Download Stability, Equilibrium, and Non

Stability, Equilibrium, and Nonequilibrium
By
Muzvondiwa J.V.
Guiding Principles
• Community stability includes resistance to change and
resilience—the ability to rebound from change.
• For a long time, ecologists have held that more diverse
communities are more stable.
• Laboratory experiments with simple communities have
usually failed to show a link between diversity and
stability. Field experiments with complex communities,
by contrast, have shown such a link.
• The intermediate-disturbance hypothesis suggests that
the most diverse communities, such as rain forests and
coral reefs, exist at intermediate levels of disturbance,
due to storms or some other kind of turbulence.
Community Stability
a. Equilibrium: a community where no change in
population size and number of species occurs
•
Long-term data shows constancy over time (Figure 17.2)
b. Community predictability can decrease over time
•
Eastern Wood bird community
–
•
•
c.
Adjacent years show more stability than between widely separated
ones
Temporal scale of change should be accounted for
Environmental variability can cause increased variability over
time
Importance of community stability
•
Changes in community richness may be indicative of something
wrong
–
•
Ex. Decline in species richness associated with DDT contamination
Instability can lead to the extinction of certain species
Community Stability
d. Type of species
– Number of species
• Simply counting the number of species may obscure potential
changes; i.e., some species go extinct or emigrate, and others
immigrate
• May result in a turnover of species
– Thus, definition of stability presupposes a stable equilibrium for
each population (not just a constant number of species)
e. Defining stability
– Resistance: how big a force is needed to change a community
– Resilience: the ability of the community to return to equilibrium
after a perturbation
• Elasticity: how quickly a community can return to equilibrium
• Amplitude: how much disturbance the community can return from
Community Stability
– Global stability: the ability to return from high-amplitude
disturbance
– Local stability: the ability to return from low-amplitude
disturbance
– Resistance and resilience may or may not be correlated
• Lakes are weakly resistant and weakly resilient
• Deserts are highly resistant and highly resilient
• Rivers are not particularly resistant, but may be resilient (fast-flowing
water often cleanses the rivers quickly)
– Can a community exist in multiple stable states?
• If, following an oil spill an intertidal community recovers and contains
95 of the original 100 species in the community, is it the same
community?
• Most evidence suggests no multiple stable states. Environment usually
changes from one state to another.
Is There a Link between Diversity and
Stability?
a. Linkage between diversity and stability first recognized by
Charles Elton (1958)
a.
Disturbances in large communities would be cushioned by
large numbers of interacting species
b. Evidence for a link between diversity and stability (Elton)
a.
b.
c.
Small, faunistically simple island communities are more
vulnerable to invading species than are species-rich
continental communities
Outbreaks of pests are often found on cultivated land or land
disturbed by humans. These areas have few naturally
occurring species
Tropical rain forests do not often have insect outbreaks like
those that are common in temperate forests
Is There a Link between Diversity and
Stability?
c. Evidence against a link between diversity and
stability
– Examples of invaders of continental regions that
assume pest proportions
– Agricultural systems may suffer from pest outbreaks
not because of their simple nature, but because their
individual components often have no coevolutionary
history. Native monocultures (e.g., Spartina and
Juncus) seem to be stable
– Goodman (1975) argued that the stability of tropical
ecosystems is a myth
Is There a Link between Diversity and
Stability?
