1. Symbiosis Mutualism | Commensalism | Parasitism Learning Objectives
Compare and contrast the three types of symbiosis
Evaluate relationships to determine the type of symbiosis occurring
Symbiosis - Mutualism | Commensalism | Parasitism

2. Mutualism
Symbiosis – a close ecological relationship between two species
Types of symbiosis
Mutualism
Commensalism
Parasitism
Mutualism – an ecological relationship in which both species benefit
Ex) Bees gain food, flowering plants gain gamete diversity
A close ecological relationship between two species is a symbiosis. There are three types of symbiosis: mutualism, commensalism, and parasitism.
In mutualism, both species benefit from the interaction.
Bees and flowering plants, as seen in the image here, form a classic example of mutualism. Bees come to flowers to collect nectar and pollen grains, food sources for the bees. As the bees travel from flower to flower, some of the pollen picked up earlier is deposited on other flowers. Therefore, bees serve as one of the delivery mechanisms for the male gametes of flower (pollen). This facilitates sexual reproduction within species of flowering plants. In summary, the bees use the flowering plants as a food source while the flowering plants increase gamete diversity. Since both bees and flowering plants benefit, this is an example of mutualism.

3. Commensalism
Commensalism – an ecological relationship in which one species benefits and the other is not affected
Ex) Barnacles gain nutrients, whales are not affected
In commensalism, one species benefits and the other species is not affected (neither helped nor harmed).
For example, adult whales often have barnacles attached to their skin, as shown in this photograph. As the whales move through the water, the barnacles benefit from moving past new areas of water, where they can feed and obtain nutrients. Since the whales are not affected by the presence of the barnacles, this is an example of commensalism.

4. Parasitism
Parasitism – an ecological relationship in which one species benefits and the other is harmed
Ex) Tick gains nutrients, dog loses blood and may be exposed to disease
Host – the organism that is harmed by the parasite
Parasite – the organism that benefits from the host
In parasitism, one species benefits and the other species is harmed.
The organism that is harmed is called the host.
The organism that benefits is called the parasite and is generally much smaller than the host. Typically, parasites do not kill their hosts directly. Hosts can die slowly as a result of the parasite or a secondary infection caused by the parasite.
Ticks are an example of a parasite, as seen in the photograph here. These small arachnids latch onto the outside of animals, such as dogs. The tick drinks the dog’s blood, from which it obtains nutrients. The dog suffers blood loss and possible disease from the tick. In this example of parasitism, the tick is the parasite and the dog is the host.

5. Types of Symbiosis Type of Symbiosis Species One Species Two Mutualism
Commensalism
Parasitism
The three types of symbiosis are summarized in the table shown here.

6. Determining Types of Symbiosis
To determine the type of symbiosis
Step 1 Write the names of the two species.
Step 2 Determine how the first species is affected.
Step 3 Determine how the second species is affected.
Step 4 Determine the type of symbiosis occurring.
The four steps described here can be used to determine the type of symbiosis present in a given scenario.
Step 1: Write the names of the two species.
Symbiosis occurs between two species. To determine the type of symbiosis, you must first identify the players.
Step 2: Determine how the first species is affected: helped, harmed, or not affected. If desired, draw its face as happy, sad, or not affected.
Types of symbiosis are identified by how the two species are affected.
Step 3: Determine how the second species is affected: helped, harmed, or not affected. If desired, draw its face as happy, sad, or not affected.
Step 4: Using answers from step 2 and 3, determine the type of symbiosis occurring.
With the affects identified, the type of symbiosis is defined.

7. Determining Types of Symbiosis Example
Ex) A fungus is 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?
Step 1 Write the names of the two species.
Step 2 Determine how the first species is affected.
Step 3 Determine how the second species is affected.
Step 4 Determine the type of symbiosis occurring.
Does the result make sense?
Human
Fungus
Human
Fungus
A fungus is 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, write the names of the two species.
Next, determine how the first species is affected: helped, harmed, or not affected. If desired, draw its face as happy, sad, or not affected.
The human is harmed.
Then, determine how the second species is affected: helped, harmed, or not affected. If desired, draw its face as happy, sad, or not affected.
The fungus is helped.
Then, using answers from step 2 and 3, determine the type of symbiosis occurring.
Since one species is helped while another is hurt, this is an example of parasitism.
Finally, check to make sure the result makes sense.
The fungus draws nutrients from the human so this is an example of parasitism.
Human
Fungus
Parasitism
Yes.

8. Graphing Symbiosis
Ecologists determine the type of symbiosis occurring by graphing the populations of the two species over time
Often, ecologists will graph the population of two different species over time. By comparing how the two populations change when living together and when living apart, the relationship can be determined. These graphs may come from wild populations or experiments.
The graphs shown here illustrate what would happen to two species when brought together under each type of symbiosis. In mutualism, both populations rise in number. In commensalism, species A remains constant while species B rises. In parasitism, species A falls somewhat as species B rises.

