the food web so that the entire
web can be affected
VI. Diversity and Stability
A. The number of links in a food web varies for each
ecosystem.
1. older, more mature ecosystems often have more
species than young ecosystems (and therefore,
more links)
B. Some ecologists think that webs with more links are
more stable because they can resist disturbance
better.
VII. Biological Magnification
A. definition: increasing concentration of a pollutant in
an organism at higher trophic levels in a food web
*** The effects of pollution caused by humans can
be magnified in a food web.
(ex. DDT and eagles, p. 120)
VIII. Energy in an Ecosystem
A. Energy and Food
1. biomass: total amount of organic matter present
in a trophic level
a. the biomass in each trophic level is the
amount of energy (in the form of food)
available to the next trophic level
2. Most of the energy that enters through organisms
in a trophic level DOES NOT become biomass
a. Energy is lost through activities
i. Mammals keeping a high body temp.
b. Some of the biomass is lost in organic
matter that is difficult to digest.
i. Bones, hooves, shells, fibers
3. When all of the energy losses are added together,
only about 10% of energy entering one trophic
level forms biomass in the next trophic level
 called “the 10% law”
a. The 10% law is the main reason why food
chains have 5 links or less.
i. Owls are not preyed upon by a high
level of carnivores. There is not
enough biomass or owls to support
another level of consumers.
B. Ecological Pyramids
1. ecological pyramid: diagram that shows the
relative amounts of energy in different trophic
levels in an ecosystem
a. the pyramid is divided into sections, each
representing one trophic level
b. can show energy, biomass, or number of
organisms in a food web
i. figure 9, page 123
ii. generally follows the 10% law
C. Matter moves between trophic levels
1. carbon, hydrogen, oxygen, and nitrogen are the
most common elements in food
2. with any of these elements missing, food cannot
be produced
3. for this reason, the growth of producers in most
ecosystems is limited by the lack of one or more
of these elements, not by the amount of energy
from the sun
4. in the next section, we’ll examine how these
elements are cycled through the environment and
reused
Ecological Succession
Change is always occurring in ecosystems. Organisms affect their
environments and so do other forces – fire, flood, and other natural
events. As the environment changes, the organisms living in that
environment change as well. Many times, different communities
follow one another in a definite pattern, and this is called ecological
succession.
I.
Primary Succession
A. The sequence of communities forming in an
originally lifeless habitat is called primary
succession.
1. occurs in new habitats without life such as cooled
lava fields, sand dunes, exposed rock from a
retreating glacier
B. The first step in primary succession is the
formation of soil from exposed rocks.
1. Rocks are first colonized by organisms called
lichens.
2. lichen: a fungus and an alga living in a symbiotic
relationship
a. Lichens secrete acids that break down the
rock and form organic material by
photosynthesis.
b. The actions of lichens, combined with
weathering, form soil. It may take several
hundred to several thousand years to form
soil.
3. Scientists call the lichen community a pioneer
community because it is the first community to
colonize a new habitat. (Plants that first colonize
disrupted areas can also be considered pioneer
species.)
C. Once soil has formed, grasses and other small
plants such as mosses begin to grow.
1. Seeds come from wind or animals.
2. Plants eventually block sunlight to the lichens
and the lichens die.
D. The grass and moss community makes the soil
deeper and more fertile; taller weeds and shrubs
with deeper roots begin to grow.
1. As the shrub community grows, it further
deepens the soil and makes it richer in nutrients.
E. Once the soil is deep enough, pines and other
shallow rooted trees move in.
1. As the trees block the sunlight, the shrubs are
replaced by the trees and the plants of the forest
floor.
F. Pine forest is replaced by hardwood forest with
trees such as oak, hickory, birch, white spruce.
Hardwood forest is often the last step in primary
succession.
1. If the ecosystem is left undisturbed, no more
drastic changes in the habitat will occur.
2. climax community: a community that does not
undergo further succession
II. Secondary Succession
A. secondary succession: occurs where a community
has been cleared by a disturbance that does not
destroy the soil, such as fires, storms, human
activity
1. Secondary succession resembles the later stages
of primary succession.
a. Fast growing grasses, mosses
b. Shrubs and tall weeds
c. Pines
d. Slow growing hardwood trees
2. Succession is complete with a climax community.
B. Old Field Succession – occurs when farmland is
abandoned.
1. Once a field is no longer cultivated, grasses and
weeds start to take over the land. Taller plants
follow, (killing the grasses and weeds with shade
and a lack of water), and then trees move in,
continuing the pattern (pines to slower growing
trees).
2. Old field succession happens much faster than
other types of succession; after about 100 years
the land can be returned to the climax
community that existed before the land was
cleared for farming.
C. Some habitats never develop climax communities.
1. Some frequently disturbed communities simply
do not last long enough – it takes several
hundred years.
2. Common example – grasslands that have
frequent fires. Ironically, disruption is the key to
keeping these communities balanced and stable.
III. Aquatic Succession
A. Aquatic succession is very similar to other forms of
succession.
1. Lakes often begin like barren rock; water is low
in nutrients and supports only a few organisms.
2. More and more water plants colonize the shore
and the surface of the lake.
