Content Benchmark L.8.C.4
Students know inter-related factors affect the number and type of organisms an ecosystem can
support. E/S
Ecology is the study of interactions between living organisms in a given area and their
surroundings. Organisms are impacted by their environment and in turn, impact their
environment. The living part of this world is referred to as the biotic community. The
surrounding nonliving environment is known as the abiotic world. The primary interest of
ecologists is how the biotic and abiotic factors affect the number and types of organisms that are
or can live in a given environment.
Biomes of the World
Biomes are recognized as the broadest area of interactions in ecology. These areas such as
deserts, rainforests or grasslands are defined by two major climatic factors: precipitation and
temperature. The chart below shows how these two factors determine the biome type in any
region of the world.
Figure 1. This pyramid shows the relationship between temperature and precipitation in the formation of biomes.
(From http://www.bbc.co.uk/schools/gcsebitesize/biology/livingthingsenvironment/0habitatsandpopsrev3.shtml)
Other local factors such as topography, wind patterns and ocean currents will have modifying
effects on regions. The world distribution of biomes can be seen on the map below.
Figure 2. Distribution of World Biomes.
(From http://www.bbc.co.uk/schools/gcsebitesize/biology/livingthingsenvironment/0habitatsandpopsrev3.shtml)
In addition to particular climate factors, each biome is defined by its major vegetation type,
which influences the types of animals and other organisms living in that biome.
For more information on particular characteristics of each major biome, visit the following
websites.
Missouri Botanical Gardens has student friendly descriptions of the plants and animals of each
of the major world biomes along with other characteristics. The webpage is
http://www.mbgnet.net/index.html
Major Biomes of the World contains detailed information on each of the major world biomes.
This is a site for higher level students and teachers. The website is available at
http://www.runet.edu/~swoodwar/CLASSES/GEOG235/biomes/main.html
Blue Planet World Biomes is another site that contains detailed information on specific plants
and animals found within each biome. The website is available at
http://www.blueplanetbiomes.org/world_biomes.htm
Biomes of North America describes biomes specific to North America. The website can be
accessed by linking to
http://www.utexas.edu/depts/grg/huebner/Spring2003/grg306c/biomes1.htm
Biomes can be subdivided into smaller units called ecosystems.
For detailed information on ecosystems, refer to MS TIPS Benchmark L.8.C.2
Growth of Populations
An organism’s maximum ability to increase its population size is referred to as its biotic
potential. This occurs under optimal conditions such as no limits to food, water, living space,
diseases and predation. This type of growth pattern is called exponential growth and is illustrated
with a J-shaped curve.
Figure 3. J-shaped curve showing exponential growth.
(From http://www.biologycorner.com/bio4/notes/popgrowth.php)
While populations may experience this type of growth, it is often only for brief periods. Seldom
do all of these optimal conditions exist at any one time. More often, an organism’s growth meets
environmental resistance which results in decreases in birth rates or increases in death rates.
These decreases or increases are related to environmental conditions such as availability of food
or predation. In this case, the growth of the population is represented by an S-shaped curve.
Figure 4. S-shaped curve showing a population has reached its carrying capacity.
(From http://www.biologycorner.com/bio4/notes/popgrowth.php)
Abiotic and Biotic Factors Affecting Population Growth
In ecological terms, this environmental resistance produces the carrying capacity of an
environment. The carrying capacity is the size of the population or community that can be
supported by the surrounding environment. In determining the carrying capacity of any
environment, biologists identify limiting factors to population growth. These either promote or
hinder growth and can be divided into abiotic or biotic factors.
Abiotic factors are nonliving aspects of the environment that affect the ability of organisms to
survive and reproduce. These often refer to the chemical or physical factors in an ecosystem as
seen in the list below.
Abiotic factors:
♦ Temperature
♦ Precipitation
♦ Soil pH
♦ Soil nutrients
♦ Space availability
♦ Length of growing season
♦ Light availability
♦ Water availability
Abiotic factors cannot only affect the growth or size of a population, but can also affect the
growth of individuals as seen in the illustration below.