d. Mathematical argument
– Increasing complexity actually decreases stability
in models
– Food webs are likely to be stable only if the
following inequality is satisfied:
• b (SC)_ < 1 ; where
– _ = interaction strength
– S = number of species
– C = connectance
• Any increase in S or C would increase instability
Experimental Tests of DiversityStability Hypothesis
a. Nelson Hairston and colleagues' experiments with
microorganisms (1968)
– Various combinations of one, two, or three species of
bacterial prey; one, two, or three species of Paramecium
predator; and sometimes one or two species of protozoan
that fed on Paramecium
– Cultures were monitored for 20 days
– Results: Two-trophic level systems with two species of
Paramecium, the least abundant species of Paramecium
showed a tendency to become extinct when only one
species of prey was present than with two or three prey
species (Figure 17.4a)
• Supported a link between diversity and stability
Experimental Tests of DiversityStability Hypothesis
– Results: Number of bacteria species held constant at
three, and the number of Paramecium species was
increased to two or three, extinction rates increased
• Does not support link between diversity and stability
– Which of the species went extinct was dependent on
which species were together in the culture
• Species were not interchangeable numerical units
• Results argue against a simple link between diversity and
stability
• When a third trophic level is added, in the form of the
predatory protozoan, there was a further decrease in
stability
Experimental Tests of DiversityStability Hypothesis
b. Sharon Lawler and Peter Morin (mid-1990s)
– Experiments with microorganisms, to examine
changes in population dynamics that occur as the
food chain increases in length
• Bacteria were the prey at trophic level 1
• Bacteriovorous protists occupied trophic level 2
• Protozoans occupied trophic level 3, and could feed on
either trophic level 1 or 2
– Consensus focused on the variability of trophic
level 2 as a measure of stability
Experimental Tests of DiversityStability Hypothesis
– Protists (Colpidium and Tetrahymena) displayed
constant abundance in shorter food chains, where
they occupied the top trophic level
– Addition of predators significantly increased the
variability of their prey populations
– The degree of reduction in stability depended on
the identity of both prey and predator species
– Species could not be regarded as simply
interchangeable units, casting doubts on
mathematical models
Experimental Tests of DiversityStability Hypothesis
c. Laboratory cultures involving many species are
difficult to conduct and maintain, because it is
virtually impossible to duplicate the full array of
environmental conditions that exist in nature
d. Field tests of the diversity-stability hypothesis
– Sam McNaughton working with large vertebrates in
Africa for 20 years
• Disturbed savanna communities in the Serengeti by allowing
buffalo to graze in certain areas
• Experiments were repeated in species-rich and species-poor
communities
Experimental Tests of DiversityStability Hypothesis
• Results: Plant biomass changes more in the species-poor
community. In the species rich plots the non-eaten species
proliferate, maintaining overall plant biomass
• McNaughton felt that this demonstrated a clear link between
diversity and stability
– McNaughton and Frank's (1991) work in Yellowstone Park
• Examined the effect of severe drought on the composition of
species in the grassland plant community
• Calculated diversity with the Shannon index, then calculated an
index of resistance
– R = 1 - Σ (Δpq/2); where
» _pq/2 = change in abundance of the ith of n species in community j,
between 1988 (drought year) and 1989 (normal year)
– R was greater in more diverse communities
Experimental Tests of DiversityStability Hypothesis
– David Tilman (1996): biodiversity and stability in
Minnesota grasslands
• Examined both population and ecosystem traits
• Used many different grassland plots in a long-term
study
• Demonstrated that, in more diverse plots, biodiversity
stabilizes community and ecosystem processes, but not
population processes
– Year to year variability in aboveground biomass was
significantly lower in plots with greater plant species richness
– Year to year variability in species abundance was not
stabilized by plant species richness
Experimental Tests of DiversityStability Hypothesis
• Differences between species and community biomass
was most likely due to interspecific competition
– When climatic variables harm some species, unharmed
competitors will increase
– Such an increase stabilizes the total community biomass, but
individual species biomass is more variable
– Dan Doak suggested that the diversity-stability
relationship was caused by a statistical averaging
effect. More species in a community increases the
likelihood of including a very productive species.
This is called the Portfolio Effect
Intermediate-Disturbance Hypothesis
a. Joe Connell (1978)
– Highest local diversities are maintained in
communities of intermediate levels of disturbance
– High levels of disturbance, only colonists (rselected) will survive, giving rise to low diversity
– Low levels of disturbance, competitively dominant
species will outcompete other species, only a few
K-selected species will persist, giving rise to low
diversity
Intermediate-Disturbance Hypothesis
– Examples
• Richest tropical rain forests occur where disturbance by
storms is common
• Coral reefs maintain their highest diversity in areas
disturbed by hurricanes
• Richest plant communities in the southeastern United
States occur on Army bombing ranges
– Many communities therefore exist in
nonequilibrium states
Intermediate-Disturbance Hypothesis
b.Wayne Sousa (1979)
– Intermediate-disturbance hypothesis experiment
in a marine intertidal situation
– Small boulders were easily disturbed and only had
a mean 1.7 sessile plant and animal species
– Large boulders were rarely moved by waves, and
had a mean of 2.5 species
– Intermediate-sized boulders had the most species,
with a mean of 3.7 species (had a mixture of rand K-selected species)
Intermediate-Disturbance Hypothesis
c. Hiura (1975) tested intermediate-disturbance
hypothesis on a larger scale
– Beech forest in Japan over 10° latitude
– Areas with intermediate disturbance maintained
the highest species diversity
– Results are confounded by latitudinal diversity
gradients (temperature increases as one
progresses south, which will increase species
diversity)
Applied Ecology
a. Can marine communities recover after oil spills?
