Community Ecology
A community is an assemblage of species’ populations which occur together in
space and time and therefore have the potential for interaction.
Communities can be recognized and studied at any number of levels, scales and
sizes.
Begon et al 1986
In terms of trophic (feeding) relationships, species
comprising communities “function” as:
•photosynthesizers -- producers
•herbivores -- primary consumers
•carnivores -- secondary, tertiary consumers)
•decomposers
•omnivores -- obtain food from more than one trophic
level)
Emergent properties of communities include:
•species diversity
•limits of similarity of competing species
•food web structure
•community biomass & productivity
Coral reef community, Indian Ocean, Figi
Community structure: patterns and
underlying processes
A central goal in the field of community
ecology is to understand the processes
that explain the “structure” (pattern) of a
community, ie the composition of species,
and their abundances and distributions?
Processes underlying those patterns
involve interactions between species and
the influence of abiotic factors.
Important questions in community
ecology address the degree to which
organismal and population level
interactions explain community structure
Coral reef community, Indian Ocean, Figi
Redwood Community
Raven and Johnson 1999
The redwood forest of coastal
California and southwestern
Oregon is “dominated” by the
redwood trees – this
community is named for its
most conspicuous member
species
Sword Fern
Ground beetle feeding on
slug on sword fern
Redwood Sorrel
Raven and Johnson 1999
Communities can be
characterized in terms of
their Species richness and
species diversity
Species richness; total
number of species
Species diversity; relative
abundance of species in a
community
The relative abundance of species
is very significant in terms of
community structure and function
Species diversity includes
information both on number of
species and on their relative
abundance
By what, if any, processes does
community structure arise?
http://home.carolina.rr.com/httpd/hikepisgah/hikepisgah.html
Observation: Particular associations of
species (especially plants) co-occur
over extensive geographic areas;
examples in eastern deciduous forest
include:
•beech maple forests
•oak hickory forests
Early thoughts focused,
somewhat narrowly, on two
hypotheses
•Individualistic hypothesis (Gleason):
chance assemblage of species that
occur together because of similar
abiotic requirements
•Interactive hypothesis (Clements):
closely linked assemblage of species
linked into association by mandatory
biotic interactions that cause community
to function as an integrated unit
Pisgah National Forest is in Transylvania County, near
Brevard, North Carolina.
Predictions of the individualistic and interactive
hypotheses, and Whitaker’s test
Individualistic hypothesis predicts
communities should generally lack discrete
geographic boundaries because species each have
independent distributions along environmental
gradients (tolerance ranges for abiotic factors)
Interactive hypothesis predicts species should
be clustered into discrete communities with distinct
boundaries because presence/absence of species is
strongly influenced by presence/absence of other
species; species should usually co-occur
Results of Robert Whitaker’s research
Each tree species at one elevation in Santa Catalina
Mountains of Arizona supports individualistic
hypothesis; independent distributions for species –
apparently due to moisture tolerances; species cooccur where environment meets their moisture
tolerances
each colored curve represents abundance of a
single species
Contemporary thinking on explanations of
community structure
•Empirical evidence indicates that in most cases
composition of plant communities appears to change
on a continuum; species’ distributions seem to be
independent by and large
•Especially true for plant species over large
regions over which environmental variation
occurs on a smooth gradient, not in abrupt steps
•Individualistic hypothesis is probably not as
broadly applicable to animal species as it is to
plant species - often linked more closely to
other organisms
•Simple generalizations on processes governing
community structure do not have broad explanatory
power; distributions of most populations in
communities are affected to some extent by both
abiotic factors and biotic interactions
•Processes that disturb and destabilize existing
relationships among organisms (eg fire, flood, storm)
are probably among the most significant abiotic
factors affecting community structure; disturbance
may be the single most important factor affecting
structure of many communities
Sharp community boundaries
may exist where
environmental factors change
abruptly – Mg content of soil
explains this abrupt change in
coastal California
The eastern grey
squirrel is not linked
strongly to a single
food; its found in
eastern deciduous
forests from Florida to
Canada in many habitat
types including pine
dominated forests, but
is most common in
mature hardwood
forests
The limpkin is widespread in
the American tropics, but
occurs in the U.S. only in
Florida, where it can satisfy
its dietary requirement for a
certain freshwater snail.