Figure 5. Wind effect on tree growth.
(From http://www.thewildlandpress.com/images/wind_tree.jpg)
Biotic factors are aspects or actions of living organisms that affect the ability of organisms to
survive and reproduce. Disease, competition, predation or behaviors of living organisms are
examples of biotic factors.
Once a population reaches its carrying capacity, the size of a population might fluctuate due to
interplay of the limiting factors within the environment.
Figure 6. A population can fluctuate around the carrying capacity.
(From http://www.gov.mb.ca/conservation/sustain/carcap.pdf)
An environmental carrying capacity is subject to change. Environments may degrade resulting in
the shrinking of the carrying capacity. In this case, the environment is no longer able to support a
population of organisms that formerly have lived in an ecosystem on a sustainable basis. No
population can live beyond the environment's carrying capacity for very long. Over grazing by
herbivores or the introduction of a new disease or predator may reduce the carrying capacity.
This may lead to a drop in the number of organisms in the ecosystem. After this drop in the
population, a new carrying capacity may be established. Large game parks in Africa face this
problem. Protection of large animals like elephants can cause severe damage to the environment
when elephant populations become too high.
Figure 7. This graph illustrates a change in the carrying capacity over time.
(From http://www.gov.mb.ca/conservation/sustain/carcap.pdf)
Since ecology focuses on the interaction of organisms with their environment and to other living
organisms, these factors (abiotic and biotic) cannot be viewed in isolation. Both types are also
referred to as limiting factors, since they determine the types and number of organisms in the
environment. For example, plant growth is restricted in the tundra due to the short, cool
summers, coupled with harsh winters and permafrost in the soil. Trees cannot survive under
these conditions. However, the plants that are present grow low to the ground and tend to be
annuals.
The actions of living organisms will have similar effects. Take predation as an example. In
certain environments, keystone predators are important in restricting the population sizes of
some organisms and preventing the disappearance of other populations. This can be seen in the
kelp forests along the Pacific coastline in California. Young kelps are grazed by sea urchins. If
left unchecked, the sea urchins can overgraze the kelp beds leading to their reduction in size.
This normally does not happen because in this situation, the sea otter acts as a keystone predator
and preys upon the sea urchins keeping their populations in check. This allows a kelp forest to
maintain itself and provide a habitat for many other organisms.
Figure 8. California Sea Otter.
(From http://upload.wikimedia.org/wikipedia/commons/thumb/f/f8/Sea-otter-morro-bay_13.jpg/800px-Sea-ottermorro-bay_13.jpg)
Figure 9. Purple Sea Urchin.
(From http://www.scuba-equipment-usa.com/marine/NOV04/images/Heliocidaris_erythrogramma.jpg)
Figure 10. Pacific Kelp Forest.
(From http://www.fossweb.com/CA/modules3-6/Environments/activities/monterey/content_old.html)
Another keystone predator along the California coastline is the sea star (Piaster) which helps to
keep the population of mussels (Mytilus) in intertidal zones in check. By limiting the mussel
populations, other organisms can maintain a presence in the tidal pools. In many habitats in
North America, the wolf and coyote are also considered to be keystone predators by keeping
rodent populations in check.
Teachers can find other examples of keystone predators at the following website
http://science.jrank.org/pages/3758/Keystone-Species-Keystone-predators-herbivores.html
Predation can also lead to fluctuations in population size of both predator and prey. The
Canadian lynx and snowshoe hare is a classic example of how populations of predators and prey
can fluctuate. As seen in Figure 11, the population peaks for these two organisms is offset. The
rise in the lynx population occurs only after the hare population has begun to rise. Then as the
lynx population increases, predation upon the hare reduces its population. Other limiting factor
like climate or disease could also affect the numbers of both organisms.
Figure 11. Fluctuation of Canadian Lynx and Snowshoe Hare.
(From http://cache.eb.com/eb/image?id=6543&rendTypeId=4)
Figure 12. Lynx chasing snowshoe hare.