3. Sediments and organic matter begin to collect
and with additional plants, a marsh is formed.
4. Land plants colonize the marsh and the lake
becomes a meadow (which may later undergo
land succession and become a forest).
IV. Island Succession
A. Occurs much the same way as succession on land.
1. Volcanic eruptions often create new islands and
they are colonized very quickly.
2. Any organism on an island must have ancestors
that were carried there by wind, water and other
organisms.
3. Islands often have large bird populations
because birds can reach islands much more
easily than land animals.
B. Unfulfilled Roles
1. Because many organisms from the mainland
have no way of getting to an island, many roles
are unfilled in island ecosystems.
2. The “lucky” organism that finds a mate on the
island can produce offspring that (over many
generations) can evolve to fill several different
roles because there is no competition.
3. The result: it’s possible to get many different
species from only a few ancestors.
Interactions in the Ecosystem: Habitats and Niches
I.
Niches
A. niche – the role of an organism in the ecosystem
1. includes both biotic and abiotic factors
a. biotic: food sources, predators
b. abiotic: temperature, sunlight, water
2. In addition to an organism’s habitat, a niche
includes what the organism does with its habitat.
B. All the members of a species are adapted to the
same niche.
1. A species’ niche is unique to that species.
a. Two species can occupy niches that are very
similar, but no 2 species can occupy the
exact same niche.
b. If two species try to share the same niche,
they will compete for resources.
i. Whichever species is best adapted and
gets the resources will force the second
species to move to another area or die
out (in that area).
c. Competitive exclusion: the extinction of a
population due to direct competition with
another species for a resource
2. Sometimes one species’ activity helps to define
the niche of another species.
a. Example:
Two species of barnacles living on the coast of Scotland have very similar niches. Both species
live on rocks in the intertidal zone of the ocean shore – Chthamalus stellatus (species A) and
Balanus balanoides (species B). Species A occurs on higher rocks that are usually exposed to
air and Species B lives on lower rocks usually covered by water except during low tide when
they are exposed. Species B survives in lower zones because it is vulnerable to drying out
while Species A is more resistant to drying out so it can survive on the higher rocks. Scientist
J.H. Connell performed this experiment: He removed all of Species B from a small area of
shore. He found that Species A began to grow on the lower rocks. The hypothesis – Species A
could live on all parts of the rocky shore, but Species B drove out Species A in the places
where Species B could survive. The result: Species A was limited to the higher rocks by
Species B, even though it could live on all of the rocks if Species B were absent.
3. fundamental niche: the theoretical niche of a
species; where/conditions the organism could live
4. realized niche: the niche that the organism
actually uses
5. Note – the fundamental niche is always equal to
or larger than the realized niche.
II. Niche Diversity
A. Niche diversity is often determined by the abiotic
factors in the habitat.
1. example – Marsh: lots of organisms, but overall
diversity of low because the physical conditions
of the marsh are generally the same from place
to place. In a desert, there are fewer organisms
but the diversity is large due to vast differences
between moisture and temperature.
2. One biotic factor that has a huge influence on
niche diversity is predation.
B. Predator: an organism that actively hunts or feeds
on other organisms
1. Predators increase niche diversity in an
ecosystem.
2. By decreasing the population of the prey species,
predators make resources available for other
species. The actions of a predator can create
other niches.
3. keystone predator: a predator that causes a large
increase in the diversity of its habitat
a. example: sea stars in tidepools – Robert
Paine, Washington state
b. When sea stars were removed from
tidepools, the number of mussels increased
dramatically. The mussels outcompeted
many of the other species in the tidepools
for resources. The number of species in the
tidepools dropped from 15 with the sea stars
present to 8 without the sea stars. By
keeping the population of mussels in check,
the sea stars enabled other species to
survive.
III. Evolving to the Niche
A. Populations evolve by adapting to niches in the
environment.
1. In the same idea, populations can also evolve to
avoid competition with another species.
2. example: warblers (type of bird)
a. five different species, all feed on insects in
the branches of spruce trees
b. Although it seems that they compete with
each other for food, each species looks for
food in a slightly different part of the tree –
outer edges of high branches, inside high
branches close to the trunk, middle of the
tree close to the trunk, etc.
c. The niches overlap a little but are
sufficiently different to allow the five species
to coexist. Each niche is very narrow.
B. Specialized vs. Generalized Species
1. specialized species – organism with a very small
or narrow niche
a. examples – giant panda (one main food
source – bamboo), warblers
b. Specialized species are very vulnerable to
extinction because they can’t respond well
to changes in the environment. If something
happens to the bamboo, the giant panda will
not survive.
2. generalized species – organism with a very wide
niche
a. examples – mice, roaches
b. Generalized species often have several
alternative food sources. Sometimes
generalized species are described as being
opportunistic.
c. These species are able to adapt to
environmental changes with relative ease.
C. Convergent Evolution
1. convergent evolution: the development of similar
adaptations in two species with similar niches
2. Similar ecosystems would have similar niches,
which in turn place similar demands on the
organisms fulfilling those niches.
