BIOL 4120: Principles of Ecology
Lecture 16: Community
Ecology
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Outline (Chapter 17)
Factors influence community structure
17.1 Fundamental niche constrains community
structure
17.2 Species interactions are diffuse
17.3 Food web illustrate indirect interactions
17.4 Food web suggest controls of community
structure
17.5 Species interactions along environmental
gradients involve both stress tolerance and
competition
17.6 Environmental heterogeneity influences
community diversity
17.7 Resource availability can influences plant
diversity within a community
17.1 Fundamental niche constrains
community structure
All organisms can live and reproduce over a specific range of
environmental condition
These conditions differ from organism to organism
The conditions under which an organism function well are the
consequence of physiological, morphological and behavioral
adaptations.
These same adaptations also limit its ability to perform equally well
under different conditions.
Plants adapted to shade, high nutrient
The environmental conditions vary in time and space, and fundamental
niche of species vary, thus fundamental niche difference among
species help explore the processes that structure communities.
Fundamental niche constrains
community structure
•All species have bellshaped niches
•Niche overlap
•Each species has
limits beyond which it
can’t survive
•For any given range
of environment, only a
subset of species can
survive
Fundamental niches of hypothetical species along
environmental gradient (e.g. T, Moisture, elevation)
•As environments
change, abundance of
species will change.
Distribution of three species
Geographic distribution of three tree species that are part of the two
forest communities presented in Tables 16.1 and 16.2.
Distribution of these three species overlap in West Virginia.
As we move across eastern North America, the set of tree species
whose distributions overlap will change, and therefore so will the
species composition of the forest communities.
Null model
Null model:
assume that presence and abundance of the individual species found in a
given community are a result of the independent responses of the
individual species to the prevailing physical environment.
Interactions among species have no significant influence on the structure
Null model provides basis for experimental study
Physically remove one species and examine the population response of
the other (competition, predation, parasitism, mutualism)
If no change, accept null model (no interaction).
But many evidences show that species interactions do influence both the
presence and abundance of species within communities.
Example, competition influence realized niches. Mutualism enhances the
presence and distribution.
17.2 Species interactions are
diffuse
Species interactions exist, but the importance are often
underestimated.
because
• such interactions are many and relatively diffuse
• Involve a number of species
Competition Experiment by Norma Fowler (UT Austin)
Usually the removal of a single species will have very limited
effects
Removal of group of species can have large effect
this makes it difficult to determine the effect of any given species
on another.
But collectively, competition may be an important factor limiting
the abundance of all species involved.
Species interactions are diffuse
Pollination:
A single plant species may be dependent on a variety of animals species
for successful reproduction.
Predator-prey:
Predator species (lynx, coyote, horned owl etc.) and snowshoe hare
Food web chart (on next slide), 11 of 12 predators prey on snowshoe
hares.
Any single predator species may have a limited effect on the snowshoe
hare population; together, they regulate its population.
17.3 Food webs illustrate indirect
interactions
Food webs
Provide
information on
indirect effects
See lynx and
white spruce
Presence of lynx
is good for
white spruce
due to survival
of seedlings
Another example of predation in shaping structure of communities
Starfish prey on
mussels, barnacles,
limpets, and chitons
Remove starfish,
what would happen?
Species diversity
increase or decrease?
Why?

Keystone predation
• Predation can also influence outcome of
interactions between prey species
• The starfish preys on many species of mussels,
barnacles, limpets, etc
• Remove starfish from experimental plots and
compare to normal situation
• Number of prey species in experimental plots
was halved
• Diversity was decreased as better competitors
excluded other species

Apparent competition
• In the absence of predator, the population of
each prey is regulated by purely intraspecific
density-dependent mechanisms
• Neither prey species compete, directly or
indirectly, with each other
• Predator abundance depends on the total
abundance of prey
• Under these conditions, the combined
population abundance of two prey species will
support a higher predator density.
Apparent competition
Combined populations of
two prey species support a
larger predator population
neither can support alone.
As a result, two prey
populations reduced, gives
outward appearance of
interspecific competition.
Experimental supports:
Nettle aphid, grass aphid and ladybug beetle (page 359, textbook)
Brought nettle aphid plants to grass aphid plants together
suppressed both population, as a results of larger ladybug beetle
population.
Indirect commensalism
Two species of
herbivorous species
Daphnia (water fleas)
Two predators: Midge
larva and Larval
salamander
Each predator prey on
one species
Benefit midge larva, neutral to
salamander
In a pond Where
salamander larval were
present, # of large
Daphnia was low, small
was high; where
absent, small Daphnia
were absent, midges
could not survive

Indirect mutualism
• When indirect interaction is beneficial to both
(predator) species.
Indirect interactions play an important role
Removing top predators from community could
have some unforeseen consequences
(conservation and management)
An Example: Predator control in Greater
Yellowstone ecosystem (Joel Berger from UN
Reno)
Predator control, decrease in Grizzly bear and
wolf
increase in moose population
decrease in willow and other woody species
along riverline
decrease in birds (even local extinction for
some species)
17.4 Food webs suggest controls of
community structure
Wealth of experimental evidence illustrates the importance of both
direct and indirect interactions on community structure.
How do you tell which ones are important in controlling community
structure? Are all interactions important?
Hypothesis one:
all species interactions are important; remove any one of these
species may have a cascading effect on all others.
Hypothesis two:
only a smaller subset of species interactions are controlling
community structure.
System stable until enough species are lost to make whole
system collapse


