Download Ecology3e Ch19 Lecture KEY

19
Species Diversity in
Communities
Chapter 19 Species Diversity in Communities
CONCEPT 19.1 Species diversity differs
among communities due to variation in
regional species pools, abiotic conditions,
and species interactions.
CONCEPT 19.2 Resource partitioning is
theorized to reduce competition and
increase species diversity.
Chapter 19 Species Diversity in Communities
CONCEPT 19.3 Processes such as
disturbance, stress, predation, and
positive interactions can mediate
resource availability, thus promoting
species coexistence and species
diversity.
CONCEPT 19.4 Many experiments show
that species diversity is positively related
to community function.
Powered by Prairies? A Case Study
Dwindling supplies of fossil fuels has led
to development of biofuels—liquid or
gas fuels from plant material
(biomass).
In the United States, ethanol is made
from corn, and biodiesel is made from
soybeans.
Powered by Prairies? A Case Study
Ideally, biofuels are carbon neutral—
the amount of CO2 produced by
burning them is equal to the amount
taken up by the plants from which they
are made.
They are a nearly limitless renewable
resource, as long as the crops can be
grown.
Powered by Prairies? A Case Study
Biofuels have many downsides as well.
Growing corn and soybeans for biofuels
competes for land and water that could
be used for growing food.
Fossil fuels, in the form of fertilizers and
pesticides and used for farm work, are
required to grow these crops.
Powered by Prairies? A Case Study
Another option is to use non-edible
plants (or parts), such as corn stalks,
straw, or waste wood, to make
biofuels.
Biofuel crops could be grown on
degraded land that is no longer
suitable for high-yield food crops.
Powered by Prairies? A Case Study
David Tillman has studied prairie plant
species diversity in abandoned
agricultural land at Cedar Creek,
Minnesota.
He has shown that plots with more
species produce more biomass than
plots with few species.
CONCEPT 19.1
Species diversity differs among
communities due to variation in regional
species pools, abiotic conditions, and
species interactions.
Concept 19.1
Community Membership
Membership in a community depends on:
1. Regional species pools and dispersal
ability
2. Abiotic conditions
3. Species interactions
These factors act as “filters,” which exclude
or include species in particular
communities.
Figure 19.4 Community Membership: A Series of Filters
Concept 19.1
Community Membership
1. The regional species pool provides an
upper limit on the number and types of
species that can be present in a
community.
The importance of dispersal can be seen
in cases of non-native species
invasions.
Concept 19.1
Community Membership
Humans have greatly expanded regional
species pools by serving as vectors of
dispersal.
Example: Aquatic species travel around
the world in ballast water carried by
ships.
Ships are now larger and faster, so transocean trips take less time—species are
more likely to survive.
Concept 19.1
Community Membership
Zebra mussels (Dreissena polymorpha),
arrived in the Great Lakes in ballast
water in the late 1980s and have had
major impacts on aquatic communities.
The comb jelly Mnemiopsis leidyi was
also introduced via ballast water, into
the Black Sea.
Figure 19.5 Humans Are Vectors for Invasive Species
Concept 19.1
Community Membership
2. Abiotic conditions:
A species may be able to get to a
community but be unable to tolerate the
abiotic conditions.
For example, a lake might not support
organisms that require fast-flowing
water.
Concept 19.1
Community Membership
3. Species interactions:
Coexistence with other species is also
required for community membership.
Other species may be required for
growth, reproduction, or survival.
Species may be excluded by competition,
predation, parasitism, or disease.
Concept 19.1
Community Membership
Some non-native species do not become
part of the new community.
Biotic resistance occurs when
interactions with the native species
exclude the invader.
Example: Native herbivores can reduce
the spread of non-native plants.
CONCEPT 19.2
Resource partitioning is theorized to
reduce competition and increase species
diversity.
Concept 19.2
Resource Partitioning
Resource partitioning: Competing
species are more likely to coexist if they
use resources in different ways.
We can think of each type of resource as
varying along a “resource spectrum,”
representing different nutrients, prey
sizes, habitat types, etc.
Concept 19.2
Resource Partitioning
In a simple model, each species’
resource use falls on a spectrum of
available resources.
Figure 19.7 A Resource Partitioning
Concept 19.2
Resource Partitioning
Assumption: The greater the overlap of
resource use, the more competition
between species.
The less overlap, the more specialized
species have become, and the less
strongly they compete.
Concept 19.2
Resource Partitioning
If species have a high degree of
specialization, it can result in less
competition and high species richness.
More species can be “packed” into a
community if overlap is small.
Or, if the resource spectrum is broad, a
diversity of resources would be available
for use by a wide variety of species.
