community interactions chapter 47. forest of new guinea community includes nine species of pigeons...
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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
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
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