Download Interactions 1 in Ecosystems - Kossmann

• Interactions
1 in Ecosystems
Habitat and Niche
14.2
Community Interactions
14.3
Population Density and
Distribution
14.4 Population Growth Patterns
Data Analysis
READING COMBINATION GRAPHS
14.5
Ecological Succession
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PREMIUM CONTENT
Environmental Stress Explore how healthy
parrot populations have weathered hurricanes
in the past. Then determine whether an
endangered parrot population can recover.
° Why are these
zebras fighting?
For the zebra, life on the African
savannah is about survival. Whether
escaping th e ambush o f a pride o f
lions, walking vast distances t o drink
fresh water, or competing f o r the right
t o mate with females, only the best
adapted zebras will survive and pass
on their genes. The interactions
among organisms, and between
organisms and their environment,
make ecosystems function.
READING TOOLBOX
This reading tool can help you learn the material in the following pages.
USING LANGUAGE
YOURTURN
Predictions Some predictions are conditional:
In t h e following sentences, identify t h e condition and t h e
Something might happen, but only i f something else
prediction.
happens first. For example, i f the temperature drops below
1.
freezing, snow might fall. The prediction is that snow might
fall tonight. But snow might fall under one condition. First,
the temperature has t o drop below freezing.
A f t e r t h e deer population reaches 600 individuals o n
t h e island, t h e deer will eat most o f t h e vegetation,
and t h e number o f deer will decrease.
2. If t h e otters are removed f r o m t h e ecosystem, t h e sea
urchins will eat all o f t h e kelp.
Chapter 14: Interactions in Ecosystems
401
Habitat and Niche
VOCABULARY
KEY C O N C E P T
Every organism has a habitat and a niche.
habitat
M A I N IDEAS
ecological niche
O A habitat differs from a niche.
competitive exclusion
O Resource availability gives structure t o a community.
ecological equivalent
Connect to Your World — —
T h e ways in which a zebra interacts w i t h its e n v i r o n m e n t a n d o t h e r organisms are
o n ly a small p a r t o f t h e e c o l o g y o f t h e African plains. To u n d e r s t a n d w h a t individuals,
populations, a n d c o m m u n i t i e s n e e d t o survive, ecologists s t u d y t h e interactions
a m o n g species a n d b e t w e e n species a n d t h e i r environment. W h y d o e s a zebra fit so
well into t h e African savannah?
Q M A I N IDEA
A habitat differs from a niche.
FIGURE 1.1 A lion must hunt
and kill its prey in order t o survive
on the African savannah. Its role
as a t o p predator is part o f t h e
lion's niche.
On the vast plains of Africa, tall grasses grow among trees and shrubs, and
small pools of water surrounded by thirsty animals dot the landscape. This
challenging environment is the home of the African lion, shown in FIGURE 1.1.
Here, lions stalk through tall grass to hunt zebras and antelope, find places to
rest in the shade of trees, and never stray far from valuable pools of water.
These are just a few of the environmental features that make up the lions
habitat. A habitat can be described as all of the biotic and abiotic factors in the
area where an organism lives. These factors include all aspects of the environ­
ment, including the grass, the trees, and the watering holes.
Each species interacts with its environment in a
different way. Within an ecosystem, each species has
an ecological niche. An ecological niche (nihch) is
composed of all of the physical, chemical, and biological
factors that a species needs to survive, stay healthy,
and reproduce.
You can think of a habitat as where a species lives and
a niche as how it lives within its habitat. A niche includes
•
Food The type of food a species eats, how a species
competes with others for food, and where it fits in the
food web are all part of its niche.
• Abiotic conditions A niche includes the range of
conditions, such as air temperature and amount of
water, that a species can tolerate.
• Behavior The time of day a species is active as well as
where and when it reproduces are factors in the niche
of a species.
402
Unit 5: Ecology
Looking closely at all of these factors, we can see that while an antelope may
use the tall grasses of the African plains as a food resource, a lion may use the
s a m e grasses as camouflage for hunting. A lion uses the antelope as a food
resource and hunts primarily during low-light times like dawn or dusk. In order
to avoid the intense heat of the savannah, lions often spend afternoons in the
shade. These examples are only a few parts of the lions ecological niche, but
they help to give a picture of how a lion fits into the African savannah.
Connect
W h a t are some o f the abiotic and biotic factors o f your habitat?
a
READING TOOLBOX
TAKING NOTES
Define ecological niche by
organizing your notes into
a chart.
Ecological N \ c h e
food
Abiotic condition^
Q M A I N IDEA
Resource availability gives structure
to a community.
You have learned that the ability of an individual to survive and reproduce is
the driving force behind natural selection. A species needs resources such as
food, water, and shelter to be successful in its habitat. The organism that is
best suited to obtain these resources is most likely to survive and reproduce.
But what if two species are competing over limited resources?
fyeJnavior
Virtuai _.-Aki
Population Niches and
Competition
Competitive Exclusion
We have already seen that many species can share
similar habitats and that they may use some of the
same resources, as shown in FIGURE 1.2. But when two species
use the same resources in the same ways, one species will
always be better adapted to the environment. The principle
of competitive exclusion states that when two species are
competing for the same resources, one species will be better
suited to the niche, and the other species will be pushed into
another niche or become extinct.
The North American gray squirrel was introduced to
Great Britain in the late 1800s. The native European red
squirrel was forced to compete with the newcomer for the
same food resources, habitat, and space. In this case the
gray squirrel was better adapted to the niche and pushed
out its smaller competitor. Currently, the red squirrel
population is declining due to competition with its larger,
more aggressive cousin. But competitive exclusion can also
result in other outcomes.
• Niche partitioning The two squirrel species could have naturally divided
different resources based on competitive advantages. If one type of squirrel
ate nuts from the tops of trees while others ate nuts from the ground, the
niche would have been divided.
