Performance Benchmark L.12.C.2
Students know how changes in an ecosystem can affect biodiversity and biodiversity contribution
to an ecosystem. E/S
A simple definition of biodiversity is the number and types of organisms in an environment. A
desert has a much lower relative biodiversity than a tropical rainforest due to limited resources.
Therefore, comparing biodiversity between biomes isn’t informative, but changes in an
ecosystem’s biodiversity can be a red flag to detrimental environmental damage or of climatic
shift. The number of species in an ecosystem can influence that system’s stability, productivity,
and value to humans. Sometimes the loss of a single species in an ecosystem can completely
shift the balance of the ecosystem.
Figure 1: An illustration of three levels of biodiversity. http://www.scq.ubc.ca/?p=528
There are several factors that affect biodiversity. These include but are not limited to succession
and density independent and dependent limiting factors.
But what is biodiversity’s contribution to an ecosystem?
According to Dr. Mathew Williams at the University of Edinburgh, high biodiversity increases
an ecosystem’s resilience and resistance. Resilience “describes the speed with which a
community returns to its former state after perturbation.” In other words, high biodiversity
reduces the amount of time it takes for secondary succession to proceed from disturbance until
the climax community is reached. See below for a greater explanation of succession.
Resistance “describes the ability of a community to avoid displacement in the first place.”
Species rich communities have high resistance to change from invasive species or environmental
fluctuations like drought. Species-poor communities have empty niches which make them
susceptible to invasion by an alien species.
High biodiversity is like an insurance policy for the
community. More diverse communities can respond to
disruptions with little overall change in population make
up. David Tilman & John A. Downing conducted a study
on grassland and biodiversity. They concluded that their
study “implies that the preservation of biodiversity is
essential for the maintenance of stable productivity in
ecosystems.” More information can be found at:
http://canadianbiodiversity.mcgill.ca/english/theory/ecosy
stemfunction.htm
http://www.nature.com/nature/journal/v367/n6461/abs/36
7363a0.html
Figure 2: Another theory, the rivet hypothesis,
shown to the above figure claims that each species
added to an ecosystem increases ecosystem
functions.
http://canadianbiodiversity.mcgill.ca/english/theory/
ecosystemfunction.htm
Succession
In the short term, ecosystems may appear to be stable but are in a constant state of flux. Minor
changes occur as one species slowly replaces another in response to such factors as climate
changes, human impact, or introduction of alien species. This process is called ecological
succession. In land environments, succession is driven by the plants. If the plant species change,
the herbivores respond, and the predators follow. The species that have the greatest impact on
the overall shift in species are called the dominant species. The changes caused by the dominant
species determine the type of other species that can survive in each successive community.
Succession continues until the most mature, stable community evolves. This community is
called the climax community. The climax community is the most stable and will remain unless
upset by a catastrophic event like a flood, fire, clear cutting, volcanic eruption, etc.
There are two main types of succession, primary and secondary. Primary succession occurs
when no soil is present. Therefore, the organisms that migrate into the area must contribute to
soil formation before other, larger plants can move in. Weathering begins breaking down the
bare rock into smaller particles, and then pioneer species can move in. Lichens are a good
example of a pioneer species. Lichens attach themselves to rocks with root like rhizoids. They
secrete acids onto the rock surface, dissolving the rocks. This begins the formation of soil. Once
there is a thin layer of soil, small herbaceous plants (like grasses) move in and further the
formation of soil. This process continues until a mature soil profile is created and the climax
community is established.
Figure 3: An example of
primary succession.
http://www.life.uiuc.edu/bio1
00/lectures/s06lects/03s06succession.html
Secondary succession is succession that takes place when soil is already present, usually
following a catastrophic event that partially or completely removes the existing vegetation.
Secondary succession occurs much faster than primary succession because soil already exists, a
healthy seed bank in the soil may be present, or root stocks of previous plants may still be viable.
