BIOL 1911 Chapter Notes - Chapter 54.1: Herbivore, Commensalism, Parasitism
29 Apr 2018
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54.1 – Community interactions are classified by whether they help, harm
or have no effect on the species involved
• A biological community = group of populations of different species living in close enough proximity to interact
o Ecologists define boundaries of a particular community to fit their research question
• I = ’
o Ex: competition, herbivory, parasitism, mutualism, commensalism
Competition
• -/- interaction: occurs when individuals of different species competes for a resource that limits the survival and
reproduction of each species
Competitive Exclusion
• G. F. Gause (1934) studied what happens in a community when two species compete for limited resources
o Found that two species competing for the same limited resources cannot coexist permanently in the same place
o In the absence of disturbance, one species will use the resources more efficiently and reproduce more rapidly
than the other
o Even a slight reproductive advantage will eventually lead to competitive exclusion
• Competitive exclusion = the total elimination of the inferior competitor
Ecological Niches and Natural Selection
• Competition for limited resources can cause evolutionary changes in population
• Ecological niche = the specific set of biotic and abiotic resources that an organism uses in its environment
o Ex: temperature range the organism tolerates, time of day when it is active, size and kinds of prey, location and
type of habitat
• Two species cannot coexist permanently in a community if their niches are identical (principle of competitive exclusion)
o Evolution by natural selection can result in one of the species using a different set of resources or similar
resources at different times of the day or year
• Resource partitioning = differentiation of niches that enables similar species to coexist in a community
• Fundamental niche: niche potentially occupied by that species
o E ’ conditions it grows and reproduces
in in the absence of competitors
• Realized niche: the portion of the fundamental niche that it actually occupies
o Realized niche is different from fundamental niche because of competition
o Ecologists can test whether a p ’
and seeing if the first species expands into the newly available space
• Species can partition their niches not only in space, but in time as well
Character Displacement
• Closely related species whose populations are sometimes allopatric and sometimes sympatric provide more evidence for
the importance of competition in structuring communities
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