FIGURE 1.2 Even though bees and
butterflies both use these flowers
f o r food, they occupy different
niches. Many species with similar
niches can coexist.
• Evolutionary response The two species of squirrel could have experienced
divergent evolution. Selection for larger teeth might have allowed one type
of squirrel to become better at cracking large nuts, while selection for
smaller teeth might have allowed the other to eat small seeds.
Chapter 14: Interactions in Ecosystems
403
FIGURE 1.3 E c o l o g i c a l Equivalents
Ecological equivalents are two species that occupy
similar niches in geographically separate areas.
Survive within a Niche
South America
Madagascar
The mantella frog (left) and the poison dart frog (right) have evolved similar defense mechanisms.
The bright coloration o f each is a warning t o predators. Each frog secretes a highly poisonous
toxin through its skin that makes i t an unpleasant meal f o r a predator.
Synthesize Explain how natural selection resulted in t h e evolution
o f t w o similar frog species in t w o similar niches.
Ecological Equivalents
The competitive exclusion principle involves species competing for resources
in the same community. In different communities, ecological equivalents
occur in very similar niches. In mathematics, numbers that are equal are
called equivalents. Similarly, ecological equivalents are species that occupy
similar niches but live in different geographical regions. Pictured in FIGURE 1.3,
the mantella frog of Madagascar and the poison dart frog of South America
have much the same niche in similar habitats. They both have brightly colored
skin that secretes a highly poisonous toxin to ward off predators. Both prey on
similar insects and live in a similar habitat, but because they live in different
regions of the world, they never compete for the same resources.
!X£2SaXZ£I~
AMPHIBIANS
Amphibians were the first
vertebrates t o move out o f the
water and onto land. In
Vertebrate Diversity, you will
learn more about amphibians.
Apply Are these frogs experiencing competitive exclusion? Explain.
Formative Assessment
REVIEWING
O M A I N IDEAS
1. W h a t are t h e t h r e e parts o f an
organism's ecological niche?
2. W h a t d o e s t h e principle of
competitive exclusion say will
happen when t w o species c o m p e t e
for t h e same resource?
CRITICAL T H I N K I N G
3. Predict If a group o f mantella
frogs were transported t o t h e
ecosystem o f t h e poison dart frogs,
what might happen t o t h e t w o
species' populations?
4. Analyze A bison and an elk live in
t h e same habitat and feed o n t h e
same grasses. Does this mean t h a t t h e
competitive exclusion principle d o e s
CONNECT TO
EXOTIC SPECIES
5. Considering t h e c o m p e t i t i v e
exclusion principle, why may |
it be harmful t o t r a n s p o r t a
species, such as a rabbit, t o
another habitat where it
currently does n o t exist?
14.2
Community Interactions
^VOCABULARY
competition
Organisms interact as individuals and
as populations.
KEY C O N C E P T
predation
M A I N IDEAS
symbiosis
O Competition and predation are t w o important ways in which organisms interact.
mutualism
commensalism
O Symbiosis is a close relationship between species.
parasitism
Connect to Your World
Each day, t w o h o t d o g v e n d o r s sell virtually identical p r o d u c t s t o a n y o n e w h o is
hungry. They may b e o n different sides o f t h e street, b u t t h e y are still trying t o sell
h o t d o g s t o t h e s a m e hungry consumers.
A v e n d o r selling h o t pretzels may also b e
trying t o sell t o t h e s a m e customers, b u t w i t h a slightly varied product. Just like t h e s e
vendors, organisms c o n s t a n t l y c o m p e t e w i t h o n e another.
Q M A I N IDEA
Competition and predation are two important
ways in which organisms interact.
Two birds may fight over territories. A fish may prey on insects floating on the
water. These are just two examples of the many interactions between species in
an ecosystem.
FIGURE 2.1 Snakes are predators
that swallow their prey whole.
The hollow fangs o f this timber
rattlesnake inject venom t o para­
lyze and kill its prey.
Competition
C o m p e t i t i o n occurs when two organisms fight for the same limited resources.
There are two different types of competition: interspecific competition and
intraspecific competition.
Even though they may have different niches, two species may still use
similar resources. Interspecific competition occurs when two different species
compete for a limited resource, such as space. In a lawn, for example, grass,
dandelions, and many other plants all compete for nutrients and water.
Competition also occurs among members of the same species. This is
known as intraspecific competition. Individuals of a particular species struggle
against one another for limited resources. You can observe intraspecific
competition during the spring breeding season of birds. A typical male will
share a particular territory with males of different bird species but will not
tolerate another male of its own species in the same area.
Predation
Another way species interact with one another is through predation.
Predation is the process by which one organism captures and feeds upon
another organism. Many organisms, such as the snake in FIGURE 2.1, have
become highly adapted to hunting and killing their prey.
Chapter 14: Interactions in Ecosystems
405
Predator/Prey Relationship
The timber rattlesnake, for example, is a predator that preys on small
animals such as mice, voles, rabbits, and squirrels. Lying silent, hidden among
leaf litter on the forest floor, the rattlesnake has found a niche as an ambush
predator. A swift bite from the snakes fangs injects its venom. The venom
attacks the nervous system and eventually paralyzes the prey. The snake
swallows the paralyzed animal whole.
Herbivores can also be considered predators. The deer that eats grass in
fields and leaves from trees is preying on the plants.
Evaluate How does natural selection shape p r e d a t o r - p r e y relationships?
© M A I N IDEA
Symbiosis is a close relationship between species.
a
READING TOOLBOX
VOCABULARY
The word symbiosis comes
from the Greek word sumbios,
which means "living together."
A honeybee buzzes away from a flower with its reward of nectar. Small pollen
grains have become attached to the bee's back. When the bee arrives at the next
flower, the pollen fertilizes the egg of the next plant. In this way, a relationship,
or symbiosis, between the bee and the flower has evolved. Symbiosis is a close
ecological relationship between two or more organisms of different species
that live in direct contact with one another. There are three major types of
symbiosis: mutualism, commensalism, and parasitism.
