1. Ecological
Relationships
12(A)

2. Ecological Relationships
Learning objectives
Know the types of ecological relationships
Determine the ecological relationship given information about the interaction
After this lesson, you will know the different types of ecological relationships. You will also be able to determine which ecological relationship is present given information about interactions between organisms.

3. Organisms interact with one another
Ecosystems
Ecosystems are composed of multiple populations of different species and their environment
Organisms interact with one another
Predation
Commensalism
Parasitism
Mutualism
Competition
Remember that an ecosystem is made up of multiple populations of different species and their environment. These organisms do not live in a bubble – they interact with other members of their species and organisms of different species.
There are many different types of interactions, including predation, commensalism, parasitism, mutualism, and competition.

4. Image by Steve Hillebrand [Public domain]
Predation
Predation – relationship in which one organism hunts and kills another for nutrition
Bears & Fish
In predation, one organism hunts and kills another for nutrition. The organism hunting is called the predator and benefits from nutrition. The organism being hunted is the prey and is killed from the interaction.
For example, brown bears hunt and kill fish. The bears are the predators, which benefit, and the fish are the prey, which are killed.
Predator
Prey
Image by Steve Hillebrand [Public domain]

5. Image by By Robert Flogaus-Faust (Own work) [CC-BY-3.0]
Predation
Predation – relationship in which one organism hunts and kills another for nutrition
Spiders & Insects
Another example of predation is spiders and insects. Here, spiders are the predators and insects are the prey. The spiders benefit and the insects are killed.
Predator
Prey
Image by By Robert Flogaus-Faust (Own work) [CC-BY-3.0]

6. Image by Martin St-Amant (S23678) (Own work) [CC-BY-SA-3.0]
What about this?
Are herbivores and grass and example of predation?
No, there is no prey hunted and killed
Herbivores & Grass
In predation, one organism, the prey, must be hunted and killed. A common mistake is to classify herbivores, like deer, as predators. However, herbivores don’t hunt and kill grass. The herbivores graze on the grass and the grass can regrow.
This is not predation.
Image by Martin St-Amant (S23678) (Own work) [CC-BY-SA-3.0]

7. Commensalism
Commensalism – relationship in which one organism benefits and the other is not affected
Whales & Barnacles
In commensalism, one organism benefits and the other is not affected.
For example, barnacles live on whales. The barnacles feed by filtering nutrients out of the water. As the whales move through the water, the barnacles benefit from moving past new areas of water. The whales are not affected by the presence of the barnacles.
Whale Image by Jan Roletto.
(NOAA Photo Library: sanc0118) [Public domain]
Barnacle image by Kim Hansed [GFDL]

8. Image by Kevin Havan [CC-BY2.0]
Commensalism
Commensalism – relationship in which one organism benefits and the other is not affected
Hermit Crabs &
Sea Snails
The body of a hermit crab is soft. To protect itself from predators, a hermit crab enters a discarded sea snail shell. When the hermit crab outgrows a shell, it finds a larger discarded shell. Since the sea snails all already dead, the hermit crab using their old shells does not affect them.
Image by Kevin Havan [CC-BY2.0]

9. Image by Borislav Dopudja [CC-BY-SA-3.0]
What about this?
Are ticks and dogs an example of commensalism?
No, the dogs are hurt
Ticks & Dogs
Ticks feed on the blood of animals like dogs. Nutrition is transferred from the dogs to the ticks. This benefits the ticks, but hurts the dogs. Since the dogs are harmed, this is not an example of commensalism.
Image by Borislav Dopudja [CC-BY-SA-3.0]

10. Image by Borislav Dopudja [CC-BY-SA-3.0]
Parasitism
Parasitism – relationship in which one organism benefits and the other is harmed
Ticks & Dogs
In parasitism, one organism benefits and the other is harmed. The organism that benefits is called the parasite. The organism that is harmed is called the host.
Ticks are small arachnids that latch onto the outside of animals, such as dogs. The tick drinks the dog’s blood, which contains nutrients.
In this example, the tick is the parasite and the dog is the host.
Parasite
Host
Image by Borislav Dopudja [CC-BY-SA-3.0]

