Chapter 16-Biodiversity and the Balance of Nature
S.L. Pimm
16.1 What Biodiversity Is Good for
I. What does the environment do for us?
A. “Ecosystem services”- movement of energy and nutrients through the
air, water, and land, and also through the food chains.
II. How much biological diversity is needed to keep movement at approximately
natural levels?
B. Theories on the dynamics of nutrient cycling devote little space to
consequences in changes in numbers of species per trophic level.
1. It is the number of trophic levels that receives attention.
16.2 A History of Ecological Stability
I. Elton from his 1958 book, The Ecology of Invasions by Animals and Plants
A. Elton provides evidence for simple communities are less stable than
complex communities.
1. “Mathematical speculation about population dynamics
a. Simple models of a predator and its prey or a single host
and its parasite have densities that “fluctuate in numbers
considerably.
b.Fluctuations would have to mean that in simple
communities, populations would never have constant
population levels, but instead would be subject to periodic
“outbreaks” of each species.
2. Lab experiments
3. Natural habitats on small Islands
a. Seem to be much more vulnerable to invading species
than those of continents.
4. Invasions and pest outbreaks most often occur on cultivated or
planted land.
5. Insect outbreaks were a feature of simple temperate forests but
not of complex tropical forests.
6. Insect pests are more likely to invade and become a nuisance in
orchards, especially following spraying designed to eliminate
pests.
Question: Why would insects want to invade after spraying designed for
elimination?
B. Elton’s arguments cause extensive discussion.
1. Models and laboratory experiments available looked only at
simple systems. Models of or experiments with complex systems
were not available as a control.
a. Data provided about invasions was compelling, but
lacking comparative studies on population fluctuations.
2. Discussed a range of very different phenomena.
a. population variability
b. potential abundance of a species
c. rate at which pest species might increase in numbers
after the removal of it’s predators
d. how readily communities may be invaded.
e. extent of damage the invading species cause
3. Ecologists should be thinking about different kinds of stability.
a. population variability
b. population recovery
c. ease of invasion
d. consequences of invasion
4. Elton considers factor called “complexity” which should
determine “stability” and states the importance of their
interdependence.
Question: What effect would humans have on this?
16.2.1 Controversy
I. Relationships between stability and complexity have created large literature.
A. The controversy arose because of multiple meanings of the words
“stability” and “complexity”.
1. Theoretical ecologists such as Elton have used the word
“stability” to mean at least five different things.
2. Theoreticians have considered stability in strict mathematical
sense.
3. May’s (1974) argument- What we observe most often in nature
will be stable systems.
a. Theoretical results make predictions about the kind of
communities we observe and those that we do not.
b. Other four definitions do permit comparisons of different
systems-which were the concerns of empirical ecologists.
C. More complications-different studies looked at “stabilities” at different
levels of ecological organization.
1. Theoretical studies ask how long a community persists before it
is invaded, or how long the community composition lasts.
a. These discussions ignore fluctuations in the abundance
of species and only look at the list of species itself.
D. Overall Elton provides us with a prescription for research into
ecological stability.
16.3 The Stability of Population
I. What happens to the dynamics of populations when we simplify the
environment?
A. Two ways of characterizing population stability
1. Resilience
2. Variability
16.3.1 Resilience: The Example of Pest Outbreaks
I. High resilience means quick return to normal conditions following some
disturbance.
II. For desirable species, high resilience is equated with “stability”.
A. Paradox: Insect herbivore population increasing toward high density
that involves more pests and few plants.
1. Species with resilient population will reach unacceptably high
levels faster than a species that is not resilient.
2. This makes resilience also a measure of instability and how
often pests are controlled by chemical means.
III. Insects are pests because of their high resilience.
A. Across animal species, a small body size is coincident with high
population growth rates. Insects are small.
IV. Resilience relies on more than just features of the species itself.
A. Depends on ecosystem in which species is embedded.
1. When disturbing densities of a set interacting species, no
species’ density can return to normal value until all others have
done so.
a. Resilience of the species will depend on least resilient
one.
B. Simple systems may be more resilient than complex systems. Or
systems may be more resilient when simplified.
16.3.2 Year-to-Year Variability in Densities
I. Highly variable species more likely to occur at high enough densities to be
considered pests. For desirable species, high variability may lead to species
encountering low densities that they become extinct.
