Topic: Interdependence in Ecosystems

Below is a list of key ideas related to Interdependence in Ecosystems. For each key idea, you will find a list of sub-ideas, a list of items, results from our field testing, and a list of student misconceptions. After clicking on a tab, click on it again to close the tab.

All organisms, both land-based and aquatic, are connected to other organisms by their need for food. This results in a global network of interconnections, which is referred to as a food web.

Students are expected to know that:

  1. When organisms eat or are eaten by other organisms, there is an effect on the two organisms involved and on the populations to which they belong.
  2. When organisms eat or are eaten by other organisms, there may be an effect on other populations that are not eating or being eaten by those organisms. This is because once an individual organism (or part of an organism) is eaten, it is no longer available as food for other populations of organisms and/or will no longer eat other organisms from other populations.
  3. Changes in the size of a population may result from changes in the size of the populations of organisms that it consumes, that consume it, or both.  Specifically, if the size of a population increases  (or is introduced), the size of a population of organisms that consumes it may increase because there will be more food available to that population, and if the size of a population increases (or is introduced), the size of a population of organisms that it consumes may decrease because more of them may be consumed. If the size of a population decreases (or the population disappears), the size of a population of organisms that consume it may decrease because less food is available to that population, and if the size of a population decreases (or the population disappears), the size of a population of organisms that it consumes may increase because there are fewer organisms to consume them.
  4. Changes to the size of populations of organisms due to changes in the size of populations it consumes or that consume it take time, and the relative sizes of the starting populations involved may affect the outcome.
  5. The network of populations of organisms being eaten by other populations can be thought of as a single global food web encompassing all populations, but food webs can also be described for populations in particular environments.
  6. Feeding interactions among selected populations of organisms in food webs can be represented by diagrams with arrows from populations being eaten to populations doing the eating. If an arrow is not present in a diagram, there is no feeding interaction between two populations, and if an arrow is present, there is a significant feeding interaction between the two populations.

Boundaries:

  1. Students are not expected to know that microorganisms are part of food webs. This is addressed in a later idea.
  2. Students are not expected to know the terms producer, consumer, or trophic level.
  3. Students are not expected to know what any particular organism eats apart from what can be determined from food web diagrams they are given.
  4. Students are not expected to know that although a population may be connected by arrows to more than one population, organisms may rely on some organisms more than others for food.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number

Knowledge Being Assessed

Grades
6–8

Grades
9–12

Select This Item for My Item Bank

IE029006

Changes in a population of organisms in a food web (worms) can affect the population of its predator (robins).

84%

80%

IE032005

Changes in the number of worms in an area can affect the number of foxes in that area even though foxes do not depend directly on worms for food.

73%

79%

IE031007

Changes in a population of organisms in a food web (worms) can affect other populations of organisms (foxes) even if they are not directly connected in a feeding relationship.

70%

81%

IE027005

Changes in a population in a food web (grasshoppers) can affect the populations of both its predator (frogs) and its prey (grass).

66%

78%

IE026005

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (no specific organisms indicated).

72%

74%

IE026004

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (ocean ecosystem).

69%

77%

IE014004

Changes in a population in a food web (robins) can affect the population of its prey (caterpillars).

66%

74%

IE029005

Changes in a population in a food web can affect the population of its predator (no specific organisms identified).

63%

74%

IE031006

Changes in a population in a food web can affect other populations of organisms even if they are not directly connected in a feeding relationship (no specific organisms identified).

63%

72%

IE019016

Changes in a population in a food web (large fish) can affect the population of its prey (tadpoles).

61%

69%

IE073004

Changes in a population in a food web can affect the population of its prey (no specific organisms identified).

59%

68%

IE032004

Changes in a population in a food web can affect populations of other organisms even if they are not directly connected in a feeding relationship (no specific organisms identified).

56%

68%

IE027004

Changes in a population in a food web can affect the populations of both its predator and its prey (no specific organisms identified).

55%

66%

IE012012

Changes in a population in a food web (wolves) can affect populations of other organisms (grass) even if they are not directly connected in a feeding relationship (text only, no diagram).

55%

58%

IE034005

Changes in a population (rabbits) may affect populations that are not directly connected by a feeding relationship (crickets) even if they are several steps away and not within the same chain in a food web.

51%

62%

IE012011

Changes in a population in a food web (wolves) can affect populations of other organisms (grass) even if they are not directly connected in a feeding relationship.

53%

57%

IE020004

Changes in a population (fish in a pond) may affect populations that are not directly connected in a feeding relationship (large birds).

52%

57%

IE025005

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (no specific organisms identified).

50%

53%

IE025004

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (woodland ecosystem).

44%

53%

IE028006

Changes in a population (frogs) may affect populations that are not directly connected by a feeding relationship (mice) even if they are several steps away and not within the same chain in a food web.

34%

45%

IE019014

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (pond ecosystem).

37%

46%

IE028005

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away and not within the same chain in a food web (no specific organisms identified).

35%

42%

IE019015

Changes in a population (fish in a pond) may affect populations that are not directly connected by a feeding relationship (insects) even if they are several steps away in a food web.