Mutualism
3
READING TOOLBOX
TAKING NOTES
Mutualism, commensalism,
and parasitism are distinct
types o f symbiosis. Add details
with examples o f your own.
Symbiosis
N u t u a lifm
Cory)rY)er)Sa\lSm
Pof-oyitifm
Mutualism is an interspecies interaction in which both organisms benefit
from one another. The relationship between the lesser long-nosed bat and the
saguaro cactus is another example of mutualism. During the spring, bats help
pollinate the cacti through the indirect transfer of pollen as they fly from one
cactus to another to feed on flower nectar. When the fruit ripens in the sum­
mer, bats become fruit eaters, as shown in FIGURE 2.2. The cactus benefits when
bats spread its indigestible seeds across the desert.
Commensalism
Another type of symbiotic relationship is commensalism. Commensalism is
a relationship between two organisms in which one receives an ecological
benefit from another, while the other neither benefits nor is harmed. Right
now you may be a part of a commensal relationship. Buried deep in the hair
follicles of your eyelashes are microscopic mites that feed on the secretions
and dead skin cells of your body. These harmless organisms are called demodicids, and they have found their highly specialized niche in your hair follicles.
Parasitism
Parasitism is a symbiotic relationship involving a species that directly harms
its host. Parasitism is a relationship similar to predation in that one organism
benefits while the other is harmed. But unlike a predator, which quickly kills
and eats its prey, a parasite benefits by keeping its host alive for days or years.
For example, the braconid wasp lays its eggs inside a caterpillar. When the
larvae hatch, they eat the caterpillar from the inside out, consuming the
nutrients they need to grow into adults.
406
Unit 5: Ecology
FIGURE 2.2 S y m b i o t i c R e l a t i o n s h i p s
l he interactions between species in an ecosystem can t a k e many d i f f e r e n t
forms- A symbiotic relationship involves interactions between organisms o f
different species t h a t live in direct contact.
Parasitism
0
Organism is harmed
Organism is n o t affected
Organism benefits
i
Braconid wasp
I Hornworm caterpillar
Braconid larvae feed
on their host and
release themselves
shortly before reach­
ing the pupae stage o f
development.
The host hornworm
will eventually die
as its organs are con­
sumed by wasp larvae.
Commensalism
| Human Our eyelashes
Demodicids Eyelash
mites f i n d all they
need t o survive in
t h e t i n y follicles o f
eyelashes. Magnified
here 225 times, these
creatures measure
0.4 m m in length and
can be seen only w i t h
a microscope.
are home t o t i n y mites
t h a t feast on oil secre­
tions and dead skin.
W i t h o u t harming us,
up t o 20 mites may be
living in one eyelash
follicle.
:
colored SEM; magnificatio
Mutualism
Lesser long-nosed bat The bat
_) depends on night-blooming
cacti as its primary source o f
food. Cacti are a rich source o f
f r u i t and nectar, staples o f t h e
bat's diet.
• a
L'ITICA
R
C
VIEWING
( Saguaro cactus As t h e
bat feeds on the cactus'
fruit, i t also ingests t h e
seeds. These indigestible
seeds are dispersed t o
new locations as the bat
flies across t h e desert.
How might the symbiotic relationship chai
i f eyelash mites desti^yed hair follicles?
14: Interactions in Ecpsystems
407
FIGURE 2.3 Human Parasites: Inside a n d O u t
Humans can get parasites in many ways. Leeches attach to the exposed skin of humans.
By penetrating human skin, hookworms find their home in the digestive tract.
Many leeches feed on the blood o f a host organism.
Freshwater leeches such as this one can grow t o
lengths o f 12 cm o r more.
Hookworms are endoparasites with sharp teeth that
attach t o the intestinal wall o f a host organism and
absorb nutrients f o r food, (colored SEM; magnification
200x)
Hypothesize Why is it important for ectoparasites to stay undetected by their hosts?
>,s-
CONNECT T O
INVERTEBRATES
Leeches a n d hookworms are
c l a s s i f i e d a s i n v e r t e b r a t e s . In
Invertebrate Diversity,
y o u will l e a r n m o r e a b o u t
t h e diversity o f invertebrates.
The needs of a parasite are met by a host—the victim of the parasite. There
are two different ways that parasites can use their host. An ectoparasite makes
its home on the exterior of an organism, attaching itself to the outside of the
host and usually feeding on its fluids. Common ectoparasites include fleas,
ticks, and leeches, such as the one seen in FIGURE 2.3. Many types of ectoparasites
are also known to carry a wide variety of diseases that can affect their host.
Parasites can also be found inside of living organisms. Endoparasites live in the
tissues and organs of a host where, safely hidden, they feed on the nutrients
ingested by their host. Large endoparasites, such as tapeworms and hook­
worms, and smaller protozoan endoparasites can kill their host if not treated.
Connect What type of symbiosis is the relationship between a dog and its owner?
SELF-CHECK Online
Formative Assessment
R E V I E W I N G O M A I N IDEAS
CRITICAL T H I N K I N G
1. During t h e fall spawning o f salmon,
grizzly bears fight over space o n t h e
banks o f a river. W h a t t y p e of
>n is this?
Compare and Contrast How are
predation and parasitism similar?
2. Describe and give examples o f t h e
t h r e e types o f symbiosis.
a kill, birds will sometimes arrive t o
pick a t t h e leftover carcass. Which are
t h e predators: t h e birds, t h e lion, o r
both? Why?
How d o they differ?
Synthesize After a lion has m a d e
HMD5cience.com
PREMIUM CONTENT
r m v m w E
A N I M A L BEHAVIOR
5. You have probably heard the j
saying "There is safety in
numbers." Why might
traveling in a large group be
beneficial t o prey s p e c i e s ?
u
14.3
Population Density and Distribution
VOCABULARY
population density
Each population has a density, a dispersion,
and a reproductive strategy.