11. Image by Borislav Dopudja [CC-BY-SA-3.0]
Parasitism
Parasitism – relationship in which one organism benefits and the other is harmed
Nematodes & Animals
Some species of nematodes are parasitic and live within other organisms. Some of these worms simply absorb nutrients in the digestive tract while others slowly kill their hosts.
In this example, the nematode is the parasite and the animal is the host.
Parasite
Host
Image by Borislav Dopudja [CC-BY-SA-3.0]

12. Why aren’t whales and barnacles an example of parasitism?
What about this?
Why aren’t whales and barnacles an example of parasitism?
The whales aren’t really hurt
Whales & Barnacles
Why aren’t whales and barnacles an example of parasitism? While the barnacles live on the whales, they aren’t withdrawing any nutrients from the whales. Since they aren’t really hurting the whales, they aren’t parasites. The relationship is an example of commensalism.
Whale Image by Jan Roletto.
(NOAA Photo Library: sanc0118) [Public domain]
Barnacle image by Kim Hansed [GFDL]

13. Mutualism – relationship in which both organisms benefit.
Bees & Flowering Plants
In mutualism, both organisms benefit.
A classic example is bees and flowering plants. Bees come to flowers to collect nectar and pollen grains. The bees use the nectar and pollen to feed their young. As the bees travel from flower to flower, some of the pollen picked up earlier is deposited on the flowers. Because pollen contains the male gametes, arrival of pollen to allows sexual reproduction between two different individual flowering plants. In summary, the bees get a food source and the flowering plants get a variety of gametes to produce offspring.
Image by Jessie Eastland aka Robert DeMeo
(Own work) [CC-BY-SA-3.0]

14. Image by Nhobgood Nick Hobgood
Mutualism
Mutualism – relationship in which both organisms benefit.
Clownfish &
Sea Anemone
At coral reefs, clownfish live between the tentacles of sea anemone. The clownfish are immune to the stings of the sea anemone, but their predators are not. Because of this, the clownfish receive protection from being eaten. In return, the sea anemone gets nutrients from the clownfish waste and can sometimes sting and capture potential predators of the clownfish.
Image by Nhobgood Nick Hobgood
(Own work) [CC-BY-SA-3.0

15. Why aren’t whales and barnacles an example of mutualism?
What about this?
Why aren’t whales and barnacles an example of mutualism?
The whales aren’t helped
Whales & Barnacles
Why aren’t whales and barnacles an example of mutualism? While the barnacles do not hurt the whales, the barnacles do not help the whales either. The relationship is an example of commensalism.
Whale Image by Jan Roletto.
(NOAA Photo Library: sanc0118) [Public domain]
Barnacle image by Kim Hansed [GFDL]

16. Image by openimagebank.com
Competition
Competition – relationship in which both organisms are harmed from fighting over resources.
Zebra & Antelope
In competition, both organisms are harmed from fighting over resources. For example, both deer and antelope graze on grasses in African plains. In times of drought, there is limited grass. With both populations eating the same resource, they both receive less food. This reduction in food hurts members of both populations.
Image by openimagebank.com

17. Image by Heinz Seehagel [Public Domain]
Competition
Competition – relationship in which both organisms are harmed from fighting over resources.
Male Deer
During the mating season, male deer fight over females, a limited resource for producing offspring. Males often get hurt during these fights, sometimes resulting in death. This is an example of competition occurring within one population.
Image by Heinz Seehagel [Public Domain]

18. Are rabbits and squirrels an example of competition?
What about this?
Are rabbits and squirrels an example of competition?
No, they don’t directly compete for resources
Rabbits & Squirrels
Why aren’t rabbits and squirrels an example of competition? They are both herbivores, but rabbits feed mainly on grasses and squirrels mainly eat nuts. When deciding if competition is present, make sure to check that both organisms are fighting over the same resources.
Rabbit image by I, EIC [GNU]
Squirrel image by Hernán De Angelis Campephilus (Own work)
[CC-BY-SA-3.0]

19. Predation Commensalism Parasitism Mutualism Competition
Types of Interactions
Predation
Commensalism
Parasitism
Mutualism
Competition
Here is a summary of the different types of interactions. In predation, the predator hunts and kills the prey. The predator is helped and the prey is killed.
In commensalism, one organism benefits and the other is not affected. In parasitism, one organism benefits while the other is harmed. In mutualism, both of the organisms benefit.
In competition, both of the organisms are hurt from the interaction.

