Community Ecology

Differences within a Community

• Community - an assemblage of species living close enough for the potential of interaction– Species richness - number of species within a

community.– Relative abundance - the number of common

species as compared to rare species.– Species diversity - species richness+relative

abundance

Interspecific Interactions between Populations of Different Species

• The adaptation of one species to the presence of another may lead to coevolution (a change in one species acts as a selective force on another).

• Example– predator/prey– mutualism– commensalism

Predation/Parasitism

• Predation - a predator eats a prey

• Parasitism - parasites live in or on a host, usually killing them outright.

• Parasitoidism - small insects such as wasps lay eggs on hosts; the larvae feed within the body of the host, killing it.

• Herbivory - animals eat plants

Plant Defenses Against Herbivores

• Thorns/hooks/spines in or on leaves and stems

• chemicals that produce distasteful foliage such as strychnine, morphine, nicotine

• production of analogous (same in appearance not function) hormones that causes abnormal insect development when eaten

Animal Defenses Against Predators

• Hiding, fleeing, alarm calls, distraction displays, escaping, combat tactics.

• Cryptic coloration - passive defense that makes potential prey difficult to see (camouflage)

• Batesian mimicry - palatable prey resembles the appearance of a harmful or unpalatable species

Predation

• Parasitism - one organisms derives nourishment from another– Endoparasites - live within the host tissue or

cavities (tapeworms)– Ectoparasites - attach or briefly feed on external

surfaces ( mosquitoes)

Interspecific Competitions

• Competitive Exclusion Principle - two similar species in the same area with similar resources can not coexist.

Ecological Niche

• What is your niche?

• Ecological niche - how an organisms fits in to its environment by using biotic and abiotic resources

• Two species can not coexist if they have identical niches.

Evidence for Competition

• The weaker individual will become extinct.

• One of the species will evolve to the point of using a different set of resources.– Resource partitioning

Commensalism

• Symbiotic relationship where one organism benefits without significantly affecting another; unlike parasitism.– Cattle egrets

• Difficult to find a true commensalistic relationship when most relationships will benefit both species to some degree.

Mutualism

• Symbiotic relationship where both organisms benefit.– Nitrogen fixing bacteria and legumes.

Community Structure - Food Webs

Primary Producers

Primary Consumers

Secondary Consumers

Disturbance and Nonequilibrium

• Disturbance - anything that disrupts a community– change in resource availability allowing for

disappearance or emergence of new species– natural disasters– human intervention

• clear cutting

• logging

• pollution

• grassland destruction

Succession

• Succession - transition of species composition over time– Primary succession - succession of barren areas

due to lack of soil formation, rubble, or barren rock (colonization of new lands)

• pioneering species - species that will first colonize areas in primary succession (mosses, algae)

Succession (con’t)

• Secondary succession - occurs when an existing community has been cleared by some disturbance that leaves the soil intact which will be recolonized by a fugitive species (weeds).