KEY C O N C E P T
population dispersion
M A I N IDEAS
survivorship curve
O Population density is the number o f individuals that live in a defined area.
O Geographic dispersion o f a population shows how individuals in a population
are spaced.
O Survivorship curves help t o describe the reproductive strategy o f a species.
•>!<• Connect to Your World
If y o u have ever traveled f r o m a rural area into a city, y o u may have n o t i c e d a change
in p o p u l a t i o n density. Cities have m o r e d e n s e populations, while rural areas have
m o r e widely dispersed populations. Scientists measure species p o p u l a t i o n s in a
similar way. W h a t c a n w e learn f r o m p o p u l a t i o n d a t a ?
Q M A I N IDEA
Population density is the number o f individuals
that live in a defined area.
The wandering albatross may fly over open ocean waters for days or weeks
at a time without ever encountering another bird. In contrast to this solitary
lifestyle, elephant seals may gather in groups of a thousand or more on Cali­
fornia beaches. By collecting data about a population in a particular area,
scientists can calculate the density of a population. Population density is a
measurement of the number of individuals living in a defined space.
,c CONNECT T O
GENE FLOW
Recall that in The Evolution o f
Populations you learned about
gene f l o w and geographic
isolation. Population dispersion
patterns influence the rate o f
gene flow among and between
species.
Calculating an accurate population density can tell scientists a great deal
about a species. When scientists notice changes in population densities over
time, they work to determine whether the changes are the result of environ­
mental factors or are simply due to normal variation in the life history of a
species. In this way a wildlife biologist can work to make changes that will help
to keep the population healthy. One way to calculate population density is to
create a ratio of the number of individuals that live in a particular area to the
size of the area. This formula is simplified as follows:
# of individuals
2
area (units
)
= population density
For example, if scientists sampling a population of deer counted 200
individuals in an area of 10 square kilometers, the density of this deer popula­
tion would be 20 deer per square kilometer.
Connect W h a t might a decrease in the density o f a deer population over a specif''
t i m e period tell scientists about the habitat in t h e area?
410
Unit 5: Ecology
FIGURE 3.1 D i s p e r s i o n P a t t e r n s
pispersion patterns help us understand species interactions
by showing h o w populations group together.
Many species o f f i s h swim together
in large groups called schools. By moving
as a large mass, individuals have an
advantage in avoiding predators.
Nesting sites o f the gannet show uniform
distances f o r protection o f eggs from
other males. Territorial organisms
generally display uniform dispersion.
The three-toed tree sloth, a solitary
animal, spends most o f its life in the
canopy o f tropical forests. The sloth
has almost no competitors and has
few natural predators.
Q M A I N IDEA
Geographic dispersion o f a population shows
how individuals in a population are spaced.
Other information can be gained from population density measurements.
Patterns of geographical dispersion give us ideas of how individuals of
the same species interact and how different species interact with one another.
Population dispersion is the way in
VISUAL VOCAB
which individuals of a population
are spread in an area or a volume.
Population dispersion is t h e way
in which individuals o f a p o p u l a t i o n
FIGURE 3.1 shows the three types of
are spread in an area o r a volume.
population dispersion.
•
•
•
Individuals
may live close together in groups
in order to facilitate mating, gain
protection, or access food
resources.
C l u m p e d dispersion
Territoriality
and intraspecies competition for
limited resources lead to
individuals living at specific
distances from one another.
Uniform dispersion
Clumped
dispersion
Uniform
dispersion
Random
dispersion
Individuals
are spread randomly within an
area or a volume.
Random dispersion
Infer W h a t t y p e o f intraspecies interaction might cause uniform dispersion?
Chapter 14: Interactions in Ecosystems
411
I N T E R P R E T I N G DATA
Survivorship Curves
MATERIALS
In this lab, you will make a type 1 survivorship curve using data from the
obituary section o f a newspaper.
• obituary section o f a newspaper
• graph paper
PROBLEM What is t h e trend in data for type 1
survivorship curves?
PROCEDURE
TABLE 1. SURVIVORSHIP DATA
Survivors
% Surviving
0-5
35 - 1 = 34
97
6-10
34 - 1 = 33
94
11-15
33 - 0 = 33
94
16-20
33 - 4 = 29
83
21-25
29 - 1 = 28
80
Age (years)
1. Obtain the obituary section of the newspaper.
2. Create a data table like the one at right that
extends t o include five-year age groups up t o
91-95 years.
3. For 35 obituaries, place a tally next t o the age
group in which t h e individual died.
4. Subtract the number of individuals that died from
Deaths
the number o f remaining survivors, and record the
answer in the third column of your data table.
Calculate the percent surviving in each age group
by dividing the number o f survivors by 35 and multiplying
by 100. Repeat this step for all age groups.
ANALYZE AND CONCLUDE
1. Graph Data Draw a survivorship curve by plotting t h e age group on t h e
x-axis, and the percent survivors on they-axis.
2. Analyze Explain t h e trend in the data.
© M A I N IDEA
Survivorship curves help to describe the
reproductive strategy o f a species.
The California red-legged frog of the western United States is an amphibian
that reproduces by laying 2000 to 5000 eggs in late winter and early spring. In
one to two weeks, these eggs hatch, and over the next four to seven months,
the tadpoles grow into frogs. If so many eggs are laid, why is this frog a threat­
ened species in much of the western United States?
Many predators feed on the eggs of the red-legged frog, so of the thousands
of eggs laid, only a small number of offspring will survive to adulthood. This
type of reproductive strategy is to produce a lot of offspring. Species use many
other reproductive strategies as well. Survivorship curves illustrate how
offspring survival from birth to death fits in with the survival strategies of a
particular species.