Secondary succession usually begins with fast growing herbaceous ground cover that stabilizes
the soil. Ultimately, the slower growing climax shrubs and trees re-grow and dominate the
community again.
Figure 4: An example of
secondary succession.
http://www.life.uiuc.edu/
bio100/lectures/s06lects/
03s06-succession.html
For additional information on succession, go to
http://www.nk2.psu.edu/naturetrail/succession.htm
http://www.countrysideinfo.co.uk/successn/index.htm
Limiting Factors
Limiting factors are resources in the environment that limit the maximum number of organisms
that can survive in an ecosystem. Therefore, limiting factors define the carrying capacity of the
ecosystem. Limiting factors can be divided into two categories, density dependent and density
independent.
Density dependent factors are factors that affect large, dense populations more strongly that
small, less dense populations. These include competition for food, water, and living space as
well as disease and predation. Large, dense populations require that individuals compete for
water, food, sunlight, space, pollinators, or other resources essential for life. There just isn’t
enough to go around. Some individuals will obtain what they need to survive, others will obtain
enough to stay alive but not enough to reproduce and/or raise offspring. Still others will not
obtain enough to survive and will die.
Why is competition considered density dependent? The more individuals present, the faster
resources will be exhausted. The fewer individuals present, the longer resources will last.
Density Dependent
3.8
Clutch Size
3.6
Figure 5. This graph shows that
the number of eggs laid per
female decreases as the density of
females increases.
3.4
3.2
3
2.8
0
10
20
30
40
50
60
70
80
Density of Females
Predation is another density dependent limiting factor. The more prey animals are present the
higher the predator count will be and conversely the lower the prey animals the lower the
predator count. This is a classic predator/prey relationship. The predator population relies of the
prey population for its very survival. So as the prey population increases, so does the predator.
Eventually, the predator population will be so large that it will over hunt its prey, reducing the
overall population, and drive its own population down.
Figure 6. A graph
showing the linked
oscillation of a
predator/prey relationship
http://www.sfws.auburn.e
du/ditchkoff/images/Lect
ure%20Images/Carnivore
s/lynx-hare_cycle.gif
Lastly, disease and parasites can spread much faster through a dense population than through a
spread out or less dense population. The individuals in a dense population are more likely to
come into contact with each other and transfer viruses, bacteria, and parasites.
Density independent limiting factors are things that will affect a population regardless of its size
or density. Weather is usually the most dramatic density independent limiting factor. For
example, a hurricane, flood, or extensive drought can destroy an entire population regardless of
the raw number of individuals in the population. Fire, earthquakes, landslides and avalanche are
other natural independent limiting factors. Human activities, like clear cutting, pollution, and
pesticide/herbicide spraying, can also have a dramatic effect on any population.
Food Web Example Activity
A good exercise to work through with students to help them understand how changes in
biodiversity affect other species is to construct a food web and then change one thing and see the
cascading changes that could occur. The left side of the food web represents an open ocean
ecosystem and the right side is a rocky shore/kelp forest ecosystem. Even though these two
ecosystems are miles apart they are still connected. Can you describe how overfishing of perch
that cuts the population by 50% will affect both ecosystems?
Orca
Seals
Perch
Phytoplankton
Sea Gulls
Otters
Purple Sea Urchins
Kelp
Figure 7. A starting food
ocean food web
Orca
Seals
Perch
Phytoplankton
Sea Gulls
Otters
Purple Sea Urchins
Figure 8. This is only one scenario
of what could happen in response
to the fish population dropping by
50%. Blue arrows reflect whether
the population increased or
decreased in response to the perch
population (red arrow) dropped by
one half.
1/2
Kelp
The following is a possible explanation of the changes in this ecosystem due to a reduction in the
number of perch. With the decrease in perch population first the seal then the Orca population
may decline due to the lack of prey. With the decrease in Orca the otter population may rise due
to the lack of predators. As the otters increase in number, they prey more heavily on the sea
urchins, causing a decline in their numbers. As the sea urchins decline in number the sea gulls
numbers fall due to a reduction in their food source. This is only one possibility of what could
happen if the fish population dropped by 50%. It is open to class discussion to create other
resulting food webs.
