Species Diversity and Community Stability

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Species Diversity and Community Stability
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Some initial observations

• When we sample species in a community, we usually
  find a few species that are very common, while
  many species are rare.
• In our trapping efforts this semester in Ecology
  and Mammalogy, we have trapped primarily B.
  carolinensis, followed by P. leucopus, M.
  pinetorum, and O. nuttalli.

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Initial observations.

• In fact, we usually find the data follow a
  logarithmic series

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Initial observation.

• Here, ax number of species in the total catch
  represented by one individual, ax2 number of
  species in the total catch represented by two
  individuals, and so on. Then, the number of
  species in a sample is S

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Relative abundance of butterflies in Rothamsted,
England, in 1935.
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Initial observations

• What does this tell us about the structure of
  communities?

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Diversity Indices

• We need some index to evaluate the diversity of
  species in a community.
• A common, and reasonable index is the
  Shannon-Wiener Diversity Index.

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Diversity Indices

• Here, pi proportion of the ith species in the
  total sample of S species.
• This index has the pleasing property, that
  communities with uneven abundances of species
  have lower diversity.

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Diversity Indices

• Compute H for each of the following
• Community 1 with 90 individuals of species A and
  10 individuals of species B.
• Community 2 with 50 of species A and 50 of
  species B.
• Community 3 with 80 of species A, 10 of species
  B, and 10 of species C.
• Community 4 with 33.3 of species A, 33.3 of
  species B, and 33.3 of species C.

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What do you get?

• Community 1) H 0.33
• Community 2) H 0.69
• Community 3) H 0.70
• Community 4) H 1.10
• These results are exactly what we would expect
  intuitively.

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Evenness

• We can estimate the evenness of the community by
  using J, where

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Evenness

• Here, Hmax is the maximum possible diversity,
  assuming all species in the community have equal
  representation.
• Of course, these estimates are valid only within
  the context of any given study, and are difficult
  to compare across studies. Do you know why?

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What if you need to compare indices across
studies?

• The best bet is to rely on Species Richness.
  This is simply the number of species observed.

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Gradients of Species Diversity

• In a very general way, we know that the tropics
  contain more species than the temperate zones.
  For example,
• there are more than 1000 species of fish in the
  Amazon, 456 in Central America, and only 172 in
  the Great Lakes.
• There are 7 ant species in Alaska, 73 in Iowa,
  101 in Cuba, 134 in Trinidad, and 222 in Brazil.

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Gradients of Species Diversity

• There are examples where the pattern is opposite
  of what we expect
• Sandpipers
• Aphids
• Overall however, the patterns appear to be clear.

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Diversity of mammals
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Isoclines of mammal diversity in North America
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Isoclines of avian diversity in North America
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How do we explain these patterns?

• Time Hypothesis
• Spatial Heterogeneity Hypothesis
• Competition Hypothesis
• Predation Hypothesis

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How do we explain these patterns?

• Climatic Stability Hypothesis
• Productivity Hypothesis
• Area Hypothesis in larger areas, the chances of
  isolation between populations increase, with
  corresponding increases in the chances of
  speciation.

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How do we explain these patterns?

• Animal Pollinators Hypothesis in the tropics and
  other humid parts of the world, winds are less
  frequent and of lower intensity than in temperate
  regions. This effect is accentuated by dense
  vegetation cover. Therefore, most plants are
  pollinted by animals.
• Can you think of others?

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Community Stability

• This is the ability of a community to resist
  change following a disturbance (community
  resistance), or
• the ability of a community to return to its
  original configuration after a perturbation(commu
  nity resilience).
• Here it is worthwhile thinking about equilibria
  from the Lotka-Volterra analyses.

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Community Stability

• Deserts have high resistance.
• Estuaries have low resistance, but high
  resilience.
• Does diversity cause stability?
• Laboratory experiments by Gause confirmed the
  difficulty of achieving numerical stability in
  simple systems.

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Community Stability

• Small, faunistically simple islands are much more
  vulnerable to invading species than are
  continents.
• Outbreaks of pests are often found on cultivated
  land or land disturbed by Humans both of which
  contain few species.
• Tropical rain forests do not have insect
  outbreaks like those common in temperate forests.

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Community Stability

• Pesticides have caused pest outbreaks by the
  elimination of predators and parasites from the
  insect community of crop plants.
• In a review of 40 food webs, the complexity of
  food webs in stable communities has been found to
  be greater than the complexity of food webs in
  fluctuating environments.

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Community Stability

• If diversity is equated with stability, then
  stability

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Community Stability

• In a food web with 4 links (1 predator and 4
  prey), each link caries 0.25 of the total energy
  in the food web, and stability -(4 x 0.25 x
  log(0.25)) 1.38.
• Adding another predator that eats all the prey
  doubles the number of links to 8, and stability
  2.08.

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What are the stability implications of these webs?
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Community Stability

• We can also get a given stability by having a
  large number of species, each with a restricted
  diet (specialists), or a smaller number of
  species each with a broader diet (generalists).
• Maximum stability occurs when there are m species
  and m trophic levels, with each trophic level
  containing 1 species.