A survivorship curve is a generalized diagram showing the number of
surviving members over time from a measured set of births. By measuring
the number of offspring born in a year and following those offspring through
until death, survivorship curves give information about the life history of a
species. For example, we will begin with 100 coyotes born in year zero. After
one year, 10 of those baby coyotes died from disease or predation. Of the
original 100, 90 are left. During year two, 4 more coyotes die, leaving 86 of
412
Unit 5: Ecology
the original 100. In year three, 3 more die, leaving 83 of
FIGURE 3.2 SURVIVORSHIP CURVES
the original 100. The number of individuals surviving
from year to year decreases, but a substantial portion of
the group will live a full life and reproduce. In FIGURE 3.2,
you can see the three basic patterns of animal survivor­
ship curves.
Type I The graph shows a type I survivorship curve in
orange. Type I survivorship represents a life history that is
common among large mammals, including humans. The
curve shows a low level of infant mortality and a popula­
tion that generally will survive until old age. A behavior
10 20 30 40 50 60 70 80 9 0
Percentage o f maximum life span
that most organisms showing type I survivorship share is
parental care for the young. Most infant organisms are
unable to care for themselves. By protecting their young, parents are better able
to ensure that their offspring stay alive until they can survive on their own.
Organisms such as birds, small mammals, and some reptiles show
a survivorship rate that is roughly equal at all ages of an organisms life. At all
times, these species have equal chances of living and dying, whether from
disease or as a result of predation. A type II survivorship curve is shown in
green on the graph.
Type II
Organisms with type III survivorship (shown in blue) have a very high
birth rate and also a very high infant mortality rate. Species with type III
survivorship are generally invertebrates, fish, amphibians, and plants. Many of
their offspring will die from predation, but inevitably a few will survive to
adulthood and be able to pass their genes on to the next generation. Though
the California red-legged frogs are threatened largely because of habitat loss
and pollution, the frogs are also targets of high levels of predation at an early
age, making recovery for this species especially difficult.
Type III
Synthesize Is there any connection between survivorship curves and reproductive
strategies? Explain.
14.3 Formative Assessment
REVIEWING O
M A I N IDEAS
CRITICAL T H I N K I N G
1- A shoreline mussel species has a
population density o f o n e organism
per square meter. Will all mussels b e
found o n e m e t e r apart? Explain.
2. Draw and label a diagram showing t h e
three population dispersion patterns.
r
How d o survivorship curves show
three types o f reproductive strategies?
•'JJ
4. Analyze W h a t might be t h e
advantages o f having a clumped
dispersal pattern?
5. Infer An organism has t e n offspring.
Two o f t h e s e offspring die each year
over a five-year period. Is t h e
organism more likely t o be a bird or
an insect? Explain.
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CONNECT TO
ABIOTIC FACTORS
6. O n t h e African savannah,
what types o f abiotic factors
may lead t o high population
density and clumped
dispersion patterns?
Chapter 14: Interactions in Ecosystems
413
Population Growth Patterns
I KEY C O N C E P T
VOCABULARY
Populations grow in predictable patterns.
immigration
M A I N IDEAS
emigration
O Changes in a population's size are determined by immigration, births,
emigration, and deaths.
exponential growth
logistic growth
O Population growth is based on available resources.
carrying capacity
O Ecological factors limit population growth.
population crash
limiting factor
density-dependent
limiting factor
- Connect to Your World
T h at ba na na y o u left in y o u r backpack did n o t g o unnoticed. After o n e week, you
density-independent
limiting factor
o p e n y o u r bag a n d d o z e n s o f tiny insects swarm out. T h e smell o f rotting fruit
follows c l o s e behind. O n l y a w e e k ago, t h e p o p u l a t i o n o f fruit flies in y o u r backpad
was zero. Just b e f o r e y o u o p e n e d it, t h e p o p u l a t i o n h a d grown t o several dozen.
How did this p o p u l a t i o n grow s o quickly?
© M A I N IDEA
Changes in a population's size are determined by
immigration, births, emigration, and deaths.
The size of a population is usually changing. If resources such as food and
water are abundant, or plentiful, a population may grow. On the other hand, i:
resources are in short supply, the population may decrease in size. Hopefully,
the normal fruit fly population in your backpack is zero. But if an abundance
of resources, such as an overripe banana, becomes available, the population
will increase dramatically. However, when the resources are removed, the fruit
fly population in your backpack will once again return to zero. Four factors
affect the size of a population.
Immigration When one or two fruit flies found the banana, they immi­
grated into your backpack. Immigration is the movement of individuals
into a population from another population.
• Births Additional fruit flies were born in your backpack. Births increase
the number of individuals in a population.
• Emigration After you opened your backpack, some fruit flies flew out and
left to find other rotting fruit. Emigration is the movement of individuals
out of a population and into another population.
• Deaths You might have squashed a couple of unlucky fruit flies as you
were opening your backpack. The size of a population decreases when
individuals die.
•
1 R E A D I N G TOOLBOX
VOCABULARY
The word
immigrate c o m e s
immigrare,
f r o m t h e Latin w o r d
m e a n i n g " t o g o into," a n d t h e
word
emigrate c o m e s f r o m
emigrare,
t h e Latin w o r d
meaning "to move."
~ \
Apply When a population is declining, what t w o factors are likely outpacing what
other t w o factors?
q M A I N IDEA
population growth is based on available
resources.
population growth is a function of the environment. The rate of growth for
a population is directly determined by the amount of resources available. A
population may grow very rapidly, or it may take a bit of time to grow. There
are two distinct types of population growth.
Exponential Growth
When resources are abundant, a
population has the opportunity to
grow rapidly. This type of growth,
called exponential growth, occurs
when a population size increases
dramatically over a period of time.
In FIGURE 4.1, you can see that exponen­
tial growth appears as a J-shaped
curve.
FIGURE 4.1 EXPONENTIAL GROWTH
1000
J
i
800
j2 6 0 0
j5
2
0
i.