Interactions between populations of different species Ecologists
recognize five major classes of interactions among organisms, based on the
effect each one has on the other
•Competive interactions mutually harmful
interaction arising when two organisms use
one or more of the same resources, the
availability of which is insufficient to meet
their combined needs
•Predator-prey or host-parasite
interactions One organism benefits at the
expense of another, by eating or otherwise
using that organism as a resource
•Symbiotic Interactions
•Mutualism Interaction in which both
organisms benefit
•Commensalism (?)Interaction in which
one organism benefits and the other is
unaffected
•Amensalism (?)Interaction in which one
organism is harmed and the other is
unaffected
Coevolution of traits bearing on interactions
Interacting species may coevolve
•Coevolved species have mutually influenced
one another’s evolution in some manner;
Ecological interactions among species may
influence evolution of species’ traits
•Coevolution may result in result in striking
reciprocoal evolutionary adaptations and
counter-adaptations, but not necessarily
Adaptation of organisms to other species
is regarded as a fundamental
characteristic of life, despite difficulties in
assessing evolutionary relationships
Coevolution may be diffuse or
species-specific
•Diffuse co-evolution species traits
evolve as a consequence of interactions
multiple species, perhaps involving
multiple types of interactions
•Regarded as much more common than
species-specific coevolution
•Species-specific coevolution species
traits evolve as a consequence of
interactions with a single species
Solomon et. al., 1999
•Implicit though, is some measure of reciproal
genetic change, which has not been
demonstrated for most putative instances of
co-evolution
•Difficult to establish that evolutionary change
in one species is the selective force that
drives evolutionary change in an interacting
species
Species in mustard family produce mustard oils
Mustard oils protect cabbage from many, but not all,
herbivorous insects; the caterpillar of the cabbage
butterfly feeds on a cabbage leaf
Competition
The Concept of the Ecological Niche
•“Niche” refers to sum total of the way an
organism uses its environment, biotic and
abiotic resources, to live
•For each species of tropical tree lizard,
niche includes
•temperature, humidity range
•size, orientation of hunting branches
•hunting times
•size, species of insect prey
•many, many more “niche dimensions”
Campbell 1993
Percent nitrogen
Relative
Growth
Rates (%)
Keeton & Gould 1994
Percent water
Species’ niche is related to acceptable limits and optimum values of
variables that affect the species. Two dimensions of a caterpillar’s niche; %
water & % nitrogen content of its food
Competition and Ecological Niches
•Fundamental Niche
Entire niche a species (or
population, or organism) is
theoretically capable of
occupying
•Realized Niche
The actual niche the
organism is able to occupy
in the presence of
competitors
Green Anole
Brown Anole
•Niche Overlap
Niche overlap refers to the
degree of similarity in the
fundamental niche of two
species
Green Anoles are native to S.E. United States. The species
has become rare in parts of Florida in recent years,
apparently as a consequence of interspecific competition
with the introduced Brown Anole.
Unit 6 Ecological Patterns and Processes
Apr 17 W Population structure and models of population growth 959-965
Apr 19 F* Influence of density, disturbance &life history on populations 965-973
Apr 22 M Biotic interactions and community structure 974-987
Apr 24 W Disturbance, succession and community structure 987-990
Apr 26 F
Historical and ecological biogeography 1007-1014
Apr 29 M Conservation and decline of biodiversity 1031-1044
Effects of Competition and its Importance in Community Organization
Laboratory Experiments early last century
•Competitive exclusion experiments
More recent lines of reasoning and natural
experiments
•Resource partitioning
•Character displacement
Gause’ test of the effect of
interspecific competition
Paramecium caudatum alone
•Supports hypothesis that similar
species with similar needs for same
limiting resource can not coexist
•Later termed “competitive exclusion
principle” and reinforced with other
experimental work
Paramecium aurelia alone
Both grown in mixed culture
•When grown together with constant food
(bacteria), caudatum driven to extinction.