Performance Benchmark L.12.C.2
Students know how changes in an ecosystem can affect biodiversity and biodiversity contribution
to an ecosystem. E/S
Common misconceptions associated with this benchmark
1. Students incorrectly believe populations will increase indefinitely because the resources
are unlimited.
The carrying capacity of an environment is limited to the number of organisms it can
sustain during the environments harshest season. For the desert this would most
likely be the summer, and for the higher latitudes it would most likely be the winter.
The factors that determine the number of organisms an environment can maintain are
called the limiting factors. Limiting factors are resources, such as, water, oxygen,
food, and space, as well as, environmental concerns such as temperature, rain/snow
fall and the geographical arrangement or access to the needed resources. If a
population’s size is greater than the carrying capacity then the organisms have
exceeded the ability of the environment to support that population. The population
will potentially suffer from starvation, dehydration, exposure, as well as, diseases,
and fighting until the population is reduced in size to a level that the environment can
support. This level will probably be well below the carrying capacity. The
population is now set to rebound in number, and repeat this cycle, of fluctuating
around the carrying capacity.
The activity “Oh! Deer,” found in the Project Wild guide or the slightly modified
version found at the link below are an excellent interactive way for the students to
simulate the changes in population due to limiting factors. This simple activity has
the potential to promote an in-depth understanding of limiting factors and carrying
capacity. To access the modified version:
http://georgiawildlife.dnr.state.ga.us/assets/documents/Oh_Deer.pdf
2. Students incorrectly believe a size change in one population will not have an effect on
other populations in the same food web because the chains are spread out.
Review the food web portion of this benchmark. In the example we see
phytoplankton, perch, seals, and orca as representatives of an open water ecosystem
food chain. We also see kelp, purple sea urchins, sea gulls, otters, and orca as
representatives of a kelp forest ecosystem food chain. Together the two food chains
represent a food web spread out over hundreds of miles. The over fishing of perch in
the open ocean has a direct and significant affect on the kelp forest ecosystem. For
additional information on food webs and chains, visit
http://www.bigelow.org/edhab/fitting_algae.html
Population Size
3. Students mistakenly believe that the relative population size of prey and predator
populations have little or no affect on the overall size of the other population.
Population A
Population B
Time
The figure to the left illustrates the student’s
misconception. Population A and B
represent the predator or prey populations.
It shows that one population is fluctuating
but the other is stable. Predation is a
density-dependent limiting factor. The
predator population is dependent on the prey
population for survival. If the number of
prey decreases then the predator population
will decrease as well. If the number of prey
increases then the predator population will
also increase. Refer to the figure 6; the
graph depicts the classic predator prey
relationship.
4. Students incorrectly believe that density-dependent factors are biotic, and densityindependent factors are abiotic.
Density-dependent limiting factors are resources that individuals compete for in a
population. The greater the size or density of the population the greater the
competition for resources will be. These resources include but are not limited to food,
space, water, and sunlight. The density-dependent limiting factors are both biotic
(food) and abiotic (water, space, and sunlight). Density-independent limiting factors
are factors that effect a population regardless of its size or density. Examples of
density-independent limiting factors are floods, earthquakes, clear cutting, and
drought. Density-independent limiting factors are usually catastrophic and include
human impact such as cutting down forests. The following limiting factor activity
may help students develop a better understanding of this concept.
Perch in Lake Winnipeg Limiting Factor activity
http://www.gov.mb.ca/conservation/sustain/limfac.pdf
Performance Benchmark L.12.C.2
Students know how changes in an ecosystem can affect biodiversity and biodiversity contribution
to an ecosystem. E/S
Sample Test Questions
GrassesMiceBobcat
1. The diagram above shows a food chain. If the population of bobcats decreased, what
would eventually happen to mice population?
a. It would increase
b. It would decrease
c. It would remain constant
d. It would decrease then increase.