Difficult to study (there are some dominant species like
starfish, but majority is mystery)
One approach is splitting species into functional groups
• Each group has a similar function and perhaps can replace each
other
Trophic levels
1.Primary
producers
2.Herbivores
3.Carnivores

Bottom-up control
• Plant population control herbivore populations, which in turn
control the diversity and population density of carnivore
population

Top-down control
• Predator (carnivore) populations control the diversity of prey
species, and the prey of the prey, and so on.
Bottom-up control is very common. Mostly, community structure
is regulated by bottom-up control.

Examples support top-down control
• Large-mouth bass experiment by Mary Power at OU
• Bass (Predator) prey on minnows (herbivore), minnows graze
on algae
• Remove bass vs control
• Pools with bass had low minnow population and a luxuriant
growth of algae
• Pools with bass removed had high minnow populations and low
populations (biomass) of algae
• Top predator control plant population indirectly through their
direct control on herbivores.
“The world is green”: predators will keep herbivores in control
17.5 Species interactions along environmental
gradients involve both stress tolerance and
competition
Biological structure of a community is constrained by environmental
tolerances of the species (fundamental niche). Those tolerances are
often modified through both direct and indirect interactions with
other species (realized niche).
Competitors and predators can function to restrict a species in a
community and mutualists can function to facilitate a species’s
presence and abundance within a community.
Results: a pattern of species distribution across an environmental
gradient
This is due to trade-off
Some species that can grow well are at an advantage when
resource at best
Some species than can survive when resource is at worst can
survive where previous species cannot.
General pattern of tradeoff between a species’
ability to survive and grow
under low resource and
maximum growth achieved
under high resource
availability.
The resulting outcome of
competition will be a
pattern of zonation.







Classic examples of zonation occur in salt marshes
Salt provides
stress
Level of salt
varies with
distance from low
water mark
Lower boundary
is determined by
stress
Upper boundary
is determined by
competition
Note also that
nutrition can
change situation
Increase
fertilization
results in change
in species at
boundary
Tradeoff
• Competitive
ability
• Tolerance of
stress
17.6 Environmental heterogeneity influences
community diversity
Biological structure of a community reflects both the ability of the
component species to survive and grow in the prevailing
environmental conditions and their interactions.
As environmental changes from location to location, so will the species
and their interaction.
But how does local environmental heterogeneity within a community
influence patterns of diversity?
Late Robert MacArthur
13 communities in
northeastern US
Bird species diversity
Foliage height diversity
Relationship between bird species diversity and foliage height
diversity for deciduous forest communities in eastern North America.
(x is not height)
17.7 Resource availability can influence plant
diversity within a community
High nutrients will support high rates of
photosynthesis, plant growth, and a high
density of plants.
How does nutrient availability influence plant
diversity in communities?
Michael Huston,
ORNL, TN
(Texas State
Uni.)
Why?
Relationship of
tree species
richness to a
simple index of
soil fertility for
46 forest
communities in
Costa Rica.
Hypothesis
Inverse relationship results from reduced
competitive displacement under low
nutrient availability
Low nutrient availability reduces growth rates and supports a
lower density and biomass of vegetation. Species that might
dominate under higher nutrient availability cannot realize their
potential growth rates and biomass and as a result are unable to
displace slower growing, less competitive species.
Supported by many other experiments
(Rothamsted Experimental Station in Great Britain, 1859-)
Increasing nutrient availability has been to decrease diversity


What processes cause the decrease in diversity
with increasing nutrient availability?
J. Cahill (University of Alberta, Canada) examined
how competition in grassland communities shifts
along a gradient of nutrient availability
• A shift in the importance of belowground and
aboveground competition and the nature of
their interaction under varying levels of
nutrient availability
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Competition for belowground and
aboveground resources differs in an
important way
Competition for belowground resources is
size symmetric because nutrient uptake
is proportional to the plant’s root size
• Symmetric competition occurs when
individuals compete in proportion to
their size
Competition for aboveground resources is
size asymmetric — larger plants have a
disproportionate advantage by shading
smaller ones.


Under low nutrient availability, plant
growth rate, size, and density are low for
all species
• Competition primarily occurs
belowground  symmetric
Growth rate, size, and density increase as
nutrient availability increases
• As faster-growing species overtop the
others, creating a disparity in light
availability  asymmetric competition
Freshwater and marine communities

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Fertilization results in an increase in the
species richness of autotrophs in both
freshwater and marine communities
Why the difference between patterns in
terrestrial versus aquatic communities?
• Differences in the role of competition
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Reduced competition results from
fertilization
Limited by more than one nutrient, no single
spp has a competitive advantage.
END
14 circular mescosms,
1.6 m in diameter, 1.5 m height
Three years