Download Chapter 17 Factors Influencing the Structure of Communities

Part four
Community Ecology
What processes shape these patterns of
community structure?
How will communities respond to the addition
or removal of a species?
Why are communities in some environments
more or less diverse than others?
Chapter 17 Factors Influencing the
Structure of Communities
17.1 The Fundamental Niche Constrains Community
Structure
17.2 Species Interactions Are Diffuse
17.3 Food Webs Illustrate Indirect Interactions
17.4 Food Webs Suggest Controls of Community
Structure
Chapter 17 Factors Influencing the
Structure of Communities
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 The Fundamental Niche Constrains
Community Structure
All living organisms have a range of specific
environmental condition under which they can
successfully survive, grow, and reproduce.
The conditions under which an organism can function
successfully are the consequence of a wide variety of
physiological, morphological and behavioral
adaptations.
Environmental conditions vary in time and space.
Fundamental niches
Fundamental Niche
Fundamental niches of hypothetical
species along environmental gradient
(e.g. T, Moisture, elevation)
•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
•As environments change,
abundance of species will
change.
The distribution of fundamental niches along the
environmental gradient represents a primary constraint
on the structure of communities.
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.
Niche overlap
17.2 Species Interactions Are Diffuse
One reason many experiments tend to underestimate
the importance of species interactions in
communities is that such interactions are often
diffuse, involving a number of species.
Competition Experiment by Norma Fowler (UT Austin)
 Usually the removal of a single species will have very
limited effects.
 Removing groups of species can have large effects.
It is 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.
Diffuse interactions, where one species may be
influenced by interactions with many different species,
is not limited to competition.
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 also provide information on indirect effects.
 Presence of lynx is good for white spruce due to
survival of seedlings.
The key feature of indirect interactions is that they can
potentially arise throughout the entire community
because of a single direct interaction between only two
component species.
Another example of predation in shaping structure of communities
Starfish is a
keystone
predation. In
the absence of
predation,
what happened?
An experiment of indirect interaction
The starfish preys on many species of mussels,
barnacles, limpets, etc
Removing starfish from experimental plots
compared to normal situation
Number of prey species in experimental plots
reduced
Diversity decreased as better competitors excluded
other species
Predation can also influence outcome of
interactions between prey species
This type of indirect interactions is called
keystone predation, where the predator
enhances one or more inferior competitors by
reducing the abundance of the superior
competitor.
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 the other.
Predator abundance depends on the combined
abundance of all prey species.
Under these conditions, the combined population
abundance of the two prey species will support a
higher predator density than in situations where only
a single prey species occurs.
Apparent competition
The prey species are
less abundant when
coexisting than when
in the other species’
absence.
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
Benefit midge larva, neutral to
salamander
Two species of herbivorous
species Daphnia (water
fleas)
Two predators: Midge larva
and Larval salamander
Each predator prey on one
species
In a pond Where
salamander larval were
present, large Daphnia was
low, small was high; where
absent, small Daphnia were
absent, midges could not
survive
17.4 Food Webs Suggest Controls of 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, removing any one species may have
a cascading effect on all others.
Hypothesis two: Only a smaller subset of
species interactions are controlling community
structure. System will not collapse until enough
species are removed.
• 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.
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.
As the availability of water (or nutrients) increases along a supply
gradient (Figure 17.9) , the competitive advantage shifts from those
species adapted to low availability of water (high root production)
to those species that allocate carbon to leaf production and height
but that require higher water availability to survive (Figure 17.10).
Figure 17.10 General trends in
plant adaptations that increase
Figure 17.9 Distribution of tree
fitness along a soil moisture
species along a soil-moisture gradient. gradient.
17.6 Environmental Heterogeneity Influences
Community Diversity
As environmental conditions change from location to
location, so will the set of species that can potentially
occupy the area and the way they interact.
Environmental conditions are typically not homogeneous
even within a given community.
Variations in soil moisture
Variations in nitrogen
How does local environmental heterogeneity within a
community influence patterns of diversity?
Late Robert MacArthur
13 communities in northeastern US
Increased vertical structure
means more resources and
living space and a greater
diversity of potential
habitats.
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?
Increasing nutrient
availability has been
to decrease diversity.
Why?
Michael Huston, ORNL, TN (Texas State Uni.)
Relationship of tree species richness to a simple index
of soil fertility for 46 forest communities in Costa Rica.
Hypothesis
Low nutrient availability reduces growth rates and
supports a lower density and biomass of vegetation.
Species that might dominate under higher nutrient
availability can not 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-)
Factors Influencing the Structure of
Communities
Fundamental Niche
Diffuse and Indirect Interactions
Species’ Stress Tolerance and Its Competition
Environmental Heterogeneity
Resource Availability
How do we build a community ?