3. Organisms evolve to meet the demands that the
environment (niche) places on them, and
organisms that evolve to occupy similar niches
may also be alike.
4. Example: The wings of birds and bats are
similar in general structure. Yet these are two
different animals (bird vs. mammal). Birds and
bats evolved flight independently, but the
demands of flying are very similar for both
organisms (weight, surface area, compactness).
Similar demands  similar solutions resulting in
wings that are very much alike.
5. Example: Ichthyosaur and the dolphin – The
dolphin is a mammal and the ichthyosaur is an
extinct reptile. Both are adapted to the demands
of living in the water – streamlined body, fin
shape and placement, eye placement, narrow
snout, musculature, etc.
D. Coevolution
1. coevolution: process of two species evolving in
response to long-term interactions with each
other
2. Coevolution often occurs between predator/prey
species and species that cooperate with one
another.
3. Predator/Prey example: Plants and caterpillars
a. Many plants have evolved poisonous
chemicals to prevent insects from eating
them and some caterpillars have evolved the
ability to resist these poisons. Some
caterpillars go so far as to feed only on
poisonous plants.
b. Co-evolved: one changed in response to the
other
4. Cooperation example: Acacia tree and stinging
ants in Central and South America
a. Acacia trees have large hollow thorns that
provide a nesting site and protect the ant
colony. The tree also provides the ant with
food; the ants are totally dependent on the
tree, able to nest nowhere else and having
no other source of food.
b. The ants attack any animal landing on the
tree, killing it or driving it away. The ants
clear nearby vegetation, eliminating the
tree’s competition for sunlight and water.
c. Experiments have shown that without the
ants, the tree cannot grow properly or
successfully. The ants and the tree have
coevolved to the point of complete
codependence, neither one being able to
survive without the other.
IV. Relationships in the Ecosystem
A. Predator and Prey
1. Sizes of predator and prey populations are
closely linked.
a. The larger the prey population, the more
predators that it can support.
b. The smaller the prey population, the fewer
predators that it can support.
2. Example: snowshoe hare and the lynx – chart of
populations, p. 207
a. Always greater number of prey (hares) than
predators (lynx) – recall the 10% law.
b. There is a 1-2 year delay in response of the
predator population changing after the
peaks and dips of the prey population.
3. Population Cycles
a. Large herbivores (hare, muskrat) have
peaks in populations approximately every
10 years; often their predators have
matching cycles.
b. Small herbivore (mice) have 4 year cycles
and the predators of these animals also have
4 year cycles.
B. Symbiosis – general description of a relationship in
which 2 species live in close association
1. commensalisms: one species benefits, other
species is unaffected (neither helped nor harmed)
a. example: barnacles on whales
b. the flow of the water over the barnacles as
the whale moves benefits the barnacle (filter
feeders) and the whale is unaffected
2. mutualism: both species benefit
a. example: acacia tree and stinging ants
b. example: flowers and pollinating insects
3. parasitism: one species benefits, other is harmed
a. parasitism – relationship in which one
organism feeds on the tissues or body fluids
of another
b. host – organism on which the parasite feeds
c. Parasites are harmful to the host and may
be fatal, but most parasites do not kill their
hosts outright. Why???
d. Examples – fleas, ticks, variety of worms
e. A true parasite is adapted to living on or in
the body of its host.
i. Depends on the host for many functions
which the parasite cannot perform on
its own.
ii. Tapeworm: no sensory organs, no
means of locomotion
iii. It lives in the intestines of animals
(doesn’t really need to move).
Tapeworms spread from one animal to
another through contact with feces that
contain segments of the tapeworm that
have broken off inside the infected
animal.
V.
Populations
A. Population Growth
1. All populations have the ability to grow very
quickly in the perfect environment.
a. Exponential growth: population growth in
which the rate of growth in each generation
is a multiple of the previous generation
b. In reality, conditions are never perfect – the
resources are always limited.
c. A population CAN grow exponentially, but
not for long.
2. Carrying capacity – the number of individuals
that can be supported by an ecosystem
a. Populations generally start out small and
then increase rapidly (a short period of
exponential growth).
b. The growth rate slows as carrying capacity
is approached.
c. Growth stops at carrying capacity.
d. The growth curve of a population where the
birth and death rates have become equal is
called an “S” shaped curve.
B. Limiting Factors
1. limiting factor – a force that slows the growth of
a population
2. There are 2 types of limiting factors: densitydependent and density-independent.
3. density-dependent limiting factors: dependent on
population size
a. As a population grows, these factors act
more strongly to limit growth.
b. Examples – lack of food, predation, disease,
parasites (thrive in crowded populations
because new hosts are easily found)
4. density-independent limiting factors: affect the
same percentage of a population regardless of its
size
a. examples: natural disasters such as
hurricanes, tornadoes, floods, fires; climate;
human disturbance
5. Populations with density-dependent limiting
factors show an S-shaped growth curve.
Populations with density-independent limiting
factors show a curve called a boom and bust
curve (sharp exponential increases followed by
sharp collapse).
a. These populations with boom and bust
curves are usually adapted to take
advantage of regular occurring densityindependent factors, like seasonal warm
temperatures.
b. Common for insect populations