Population Interactions: An Overview

A population is defined in ecology as a group of individuals of the same species, living within a defined geographical area, sharing or competing for similar resources, and potentially able to interbreed. Because many different populations of different species typically occupy the same habitat at the same time, they inevitably interact with one another -- this is called an interspecific interaction (an interaction between two different species), as opposed to an intraspecific interaction (an interaction among members of the same species, such as competition for a mate).

Ecologists classify interspecific interactions by the net effect each interaction has on the two species involved, using a simple system of plus (+, benefit), minus (−, harm), and zero (0, no significant effect). The WBCHSE syllabus for this chapter focuses on four major interaction types that recur throughout the rest of this chapter:

• Mutualism (+, +) -- both species benefit from the association.
• Competition (−, −) -- both species are harmed because they draw on the same limited resource.
• Predation (+, −) -- one species (the predator) benefits by killing and consuming the other (the prey), which is obviously harmed.
• Parasitism (+, −) -- one species (the parasite) benefits by living on or in the other (the host) and drawing nourishment from it, harming the host in the process. …