Community Interactions

Chapter 47

Forest of New Guinea

Community includes nine species of

pigeons that partition the food supply

Pigeons disperse seeds of the trees that

provide their food (fruit)

These are just a few of the many

interactions that shape this community

Community

All the populations that live together in a

habitat

Type of habitat shapes a community’s

structure

Factors Shaping Community Structure

Climate and topography

Available foods and resources

Adaptations of species in community

Species interactions

Arrival and disappearance of species

Physical disturbances

Niche

Sum of activities and relationships in which a

species engages to secure and use

resources necessary for survival and

reproduction

Realized & Fundamental Niches

Fundamental niche – Theoretical niche occupied in the absence of any

competing species

Realized niche – Niche a species actually occupies

Realized niche is some fraction of the fundamental niche

Species Interactions

Most interactions are neutral; have no effect

on either species

Commensalism helps one species and has

no effect on the other

Mutualism helps both species

Species Interactions

Interspecific competition has a negative

effect on both species

Predation and parasitism both benefit one

species at a cost to another

Symbiosis

Living together for at least some part of the

life cycle

Commensalism, mutualism, and parasitism

are forms of symbiosis

Mutualism

Both species benefit

Some are obligatory; partners depend

upon each other

– Yucca plants and yucca moth

– Mycorrhizal fungi and plants

Yucca and Yucca Moth

Example of an obligatory mutualism

Each species of yucca is pollinated only by

one species of moth

Moth larvae can grow only in that one

species of yucca

Mycorrhizae

Obligatory mutualism between fungus and

plant root

Fungus supplies mineral ions to root

Root supplies sugars to fungus

Competition

Interspecific - between species

Intraspecific - between members of the same

species

Intraspecific competition is most intense

Forms of Competition

Competitors may have equal access to a

resource; compete to exploit resource more

effectively

One competitor may be able to control

access to a resource, to exclude others

Interference Competition

Least chipmunk is

excluded from piñon

pine habitat by the

competitive behavior of

yellow pine chipmunks

Yellow Pine Chipmunk

Least Chipmunk

Figure 47.3 Page 848

Competitive Exclusion Principle

When two species compete for identical

resources, one will be more successful and

will eventually eliminate the other

Gause’s Experiment

Paramecium caudatum

Paramecium aurelia Figure 47.4Page 848

Species grown together

Hairston’s Experiment

Two salamanders species overlap in parts of their ranges

Removed one species or the other in test plots

Control plots unaltered 5 years later, salamander populations were

growing in test plot

Resource Partitioning

Apparent competitors may

have slightly different niches

May use resources in a

different way or time

Minimizes competition and

allows coexistence

Figure 47.6  Page 849

Predation

Predators are animals that feed on other living

organisms

Predators are free-living; they do not take up

residence on their prey

Coevolution

Joint evolution of two or more species that exert selection pressure on each other as an outcome of close ecological interaction

As snail shells have thickened, claws of snail-eating crabs have become more massive

Predator-Prey Models

Type I model: Each individual predator will consume a constant number of prey individuals over time

Type II model: Consumption of prey by each predator increases, but not as fast as increases in prey density

Type III model: Predator response is lowest when prey density is lowest

Variation in Cycles

An association in predator and prey

abundance does not always indicate a cause

and effect relationship

Variations in food supply and additional

predators may also influence changes in

prey abundance

Canadian Lynx and Snowshoe Hare

Show cyclic oscillations Krebs studied populations for ten years Fencing plots delayed cyclic declines but

didn’t eliminate them Aerial predators, plant abundance also

involved Three-level model

Prey Defenses

Camouflage

Warning coloration

Mimicry

Moment-of-truth defenses

Predator Responses

Any adaptation that protects prey may select

for predators that can overcome that

adaptation

Prey adaptations include stealth,

camouflage, and ways to avoid chemical

repellents

Parasitism

Parasites drain nutrients from their hosts

and live on or in their bodies

Natural selection favors parasites that do

not kill their host too quickly

Kinds of Parasites

Microparasites

Macroparasites

Social parasites

Parasitoids

Fungus and Frogs

Amphibians are disappearing even in undisturbed tropical forests

Infection by a parasitic chytrid is one of the causes of the recent mass deaths

Parasitic Plants

Holoparasites– Nonphotosynthetic; withdraw nutrients and water

from young roots

Hemiparasites – Capable of photosynthesis, but withdraw nutrients

and water from host

Parasitioids

Insect larvae live inside and consume all of the soft tissues of the host

Used as agents of biological control

Can act as selective pressure on host

Price’s Sawfly Study

Number emerging when wasp attacks were experimentally prevented

Number emerging after wasp attacks

Figure 47.15  Page 855

Ecological Succession

Change in the composition of species

over time

Classical model describes a predictable

sequence with a stable climax community

Types of Succession

Primary succession - new

environments

Secondary succession -

communities were destroyed or

displaced

Pioneer Species

Species that colonize barren habitats

Lichens, small plants with brief life cycles

Improve conditions for other species who

then replace them

Climax Community

Stable array of species that persists relatively

unchanged over time

Succession does not always move

predictably toward a specific climax

community; other stable communities may

persist

Cyclic Changes

Cyclic, nondirectional changes also shape

community structure

Tree falls cause local patchiness in tropical

forests

Fires periodically destroy underbrush in

sequoia forests

Restoration Ecology

Natural restoration of a damaged community

can take a very long time

Active restoration is an attempt to reestablish

biodiversity in an area

Ecologists are actively working to restore

reefs, grasslands, and wetlands

Community Instability

Disturbances can cause a community to

change in ways that persist even if the

change is reversed

Keystone Species

A species that can dictate community

structure

Removal of a keystone species can cause

drastic changes in a community; can

increase or decrease diversity

Lubchenco Experiment

Tidepools Rocks exposed at high tide

Periwinkles promote or limit diversity in different habitats

Figure 47.17Page 898

Species Introductions

Introduction of a nonindigenous species can

decimate a community

No natural enemies or controls

Can outcompete native species

Exotic Species

Species that has left its home range and

become established elsewhere

Becomes part of its new community

Can have beneficial, neutral, or harmful

effects on a community

Endangered Species

A species that is extremely vulnerable to

extinction

Close to 70 percent of endangered species

have been negatively affected by exotic

competitors

Nile Perch in East Africa

Nile perch were introduced into Lake Victoria

as a food source

This predator ate native cichlids; drove many

species to extinction

Now Nile perch species is close to crashing

Rabbits in Australia

Rabbits were introduced for food and hunting

Without predators, their numbers soared

Attempts at control using fences or viruses

have thus far been unsuccessful

Kudzu in Georgia

Imported for erosion control No natural herbivores, pathogens, or

competitors Grows over landscapes and cannot be dug

up or burned out May turn out to have some commercial use

Diversity by Latitude

Diversity of most groups is greatest in tropics; declines toward poles

Ant diversity

Figure 47.20Page 862

Why Are Tropical Species Rich?

Resources are plentiful and reliable

Species diversity is self-reinforcing

Rates of speciation are highest in the tropics

Distance Effect

The farther an island is from a mainland, the

fewer species

Closer islands receive more immigrants

Species that reach islands far from mainland

are adapted for long-distance dispersal and

can move on

Distance Effect

Figure 47.22Page 863

Area Effect

Larger islands tend to support more species

than smaller islands

More habitats

Bigger targets

Larger populations decrease extinction risks