(From http://www-rohan.sdsu.edu/~jmahaffy/courses/f00/math122/lectures/qual_de2/images/mr285.jpg)
Another interesting organism that acts as an important biotic limiting factor in many
communities is the fungus Mycorrhizae (from the Greek meaning “fungus root”). This
filamentous fungus (the white “threads” in the illustration below) aids the plant’s ability to
absorb nutrients, especially phosphorus from the soil. The root hairs alone often do not provide
enough surface area to absorb all the nutrients needed by plants. A majority of plants have
developed a symbiotic relationship with the Mycorrhizae. Without this fungus, the ability to
sustain themselves would be reduced in many plant species.
Figure 13. Mycorrhiza hyphae surrounding root hairs.
(From http://www.extension.iastate.edu/newsrel/garden/mycorrhiza2.jpg)
Teachers can find more information on Mycorrhiza at the following links
http://herbarium.usu.edu/fungi/funfacts/Mycorhiz.htm
and, http://www.extension.iastate.edu/newsrel/2004/apr04/apr0427.html
Competition for resources is both interspecific and intraspecific. Members of the same species
(intraspecific) may compete for the same resources in an ecosystem; however members of
different species (interspecific) may also compete for these same resources. This has lead to the
ecological idea known as an ecological niche. This idea was first developed in the early part of
the 1900’s by G.F. Gause. In experiments using paramecium, Gause developed the ecological
principle known as competitive exclusion. In this situation, one species out competes another for
available resources and drives the other to extinction. Thus no two organisms can occupy the
same niche in the same community.
Today we can think of a niche has how an organism makes its living as described in this quote
by Eugene Odum.
The ecological niche of an organism depends not only on where it lives
but also on what it does. By analogy, it may be said that the habitat is the
organism's "address", and the niche is its "profession", biologically
speaking.
Odum - Fundamentals of Ecology - W B Saunders 1959
Thus no two organisms can obtain resources in exactly the same fashion, nor occupy the same
habitats in a community. Competition places selective pressures on organisms to develop traits or
adaptations that allow them to survive without directly competing with other species. This
selective pressure is one of the agents of biological evolution.
Adaptations to Selected Environments
In order to survive in various environments, plants and animals have developed numerous
adaptations to aid in their survival. Different desert plants have developed one of three main
adaptive strategies to survive. These include succulence, drought tolerance and drought
avoidance. Examples of each strategy can be seen in the table below.
Table 1. Adaptive Strategies of Desert Plants
Succulence
Absorb large amounts
of water in a short
period of time
Have extensive system
horizontal, shallow
roots
Have a thick waxy
cuticle
Drought Tolerance
Shed leaves during dry
periods
Larger, deeper roots
Develop mechanism to
absorb moisture from
dry soils
Drought Avoidance
Grow annually, grow
and die quickly. Shed
seeds for next season
Germinate when soil
moisture conditions are
optimal
Not all seeds germinate,
even under optimal
conditions
Stomata open at night
rather than the day
Teachers can find further explanation of desert plant strategies at
http://www.desertmuseum.org/programs/succulents_adaptation.html
Teachers can find additional information on desert adaptations at
http://www.desertusa.com/du_plantsurv.html
Plants and animals in each biome have developed specific adaptations for survival. To survive
the harmful effects of fire, grasses have their growing regions located below the surface of the
soil. Therefore, the growing regions are protected from the fires that periodically burn across the
grasslands. In another example, deciduous trees drop their leaves in the fall to avoid damage to
during the cold winters. Also, cold winters reduce or stop the ability of deciduous tree to carryout
photosynthesis. In a sense, these trees “hibernate” to overcome their ability to function normally
during winter conditions. In addition, some organisms may migrate or hibernate to avoid harsh
winter conditions.