9. Predator-Prey Relationships | Limiting Factors & Carrying Capacity
Community Dynamics
Predator-Prey Relationships |
Limiting Factors & Carrying Capacity
Learning Objectives
Describe predator-prey relationships
Recognize how limiting factors affect populations
Explain rates of growth in a population
Community Dynamics - Predator-Prey Relationships | Limiting Factors & Carrying Capacity

10. Predator-Prey Relationships
Predation – relationship in which one organism hunts and kills another for nutrition
Predator – an organism which hunts and kills another organism (ex: bear and snake)
Prey – the organism which is killed by the predator (ex: fish and frog)
Population size is also affected by interactions between predators and prey, such as bears feeding on fish.
In predation, one organism (the predator) hunts and kills another organism (the prey) for nutrition. Snakes feeding on frogs, and lynxes feeding on hares are also examples of predator-prey relationships.

11. Predator-Prey Relationships
Prey populations determine predator populations
As hare (prey) population increases, lynx (predator) population increases
As hare (prey) population decreases, lynx (predator) population decreases
In predation, the availability of prey determines the size of the predator populations.
For example, lynxes prey on hares. When there are abundant hares, the lynx population rises.
However, as the lynxes eat more hares, hares become scarce, and some lynxes may starve to death. With fewer lynxes, the hare population will rise again. The patterns in population growth of predators and prey rise and fall together.

12. Predator-Prey Relationships
Late 1960s
An increase in moose population causes increase in wolf population.
Increased wolf population causes decrease in moose population.
1980
Wolf population is decimated by disease.
Decreased wolf population allows moose population to increase.
Wolves and moose are another example of a predator-prey relationship. On Isle Royale in Michigan, a research study of wolves and moose has become the longest predator-prey study in the world. The population graph of wolves and moose illustrates how two populations in a community affect each other.
In the late 1960s, the moose population had doubled. This increase in prey allowed the wolf population to skyrocket, resulting in a population decrease among the moose.
Around 1980, a disease caused the wolf population to crash. Since there were very few predators to hunt the moose, the moose population grew.

13. Limiting Factors and Carrying Capacity
Biotic potential – the highest rate of reproduction possible for a population under ideal conditions
Exponential growth – extremely high rate of growth experienced by a species that reaches its biotic potential
Limiting factors keep growth rates in check
Limiting factors – external factors or environmental conditions that affect population size
In nature, populations strive to reach their biotic potential. Biotic potential is the highest rate of reproduction possible for a population under ideal conditions with no resource limitations.
If a species reaches its biotic potential, it exhibits exponential growth. For example, if two houseflies produce 50 offspring each generation, there would be millions of offspring after a few generations. Fortunately, no species achieves its biotic potential for any length of time. Populations in nature may experience exponential growth initially, but then the growth rate tapers off.
The growth rate is kept in check by limiting factors in the environment. Limiting factors are external factors or environmental conditions that affect the growth rate and size of populations and communities. Examples of limiting factors include limited resources, such as food, and environmental events, such as floods.

14. Limiting Factors and Carrying Capacity
Density-dependent factors – natural resources that affect population growth as a result of density
Ex) Food, water, and space, predation, disease, and availability of mates
Density-independent factors – environmental conditions that affect a population regardless of its density
Limiting factors can be described according to how they relate to population density. Density-dependent factors are natural resources, such as food, water, and space that affect population growth as a result of density. The denser a population, the more competition there will be for these resources. Other examples of density-dependent factors include predation, disease, and availability of mates. A low-density population is more likely to fall during a decrease in mate availability. The transmission of disease is likely to occur more rapidly in a high-density population.
Density-independent factors are environmental conditions, such as weather changes and natural disasters that affect a population regardless of its density. Density-independent factors affect all population sizes equally. For example, a flood will devastate a small, sparse population as well as a large, dense population.

15. Limiting Factors and Carrying Capacity
Density-independent factor: weather
Grey squirrels must find shelter during the winter
Does not depend on the density of the population
Members of a grey squirrel population will need to find shelter during cold, snowy winters, regardless of the density of the population.

16. Limiting Factors and Carrying Capacity
Density-independent factor: human activities
A new dam affects all densities of populations
Other examples of density-independent factors include human activities such as pollution and dam building. When a dam is built, it affects all densities of populations.

17. Limiting Factors and Carrying Capacity
Population Growth
Growing populations have a positive growth rate
Decreasing populations have a negative growth rate
Populations that are neither growing nor decreasing are in a state of equilibrium
Carrying capacity – the point at which a population reaches a state of equilibrium and there is no net gain or loss of individuals
The long-term survival of a species depends on its ability to maintain its population size.
Populations are growing if they have a positive (natural) population growth rate.
If the growth rate is negative, the population will continuously decrease.
If a population is in a state of equilibrium, it has reached its carrying capacity.
Carrying capacity is the number of individuals in a given population that the environment can actually support. At the carrying capacity, the population has no net gain or loss of individuals.