II. Variability, depends on number of factors.
A. Population variability will be greater in those ecosystems where
critical environmental variables are themselves more variable.
B. Depends on resilience
C. Depends on structure of the ecosystem in which species occurs and is
altered when we simplify the system.
III. MacArthuer (1955) was one of first to argue that more complex communities
were also more stable.
A. Defined complexity as number of pathways energy took to reach a
given population.
B. Defined Stability as extent to which the population density changed if
one of those pathways failed.
IV. Watt (1964) argued that species with ready access to more resources would be
more variable.
A. This is with exceptions to species in monocultures. Populations of
polyphagous species, will be more variable than monophagous species.
V. Both arguments are sketched by Figure 16.3. There is evidence to support
both.
Question: Which do you think is more accurate?
VI. Predator Diversity.
A. Increased predator diversity ought to reduce the variability of their
prey species.
1. Diversity of predators provides a more reliable control of their
prey’s density.
2. Species preyed upon by many species of a predator may be less
likely to escape the attentions of those predators than a species
exploited by only a very few species of predator.
3. Increased predator diversity may reduce likelihood of unusually
high population densities.
a. Empirical support for this pattern in both insects and
mammals.
VII. Summary-structure of a system affects the population variability of its
component species, but n complex ways.
A. The more generalized a species’ diet, the more or less variable they
may be.
B. The more predatory species a group of prey species suffer the less
variable they will be.
C. Simplification leads to greater variability.
16.4 The Persistence of Communities
I. The length of community composition last depends on factors.
A. How fast the community is losing species.
B. How fast the community is gaining species.
1. Gaining and losing the slowest will be the most persistent.
2. Must understand both extinction and invasion.
16.4.1 Extinction
I. Rare species are prone to extinction.
Question: Why are rare species prone to extinction?
16.4.2 Invasions
I. Communities gain species at different rates.
A. Physical features of environment play a part in determining invasion
rates.
1. Many species fail because they land in an environment
unsuitable for them.
II. Persistence depends on features of the community as well.
A. Likely to be altered significantly when the community is simplified.
B. Communities are easier to invade when they have few species, early in
the assembly process.
1. Also need to have simple patterns of interspecific
interactions.
C. Model communities become harder to invade as assembly proceeds.
1. Assembly itself increases community persistence.
III. As we destroy communities, we substitute communities that are easily
invaded by other species.
IV. Existence of alternative persistent states.
A. Different combinations of species, each of which is persistent in
presence of other combinations.
1. Evidence in both laboratory and field.
B. May not always be possible to create persistent communities from just
their constituent species.
1. Humpty-Dumpty Effect: Even with all the pieces, we cannot
put the community back together again.
V. Summary
A. Simple communities are likely to be less persistent than complex, but
simplified communities are very unlikely to persist.
1. Both are simple and are product of a short assembly process.
B. Complex communities are likely to be the most persistent.
1. Result of long assembly process.
C. We may not be able to reassemble damaged communities.
1. When trying, we might end up with another type of
community altogether.
16.5 Resistance to Change
I. Questions involving resistance
A. Which invasions will cause the most damage?
B. Which species in a community are the keystone species?
C. Complex communities should be most sensitive to the loss of species
from the top of the food web.
1. Secondary extinction propagates more widely in complex than in
simple communities.
D. Simple communities should be more sensitive to the loss of plant
species than complex communities.
2. In simple communities the consumers are dependent on only a few
species and cannot survive their loss.
II. Resistance depends on whether the system is simple or complex and from
whence the species are lot or where they are introduced.
16.6 Conclusions
I. Our effects on the environment are complicated but not incomprehensible.
A. When we destroy species, the simplified communities may more
quickly gain a new species.
1. Species may be undesirable
B. In creating communities that are “new”, we may create communities
that will persist for only short periods of time before invasions take place.
1. Invading species may cause the extinction of native species.
C. Communities are often not resistant to extinctions.
D. When we lose a community, there is a good chance that we will not be
able to restore it.
E. Species have the capacity to recover from losses.
1. Recovery appears to be faster in simpler systems.
2. Sometimes fast recovery is not wanted.
F. Sometimes the properties of natural systems may mitigate the damage
which we subject to them, but mostly the damage propagates widely.