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Content Benchmark L.12.C.2
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Life Science
Heredity
Structure of Life
Organisms and Their Environment
  L.12.C.1
  L.12.C.2
  L.12.C.3
  L.12.C.4
Diversity of Life
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

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.

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

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/ecosystemfunction.htm

http://www.nature.com/nature/journal/

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.

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.

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.

Figure 5. This graph shows that the number of eggs laid per female decreases as the density of females increases.

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.

(Click Image to Enlarge)

Figure 6. A graph showing the linked oscillation of a predator/prey relationship http://www.sfws.auburn.edu/ditchkoff/images/Lecture%
20Images/Carnivores/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?


Figure 7. A starting food ocean food web

 


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.

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.

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Content 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

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.


The figure 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 density-independent 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

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Content Benchmark L.12.C.2

Students know how changes in an ecosystem can affect biodiversity and biodiversity’s contribution to an ecosystem’s stability.  E/S

Sample Test Questions

1st Item Specification: Predict how changes in an ecosystem’s stability impact biodiversity.

Depth of Knowledge Level 1

  1. Which of the following statements regarding predator/prey relationships is MOST accurate?
    1. There is no relationship between sizes of predator and prey populations.
    2. The predator population is dependent on the prey population for survival.
    3. The relationship centers on a carnivore eating a defenseless herbivore.
    4. If a prey species is depleted, the predator will switch to another prey species.
  1. Biodiversity in an ecosystem is a measure of the
    1. number and types of organisms.
    2. climax community.
    3. limiting factors that control the population.
    4. predator/prey relationship.

Depth of Knowledge Level 2

  1. The following table lists the population size of six species in four different areas.
Species1
Species 2
Species 3
Species 4
Species 5
Species 6
Area A
20
50
70
20
20
20
Area B
0
60
80
40
20
0
Area C
0
0
20
0
0
20
Area D
40
30
80
110
0
20

If these four areas are the only habitats where these organisms exist, which species is most likely to be negatively impacted by changes in the environment?

  1. Species 1
  2. Species 3
  3. Species 5
  4. Species 6
  1. The following table lists the population size of six species in four different areas.
Species A
Species B
Species C
Species D
Species E
Species F
Area 1
20
50
70
20
20
20
Area 2
0
60
80
40
20
0
Area 3
0
0
20
0
0
20
Area 4
40
30
80
110
0
20

Which of the following areas has the GREATEST biodiversity?

  1. Area 1
  2. Area 2
  3. Area 3
  4. Area 4

2nd Item Specification:  Recognize that ecosystems change over time.

Depth of Knowledge Level 1

  1. The change in an ecosystem over time until a stable stage is reached is known as
    1. competition.
    2. homeostasis.
    3. transpiration.
    4. succession.
  1. A lava field is bare for several years when lichens begin to grow on the rocks. These are gradually replaced with grasses, shrubs, and finally trees. This process is called
    1. primary homeostasis.
    2. secondary homeostasis.
    3. primary succession.
    4. secondary succession.

Depth of Knowledge Level 2

  1. The below diagram represents a change in an environment over time.


(Diagram modified from ExamGen 4.3)

Which of the following statements BEST describes the process taking place?

  1. The community is increasing in complexity over time.
  2. The evolution of a new species filling an empty niche./li’
  3. The water cycle includes the absorption of water through the soil.
  4. The path of energy is moving through a natural food chain
  1. An area in the early stages of secondary succession would most likely be
    dominated by
    1. lichens.
    2. grasses.
    3. shrubs.
    4. trees.

3rd Item Specification: Predict consequences of an environmental change.

Depth of Knowledge Level 1

  1. Acid rain, deforestation, and soil erosion are examples of
    1. habitat fragmentation.
    2. invasion of exotic species.
    3. habitat degradation.
    4. global warming
  1. This graph shows the population trends of five populations over one year.

What happened to Population B over the year? The population showed

  1. an increased over time, followed by a decrease.
  2. a slight decrease in population over time resulting in extinction.
  3. a dramatic decrease in population resulting in extinction.
  4. a stable unchanging population over time.

Depth of Knowledge Level 2

  1. Which of the following types of animals will be MOST impacted by habitat fragmentation?
    1. Large predators and migratory animals
    2. Large herbivores and large predators
    3. Non-migratory animals and small herbivores
    4. Migratory animals and large herbivores
  1. This graph shows the population trends of five populations over time.

Which of the following statements is the MOST plausible explanation for the changes in population E?

  1. Species E started eating species A because the increase in E occurs concurrently with the decrease in A.
  2. Individuals of species E immigrated into this habitat from the surrounding areas.
  3. The decline in B and D reduced the competition for resources resulting in an increase in species E.
  4. Species E moved into the niche previously occupied by species B resulting in an increase in species E.

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Content Benchmark L.12.C.2

Students know the amount of living matter an environment can support is limited by the availability of matter, energy, and the ability of the ecosystem to recycle materials. E/S

Answers to Sample Test Questions

  1. B, DOK Level 1
  2. A, DOK Level 1
  3. C, DOK Level 2
  4. A, DOK Level 2
  5. D, DOK Level 1
  6. C, DOK Level 1
  7. A, DOK Level 2
  8. B, DOK Level 2
  9. C, DOK Level 1
  10. C, DOK Level 1
  11. A, DOK Level 2
  12. C, DOK Level 2

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Content 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

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

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-limiting-factors.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/0534-KathyParis/index.html

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Benchmark
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Sample Questions:
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Intervention Strategies & Resources:
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