Teachers can find more about specific adaptations of plants and animals to major biomes at
http://www.cotf.edu/ete/modules/msese/earthsysflr/desertP.html
The Missouri Botanical Gardens has an excellent website that discusses plant adaptations to
major biomes that is very student-friendly at
http://www.mbgnet.net/bioplants/adapt.html
As we view organisms in the environment, we cannot study them in isolation. Ecology is defined
as the study of how organisms interact with other living and nonliving things in their
environment. Thus, to understand the growth and decline of organisms in an environment, we
need to identify the abiotic and biotic limiting factors affecting those organisms. In addition, the
study of how organisms are adapted to their particular environment will give insight as to how
these limiting factors affect these same organisms.
Content Benchmark L.8.C.4
Students know inter-related factors affect the number and type of organisms an ecosystem can
support. E/S
Common misconceptions associated with this benchmark
1. Students have the misconception that all factors are limiting, except the most abundant
one. The most limiting factor is the least abundant one.
This misconception results from the misunderstanding about the interrelatedness of limiting
factors in the environment. While light is abundant and necessary for photosynthesis in
plants, magnesium is a trace element in the soil and necessary for chlorophyll production.
Without magnesium plants cannot produce chlorophyll and without chlorophyll plants cannot
absorb light. Without light chlorophyll has no function in plants. These two limiting factors
and others like water and carbon dioxide allow photosynthesis to occur in plants. Remove
any one and photosynthesis is not possible. Most processes of life are controlled by a
complex interaction of factors. Some are abundant and some are not, but as illustrated with
photosynthesis, remove any one and the process does not proceed.
More on this misconception can be found at
http://ecomisconceptions.binghamton.edu/posintro.htm
2. Students incorrectly think that adaptations are developed by individuals in response to
the needs of the individual.
Adaptive traits of organisms are a result of evolutionary changes that have evolved over time.
Individual organisms cannot will themselves to change. This process of change is a result of
natural selection where those organisms that are the best adapted, survive and pass their traits
(genes) on to the next generation. Slowly, over time a population or species becomes adapted
to a particular environment.
More on this misconception can be found at
http://ecomisconceptions.binghamton.edu/adaptation.htm
3. Students incorrectly assume some ecosystems have limitless resources and provide an
opportunity for limitless growth of a population.
Resources in nature are finite and while nature does have ability to recycle essential
elements, the process is slow. Over grazing of grasslands by herbivores may not allow time
for sufficient number of plants to regenerate. This can be brought about by excessive
numbers of herbivores. Drought or other changes in the weather can lead to a reduction in the
producers, which in turn will reduce the numbers of all other organisms. The result is a
balance in any ecosystem between the number of organisms that live there and the
availability of resources.
More on this misconception can be found at
http://ecomisconceptions.binghamton.edu/adaptation.htm
4. Students incorrectly think that populations increase until limits are reached, then they
crash and go extinct.
Populations may show periods of constant growth, decline or stabilization depending on the
limiting factors present. The initial introduction of a population into a new environment may
result in exponential growth until the carrying capacity is reached. This is often followed by a
stabilized population which may fluctuate around this carrying capacity. This stabilized
population may remain as long as the limiting factors themselves do not change. A drought
or other destruction to the environment may reduce the carrying capacity, therefore the
populations of organisms present. Extinction may only occur if the environment changes
drastically enough, so that, the organism no longer has traits that allow it to adapt to this new
environment. Normally once adapted to an environment organisms may remain for thousands
of years.
More on this misconception can be found at
http://ecomisconceptions.binghamton.edu/adaptation.htm
5. Students incorrectly believe a size change in one population will have little effect over
another population of the same food web because the chains are spread out.
While organisms often depend on more than one food source in a food web, a significant
drop in one organism could affect another. An example of this can be seen in the case of
keystone predators. The loss of a keystone predator could result in one of the prey
populations to over populate a given ecosystem. In turn the over population of the prey could
cause a reduction in its food source. The above discussion of sea otters and kelp forest is a
classic example of this occurring.
More on this misconception can be found at
http://departments.weber.edu/sciencecenter/biology%20misconceptions.htm
6. Students incorrectly believe that competition (competitive exclusion principle) leads to
less diversity in an ecosystem.
