Transcript Community Ecology - Effingham County Schools

Chapter
53
Community
Community Ecology
• A community is an assemblage of species
(populations) living close enough together
for potential interaction in a habitat
Community Ecology
• The boundaries of a community can be set
by the experimental question (the
decomposers on a rotting log, the benthic
community of Lake Erie, the trees and
shrubs in Yellowstone National Park)
• Communities vary in their species richness
and relative abundance
Interspecific Interactions
• Interspecific competition – competition for
resources between different species when
resources are in short supply
Interspecific Competition
• Competitive exclusion principle – The concept
that when populations of two similar species
compete for the same limited resources, one
population will use the resources more
efficiently and have a reproductive advantage
that will
eventually lead
to the
elimination of
the other
population
The Ecological Niche
• An organism’s ecological niche is described
as its place in an ecosystem – its habitat and
use of biotic and abiotic resources
• The competitive exclusion principle holds
that two
species with
identical
niches cannot
coexist in a
community
Resource Partitioning
• Resource partitioning – slight variations in
niche that allow ecologically similar species
to coexist; competition is a selection factor
in evolution
Predation
• Predation – a predator eating prey
• Includes herbivory and parasitism
• Eating and being eaten are prerequisite to
reproductive success
• Adaptations to increase success in
predation may include acute senses, speed
and agility, camouflage coloration, and
physical structure such as claws, fangs,
teeth, and stingers
Predation
• Plants may defend themselves with
mechanical devices, such as thorns, or
chemical compounds
• Animals can defend against predation by
hiding, fleeing, or defending
• Cryptic coloration allows animals to blend in
to their environment
• Mechanical and chemical defenses
discourage predation
• Aposematic coloration warns predators not
to eat animals with chemical defenses
Predation
Predation
• Mimicry may be used by prey to exploit the
warning coloration of other species
• Batesian mimicry – a palatable or harmless
species mimics an unpalatable or harmful
model
• Müllerian mimicry – two or more unpalatable
species resemble each other
Cuckoo Bee
Yellow Jacket
Parasites
• A parasite derives its nourishment from
another organism, the host, which is harmed
in the process
• Parasites that live within the host are
endoparasites ; those that feed on the
surface of a host are ectoparasites
• In parasitoidism, insects lay eggs
on hosts, on which their larvae
then feed.
• Parasites do not usually cause
lethal harm to the host on which
they feed
Mutualism
• In mutualism, interactions between species
benefit both participants
• Mutualistic interactions require the
coevolution of adaptations in both species
• Many cases of mutual
symbiosis may have evolved
when organisms became able
to derive some benefits
from their predator or
parasite
Mutualism between
acacia trees and ants
Commensalism
• In commensalism, only one member appears
to benefit from the interaction
• Examples include “hitchhiking” species and
species that feed on food incidentally
The Patella gets it
exposed by another
Example of
Mutualism
food from the plant,
the Euklonia, which
is not harmed or
damaged in the process.
Trophic Structure
• The trophic structure
of a community is its
feeding relations
• A food chain shows
the transfer of food
energy from one
trophic level to the
next
Food Webs
• A food web
diagrams the
complex trophic
relationships
within a
community
What Limits the Length of a Food Chain?
• Within a food web, each food chain consists
of only a few links (usually five or fewer)
• Two hypotheses
– Energetic hypothesis – food chains are limited
by inefficiency of energy transfer (only about
10%) from one trophic level to the next
– Dynamic stability hypothesis – suggests that
short food chains are more stable than long
ones because an environmental disruption that
reduces production at lower levels will be
magnified at higher trophic levels as food supply
is reduced all the way up the chain
Dominant Species
• Species in a community that have the
highest abundance or largest biomass are a
major influence on the distribution and
abundance of other species
• A species may become a dominant species
due to its competitive use of resources or
success at avoiding predation
• What happens if the dominant specie is
removed from a habitat such as the
American chestnut trees?
Keystone Species
• A keystone species has a large impact on
community structure as a result of its
ecological role
• Experiments with a predatory sea star
(Pisaster) demonstrates the importance of
keystone species
Sea Otters as Keystone
Predators in the North Pacific
Structures of Communities
• The structure of a community may be
controlled bottom-up by nutrients or topdown by predators
• According to the bottom-up model of
community organization, N → V → H → P, an
increase in nutrients yields an increase in
biomass at each succeeding trophic level
• The top-down model, N ← V ← H ← P,
assumes that predation controls community
organization; also called the trophic cascade
model
Disturbance and Community Structure
• The nonequilibrium model emphasizes the
nonstable, changing structure and
composition of communities as a result of
disturbances
• Disturbances such as fire, drought, storms,
overgrazing, or human activity change
resource availability, reduce or eliminate
some populations, and may create
opportunities for new species
• Small scale disturbances may enhance
environmental patchiness and help maintain
species diversity
Disturbance and Community Structure
Disturbance and Community Structure
Disturbance and Community Structure
Ecological Succession
• Ecological succession is the transition in
species composition in a community, usually
following some disturbance
• Primary succession occurs in areas
previously devoid of life (new volcanic island
or the moraine left by a retreating glacier)
– Autotrophic bacteria → lichens → mosses →
grasses → shrubs → trees
• Secondary succession occurs in existing
communities that have been disturbed by
fire, logging, or farming, but the soil
remains intact
– Herbaceous species → woody shrubs → trees
Biodiversity
• A communities biodiversity correlates with
its size and geographic location
• Biodiversity is greatest in the tropics and
on large versus smaller islands
• Biodiversity is measured by both species
richness and relative abundance
• Ecologists measure biodiversity as
heterogeneity, which considers both
diversity factors: richness and relative
abundance
Which Forest is More Diverse?
Biodiversity
• Tropical habitats
support much larger
numbers of species
of plants, animals,
and other organisms
than do temperate
and polar regions
Biodiversity
• A species–area curve illustrates the
correlation between the size of a community
and the number of species found there
Biodiversity
• Island biogeography states that the size of
the island and its closeness to the mainland
(or source of dispersing species) are
important variables directly correlated with
species diversity