Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved

Competition

Ecological Niches and Natural Selection

Exploitation (剝削/ 掠奪)

Predation

Prey display various adaptations to avoid being eatenBehavioral defenses include hiding, fleeing, and forming herds or schoolsAnimals also have morphological and physiological defense adaptationsFor example, mechanical and chemical defenses protect specie

Animals with effective chemical defenses often exhibit bright warning coloration, called aposematic colorationPredators are particularly cautious in dealing with prey that display such colorationCryptic coloration, or camouflage, makes prey difficult to s

In some cases, a prey species may gain significant protection by mimicking the appearance of another speciesIn Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful modelIn Müllerian mimicry, two or more unpalatable species re

Herbivory

Parasitism

Positive Interactions

Mutualism

Commensalism

Positive interactions can have significant influence on the structure of ecological communitiesFor example, the black rush (燈心草) affects community diversity by making the soil more hospitable for other plant species

Concept 54.2: Diversity and trophic structure characterize biological communities

Species Diversity

Determining the number and abundance of species that are rare, hard to identify, highly mobile, or less visible presents challengesMolecular tools can be used to help identify such species and determine diversity

Diversity and Community Stability

Communities with higher diversity aremore productive; they produce more biomass (the total mass of all organisms) more stable in their productivitybetter able to withstand and recover from environmental stressesmore resistant to invasive species, organism

Trophic Structure

Food Webs

Limits on Food Chain Length

Species with a Large Impact

Dominant species (優勢物種) are those that are most abundant or have the highest biomassOne hypothesis suggests that dominant species are most competitive in exploiting resourcesAnother hypothesis is that they are most successful at avoiding predators or dise

Keystone species (基石物種) exert strong control on a community by their ecological roles, or nichesIn contrast to dominant species, they are not usually abundant in a communityField studies of sea stars illustrate their role as a keystone species in intertid

Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structureFor example, beaver (河狸) dams can transform landscapes on a very large scale

Bottom-Up and Top-Down Controls

The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level aboveIn this case, predators limit herbivores, herbivores limit plants, and plants limit nutrient levelsEcologists have applied the top-down mode

Concept 54.3: Disturbance influences species diversity and composition

A disturbance is an event that changes a community, removes organisms from it, and alters resource availabilityThe nonequilibrium model describes communities as constantly changing after disturbance

Characterizing Disturbance

The intermediate disturbance hypothesis states that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbanceHigh levels of disturbance exclude many slow-growing speciesLow levels of disturbance allow domina

The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbanceThe Yellowstone forest is an example of a nonequilibrium community

Ecological Succession

Secondary succession begins in an area where soil remains after a disturbanceFor example, abandoned agricultural land may return to its original state through secondary succession

Human Disturbance

Concept 54.4: Biogeographic factors affect community diversity

Latitudinal Gradients

Tropical environments may have greater species richness because there has been more time for speciation to occurTemperate and polar communities have “started over” repeatedly following glaciations

Two main climatic factors correlated with biodiversity in terrestrial communities are sunlight and precipitationThey can be considered together by measuring a community’s rate of evapotranspiration, the evaporation of water from soil plus transpiration of

Area Effects

Island Equilibrium Model

Concept 54.5: Pathogens alter community structure locally and globally

Pathogens and Community Structure

Community Ecology and Zoonotic Diseases

Identifying the community of hosts and vectors for a pathogen can help prevent diseaseFor example, recent studies identified two species of shrew (鼩鼱, 食蟲目) as the primary hosts of the pathogen for Lyme disease

Avian flu is a highly contagious virus of birdsEcologists are studying the potential spread of the virus from Asia to North America through migrating birds