JO
400
Q
>
E 200
3
v
/
z
Exponential growth may occur
o
Time
when a species moves to a previously
uninhabited area. For example, in 1859
an Australian landowner returning home from England brought
24 European rabbits to the country for the purpose of sport
hunting. The rabbits were introduced into an environment that
had abundant space and food and no predators fast enough to
catch them. The initial population of 24 rabbits grew exponentially
and spread across the country. After many attempts to control the
population, today there are between 200 million and 300 million
rabbits in Australia.
y
/
FIGURE 4.2 In Australia during
the early 1900s, the introduced
European rabbit population
exhibited exponential growth.
J
1
Logistic Growth
FIGURE 4.3 LOGISTIC GROWTH
Most populations face limited
resources and thus show a logistic
growth rate. During logistic growth,
carr /ing
a population begins with a period
capa city
of slow growth followed by a brief
C
o
period of exponential growth before
JS3
X*/
leveling off at a stable size. A graph of
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Jo /y
O
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Q.
logistic growth takes the form of an
y
S-shaped curve and can be seen in
FIGURE 4.3, which models a populations
change in size over time. During initial
Time
growth, resources are abundant, and
the population is able to grow Over time, resources begin
to deplete, and growth starts to slow. As resources become limited, the
population levels off at a size the environment can support.
/
Chapter 14: Interactions in Ecosystems
415
What Limits Population
Growth?
Carrying Capacity
The environment determines how many individuals of the species can be
supported based on natural cycles and species diversity. An environment,
therefore, has a carrying capacity for each species living in it. The
carrying capacity of an environment is the maximum number of individuals of
a particular species that the environment can normally and consistently support.
In nature, a carrying capacity can change when the environment changes.
Consider a population of grasshoppers that feed on meadow grasses. If a fire
burns part of the meadow, the insects' food resources diminish, and the carry­
ing capacity declines. But during years with plentiful rain, the meadow
grasses flourish, and the carrying capacity rises.
The actual size of the population usually is higher or lower than the
carrying capacity. Populations will rise and fall as a result of natural changes
in the supply of resources. In this way, the environment naturally controls the
size of a population.
Population Crash
When the carrying capacity for a population suddenly drops, the population
experiences a crash. A population crash is a dramatic decline in the size of a
population over a short period of time. There are many reasons why a popula­
tion might experience a crash.
DATA ANALYSIS
READING C O M B I N A T I O N GRAPHS
Combination graphs s h o w t w o sets o f d a t a o n t h e
s a m e graph. O n e s e t o f d a t a may b e s how n as a
bar graph, while t h e o t h e r s e t may b e s how n as a
line graph. T h e t w o d a t a sets m u s t share t h e s a m e
i n d e p e n d e n t variable o n t h e x-axis. Scientists can
t h e n interpret t h e d a t a t o d e t e r m i n e if a
relationship exists b e t w e e n t h e variables.
GRAPH 1. FLORIDA FISH KILLS 1991-2001
12.5
20
Fish kill e v e n t s
Rainfall ( i n c h e s ) _
15
>Q
V)
c01
s
=• 10
7.5-5
c
This combination graph displays d a t a a b o u t fish kill
events, during which many fish died a t once, a n d
average m o n t h l y rainfall in Florida from 1991-2001.
• T h e y - a x i s o n t h e left side represents t h e t o t a l
n u m b e r o f fish kill events.
• T h e y - a x i s o n t h e right side represents average
m o n t h l y rainfall during t h a t t i m e .
5.0*:
2.5
Jan Feb Mar Apr May Jun Jul Aug Sep O c t Nov Dec
Months
• T h e x-axis s h o w s t h e m o n t h o f d a t a collection.
S o u r c e : T h e U n i v e r s i t y o f F l o r i d a Extension
1. Analyze An increase in fish kills and a decrease in rainfall occurs in w h a t months?:
2. Analyze Describe t h e t r e n d in t h e fish kill e v e n t s t h r o u g h o u t t h e year.
Describe t h e t r e n d in rainfall d a t a t h r o u g h o u t t h e year.
3. Hypothesize W h a t relationship might exist b e t w e e n fish kill e v e n t s a n d rainfall?
Unit 5: Ecology
0.0
I n f o r m a t i o n C i r c u l a r 107. U s e d b y permission.
T h e graph s h o w s t h a t in January t h e r e w e r e four
fish kill e v e n t s a n d an average o f 2.7 inches o f rain.
416
10.0
For example, in 1944, 29 reindeer were introduced to St. Matthew Island
off the coast of Alaska. At the time of the introduction, the entire island was
c o v e r e d with a rich mat of lichens. Plenty of good food allowed the reindeer
herd to grow at an exponential rate. By the summer of 1963, the island popula­
tion had grown to 6000 reindeer. However, over the winter, large amounts of
snow fell on food resources that had already become greatly depleted by the
large herd. By the spring of 1964, only 50 reindeer remained. The population
crash on St. Matthew Island came as the result of two factors that limited
resources: the harsh winter and the scarcity of food.
P r e d i c t W h a t would have eventually happened t o the reindeer herd i f the winter
had not made foraging so difficult? Explain.
Q M A I N IDEA
Ecological factors limit population growth.
Many factors can affect the carrying capacity of an environment for a popula­
tion of organisms. The factor that has the greatest effect in keeping down the
size of a population is called the limiting factor. There are two categories of
limiting factors—density dependent and density independent.
FIGURE 4.4 Taking down prey
as large as a moose requires that
the members o f a pack work
together. As many as ten wolves
may take hours o r even days t o
wear down this moose.
Density-Dependent Limiting Factors
Density-dependent limiting factors are limiting
factors that are affected by the number of individuals
in a given area. Density-dependent limiting factors
include many different types of species interactions.
Competition Members of populations compete with
one another for resources such as food and shelter. As
a population becomes denser, the resources are used
up, limiting how large the population can grow.