Gause’s principle, restated in the
context of fundamental and realized
niches; two species can not coexist if
their niches are identical; ecologically
similar species can coexist, given one
or significant differences in their
niches.
Competition and the Consequences of Competition in Nature
•Competition in natural communities is much more complex, and more difficult to study,
than in laboratory experiments
•The possible consequences of interspecific competition (consider two species) are
either; extirpation of one species or coexistence of both species
•Coexistence happens when competition is reduced, minimized,
•Niche differentiation is the process by which species partition the use of resources
in a shared habitat
•Catch 22 for researchers;
•competition in nature is difficult to demonstrate because its been reduced through
niche differentiation.
•What we are able to study in nature, typically, is not intense competition, but the
“ghost of competition past”
•Demonstration of niche differentiation is taken as strong evidence that competition is an
important ecological force
demonstrate resource partitioning
demonstrate character displacement; evolutionary character divergence that
reduces competition
We’ll look at comparative and experimental approaches
Competition as an Ecological and Evolutionary Force: Strong
Circumstantial Evidence I
Resource Partitioning
•Similar, coexisting species often have niche differences; implies
competition has operated in the past as an evolutionary-ecological force
•Resource partitioning refers to (often slightly) different use of resources
among similar species in sympatry.
•Differences in habitat use (location within area of the community) is also
regarded a potential consequence of resource partitioning.
Resource partitioning in sympatric Anolis lizards in the Caribbean.
•Jonathan Losos at Washington U. found that species variously use upper canopy,
lower branches, trunk, or grass below tree for hunting areas.
•When two species do occupy same part of tree, they either eat different size
insects or occupy different thermal microhabitats.
•Same pattern of partitioning independently evolved on other Caribbean Islands.
Raven and Johnson 1999
Competition as an Ecological and Evolutionary Force: Strong
Circumstantial Evidence II
Character Displacement
•Character displacement refers to presumed divergence in morphology or
other trait (behavior, etc,) and related divergence in resource use, in
sympatric populations compared to allopatric populations
•Comparison of closely related species occurring both sympatrically and
and allopatrically
Character displacement in
Geospiza. These two species
have similar bill size among
allopatric populations, but different
size when populations are
sympatric
(Raven and Johnson 1999)
Controlled Field Experiments Provide Direct Evidence of the
Importance of Competition
Controlled Field Experiments make for a strong inference regarding cause and
effect
•Barnacles (Joseph Connell)
•Sticklebacks (Donald Schluter)
Experimental Demonstration of Competitive Exclusion
•Joseph Connell
demonstrated
competitive
exclusion within a
community in a
rocky intertidal
zone
•The species
coexist by
partitioning the
habitat.
Competition among two species of barnacles limits niche use
Chthamalus can live in deep and shallow zones (its fundamental
niche), but Balanus forces Chthamalus out of the part of its
fundamental niche that overlaps the realized niche of Balanus.
Solomon et al 1999
Part of Shluter’s wet lab & one of the experimental ponds
Photo’s from Shluter’s webpage at
UBC:
Dolph Schluter,Department of Zoology,
University of British Columbia
http://www.zoology.ubc.ca/~schluter/
Benthic three-spined stickleback from Paxton Lake,
attending nest (photo by Matt Mcleod, 1996)
Predation
•ecological relationships in which one organism
(population, species) benefits and the other is
harmed
•generally, feeding relationships where individuals of
one species directly provide nourishment for
individuals of another species
•includes predation, herbivory, parasitism,
parasitoidism
Parasitoidism Insects,
usually small wasps, lay
eggs on living hosts; on
hatching, larvae feed in
boty of host, eventually
causing its death
•objectives; examine nature of these ecological
relationships especially predation, and the
consequences of these relationships on community
structure
Predation (Hunting): predator kills and eats its
prey
Herbivory Animal eats a plant, perhaps killing the
organism, eg mouse eating a seed), perhaps not,
eg, grazing by cattle – the latter being akin to
parasitism
animals have many defensive
adaptations to avoid being
eaten
•behavioral
•fighting/defense
•concealment
Bombadier beetle ejects a
noxious spray at the
temperature of boiling water at
a predator
•congregating
•fleeing
•morphological
•shells, exoskeletons
Indo-Pacific lionfish, one of
the most toxic reef fishes.
