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

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.


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

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

Blue Planet World Biomes is another sitethat contains detailed information on specific plants and animalsfound within each biome. The website is available at

Biomes of North America describes biomes specific to North America. The website can be accessed by linking to

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.


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.


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 thesurrounding 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:



♦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.


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.


An environmental carrying capacity is subject to change. Environmentsmay degrade resulting in the shrinking of the carrying capacity. In this case, the environment is no longer able to support a population of organismsthat 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 maybe 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.


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 providea habitat for many other organisms.

Figure 8.CaliforniaSea Otter.


Figure 9. Purple Sea Urchin.


Figure 10.PacificKelpForest.


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 coyoteare also considered to be keystone predators by keeping rodent populations in check.

Teachers can find other examples of keystone predators at the following website

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.


Figure 12. Lynx chasing snowshoe hare.


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.


Teachers can find more information on Mycorrhiza at the following links


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 / Drought Tolerance / Drought Avoidance
Absorb large amounts of water in a short period of time / Shed leaves during dry periods / Grow annually, grow and die quickly. Shed seeds for next season
Have extensive system horizontal, shallow roots / Larger, deeper roots / Germinate when soil moisture conditions are optimal
Have a thick waxy cuticle / Develop mechanism to absorb moisture from dry soils / 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

Teachers can find additional information on desert adaptations at

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

The Missouri Botanical Gardens has an excellent website that discusses plant adaptations to major biomes that is very student-friendly at

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 anyone 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 anyone and the process does not proceed.

More on this misconception can be found at

  1. 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

  1. 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

  1. 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

  1. 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

  1. 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

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