Predation The population of a predator can be limited
by the available prey, and the population of prey can be
limited by being caught for food. On Isle Royale in
Michigan, changes in wolf and moose populations,
shown in FIGURE 4.5, provide an example. As the moose
population grows, so does the wolf population. But at a
certain point, the wolves eat so many moose that there are
not enough left to feed all the wolves. The result is a de­
crease in the wolf population. Over time, the two popula­
tions rise and fall in a pattern, shown in FIGURE 4.5.
FIGURE 4.5 D E N S I T Y - D E P E N D E N T L I M I T I N G F A C T O R S
Parasitism and disease Parasites and diseases can spread
more quickly through dense populations. The more
crowded an area becomes, the easier it is for parasites or
diseases to spread. The parasites or diseases can then
cause the size of the population to decrease.
Analyze How does the w o l f population on Isle Royale affect
the carrying capacity o f the moose population?
S o u r c e : Isle R o y a l e R e s e a r c h D a t a
Chapter 14: Interactions in Ecosystems
417
Density-Independent Limiting Factors
Density-independent limiting factors are the aspects of the
environment that limit a populations growth regardless of the
density of the population.
Unusual weather Weather can affect the size of a population
regardless of its density. For example, along the western coast of
the United States, a lack of southerly winds can prevent nutrientpoor warm water from being replaced, as it normally is, with
nutrient-rich cold water. The lack of nutrients in the water along
the coast can prevent phytoplankton, which form the base of the
marine ecosystem, from growing in their usual large numbers. In
turn, zooplankton, tiny organisms that feed on phytoplankton,
have smaller populations. The effects are felt all the way up the
food chain, with smaller populations offish and birds.
Natural disasters Volcanoes, tsunamis, tornados, and hurricanes, shown in
FIGURE 4.6, can wipe out populations regardless of density. For example, the
FIGURE 4.6 The storm surge
accompanying a hurricane can
cause dangerous flooding.
large wave of a tsunami can damage fragile coral reefs, knock down entire
mangrove forests, and destroy sea turtle nesting beaches.
Human activities Destruction of a wetland habitat along the Platte River in
Nebraska has threatened an important feeding ground for the sandhill crane.
Urbanization in this area is depleting the resources these migratory birds need
during their trek to nesting grounds in northern Canada and in Alaska. By
clearing forests, filling wetlands, and polluting the air, land, and water, humans
threaten habitats and the organisms that live in them. As we will discuss in
Human Impact on Ecosystems, human influence as a limiting factor has had a
profound effect on populations. For example, the introduction of nonnative
species has caused population crashes in many parts of the world where
biodiversity is an important part of the ecosystems functioning.
Apply A population of algae in a pond is limited in size by the amount of
sunlight that strikes the pond's surface. Is sunlight a density-dependent or
density-independent limiting factor for the algae population?
•EQ Formative Assessment
REVIEWING O MAIN IDEAS
CRITICAL THINKING
1. W h a t four factors determine t h e
growth rate o f a population?
4. Apply W h a t might cause
exponential growth t o occur only
2. How d o e s carrying capacity affect
t h e size o f a population?
3. What is t h e main difference between
a!
factor and a density-independent
limiting factor? Give examples o f each.
for a s hort period when a new
species is introduced t o a resourcefilled environment?
5. Synthesize How might densityd e p e n d e n t limiting factors b e
affected by a flood or s o m e o t h e r
natural disaster?
Barwsisiviaia
SYMBIOSIS
6. Give an example o f how a
symbiotic relationship c o u l d
cause a population crash.
Ecological Succession
VOCABULARY
I
1
succession
primary succession
I pioneer species
Ecological succession is a process o f change
in t h e species that make up a community.
KEY C O N C E P T
M A I N IDEAS
O Succession occurs following a disturbance in an ecosystem.
secondary succession
,
/
>,<- Connect to Your World —
———
It begins with a dirty sock. T h e n a discarded h o m e w o r k assignment. But this is only
t h e start. If y o u have ever s p e n t a Saturday a f t e r n o o n cleaning y o u r b e d r o o m , y o u
may have w o n d e r e d h o w a perfectly clean r o o m c o u l d m a n a g e t o b e c o m e such a
c l u t t e r e d mess. A clean r o o m b e c o m i n g c l u t t e r e d is a gradual process m u c h like t h e
process t h a t rebuilds d a m a g e d e c os ys te ms .
© M A I N IDEA
Succession occurs following a disturbance
in an ecosystem.
After an ecosystem experiences a devastating catastrophe and begins to
regrow, the space re-forms itself through a process known as succession.
Succession is the sequence of biotic changes that regenerate a damaged
community or create a community in a previously uninhabited area.
FIGURE 5.1 The path o f a lava
flow, like this one on the island o f
Hawaii (left), leaves behind noth­
ing but solid rock. Over time, pri­
mary succession will turn this
harsh landscape into a fertile eco­
system (right).
The Hawaiian Islands began to form more than 70 million years ago.
Over time, volcanic eruptions like the one shown in FIGURE 5.1 created these
islands in the middle of the Pacific Ocean. Eventually, the bare volcanic rock
began to break down into soil, which provided a place for plants to grow. As time
passed, the process of succession created unique tropical ecosystems. Succession
from bare rock to such highly diverse vegetation takes a great deal of time.
FIGURE 5.2 Primary S u c c e s s i o n
M e l t i n g glaciers, volcanic eruptions,
landslides, and strip mines
can all begin t h e process
o f primary succession.
0 - 1 5 y e a r s Moss,
1 5 - 8 0 y e a r s Shrubs,
lichens, grasses
:
80-115 y e a r s Transi­ 1115-200 y e a r s
cottonwoods, alder tion to forest, alder, Hemlock-spruce
! forest
thicket
; spruce
Glacier Bay National Park in
Alaska has given scientists an
opportunity t o witness primary
succession as the glacier recedes.