Posion glands at base of
spines, and warning
coloration
•spines, spinous fins
•size
•chemical, coloration
Bluejay vomiting after
eating a noxious monarch
butterfly
•toxins
•cryptic coloration
•warning coloration
•mimicry
cryptic coloration in
canyon tree frog, on
granite substrate
Crows
mobbing
barn owl
Aposematic (warning) coloration is common in species that uses poisons and
stings to repel predators.
Dendrobatid frogs of Latin America
are highly toxic to vertebrates. Over
200 alkaloids isolated from
Dendrobatid mucus; some are so
toxic that 2-3 micrograms in
bloodstream will kill a human being.
Milkweed toxins are poisonous to
many herbivores, but not Monarch
caterpillars
Monarchs metabolize toxins,
thereby becoming unpalatable
themselves
Aposematic (warning) coloration
has evolved many times in
unpalatable lineages
Mullerian and Batesian
mimicry among species of
Costa Rican butterflies
Some species have a disproportionately large influence on community
composition and structure
•Through their ecological interactions, many if not all species have an effect, to
some degree, on various components of community organization, including
•species richness
•microclimate, soil structure, soil chemistry
•resource abundance and distribution
•flow of energy, nutrients
•Some species have major influences on community organization by virtue of their
number, biomass, size
•in terrestrial communities, plants constitute much of the structural environments,
strongly modify the physical environment, and are channels for input of energy and
nutrients
•Some species are “keystone” members of communities in that they have a
disproportionately large effect compared to their representation (abundance) in the
community
•Starfish (hunters)
•Bison (grazers)
By grazing preferentially on grasses, bison
increase the density of forbs and overall plant
productivity and species richness
•30 bison introduced into Konza Prairie research
Natural Area in Kansas (experimental plot); plant
community compared to control plot
•grasses fertilized by bison urine photosynthesize
faster (nitrogen becomes available quickly to
plants)
Images from Begon et al 1986, Campbell 2000, Purves et al 2000
Paine’s (1966) manipulation experiment shows
the influence a top carnivore may have on
community structure (species’ richness)
•The main influence of the starfish was to make space available
for competitively subordinate species. It created areas free of
barnacles and, most importantly, free of the dominant mussels
which would otherwise outcompete other invertebratres and
algae for space.
•Overall the removal of starfish led to a reduction in number of
species from fifteen to eight.
Disturbance, Succession, Non-equilibrium, and Community Structure
Alder, cottonwood and willow on a glacial moraine, perhaps 100
years after the glacier had retreated from this area
Succession is a process of change that results
from disturbance in communities
•Many if not most communities are characterized by
periodic disturbances that affect structure and
composition, such as fire, floods, storms, freezes, volcano
•Effects of such disturbances may variously include
- “knock back” many if not all popul’s to low levels
-remove all vegetation from terrestrial or aquatic
community
-scour the soil, streambed, etc
Succession: Vegetation colonizes
a moraine near McBride Glacier,
AK. Copyright Bruce F. Molnia/TP.
•These disturbances occur at various scales; they may
be localized and patchy or geographically extensive;
•Such disturbances create the conditions for “ecological
succession”
•disturbed area is colonized by a variety of species
(often time those with life history traits that give them a
competitive advantage in a low-competition
environment)
•with time, growth of populations of colonizers, etc.,
ecological conditions change and colonizers are
eventually replaced by a succession of other species
•“Ecological succession” refers to transitions in species
composition over ecological time
Succession: Fireweed after fire in
Yellowstone National Park.