Figure 19.7 Resource Partitioning
CONCEPT 19.3
Processes such as disturbance, stress,
predation, and positive interactions can
mediate resource availability, thus
promoting species coexistence and
species diversity.
Concept 19.3
Resource Mediation and Coexistence
If disturbance, stress, or predation keeps
the dominant competitor from reaching
carrying capacity, competitive exclusion
cannot occur, and coexistence will be
maintained.
Figure 19.12 The Outcome of Competition under Constant and Variable Conditions
Concept 19.3
Resource Mediation and Coexistence
G. E. Hutchinson considered the idea in
his paper “The Paradox of the Plankton”
(1961).
Lake phytoplankton communities have
very high diversity (30–40 species), all
using the same limited resources, in a
homogeneous environment.
Figure 19.13 Paradox of the Plankton
Concept 19.3
Resource Mediation and Coexistence
His explanation was that conditions in the
lake changed seasonally, which kept
any one species from outcompeting the
others.
As long as conditions changed before
competitively superior species reached
carrying capacity, coexistence would be
possible.
Concept 19.3
Resource Mediation and Coexistence
Intermediate disturbance hypothesis,
first proposed by Connell (1978):
Species diversity will be greatest at
intermediate levels of disturbance.
At low levels of disturbance, competition
regulates diversity. At high disturbance
levels, many species cannot survive.
Figure 19.14 The Intermediate Disturbance Hypothesis
Concept 19.3
Resource Mediation and Coexistence
There have been many tests of this
hypothesis.
Sousa studied communities on intertidal
boulders in southern California that were
overturned by waves.
Small boulders were overturned
frequently (disturbance), large boulders
were overturned less often.
Concept 19.3
Resource Mediation and Coexistence
After two years:
Most small boulders had one species
living on them (frequent disturbance).
Most large boulders had two species
(rare disturbance).
Intermediate sized boulders had four to
seven species.
Figure 19.15 A Test of the Intermediate Disturbance Hypothesis
Concept 19.3
Resource Mediation and Coexistence
Huston (1979) added competitive
displacement to the intermediate
disturbance model:
The best competitor uses the limiting
resources, reducing the weaker
competitor’s population growth to the
point of extinction.
Concept 19.3
Resource Mediation and Coexistence
Hacker and Gaines (1997) incorporated
positive interactions into the
intermediate disturbance hypothesis.
Evidence suggests that positive
interactions are more common under
relatively high levels of disturbance,
stress, or predation.
Concept 19.3
Resource Mediation and Coexistence
At low levels of disturbance, competition
reduces diversity.
At intermediate levels, species involved in
positive interactions are released from
competition and can increase diversity.
At high levels, positive interactions are
common and help to increase diversity.
Figure 19.17 Positive Interactions and Species Diversity
Concept 19.3
Resource Mediation and Coexistence
The intermediate disturbance hypothesis
considers disturbance and predation to
be similar—a dominant competitor is
killed or damaged, creating opportunities
for other species.
Menge and Sutherland (1987) argue that
because predation is a biological
interaction, it should be considered
separately.
Concept 19.3
Resource Mediation and Coexistence
Their model predicts that predation is
most important when environmental
stress is low.
As stress increases, importance of
predation decreases, and competition
increases in importance.
At high stress levels, neither are
important.
Figure 19.19 The Menge–Sutherland Model
Concept 19.3
Resource Mediation and Coexistence
The above theories assume an
underlying competitive hierarchy.
What if species have equivalent
interaction strengths?
Lottery models and neutral models
emphasize the role of chance in
maintaining species diversity.
Concept 19.3
Resource Mediation and Coexistence
In lottery models, all species have equal
chances of obtaining resources that
were made available by disturbances,
and this allows coexistence.
Species must have similar interaction
strengths and growth rates and be able
to respond quickly to disturbances that
free up resources.
Concept 19.3
Resource Mediation and Coexistence
The lottery model may be most relevant
in very diverse communities where
many species overlap in their resource
requirements.
Its relevance decreases in communities in
which species have large disparities in
interaction strength.
CONCEPT 19.4
Many experiments show that species
diversity is positively related to community
function.
Concept 19.4
The Consequences of Diversity
A central idea in ecology is that species
diversity can control community
functions, such as plant productivity, soil
fertility, water quality, etc.
Concept 19.4
The Consequences of Diversity
Many community functions also provide
valuable services to humans:
• Food and fuel production
• Water purification
• O2 and CO2 exchange
• Protection from catastrophic events,
such as floods
Concept 19.4
The Consequences of Diversity
The Millennium Ecosystem Assessment
(2005) predicts that if current losses of
species diversity continue, human
populations will be severely affected by
the loss of services those species
provide.
Concept 19.4
The Consequences of Diversity
The Diversity–Stability Theory
A long-standing idea in ecology is that
species richness is positively related to
community stability—
The tendency of a community to remain
the same in structure and function, or to
return after a disturbance.
