Community Ecology
Essential Questions What factors are most significant in structuring a community? What determines a communities species composition and the relative amount of species present?
What is an ecological community?
Community Ecology Community: Any assemblage of populations in an area or habitat.
All are members of a savanna community
Communities differ dramatically in their species richness. Species richness: number of different species in community. Communities also differ in the relative abundance of species present. Relative abundance: number of certain species in a community
What causes each community to have a certain assemblage of species? Individualistic hypothesis Interactive hypothesis Based on observations of plant distribution Forerunners to the rivet and redundancy models
Individualistic hypothesis By H.A. Gleason in the early 1900 s Chance assemblage of species found in the same area because they happen to have similar abiotic requirements.
Interactive Hypothesis By F.E. Clements in the early 1900 s Community as an assemblage of closely linked species, held together by mandatory biologic interactions that caused the community to function as a unit.
In most actual cases the composition of communities Seems to change continuously, with each species more or less independently distributed
Rivet Model vs. Redundancy Model Rivet Model- most species in a community are tightly associated with other in the web of life. Redundancy Model- Most species in a community are not tightly associated with one another and the web of life is very loose.
Understanding community ecology or species interactions is to understanding our environmental problems today. Rachel Carson s Silent Spring
Pop Quiz 3/11/13 What are the two opposing hypothesis we discussed? How do these two hypothesis differ? Who wrote the book Silent Spring? What did this book launch?
Interspecific Interaction and Community Structure
Community Interactions Populations are linked by interspecific interactions that effect the survival and reproduction on the species involved. Competition Predation Parasitism Mutualism Commensalism
Competition Interspecific competition occurs when species compete for a particular resource that is in short supply. The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place.
Gause s Experiment with Paramecium Unicellular Eukaryote
Realized niche vs. fundamental niche Realized niche- The part of fundamental niche that an organism occupies Fundamental niche- The full range of environmental conditions and resources an organism can possibly occupy and use, especially when limiting factors are absent in its habitat.
Field Experiment of Competitive Exclusion Principle
The ecological niche is the total of an organism s use of the biotic and abiotic resources in its environment. Two possible outcomes if species occupy the same niche. One become extinct Resource Partitioning
Resource Partitioning Species evolve through natural selection to use a different set of resources. A. distichus perches on fence posts and other sunny surfaces.
Character Displacement There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species.
Predation Predation refers to an interaction where one species, the predator, kills and eats the other, the prey. Herbivory and Parasitism are considered forms of predation. Predation is a large factor in adaptive evolution. Eating and avoiding being eaten are necessary for reproductive success.
Parasitism Endoparasites- tapeworm and malariel parasites Ectoparasites- Mosquitoes and aphids
Predator Adaptations Acute sense of smell Claws, Teeth, fangs, stingers, or poison Pursuant predators usually fast and agile Ambush predators usually camouflaged for their environment.
Defenses against Predators Passive defenses- Active defenses-
Mimicry Batesian mimicry- a harmless species mimics a harmful species. Mullerian mimicry-two or more unpalatable species resemble each other
(a) Unpalatable moth (b) unpalatable beetle (c) Unpalatable wasp (d) Unpalatable wasp
Aposematic coloration- brightly colored animals act as warning to potential predators.
Disease Bacteria, Viruses, Fungi
Certain species have an especially large impact on the structure of entire communities either because they are highly abundant or because they play a pivotal role in community dynamics
Dominant Species Are those species in a community that are most abundant or have the highest biomass. Exert powerful control over the occurrence and distribution of other species.
One hypothesis suggests that dominant species are most competitive in exploiting limited resources. Another hypothesis for dominant species success is that they are most successful at avoiding predators.
Keystone Species Are not necessarily abundant in a community They exert strong control on a community by their ecological roles, or niches
Field Study of Sea Stars
Sea Otter effects on communities
Foundation Species Some organisms exert their influence by causing physical changes in the environment that affect community structure Beaver dams can transform landscapes on a very large scale Figure 53.18
Some foundation species act as facilitators That have positive effects on the survival and reproduction of some of the other species in the community
Models of Community Organization The bottom-up model of community organization Proposes a unidirectional influence from lower to higher trophic levels In this case, the presence or absence of abiotic nutrients determines community structure, including the abundance of primary producers
Models of Community Organization The top-down model of community organization Proposes that control comes from the trophic level above In this case, predators control herbivores which in turn control primary producers
Long-term experiment studies have shown That communities can shift periodically from bottom-up to top-down
However, a recent emphasis on change has led to a non-equilibrium model which describes communities as constantly changing after being buffeted by disturbances.
What is a disturbance? A disturbance Is an event that changes a community Removes organisms from a community Alters resource availability
Fire Is a significant disturbance in most terrestrial ecosystems and is often a necessity in some communities
After a fire, lodgepole pines are often the first trees to reappear because the bare, sunlit soil that remains after a fire is deal for lodgepole seedling growth. Fires allow lodgepole pines to release seeds.
The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance
Ecological Succession Ecological succession is the sequence of community and ecosystem changes after a disturbance.
Pioneer Species Early-arriving species May facilitate the appearance of later species by making the environment more favorable May inhibit establishment of later species May tolerate later species but have no impact on their establishment
Retreating glaciers provide a valuable fieldresearch opportunity on succession
Succession on the moraines in Glacier Bay, Alaska follows a predictable pattern of change in vegetation and soil characteristics
Mount St. Helens http://www.livescience.com/6452-strikingimages-mount-st-helens.html