18. Limiting Factors and Carrying Capacity
Initially, populations have exponential growth.
Limiting factors slow population growth.
Population reaches its carrying capacity.
Populations reach their carrying capacity because of limiting factors. Initially, while the population is small and resources are plentiful, the population has exponential growth.
Limiting factors, such as predation and the availability of food, affect the population as it continues to grow.
The growth rate slows down and the population reaches its carrying capacity. Since limiting factors affect carrying capacity, different ecosystems can have different carrying capacities for the same species.

19. Community Dynamics External Dynamics | Competition Learning Objectives
Analyze how organisms, populations, and communities respond to external factors
Compare interspecific and intraspecific competition
Community Dynamics – External Dynamics | Competition

20. External Dynamics Populations constantly change
Factors that affect population size
Births – increase population size
Deaths – decrease population size
Immigration – increase population size
Immigrate – the movement of organisms into an area
Emigration – decrease population size
Emigrate – the movement of organisms out of an area
Remember that ecologists study multiple levels of interacting organization. Populations are continuously undergoing change.
A population’s size is directly impacted by four factors: births, deaths, emigration, and immigration.
A population’s size increases when organisms are born and decreases when organisms die.
The size of a population also changes when organisms enter or exit the community. As organisms immigrate (enter) a population, its size increases.
When organisms emigrate (leave) the population, the size decreases.

21. External Dynamics External factors and environmental conditions
Directly affect organisms, populations, and communities
Affect the availability of resources
Organisms must adapt in order to survive
Organisms, populations, and communities are influenced by external factors and environmental conditions.
Each type of organism requires certain resources such as space, food, and water.
When the availability of these resources changes, organisms must respond or adapt in order to survive. Weather and natural disasters are two external factors that affect the amount of resources.
Organisms adapt to changes in various ways. During the winter, bison adapt to the change in weather by growing thick coats of fur. Some insects produce a type of natural “antifreeze” called glycerol.

22. External Dynamics Natural disasters Community impact
Forest fires, hurricanes, floods, droughts, and earthquakes
Affect organisms and resources
Community impact
Populations are interrelated and affect one another
Natural disasters also affect the availability of resources.
Forest fires, hurricanes, floods, droughts, and earthquakes can have devastating effects on organisms, populations, and communities. Organisms can be injured and die, resulting in a population decrease. For example, forest fires can destroy massive populations of trees, shrubs, and grasses.
Changes in populations also have an impact on communities.
As seen in food webs, populations in the same community are interrelated and have an effect on one another. If a population of one species significantly decreases or disappears, other populations in the community will be affected. The decreased population may have been the only food source for another population. When this happens, members of other populations within the community must compete for resources to survive. If a forest fire destroys the plants in a community, the surviving herbivores will not have food to eat. This will cause the herbivore population to decrease as well.

23. Competition
Competition – occurs when two organisms require the same resource to meet their needs
Organisms must compete for resources to survive
Competition occurs when two organisms require the same resource to meet their needs. Since all organisms require space, food, and water, these resources become scarce.
Organisms must compete for these resources in order to live.

24. Competition Dynamics Food is scarce for deer in winter.
Competition increases in deer population.
Deer population decreases.
Wolves feed on deer.
Competition increases over scarce deer.
Competition contributes to limited population growth for both the deer and wolf populations.
During extreme weather, snow makes it harder for deer populations to find food.
This lack of food may cause deer to compete and even starve, decreasing the deer population.
Since wolves hunt deer, competition within the wolf population would increase over the scarce deer.
Competition contributes to limited population growth for both the deer and wolf populations.

25. Competition
Intraspecific competition – organisms of the same species compete for resources within the ecosystem
Ex) Diatoms grown in lab compete for space and light
Leads to population decrease
There are two types of competition: intraspecific and interspecific. Intraspecific competition occurs between organisms of the same species for resources within an ecosystem.
For example, diatoms (a type of photosynthetic protist) grown in a laboratory reproduce quickly until space becomes limited. With the diatoms crowded together, light is not available for photosynthesis. If diatoms cannot photosynthesize sugars, then they will die from starvation. Only the diatoms able to find enough space for light will survive. Other examples of intraspecific competition include competition over territory and mates. In the wild, intraspecific competition may cause some individuals to emigrate to find other resources. Intraspecific competition can lead to stress and reduced health, which may decrease populations.

26. Competition
Interspecific competition – occurs between different species for resources within an ecosystem
Keeps the growth rate of populations in check
Interspecific competition occurs between different species for resources within an ecosystem.
For example, owls and hawks compete for mice. Two different plant species may compete for sunlight on the forest floor. The species that successfully acquires the resource will be the species that thrives. The other species may emigrate or begin to use other resources, changing their niche.
Competition for these limited resources and changes in environmental conditions keep the growth rate of populations in check.