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 co-occur 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 CatalinaMountains 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