20. Determining Types of Interactions
You can use the steps below to determine the relationship
If you are asked to determine the relationship given a situation, follow these four steps.
First, identify the two species or individuals. Using the situation that the first species is in, draw the first face as helped, hurt, killed, or not affected.
Using the situation that the second species is in, draw the second face.
With both faces drawn, determine the relationship that exists.

21. Determine the Interaction
Off the coast of California, sea otters dive and collect mollusks. They return to the surface, crack open the mollusks’ shells, and eat the meat inside. What kind of interaction exists between the sea otters and the mollusks?
Sea Otters
Mollusks
Here is an example problem. Off the coast of California, sea otters dive and collect mollusks. They return to the surface, crack open the shells, and eat the meat inside. What kind of interaction exists between the sea otters and the mollusks?
First, identify the two species. In this problem, the organisms mentioned are sea otters and mollusks. Next, draw the first face. The sea otters eat the mollusks, so they receive food, which helps them. The sea otters are therefore helped. The mollusks are eaten, so they are killed.
Since one organism benefits and the other is killed, this is an example of predation.
Predation

22. Determine the Interaction
A fungus found on the bottom of some people’s feet. The fungus absorbs nutrients from the human it grows upon. The fungus is not fatal. What ecological relationship does this represent?
Fungus
Human
A fungus found on the bottom of some people’s feet. The fungus absorbs nutrients from the human it grows upon. The fungus is not fatal. What ecological relationship does this represent?
First, identify the two species. In this problem, the organisms mentioned are a fungus and humans. Next, draw the first face. The fungus receives nutrients from the human, so it benefits. The human loses these nutrients, but because the fungus is not fatal, it does not die.
Since one organism is helped and the other is hurt, this is an example of parasitism.
Parasitism

23. Determine the Interaction
In some coral reefs, cleaning stations exist. Fish swim here and open their mouths. Shrimp climb on their skin, enter their clean their mouth and gills, and eat parasites. What kind of relationship exists between the fish and the shrimp?
Fish
Shrimp
In some coral reefs, cleaning stations exist. Fish swim here and open their mouths. Shrimp climb on their skin, enter their clean their mouth and gills, and eat parasites. What kind of relationship exists between the fish and the shrimp?
First, identify the two species. In this problem, the organisms mentioned are fish and shrimp. Next, draw the first face. The fish have parasites removed, so they benefit. The shrimp eat the parasites, so they benefit from getting a meal.
Since both organisms benefit, this is an example of mutualism.
Mutualism

24. These interactions can be represented graphically
Interactions Graphically
These interactions can be represented graphically
Both populations over time
What happens as one population increases?
Sometimes you will see ecologically interactions represented graphically. These typically trace both populations over time. To identify the relationship, use the same four steps. When drawing the faces, consider what happens as one population increases.

25. Population Size (hundreds)
Interactions Graphically
What relationship is most likely occurring between the two populations in the graph below?
Commensalism
Predation
Mutualism
Parasitism
Population Size (hundreds)
Here is an example of a problem with a graph of two populations over time. As the blue population increases, the red population rapidly increases. After the red population increases, however, the blue population begins to decrease. Once the blue population has decreased, the red population falls.
Again, write each population and draw faces. Our two populations are red and blue.
It looks like the red population is helped by the presence of blue organisms.
What is probably happening to the blue population? Since the blue population falls with a rise in red, it’s probably being killed by the red population.
This is probably an example of predation.
If it was commensalism, one population would probably stay steady since it would be unaffected by the other. If it was mutualism, both populations would rise. If it was parasitism, one population would fall as another rose. That would be similar to this graph, but the changes would not be as steep and cyclical.
Red
Blue
Time (years)

26. Determining Types of Interactions
You can use the steps below to determine the relationship
Whether the information is presented in text or in a graph, these four steps can be used to determine the relationship.

27. Ecological Relationships
Learning objectives
Know the types of ecological relationships
Determine the ecological relationship given information about the interaction
You should now know the different types of ecological relationships. You will also be able to determine which ecological relationship is present given information about interactions between organisms.