Prediction of richness local vs regional curves to assess community saturation David Luther Biology

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Prediction of richness (local vs regional curves
to assess community saturation?)David
LutherBiology 255Oct. 7, 2003
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Why do we care?

• Resolving relative contributions of local vs
  regional processes may be the key to
  understanding global patterns of species
  diversity Huston 1999
• Can one comprehend the ruin of natural systems
  without understanding how they are built?.
  Ricklefs 1987

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Background

• 1st half of century regional factors were favored
  as explanations of local diversity.
• 2nd half of century local factors were favored as
  explanations of local diversity.
• Key reason local and regional diversity were
  divorced is the idea of limiting similarity.
  Ricklefs 1987

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Local-Regional Richness Plots

• If linear then the local richness is independent
  of biotic interactions and increases
  proportionally with the regional richness (type I
  community)
• If curvilinear then the local community is
  saturated and the local richness is independent
  of regional richness (type II community).

Cornell Lawton 1992
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Local Richness

• An area small enough that all species could
  encounter each other within ecological time and
  possibly interact.
• Ex. All fish species in a lake or all grass
  species in a meadow. Srivastava 1999
• Limitations are ecological that result in species
  being actively excluded from communities by local
  effects such as an upper limit of niche packing.
  There is little or no regional effect.

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How do we detect effects of local processes?
Huston 1999

• Organisms must be potential competitors, ducks
  and warblers dont interact, need similar
  functional types.
• Size of local area needs to be small enough for
  local species to interact competitively.
• Conditions suitable for competitive equilibrium
  to be achieved.

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Regional Richness

• An area that comprises all species that could
  eventually colonize a location if no competitive
  exclusion occurred.
• Ex. All fish species in Britain, all grass
  species in the Serengeti. Srivastava 1999
• Limitations are evolutionary and involve regional
  controls on local community richness such as rate
  of speciation and fixation rate of mutations.

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4 methods of testing for saturation Srivastava
1999

• REGIONAL POOL SUMS OVER HABITATS
• ANALAGOUS HABITATS IN THE SAME GEOGRAPHIC AREA
• IDENTICAL HABITATS IN GEOGRPAPHICALLY DIFFERENT
  REGIONS
• 4 COMPARISONS IN A GIVEN REGION OF A PARTICULAR
  COMMUNITY THROUGH EVOLUTIONARY TIME

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Effect of ScaleKarlson and Cornell 2002
Local
EXPECT
L
R

• Relationship is more likely to be curvilinear as
  regional influences get lost.
• Because the community is saturated or because
  species coexistence is precluded by spatial
  constraints?

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Effect of Scale cont.
EXPECT
L
R

• Relationship is more likely to be linear.
• Because it is regionally enriched or because
  there are multiple saturated habitats?

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PsuedosaturationDefinition- detection of
curvilinearity when the true relation is linear

• Reasons
• Underestimate local richness
• Overestimate regional richness but not local
  richness

Griffiths 1997
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Psuedoreplication

• Definition- when replicates used in analysis are
  not truly independent because they are spatially
  or temporally correlated. Srivastava 1999
• Confuses the differences between regions with
  differences between regional species pools

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Ricklefs 2000

• This paper addresses the problems of estimating
  regional species pools and establishing the
  independence of these pools. He also comments on
  L-R relationships as they relate to saturation.
• He used data on avifauna of the West Indies L is
  the average of species observed in each of 9
  habitats on one island R(obs) is the of species
  observed in all habitats together R(total) of
  species known to occur In the region (island or
  continental area) observed or not.

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Ricklefs 2000 cont.

• L-R plots are not enough info on there own to
  determine ecological processes.
• Interactions between species constrain
  membership in local assemblages but do not place
  an upper limit on their size.
• Ecologists should be less concerned about
  whether populations interact within local
  assemblages than with the mechanisms responsible
  for
• Generating the regional pool of species
• Distribution of those species over habitats
  within the region
• Evolutionary adjustments that accompany invasion
  of a local assemblage

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Levine 2000

• This paper addresses the classic hypothesis that
  species diversity enhances community resistance
  to invasion.
• - Support from theoretical models lab
  experiments
• - Yet natural studies show that the most diverse
  communities are most likely to be invaded
• To address this contradiction he couples
  patterns of diversity and invasion with in situ
  manipulations of diversity in a natural context,
  tussocks forming sedges in the South Fork Eel
  River, CA. Each tussock forms a discrete micro
  island.

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Levine 2000 cont.

• METHODS
• 1st surveyed similar sized tussocks along 7km of
  river, all 3 exotic species were significant
  positively related to species richness
• possibly not just due to richness but endemic
  exotic both respond to environmental conditions
• 2nd He tested the effect of diversity on
  invasions, he invaded tussocks and manipulated
  the of residential species. To do this he
  removed species from 65 equal size tussocks
  assigned 5 species richness treatments and
  covered about 90 of each tussocks with the
  natives
• 3rd he added 200 seeds of each exotic species per
  tussock

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Levine 2000 cont.

• RESULTS
• As species richness increased the proportion of
  propagule survival went down in 2 out of 3
  exotics per island the size of the largest
  invader declined thus showing an effect of
  diversity to enhance resistance to biological
  invasion.
• Results were similar after 3 weeks and 5 months
  suggesting the effect of diversity at
  germination/seedling stage.

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Closing questions issues and thoughts

• Ecological plasticity can fill all niches with
  out saturation Griffiths 1997
• Saturation is a physical limitation regardless of
  species interactions Loureau 2000
• Species interaction only limit the community from
  a given species pool, a novel invader could still
  invade and do just fine even if saturation
  occurred. Loureau 2000
• Pick systems that are undisturbed or else local
  species interactions could not be in equilibrium.
  Huston 1999
• Taxa evolve at different rates so some taxa may
  be saturated while others are not, the study
  taxa could give different results.
• Weve had a lot of extinctions in the last 10,000
  years how does this effect community equilibrium
  and saturation?
• Why isnt local richness simply a product of
  resource availability?