35%

40%

IE017005

Changes in a population (introduction of a new organism) may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (woodland ecosystem).

29%

38%

IE017006

Changes in a population may affect populations that are not directly connected by a feeding relationship even if they are several steps away in a food web (woodland ecosystem).

20%

28%

IE016008

Changes in a population (introduction of a new species) may affect populations that are not directly connected by a feeding relationship (caterpillars) even if they are several steps away in a food web.

20%

27%

Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

IEM022

If a population in a food web is disturbed, there will be little or no effect on populations that are not within the linear sequence in the food web (Webb & Boltt, 1990).

31%

30%

IEM065

Varying the size of a population of organisms will affect only those populations of organisms that are directly connected to it in a feeding relationship, not organisms that are one or more steps removed/away from it (Griffiths & Grant, 1985; Webb & Boltt, 1990).

20%

19%

IEM021

If a population in a food web is disturbed, there will be little or no effect on populations below it in the food web (e.g. if a predator is removed, no effect on prey: Webb & Boltt, 1990; Leach, 1996).

18%

15%

IEM006

Organisms higher in a food web eat everything that is lower in the food web (Griffiths & Grant 1985).

15%

12%

IEM030

If the size of one population in a food web is altered, all other populations in the web will be altered in the same way (Griffiths & Grant, 1985).

12%

8%

IEM029

A change in the size of a prey population has no effect on its predator population (Griffiths & Grant, 1985).

10%

8%

IEM061

Changes in a population in a food web do not affect the populations of any other organism in the food web (AAAS Project 2061, n.d.).

10%

8%

IEM047

The top predator in a food web will never be significantly affected by changes in the populations of organisms below it in the food web (AAAS Project 2061, n.d.).

9%

7%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.

In all environments, individual organisms that depend on the same resource may compete for that resource when it is limited. Resources that can be limited include food, space, water, shelter, and light.

Students are expected to know that:

  1. Organisms that depend on the same resource compete for that resource when it is limited.
  2. Resources that can be limited for animals include food, space, water, and shelter (including nesting sites).
  3. Resources that can be limited for plants include space, water, and light.
  4. Both plants and animals compete for resources.
  5. Competition may be between individuals of the same species or between individuals of different species.
  6. The scientific use of the term “competition” refers to the attempt of different organisms to obtain the same limited resource, whether or not the organisms interact with each other. In other words, competition for resources does not necessarily imply direct confrontation between organisms, and sometimes competition involves finding or using the resource first.

Boundaries:

  1. Students are not expected to know which resources can be limited for other organisms besides animals and plants.
  2. Students are not expected to know that animals compete for light in some instances.

Given adequate resources and an absence of disease or predators, populations of organisms in ecosystems can increase at rapid rates. Finite resources and other factors limit their growth.

Students are expected to know that:

  1. Individual organisms are added to a population by birth and removed by death so that populations increase when the number of births is greater than the number of deaths and populations decrease when the number of births is smaller than the number of deaths.
  2. Disease, predators, and availability of resources can affect how many individuals in a population die and how many offspring are produced and how the size of a population can change over time. 
  3. In the absence of factors that lead to the death of organisms in a population or a reduction in the number of offspring they produce, the size of a population will grow faster and faster (more and more between consecutive equal time points).
  4. Populations do not grow infinitely large in nature because there is always at least one factor, including disease, predators, and availability of resources, that limit the growth of a population.

Boundaries:

  1. The idea that immigration or emigration (organisms moving into or out of a population or environment) can affect the size of the population of organisms in a particular environment is not part of the scope of this idea.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number

Knowledge Being Assessed

Grades
6–8

Grades
9–12

Select This Item for My Item Bank

IE077002

Populations increase when the number of births is greater than the number of deaths.

73%

81%

IE042007

The size of a population of organisms (deer) can increase when its predators are removed from an area because the deer can live longer and have more offspring that also live longer (text only, no food web diagram).

70%

80%

IE079002

Populations decrease when the number of births is less than the number of deaths.

71%

79%

IE085001

Both disease and the number of predators can limit population growth.

65%

79%

IE086001

Disease and the availability of resources can limit population growth.

62%

81%

IE042006

The size of a population of organisms can increase when its predators are removed from an area because the organisms live longer and have more offspring that also live longer (includes a food web diagram).

67%

73%

IE045003

The size of a population of organisms (deer) can increase when its predators are removed from an area because the organisms can live longer and have more offspring that also live longer (includes a food web diagram).

63%

74%

IE084001

Both the number of predators and the availability of resources can limit the growth of a population of organisms.

54%

72%

IE081002

The size of populations of squirrels can increase when there is an increase in the food available and a decrease in the number of predators.

55%

69%

IE081003

The size of populations of robins can increase when there is an increase in the food available and a decrease in the number of its predators.

57%

64%

Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

IEM006

Organisms higher in a food web eat everything that is lower in the food web (Griffiths & Grant 1985).

16%

11%

IEM014

Populations exist in states of either constant growth or decline (Munson 1991).

9%

7%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.