Community ecology
Chapter 57
Darwin’s observation
What is community ecology?
 A community is all of the species inhabiting a common environment and interacting
with each other
 Characterized by composition
 Characterized by properties, such as species richness or primary productivity
 Community interactions affect abundance of populations and ecosystem properties
Assemblage
 Assemblage is group of species that only comprise a portion of the community
 e.g., bird assemblage



Acorn production determines the abundance of mice and deer
Abundant mice suppress gypsy moth outbreaks
Mice and deer support tick populations, potentially increasing the risk of Lyme
disease
Gypsy moth (Lymantria dispar)
Small mammal removal experiment
 6 x 3 ha oak forest open grids
 Live-trap stations
 Pupal predation
Two views of structure and functioning of communities
 Holistic concept
 Clements
 Community is “superorganism” whose constituent species have coevolved to
function as part of greater whole
 Individualistic concept
 Gleason
 Community is nothing more than an aggregation of species that happen to cooccur at one place
 Species respond independently to environmental gradients
Biological Communities
 Most ecologists today favor the individualistic concept
 In communities, species respond independently to changing environmental conditions



Abundance of tree species along a moisture gradient in the Santa Catalina Mountains
of Southeastern Arizona
Each line represents the abundance of a different tree species
Community composition changes continually along the gradient
 Sometimes the abundance of species in a community does change geographically in a
synchronous pattern
 Ecotones: places where the environment changes abruptly
What factors control where a species lives?
Critical factors and tolerance limits
 A critical factor is the single environmental factor in shortest demand and determines
species distribution
 Every living organism has tolerance limits to the environmental conditions it can
endure
 minimum, maximum and optimum
Tolerance limits
Critical factors and tolerance limits
 For many species, the interaction of several factors, rather than a single limiting
factor, determines biogeographical distribution
 For some organisms, there may be a specific critical factor that mostly
determines abundance and distribution
Saguaro cactus has one critical factor
Ecological niche
 Habitat - Place or set of environmental conditions where a particular organism lives
 Ecological niche - Description of the role a species plays in a biological community,
or the total of all the ways an organism uses the resources of its environment
 Generalists - Broad niche
 Specialists - Narrow niche
Ecological niche
 Fundamental niche - Full range of resources or habitat a species could exploit
 Realized niche - Resources or habitat a species actually uses
Barnacle distribution
 Realized niche
 Fundamental niche
Other causes of niche restriction
 Predator absence or presence
 Absence of pollinators
Competition
 Interspecific - Competition between members of different species
 Intraspecific - Competition among members of the same species
 Often intense due to same space and nutritional requirements
Law of Competitive Exclusion
 No two species will occupy the same niche and compete for exactly the same
resources for an extended period of time
Paramecium studies of Gause
Paramecium studies of Gause
Law of competitive exclusion
 No two species can occupy the same niche indefinitely when resources or limiting
 One will either migrate, become extinct, or partition the resource and utilize a
sub-set of the same resource
 Given resource can only be partitioned a finite number of times
Resource partitioning
 Apparent competitors may actually have slightly different niches
 Species may use resources in a different way or time
 Minimizes competition and allows coexistence
Character displacement
Competition experiments
 Often remove one species to see effects on other species
 Trapping
 Exclosures
 May be due to other effects
 Some species are not amenable to experimental manipulation
Experimental studies of competition
 Seed-eating rodents and Kangaroo rats
 50m x 50m enclosures
 Enclosures had openings large enough for seed-eating rodents but not the
Kangaroo rats
 Monitor the number of small rodents