Concept 19.4
The Consequences of Diversity
In the experimental plots at Cedar Creek,
Tilman and Downing (1994) showed that
plots with higher species richness had
better drought resistance than plots with
lower species richness.
High diversity plots lost less plant
biomass during the drought.
Figure 19.21 Species Diversity and Community Function (Part 1)
Concept 19.4
The Consequences of Diversity
In another experiment, they set up plots
with different numbers of species, but
the same number of individuals (equal
density).
Plant productivity (biomass) increased
and nitrogen was more efficiently used
as species richness increased, up to a
threshold.
Figure 19.21 Species Diversity and Community Function (Part 2)
Concept 19.4
The Consequences of Diversity
Four hypotheses have been proposed to
explain the relationship between species
diversity and community function.
Two variables in all the hypotheses:
• Degree of overlap in ecological
function of species
• Variation in strength of the ecological
functions of species
Figure 19.22 Hypotheses on Species Richness and Community Function (Part 1)
Concept 19.4
The Consequences of Diversity
1. Complementarity hypothesis: As
species richness increases, there will
be a linear increase in community
function.
Each species added has an equal effect.
Figure 19.22 Hypotheses on Species Richness and Community Function (Part 2)
Concept 19.4
The Consequences of Diversity
2. Redundancy hypothesis: The
functional contribution of additional
species reaches a threshold.
As more species are added, there is
overlap in their function, or redundancy
among species.
This model best fits the results of Tillman
et al.
Figure 19.22 Hypotheses on Species Richness and Community Function (Part 3)
Concept 19.4
The Consequences of Diversity
3. Driver and passenger hypothesis:
Strength of ecological function varies
greatly. “Driver” species have a large
effect; “passenger” species have a
minimal effect.
Addition of driver and passenger species
will have unequal effects on community
function.
Figure 19.22 Hypotheses on Species Richness and Community Function (Part 4)
Concept 19.4
The Consequences of Diversity
4. A variation on the driver and passenger
hypothesis: It assumes there could be
overlap between driver and passenger
functions.
Figure 19.22 Hypotheses on Species Richness and Community Function (Part 5)
Concept 19.4
The Consequences of Diversity
Experiments to test these hypotheses will
be logistically challenging.
They can tell us something about how
communities work.
They may be able to tell us what the
future holds for communities that are
both losing species by extinction and
gaining species by invasions.
A Case Study Revisited: Powered by Prairies?
Tilman et al. (2006) showed that highdiversity plots produced nearly 238%
more biomass per input of energy than
single-species plots.
They then compared three types of
biomass—soybeans, corn, and lowinput, high-diversity (LIHD) biomass
from their prairie plots.
A Case Study Revisited: Powered by Prairies?
Biodiesel, ethanol, and synfuel (synthetic
gasoline) can be made from these
crops.
Synfuel from LIHD prairie biomass had
the highest net energy balance (amount
of biofuel produced minus the amount of
fossil fuels used to produce it).
Figure 19.23 Biofuel Comparisons
A Case Study Revisited: Powered by Prairies?
Energy inputs were lower for LIHD crops
because they are perennial plants and
require little water, fertilizer, or
pesticides.
LIHD crops had a very high yield of
biomass due to diversity effects; and all
of the aboveground plant material can
be used.
Figure 19.24 Environmental Effects of Biofuels (Part 1)
Figure 19.24 Environmental Effects of Biofuels (Part 2)
A Case Study Revisited: Powered by Prairies?
Prairie plants also take up and store more
CO2 than corn and soybeans.
LIHD plots sequestered 160% more CO2
in roots and soil than single-species
prairie plots.
Greenhouse gas emission reductions
were 6 to 16 times greater for LIHD fuels
than for corn ethanol or soybean
biodiesel.
Figure 19.24 Environmental Effects of Biofuels (Part 3)
Connections in Nature: Barriers to Biofuels:
The Plant Cell Wall Conundrum
Biofuels vary in the biomass and energy
required to make them.
Biodiesel is easily produced from oils
such as soybean oil, but growing the
crops can increase soil erosion, requires
large amounts of water, and competes
with food crops.
Connections in Nature: Barriers to Biofuels:
The Plant Cell Wall Conundrum
Ethanol is commonly made from corn that
is fermented and distilled.
The energy costs associated with growing
the corn and producing the ethanol are
high, so there is only a slight energy
gain in ethanol production.
Connections in Nature: Barriers to Biofuels:
The Plant Cell Wall Conundrum
It also competes with food crops.
An acre of corn produces about 440
gallons of ethanol.
This is 4–5 months of driving for the
average individual in the United States.
The same amount of corn could feed one
person for 20–27 years.