Copyright Jon Mark Stewart/BPS.
Solomon et al 1999, Purves et al 200
barren landscape
exposed after retreat
of the glacier is
initially colonized by
lichens, then
mosses
Alder and Dryas (an herbaceous plant)
have nitrogen-fixing bacteria in root
nodules, which “improves soil for other
species
at a later time, dwarf
trees and shrubs
(alder, cottonwood,
willow) colonize the
area
Still later, hemlocks
and spruces
dominate the
community
Primary succession after
retreat of glaciers
In the 1960’s and 1970’s, community structure
was explained in terms of “stability”,
“equilibrium” and climax communities
•By the early 1900’s, the idea of “climax
communities” began to gain acceptance; succession
leads to a stable endpoint, a climax community;
stable climax predicted when web of interspecific
interactons became so intricate that the community
was”saturated” -- no more species could colonize
except after a localized or extensive extinction of
species
•The “balance of nature” view held that communities
exist, normally, in a “state of equilibrium”, unless they
are significantly “disturbed”
Mature northern hardwood forest
dominated by sugar maple; Upper
Peninsula, MI. Copyright J. Robert
Stottlemyer/BPS.
•“Stability” was regarded as tendency for community
to reach and maintain equilibrium (relatively constant
condition) in spite of disturbance
•Interspecific interactions were regarded as agents
of stability; maintained stability, or returned
communities to equilibrium following disturbance
•This “balance of nature” model is now regarded as
having limited explanatory power with respect to
community structure
Tropical evergreen forest (rainforest),
Mt. Spec National Park, Australia.
Copyright BPS.
Contemporary nonequilibrial model views
communities as mosaics of patches at different
stages of succession
•Succession is a highly variable and virtually
perpetual process – no longer understood as an
orderly, linear progression driven mainly by
interspecific interactions;The course of successional
change depends on size, frequency and severity of
disturbance.
•Most communities are routinely disturbed by outside
factors during the course of succession; few if any
communities “reach”, much less persist in a climax
state; growing body of research indicates that
disturbance is the main force driving successional
changes.
•Disturbance keeps communities in a constant state
of flux, rendering them mosaics of patches at
different successional stages and preventing them
from ever achieving a state of “equilibrium”
Chaparral biome in Monterrey, CA.
Copyright Edward Ely/BPS.
Community Ecology
Communities are composed
of populations living and
interacting in a given
environment
Producers
Primary Consumers
Primary consumers
Death
Death, waste products
Secondary Consumers
Death, waste products
Decomposers
Ecological Interactions among organisms
Interaction
Effect on Species 1
Effect on Species 2
Competion
between sp. 1 and sp. 2
harmful
harmful
beneficial
harmful
Mutualism of sp. 1 and sp. 2
beneficial
beneficial
Commensalism of sp. 1 w/ sp. 2
beneficial
no effect
Parasitism by sp. 1 on sp. 2
beneficial
harmful
Predation
by sp. 1 on sp. 2
Symbiosis
Competion among
two species of
barnacles limits niche
use
Chthamalus can live in
deep and shallow zones
(its fundamental niche),
but Semibalanus forces
Chthamalus out of the
part of its fundamental
niche that overlaps the
realized niche of
Semibalanus.
Predation
Predator-Prey Interactions
•Interactions in which one organism
uses another one for food
•Animals that capture live animals
and eat them
•Animals that eat live plants
•Among most conspicuous of
community interactions
•Interactions drives evolution of
adaptations in both prey and predator
species
•Interactions drive causally related
patterns of population dynamics
North American bobcat, a solitary
hunter, feeding on a mouse.