2. Which of the following statements regarding predator/prey relationships is accurate?
a. There is no relationship between sizes of predator and prey populations.
b. The predator population is dependent on the prey population for survival.
c. Predator/prey relationships center around a carnivore eating a defenseless
herbivore.
d. If a prey species is depleted, the predator will automatically switch to another
prey species.
3. A resource that is competed for within a population is a
a. Density-independent limiting factor
b. Density-dependent limiting factor
c. Ecological Succession
d. Carrying Capacity
4. Biodiversity in an ecosystem is a measure of
a. the number and types of organisms that live in the ecosystem.
b. the climax community.
c. the limiting factors that control the population.
d. the predator/prey relationship.
5. The change in an ecosystem over time until a stable stage is reached is known as
a. competition.
b. homeostasis.
c. transpiration
d. succession
6. An ecosystem will most likely remain stable if
a. It has a high level of biodiversity
b. It has more predators than prey
c. Biotic factors decrease
d. Finite resources decrease
7. The above diagram represents a change in an environment over time. The changes over
time provide an example of
a. Communities replacing each other in an orderly fashion
b. The evolution of a new species.
c. The stages of the water cycle.
d. The path of energy through a natural food chain
Performance Benchmark L.12.C.2
Students know how changes in an ecosystem can affect biodiversity and biodiversity contribution
to an ecosystem. E/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
7.
(a)
(b)
(b)
(a)
(d)
(a)
(a)
Performance Benchmark L.12.C.2
Students know how changes in an ecosystem can affect biodiversity and biodiversity contribution
to an ecosystem. E/S
Intervention Strategies and Resources
1. “The Virtual Nature Trail…” Reading on Ecological Succession
This is a good reading assignment to assign students about ecological succession. The home
page for this site is a virtual trail through a deciduous forest in Pennsylvania. Students can
“walk” the trail where “ecosystem change ("succession") is the major theme of this nature trail.”
To access this resource: http://www.nk2.psu.edu/naturetrail/succession.htm
2. Environmental Science Webquest
The website states that this webquest is designed for 12th grade environmental science students.
However, it may be appropriate as is or modified for any secondary biology class. The author
states that “during this computer lab session, students will explore some of the most threatened
species in the world, several threats to species preservation and biodiversity, and many of the
reasons for preserving the biodiversity of all of the world's communities.”
To access this resource: http://web.syr.edu/~jfboswor/biodiversity/biodiversity.html
3. A population simulation
This is an interactive activity where students set population parameters for guppies and their
potential predators. Then a simulation will run for many generations and students can observe
how the guppy population changes in response to competition and predator/prey relationships.
This website simulation is also appropriate for the evolution benchmarks.
To access this resource: http://www.pbs.org/wgbh/evolution/sex/guppy/index.html
4. Lesson plan resource on Limiting Factors
“This unit exposes students to the concept that limiting factors control all parts of an ecosystem.
These limiting factors will differ from system to system and from species to species. Discussion
will involve fire, predator/prey relationships, habitat, and adaptation. Activities are designed to
help students understand and identify these factors and the responsible role of humans within the
ecosystems.”
To access this resource: http://www.nps.gov/wica/forteachers/seventh-and-eight-grade-limitingfactors.htm
5. Biodiversity Activities
“The first activity illustrates how to use math to calculate the diversity index of a selected
habitat. The closer to 1 the diversity index is the more diverse and healthy the habitat is. This is a
very simplified version of diversity index.” Activity 2 is a simulation. “Side one of a card
represents the monoculture (the opposite of diversity) of second growth forests. In this case,
Douglas Fir trees were planted after an old growth forest was cut down. A disease hits one of the
Douglas Firs, and because of the proximity of the other Douglas Firs, disease spreads quickly.”
To Access this resource: http://www.accessexcellence.com/AE/ATG/data/released/0534KathyParis/index.html