Apply W h a t function might t h e mosses and lichens serve in primary succession?
Primary Succession
One of the best ways to understand succession is to watch it progress.
Primary succession is the establishment and development of an ecosystem in
an area that was previously uninhabited. The first organisms that live in a
previously uninhabited area are called pioneer species. Typical examples of
pioneer species are lichens and some mosses, which can break down solid rock
into smaller pieces. The process of primary succession, which is illustrated in
FIGURE 5.2, follows this basic pattern:
CONNECT TO
SYMBIOSIS
A lichen is actually t w o
completely different species.
Fungus and algae form a
symbiotic relationship in which
the fungi collects water, while the
algae uses chlorophyll t o conduct
photosynthesis and synthesize
food for the lichen community.
420
Unit 5: Ecology
• Bare rock is exposed by a retreating glacier or is created when lava cools.
Wind, rain, and ice begin to break down the surface of the rock, forming
cracks and breaking the rock into smaller pieces.
• Lichen and moss spores are blown in by wind. As they grow, they break up
the rock further. When they die, their remains mix with the rock pieces to
form a thin layer of soil.
• Over time, seeds are blown into the area or are dropped by birds. Small
flowers and hardy shrubs grow from these seeds. These new plants provide
a habitat for small animals, break up the rock with their roots, and add
material to the soil when they die.
• As the soil continues to grow thicker, small trees take root, and different
animals move into the area. These trees provide shade.
• Different tree species take root in the shade and eventually replace the
original trees, which need direct sunlight to thrive.
following a f l o o d o r a fire,
is given a chance f o r n e w life. Plants
r e m a i n i n g a f t e r t h e disturbance
r e e s t a b l i s h t h e ecosystem.
0 - 2 y e a r s Horse
weed, crabgrass,
asters
2 - 1 8 y e a r s Grass,
shrubs, pine
seedlings
1 8 - 7 0 y e a r s Pine
7 0 - 1 0 0 y e a r s Oak-
forest and young
hardwood seedlings
hickory forest
forests return nutrients t o t h e
soil. Secondary succession uses
these nutrients t o grow.
Analyze W h y does secondary succession take less t i m e than primary succession?
Secondary Succession
Succession does not always begin from bare rock. More often, a disturbance,
such as a fire or hurricane, halts the progress of succession or destroys an
established community. Secondary succession, which is illustrated in
FIGURE 5.3, is the reestablishment of a damaged ecosystem in an area where the
soil was left intact. Plants and other organisms that remain start the process of
regrowth. There is no end to secondary succession. Small disturbances, such
as a tree falling, start the process again and again. The dynamic processes of
succession are always changing the face of an ecosystem.
Connect Where might succession occur in the ocean?
Formative Assessment
REVIEWING O
1. How is
M A I N IDEAS
primary succession different
from secondary succession?
2. Why are pioneer species so impor­
t a n t for primary succession?
CRITICAL T H I N K I N G
3. Infer Does t h e process o f primary
succession take longer in tropical o r
arctic areas? Explain.
4. Predict During succession, w h a t
might b e c o m e t h e limiting factor for
sun-loving mosses as taller plants
begin t o grow?
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s
, CONNECT TO
BIOLOGICAL NICHE
5. At what point during primary
succession does an ecosys­
t e m provide t h e fewest
habitats for organisms?
Explain your reasoning.
14 Summary
Review Games • Concept Map • Section Self-Checks
KEY CONCEPTS
14.4 Population Growth Patterns
14.1 Habitat and Niche
Every organism has a habitat and a niche. Each organ­
Populations grow in predictable patterns.
ism in an ecosystem has an ecological niche, which
includes t h e t y p e o f food it consumes, its behavior,
and its habitat—the place where it lives. Competitive
exclusion prevents t w o species from sharing t h e
same niche. In different geographical regions, ecologi­
cal equivalents may have similar ecological niches.
Population growth accommodates changes in
population size d u e t o births and deaths as well as
immigration and emigration. Populations experiencing
exponential growth increase dramatically overtime.
When resources become a limiting factor, a
population will grow logistically until it reaches the
environmental carrying capacity, or t h e maximum
population size t h e environment can support.
Density-dependent
limiting factors
affect dense
populations,
but densityindependent
limiting factors
affect populations
regardless o f density.
14.2 Community Interactions
Organisms interact as individuals and as populations.
Interactions between species include competition
and predation. Interactions shape ecosystem dynam­
ics. Parasitism, commensalism, and mutualism are
symbiotic relationships involving t w o species living in
direct c o n t a c t with o n e another.
14.3 Population Density and Distribution
Each population has a density, a dispersion, and a
reproductive strategy. The distribution o f a popula­
tion can be measured by population density. Species
can have clumped, uniform, or random dispersion
patterns. Survivorship curves describe t h e reproduc­
tive strategies o f different species.
•
e®
s
e
•
® o ® |
Q
©
•
e
•
e
•
@
Ecological succession is a process o f change in the
species that make up a community. Succession
refers t o t h e progression o f plants and animals that
repopulate a region after an ecological disturbance.
Primary succession begins in a previously uninhabited
area, such as bare rock exposed by t h e receding of a
glacier or created by a volcanic eruption. Secondary
succession occurs in a previously inhabited area that
is damaged by an ecological disturbance, such as a
fire o r a flood.
Random
dispersion
Uniform
dispersion
Clumped
dispersion
14.5 Ecological Succession
•
•
®
READING TOOLBOX
SYNTHESIZE Y O U R N O T E S
Concept Map Use a c o n c e p t map t o display t h e differences b e t w e e n exponential and logistic growth.
Main Idea Chart Use t h e main idea chart t o explain and
give examples o f density-independent and density-depen­
d e n t limiting factors.
Density Independent
when
Density Dependent
u n t i l i t reaches
J
J
422
Unit 5: Ecology