Module 8

Module 8—Populations, Individuals, and Gene Pools

 

Explore

 

What is meant when it is said that two organisms have a “relationship” in a biological community? It might be tempting to say that, like human relationships, ecological relationships can be “good” or “bad.” However, all relationships must be positive in some way or, through the evolutionary process, the relationship would have led to species extinction.

 

The ladybug—a major predator in the insect world—is the archenemy of aphids. Aphids are parasites that suck the nectar, the sugary juice that runs through phloem tubes, from plants. Aphids benefit at the expense of the host plant. If the infestation of aphids is severe enough, the host plant will weaken and may die.

 

You may think of the ladybug as a simple predator preying on the aphids; however, by eating the aphids, the ladybug is also protecting the plant. In return, the plant indirectly provides the ladybug with a steady supply of food. This is an example of mutualism, a relationship in which both organisms benefit.

 

Reflect on the relationship between humans and the ladybug. People benefit when a ladybug protects food crops from aphids, but humans do nothing to harm or benefit the ladybug in return. This is an example of commensalism.

 

 

The example involving a crop plant, aphids, ladybugs, and humans demonstrates all three kinds of symbiotic relationships that can bring species together for life.

 

The relationship between predator and prey is complex. Although the relationship does not sound particularly positive for the prey organism, predators do perform a service for the prey population. Recall the cheetah in the Big Picture section of this module. As a predator, the cheetah is much more likely to kill the old, weak, sick, and young simply because it is easier for the cheetah to catch those kinds of animals. By selecting poorly adapted individuals or individuals with inferior alleles, predators improve the genetic stock of the prey population. Because superior prey contribute to the development of superior predators, prey and predators co-evolve. The predator-prey population cycles described under the heading “Producer-Consumer Interactions” on pages 719 to 723 of your textbook show how intimately the two populations control one another.

 

Read

 

Read from the beginning of “Section 20.2” to “Succession: Community Change over Time” on pages 717 to 725 of the textbook. You might create a table to record terms, definitions, and examples as a way to summarize this material for your course folder.

 

Watch and Listen

 

As you watch the video “Interactions and Relationships Among Organisms: 'The Intricate Web of Life,'” you will see many excellent examples of the relationships that shape the interactions of organisms in nature. You may wish to supplement your notes as you consider these questions:

• What happens when human activity, such as habitat destruction, results in the removal of a partner from a mutualistic or commensal relationship? How many species are affected?
  
  
• What happens if a parasite is too successful? How does the parasite limit its own survival?
  
  
• How can existing relationships in nature be used as technologies to serve human needs (e.g., introducing ladybugs into crops as a biological control mechanism)?
  
  
• How does competition between members of a species during times of scarcity act to improve the species’ gene pool?
  
  
• What happens when human activity removes predators from ecosystems? How does the gene pool of the prey species change?
  
  
• What relationship exists between humans and the thousands of plant and fungal species used to make medicine?

Self-Check

 

SC 1. Both human hunters and animal predators keep prey populations in check, but human hunters tend to have a negative effect on the prey gene pool, whereas animal predators have a positive effect. Write a comparative explanation of why this is the case. 

 

SC 2. Create a chart like the following and fill in the empty boxes using examples that you read on pages 712 to 725 of the textbook.

 

Example	Relationship	Population That Increases	Population That Decreases	Population That Remains Stable
	commensalism
tapeworm and human
plant species that produces root toxins that kill plants of the same species nearby
	interspecific competition
	predator/prey
Acacia tree and stinging ants of Latin America
viceroy and monarch butterflies

 

Biology 30 Learn EveryWare © 2009 Alberta Education