Download Lecture K6 – Community Ecology – Dr

Lecture K6 – Community Ecology – Dr. Kopeny
Delivered 4/24
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
Sharp community boundaries
may exist where
environmental factors change
abruptly – Mg content of soil
explains this abrupt change in
coastal California
•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
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
•Symbiotic Interactions
•Predator-prey or host-parasite
interactions One organism benefits at the
expense of another, by eating or otherwise
using that organism as a resource
•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
Effects of Competition and its Importance Community Organization
Laboratory Experiments early last century
•Competitive exclusion experiments
More recent lines of reasoning and natural
experiments
•Resource partitioning
•Character displacement
Relative population densities
Paramecium caudatum alone
Gause’ test of the effect of
interspecific competition
•When grown together with
constant food (bacteria), aurelia
driven to extinction.
•Supports hypothesis that
similar species with similar
needs for same limiting
resource can not coexist
Paramecium aurelia alone
Both grown in mixed culture
•Later termed “competitive
exclusion principle” and
reinforced with other
experimental work