Lesson 4 Text

1 Species Interactions

Lesson 4: Species Interaction
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Species interactions can be classified into two distinct groups: interspecific interactions and intraspecific interactions. Interspecific interactions are interactions between two species while intraspecific refers to the interactions between two individuals of the same species.


Gerald J. Lenhard, , www.forestryimages.org
LESSON 4
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2 Competition

Lesson 4: Species Interaction

Competition occurs when individuals are attempting to utilize the same resource. The outcome is a reduced fitness in the competing individuals. Competition can occur between individuals of the same species or between individuals of two different species. When competition occurs between members of the same species, it is called intraspecific competition. In such a case, the competition is more likely to be intense because both individuals have the same resource requirements. Intraspecific competition is of major importance in ecology since it can play a role in mortality and recruitment rates.

When two individuals from different species compete, we say there is interspecific competition. The classic example of interspecific competition is Darwin’s finches. The competition for a limiting resource, in this case food, eventually caused the two species of finches to find their own niche for survival. A niche is the conditions under which an individual is found including the resources it uses (Mackenzie et al 1998). In the case of Darwin’s finches, one population utilized small seeds while the other utilized large seeds in response to competition.


Terry Spivey, USDA Forest Service, www.forestryimages.org
LESSON 4

2 Competition (Cont.)

Lesson 4: Species Interaction

Competition can either have direct or indirect effects on individuals. Indirect effects occur through the depletion of a limiting resource; this is often called exploitation competition. In this situation, individual fitness is reduced due to a lack of resource availability.

Individuals may also interact directly with each other, called interference competition. A common case of this type of interaction occurs when animals fight over territory. Another example of direct competition is the use of toxins by one of the competitors. For example, black walnuts release quinine into the soil, killing other individual plants and thereby limiting the amount of competition (Mackenzie et al. 1998).


Paul Wray, Iowa State University, www.forestryimages.org

The effects of competition may differ for the individuals involved. Often, the cost for one of the individuals is much greater than the other. For example, one individual may die while the other one lives

LESSON 4

3 Predation

Lesson 4: Species Interaction

Predation is defined here as the consumption of all or part of another individual (the prey) by another organism (the predator) (Mackenzie et al 1998). We typically classify predators into one of three groups: herbivores (eat plant tissue only), carnivores (eat animal tissue only), and omnivores (eat both plant and animal tissue).


Terry Spivey, USDA Forest Service, www.forestryimages.org

In addition to these categories a predator may either be a generalist or a specialist. Specialists tend to concentrate their feeding on one prey species, while generalists often will feed on many different species.

Additional Information:
A Predation model of Sheep and Wolves

LESSON 4

4 Parasitism

Lesson 4: Species Interaction

Parasites are a subgroup of predators that live in close association with their host. The parasite benefits from the host but typically does not kill the host (note that in many cases, the long-term effects of parasitism will cause mortality). The reason we say that the relationship between the two organisms is particularly close is due to the fact that the parasite typically lives within or upon the host, making it a form of symbiosis.


Susan Ellis, , www.forestryimages.org
LESSON 4

5 Types of Parasitism

Lesson 4: Species Interaction

Parasites that live inside the body of the host are called endoparasites and those that live on the outside are called ectoparasites. An epiparasite on the other hand is a parasite that feeds on another parasite.

Social parasites do not directly feed on the tissue of their hosts like true parasites instead they gain benefit from their host by convincing the host to provide food or other benefits. A good example of a social parasite occurs when there are generalized non-specific mutualisms between classes of organisms. For example some plants can behave as "mycorrhizal cheaters", establishing mycorrhiza-like interactions with a fungal symbiont, but taking carbon from the fungus (which the fungus, in turn, gets from other plants) rather than donating carbon. Another example is the cockoo bird which leaves its young with a host to raise. This type of parasitism is often called brood parasitism.


Terry Spivey, USDA Forest Service, www.forestryimages.org
LESSON 4

6 Parasitism Dynamics

Lesson 4: Species Interaction

Due to the nature of the interaction between a host and a parasite many host populations will develop some form of immunity. The development of host population immunity causes either a decline in the parasite population or a genetic mutation in the parasite population to overcome the immunity. This dynamic tends to form a cycle in the parasite population where there are periods of population increases and periods of population decreases.


Maja Jurc, University of Ljubljana, www.forestryimages.org
LESSON 4

7 Mutualism

Lesson 4: Species Interaction

Due to the nature of the interaction between a host and a parasite many host populations will develop some form of immunity. The development of host population immunity causes either a decline in the parasite population or a genetic mutation in the parasite population to overcome the immunity. This dynamic tends to form a cycle in the parasite population where there are periods of population increases and periods of population decreases.


Maja Jurc, University of Ljubljana, www.forestryimages.org
LESSON 4