Question 1

How do signs represent population interactions?

Accepted Answer

A plus sign means the species concerned benefits, while a minus sign means it is harmed. The first sign always refers to the first species named and the second sign to the second species.

Question 2

Why is mutualism represented as +/+?

Accepted Answer

In mutualism, both interacting species benefit. A mycorrhizal association is an example: the fungus helps the plant obtain minerals and water, while the plant supplies organic food to the fungus.

Question 3

Why are both species harmed in competition?

Accepted Answer

When two species require the same limited resource, each can reduce the other's access to it. Compared with being able to use the resource without the competitor, both species may suffer reduced fitness, giving the pattern -/-.

Question 4

How does predation differ from parasitism?

Accepted Answer

Both commonly have the sign pattern +/-, but their relationships differ. A predator usually kills and consumes its prey, whereas a parasite obtains nourishment from a living host while living on or in its body.

Question 5

Why can predation and parasitism not be merged?

Accepted Answer

The signs alone can look identical, but the ecological arrangement and feeding relationship are different. Direct killing and consumption characterize the usual predator–prey relationship; sustained nourishment from a living host characterizes parasitism.

Population Interactions — Benefits, Costs and Signs

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Practice