Chapter 21Community Ecology

21.1 Species InteractionsCommunities contain populations that interact in many ways

There are five major types of interactions (symbioses): predation, parasitism, competition, mutualism, and commensalism

PredationA predator captures, kills, and consumes the prey

Predation determines where and how a species lives

Regulates population size

Predators and Natural SelectionNatural selection favors adaptations that improve the efficiency of predators (the better the hunter, the more food to survive)

Examples: predator adaptations such as coat color, sharp teeth or claws, able to find prey (sense of smell, accurate vision)

Prey and Natural SelectionThe survival of prey animals depend on their ability to avoid being eaten

Examples: camouflage, poison, behavior, ability to hide, able to outrun predator

MimicryA harmless species resembles a poisonous one

The mimic is protected since it is avoided

Mullerian MimicryWhen several dangerous/poisonous species have similar warning coloration

Example: the black and yellow striped pattern on bees and wasps

Batesian MimicryWhen a harmless species mimics the warning coloration of a dangerous species

Example: coral snakes and scarlet king snakes

Plant-Herbivore InteractionsPlants have evolved adaptations that protect them from being eaten by herbivores

examples: thorns, spines, stinging hairs, tough leaves, chemical defenses

Secondary CompoundsSome chemicals in plants are poisonous, irritating, or bad-tasting

Many plant chemicals are used for medicines

There are many medicinal cures in rainforest plants, but they are being destroyed due to habitat destruction!

ParasitismOne individual is harmed while the other benefitsThe parasite feeds on the hostEctoparasites (external)Endoparasites (internal)

Parasitism and EvolutionParasitism has caused an evolution of a variety of host defenses

Natural selection favors adaptations that allow a parasite to exploit its host (very specialized bodies and lifestyles)

CompetitionThe use of the same limited resource by two or more species

Competitive exclusion: one species can be eliminated from a community because of competition

Competition in ParameciaG.F. Gause studied competition between two species of paramecia in the lab

Character DisplacementCompetitors may evolve niche differences or anatomical differences that lessen the competition

Example: Darwins finches

Resource PartitioningWhen similar species coexist, each species uses only part of the available resources

Example: warblers forage in different types of trees

MutualismA cooperative relationship in which both species benefit

example: pollination (pollinators are attracted to a food source and transfer pollen to other flowers while feeding)

CommensalismOne species benefits while the other is not affected

Example: cattle egrets feed on insects that fly out of the grass when the Cape buffalo is walking

21.2 Properties of CommunitiesSpecies richness: the number of species a community contains

Species diversity: relates the number of species to the relative abundance of each species

Patterns of Species RichnessVaries with latitudeThe closer to the equator, the more species (greatest in tropical rainforests)E.O. Wilson found more ant species in a single tree in Peru than there are in the entire British Isles

Tropical RainforestsClimate is more stable than a temperate area Plants can photosynthesize year-roundTropical rainforests have the highest biodiversity on Earth

The Species-Area EffectLarger areas usually contain more species than smaller areas (more habitats)

Most often applied to islands

Reducing the size of a habitat reduces the number of species

Community StabilityStability refers to a communitys resistance to change and is directly related to species richness

The more species, the more interactions and the better a habitat can withstand a disturbance

21.3 SuccessionA gradual process of change and replacement of the types of species in a community

Ecosystems are constantly changing

Primary SuccessionOccurs on a surface where no ecosystem existed before

Example: rocks, volcanic islands, cliffs, sand dunes

Primary SuccessionHappens very slowly

Bare rock lichens moss grasses shrubs trees

Example: Galapagos Islands

Secondary SuccessionMost common type of succession

Occurs on a surface where an ecosystem previously existed

Occurs after an ecosystem is disturbed by humans or by storms, floods, earthquakes, or volcanoes

Secondary SuccessionPioneer species- the first organisms to colonize any newly available area

Grasses and weeds shrubs shade intolerant trees shade tolerant trees climax community

Example: eruption of Mt. St. Helens destroyed 44,460 acres of forest

Old-field SuccessionWhen farmland is abandoned, it will start to turn back into a forest ecosystem

Pioneer plants such as grasses and weeds will start to grow

After about 100 years, the field will return to a climax community

Old-field Succession