Giant Panda of
mountainous China
feeding on bamboo
Plants evolve defenses against herbivores
Morphological defenses
•Thorns, spines
•Hairs; glandular and sticky distally, deters herbivorous insects
•Intracellular silica deposition; renders plant (esp. grasses) tough to eat
Plants evolve defenses against herbivores
Chemical defenses: secondary chemical compounds
• Widely occurring among plant lineages
•Generally either toxic or impede development by disrupting metabolic
pathways
Plants evolve defenses against herbivores
Chemical defenses: secondary chemical compounds
•Milkweed family and the related dogbane family produce milky sap that
deters herbivores; also produce cardiac glycosides, which impair
vertebrate heart function
•Species in mustard family produce mustard oils
Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar
of the cabbage butterfly feeds on a cabbage leaf (Solomon et. al., 1999)
Animal defenses against predators
Aposematic (warning) Coloration. Dendrobatid frogs of Latin America
are highly toxic to vertebrates. Over 200 alkaloids isolated from
Animal defenses against predators
Milkweed toxins are
poisonous to many
herbivores, but not
Monarch caterpillars
Monarchs metabolize
toxins, thereby becoming
unpalatable themselves
Aposematic (warning)
coloration has evolved
many times in unpalatable
lineages
Animal defenses against predators
Aposomatic (warning) coloration; common in species that uses
poisons and stings to repel predators.
(Solomon et. al. 1999)
Mullerian and
Batesian
mimicry among
species of Costa
Rican butterflies
Ecological Interactions among organisms
Interaction
Effect on Species 1
Effect on Species 2
Competion
between sp. 1 and sp. 2
harmful
harmful
beneficial
harmful
Mutualism of sp. 1 and sp. 2
beneficial
beneficial
Commensalism of sp. 1 w/ sp. 2
beneficial
no effect
Parasitism by sp. 1 on sp. 2
beneficial
harmful
Predation
by sp. 1 on sp. 2
Symbiosis
Competiton
•Competition occurs when two organisms attempt to use the same limiting resource;
resource availability can not satisfy both individuals
•Interference competition; individuals “fight” over resource
•Exploitative competition; individuals simply consume resource
•Interspecific competition; among individuals of differing species
•intensity correlates with similarity of organisms (similar niches)
•Intraspecific competition; among conspecifics (very similar niches!)
Competiton
•Competition occurs when two organisms attempt to use the same limiting resource;
resource availability can not satisfy both individuals
•Interference competition; individuals “fight” over resource
•Exploitative competition; individuals simply consume resource
•Interspecific competition; among individuals of differing species
•intensity correlates with similarity of organisms (similar niches)
•Intraspecific competition; among conspecifics (very similar niches!)
Predator-Prey Interactions
Pelican
catching
surface
feeding fish
•Interactions in which one organism
uses another one for food
•Animals that eat live plants,
fungi, etc.
•Animals that capture live
animals and eat them
•Interactions drive evolution of
adaptations in both prey and predator
species
•Interactions drive causally related
patterns of population dynamics
Giant Panda of
mountainous China
feeding on bamboo
North American bobcat,
a solitary hunter,
feeding on a mouse.
(Solomon et. al., 1999)
Plants defenses against herbivores
Morphological defenses
•Thorns, spines
•Hairs; glandular and sticky distally, deters herbivorous insects
•Intracellular silica deposition; renders plant (esp. grasses) tough to eat
Plants evolve defenses against herbivores
Chemical defenses: secondary chemical compounds
•Milkweed family and the related dogbane family produce milky sap that
deters herbivores; also produce cardiac glycosides, which impair
vertebrate heart function
•Species in mustard family produce mustard oils
Mustard oils protect cabbage from many, but not all, herbivorous insects; the caterpillar
of the cabbage butterfly feeds on a cabbage leaf (Solomon et. al., 1999)
Predators can alter community structure by moderating
competition among prey species
Herbivory
Some lineages of plants defend against feeders
with chemical protection
Predation
Predators moderate competition among
prey species
•One important effect of a predator on
community structure
Plants evolve defenses against herbivores
Chemical defenses: secondary chemical compounds
• Widely occurring among plant lineages
•Generally either toxic or impede development by disrupting metabolic
pathways
Animal defenses against predators
Aposomatic (warning) coloration;