While over the short term competition may lead to the reduction in numbers of some species,
over time, diversity increases. Competition in nature can act as a selective pressure, with the
result that over time species may develop a change in their fundamental and/or realized
niches.
More on this misconception can be found at
http://www.esapubs.org/bulletin/backissues/087-2/pdfweb87_2/ecol101_misconceptions.pdf
Content Benchmark L.8.C.4
Students know inter-related factors affect the number and type of organisms an ecosystem can
support. E/S
Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark L.8.C.4
Students know inter-related factors affect the number and type of organisms an ecosystem can
support. E/S
Answers to Sample Test Questions
Questions and Answers to follow on separate document
Content Benchmark L.8.C.4
Students know inter-related factors affect the number and type of organisms an ecosystem can
support. 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. Biome Adaptation Games
There are several websites that have quizzes or games that allow students to test their
knowledge of how plants and animals are adapted to various biomes around the world.
Interactive adaptations games can be found at
http://ecokids.earthday.ca/pub/eco_info/topics/climate/adaptations/index.cfm
and
http://ecokids.earthday.ca/pub/eco_info/topics/climate/adaptation_quiz/index.cfm
Students can test their ability to matching biomes to the plants that are found there by taking
the matching quiz at http://www.mbgnet.net/teachers/plants.html
2. The Biology of Plants
The Missouri Botanical Gardens website has several lab activities that teachers can download
which demonstrate various plant adaptations. The “Biology of Plants” activity is available at
http://www.mbgnet.net/bioplants/downloads/places.pdf
3. Animal Adaptations
This link will take you to the “Animals Adaptations” activity which is a web-based exercise
that has students discover animal adaptations to selected biomes.
http://www.sciencenetlinks.com/pdfs/adapt_actsheet.pdf
4. Science Netlinks: Yellowstone Wolves
The activity at this website explores the introduction of wolves back into Yellowstone
National Park. Students use the Internet to explore relationships between habitats and species
(specifically the gray wolf and those species with which it must coexist) as well as the effect
of physical and human forces on living things and their environment.
http://www.sciencenetlinks.com/lessons.cfm?Grade=6-8&BenchmarkID=5&DocID=78
5. Graphing Exercises to Teach Carrying Capacity
There are a number of activities that teachers can do to help teach their students about the
idea of carrying capacity and limiting factors. The activities listed below involve graphing
and mathematical manipulation.
Biology Graph: Carrying Capacity involves answering questions about a graph illustration
on carrying capacity. The activity is available at
http://www.algebralab.org/practice/practice.aspx?file=Reading_CarryingCapacity.xml
An activity at this website asks student to make interpretations about several graphs
illustrating various situations involving carrying capacity. To view the activity, go to
http://www.gov.mb.ca/conservation/sustain/carcap.pdf
6. Carrying Capacity
This link will take you to an activity that requires students to formulate and test hypotheses
related to the carrying capacity of wildlife populations. The only materials needed are a bag
of dried beans and a kitchen timer. The activity is available at
http://www.na.fs.fed.us/spfo/pubs/misc/eco/Carrying_Capacity.pdf
7. Murder Ewe Wrote
In this activity, students are asked to solve an ecological mystery about the disappearance of
bighorn sheep. This activity is available at
http://www.na.fs.fed.us/spfo/pubs/misc/eco/Murder_Ewe_Wrote.pdf
8. Environmental Check-up
This simulation asks students to discover how limiting factors make bighorn sheep an
indicator species. This activity is available at
http://www.na.fs.fed.us/spfo/pubs/misc/eco/Environmental_Checkup.pdf
9. My Science Box: Lynx and Hare Populations
Students can explore real data that effects the population cycles of the Canadian lynx and
snowshoe hare. To access this activity, follow the link
http://www.mysciencebox.org/harelynx
10. Project Wild
Project Wild contains a number of activities that will help teachers with concepts from this
benchmark. More information on Project Wild can be found at their website or the Nevada
Division of Wildlife at their Learn & Participate webpage.
Project Wild - http://www.projectwild.org/
Nevada Division of Wildlife - http://www.ndow.org/learn/programs/