–
–
Crude oil is a common contaminant of marine systems
Water-soluble fractions can be lethal to fish and
invertebrates and may disrupt the body insulation of
birds
b. Wreck of the Exxon Valdez near Valdez, Alaska (March
24, 1989)
–
–
–
Worst oil spill in U.S. waters: ~11 million gallons spilled
By international standards, not that big
A week after the spill, the slick covered 900 square miles,
and hundreds of miles of shoreline. Officially, 27,000
birds, 872 sea otters, and an untold number of fish died
Applied Ecology
– Effects spread to the terrestrial environment, with 100
dead eagles and most pairs failing to produce young
that year
– Exxon spent $3.2 billion on cleanup
– Was cleanup a success?
• State viewed recovery as successful if the affected
communities returned to their pre-spill condition
– Probably unattainable because we cannot know what the
community was like before the spill
• Exxon view was that recovery occurs by the reestablishment
of a healthy biological community in which the plants and
animals characteristic of that community are present and
functioning normally
– This view pays little attention to densities and age structure
Applied Ecology
• If populations do not recover, it may not be possible to
determine if it is the blame of the oil spill, or other
environmental variables, such as a severe freeze
– Examination of long-term consequences
• Compared nine oiled areas that were set aside and not
cleaned, with (a) areas that were oiled and cleaned,
and (b) areas that were not oiled
• Percent cover of rockweed
– 50% on unoiled areas
– Returned to normal values by 1991 on oiled, non-cleaned sites
– Returned to normal values by 1992 on oiled, cleaned sites
Applied Ecology
• Conclusion: cleaning reduced diversity of species found
in soft sediment cores for two years after the spill.
Oiled sites that were not cleaned showed a reduction in
abundance but not diversity
• Best cleaning: Leave the beach alone!
Summary
• Stability can be thought of in different ways—as resistance to
change or as resilience, which refers to the ability of the community
to return to equilibrium after perturbation. Resilience in turn can be
divided into two concepts: elasticity, which measures how quickly a
community can return to a former state, and amplitude, which
measures how big a disturbance it can return from.
• A community could exist in more than one form or stable state, but
the evidence for this hypothesis is weak.
• The anecdotal evidence for a link of diversity with stability, first
suggested by Elton (1958), has not held up well under scrutiny.
• Experimental studies of communities of microorganisms have not
shown a strong link of diversity with stability. Large-scale field
experiments with plants have shown a link between diversity and
stability.
Summary
• In 1996, Tilman reconciled the opposing viewpoints in the diversitystability debate by showing how communities of prairie plants
exhibit stability in biomass and ecosystem function, but not in
species abundance.
• The diversity-stability link may be caused by a "portfolio," or
statistical averaging, effect.
• The older, more conventional view called the equilibrium
hypothesis proposes that most communities are stable. The more
modern nonequilibrium viewpoint argues that disturbances are
frequent and species composition is constantly changing, so that
stability is elusive.
• The intermediate-disturbance theory suggests that the most
diverse communities, such as tropical forests and coral reefs, exist
at intermediate levels of disturbance.
Discussion Questions
1.
2.
3.
4.
5.
6.
Classify the biomes (temperate and tropical grasslands and forest,
deserts, tundra, rivers, lakes, estuaries, coral reefs, and the open sea) in
terms of their resistance and resilience.
Do you think that pesticides might reduce the stability of agricultural
systems? Explain how such an effect would work.
If the intermediate-disturbance hypothesis is correct, how does that
influence the conservation movement? Should conservationists promote
disturbance to maximize diversity?
What differences might you expect between equilibrium and nonequilibrium communities in terms of numbers of species, stochastic
effects, and life history strategies of species?
How would you set about establishing a link between diversity and
stability in nature?
Do you believe in the idea of multiple stable states? Does this idea help
or hinder the restoration of natural habitats by polluters in terms of how
far they are expected to go to restore communities?