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Community Stability

• Does this make sense?
• Restricted diets lower stability in general, but
  in practice specializations may be essential for
  efficient exploitation of prey.
• In arctic systems with few species, it is
  difficult to have a specialized diet, and species
  are generalists (greater stability), but there
  are few species and thus populations fluctuate
  considerably.

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Community Stability

• In the tropics, with many species, stability can
  be achieved with restricted diets, and species
  specialize, feeding on only 1 or 2 trophic levels.

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Is there convincing evidence that diverse
communities are more stable than simple ones?

• 1 fluctuations of microtine rodents are as great
  in simple arctic communities as they are in
  complex temperate communities.
• 2 Some field data suggests tropic stability is a
  myth (Robin Andrews).

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Is there convincing evidence that diverse
communities are more stable than simple ones?

• 3 Rain Forests seem particularly susceptible to
  human perturbations.
• 4 Agricultural systems may suffer from outbreaks
  not because of their simplicity, but because
  their components have no co-evolutionary history.

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Two alternative views

• Equilibrium Hypothesis
• Local population sizes fluctuate little from
  equilibrium values, which are determined by
  predation, competition, and parasitism.
  Communities are stable and perturbations are
  damped out.

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Two alternative views

• Non-equilibrium Hypothesis
• Species composition is constantly changing, and
  never in balance. Stability is elusive, and
  persistence and resilience are key measures of
  community behavior.
• Key mechanism for this hypothesis is the
  intermediate disturbance hypothesis

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Community Change

• Succession
• If we were to burn the I.R. Kelso sanctuary and
  then leave it alone, we could predict fairly
  accurately what would happen.
• In the first few years it would be covered by
  weeds and grasses.
• Shrubs would be established.
• Maple seedlings and other pioneer species would
  be establsihed.
• Finally, after 30 or 40 years it would look like
  a young Oak-Hickory forest.

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Community Change

• After 300 years, it would be a climax community
  an old growth forest with little understory.
• If we then burned it again, it would repeat the
  sequence. However, once in the climax, it will
  stay there unless perturbed.

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Community Change

• The same pattern can be seen in the coastal
  habitats of California, the Mesas of New Mexico,
  the rocky intertidal, or even hot-spring algal
  communities in Yellowstone.
• Why does this happen?

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Rate of ice recession in Glacier Bay, Alaska
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Community Change

• Glacier recession results in significant
  disturbance, with the newly exposed habitats
  undergoing successional change.
• But, do the communities ever reach the climax
  stage? There are many examples where
  environmental perturbations are frequent and
  prevent attainment of the climax condition.

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Community Change

• There are 3 models for succession
• Facilitation model
• Inhibition model
• Tolerance model

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Facilitation model each species makes the
environment more suitable for the next.
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Inhibition model Initial colonists tend to
prevent subsequent colonization by other species.
Succession depends on chance events (who invades
first). Succession proceeds as colonists die,
but it is not in an orderly or predictable
fashion
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Tolerance model Any species can start the
succession, but the eventual climax is reached in
a somewhat orderly fashion.
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Succession

• Succession can be modified by a number of
  factors
• Stochastic events
• Life history
• Facilitative events
• Competition
• Herbivory

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Influence of succession and environmental
severity on major successional processes that
determine change in species composition during
colonization (C), maturation (M), or senescence
(S).
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Succession

• What does all this mean?
• Succession is a complex process, influenced by
  many factors.

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Percent vegetative cover vs. field age and
nitrogen conc. for a intruduced plants, b
non-prairie natives, c true prairie natives.
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Island Biogeography

• Studies of succession have benefited greatly from
  studies of island recolonization Krakatau in
  1883, Mt. St. Helens in 1980.
• When we study how islands are recolonized, we
  begin to understand a great deal.

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Island Biogeography

• Area Effects
• What is the likelihood that an area will be
  colonized? It depends on distance from source
  pool, but also on size of the target.
• Also, a small habitat is unlikely to support as
  many different types of colonists as a big area.

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Island Biogeography

• This can be expressed as the following, where S
  number of species, c is a constant measuring
  number of species per unit area, A area, and z
  a constant measuring the slope of the line
  relating S and A.

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Island Biogeography

• Remarkable, for a wide range of species and
  island situations, z tends to be about 0.3
  (amphibians and reptiles of the west Indies,
  beetles in the West Indies, Ants in Melanesia,
  Vertebrates in Lake Michigan, and plants on the
  Galapagos.

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Amphibians and Reptiles of the Antilles.
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Flowering plants in England.
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Insects of British trees open dots are
introduced species.
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North American Birds
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Island Biogeography

• Actual parameter values depend on whether a true
  island is being considered, size of the island
  relative to number of possible colonists, and
  colonizing ability of species.
• The pattern also holds for non-traditional
  islands like mountain-tops in the Great Basin.

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Boreal birds and mammals in the Great Basin.
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Island Biogeography

• Can we use these ideas to build a model of island
  diversity?
• This work was done by MacArthur and Wilson back
  in the 1970s, and constitutes some of the most
  ground-breaking ecological work ever. Now of
  course, it seems intuitively obvious. I wonder
  why it was not obvious earlier?

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