Spatial Turnover and Beta Diversity: What causes species turnover and why is this important from a c

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Spatial Turnover and Beta DiversityWhat causes
species turnover and why is this important from a
conservation standpoint?

• Amanda Senft
• Bio 255

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Beta Diversity (species turnover through space)

• Previously, we discussed how to measure beta
  diversity. but what processes are we measuring?
  In other words, what causes beta diversity?
• When we conserve areas of high beta diversity
  (biogeographic crossroads), are we conserving
• Areas of high species turnover?
• OR
• The processes causing this high species turnover?

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Whittaker (1960)
NON - GRADIENT
GRADIENT
ß?/a
Half changes Similarity v. distance
Harrison Wilson Schmida Cowling Willig
Sandlin Blackburn Gaston Oliver Young Pharo..
Cowling Rey Benayas Porembski Harrison Nekola
White Cody Syuzo Givnish..
Slide stolen from Meghan with an H
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What causes beta diversity?
From Duivenvoorden et al 2002
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Dispersal limitation
I seeds per area P probability that a seed
predator wont get the seed PRCpopulation
recruitment curve distance from adult that its
offspring is likely to appear
From Janzen 1970
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Climatic and Edaphic factors?
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How did Condit et al consider beta diversity?

• Similarity in species composition as a function
  of distance.
• All pairs of plots considered together.
• Spatial heterogenity (e.g. ecotones) not
  considered in null model
• Tested Dispersal Limitation as a major driving
  force

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How did Spector consider beta diversity?

• Caused by environmental factors geological,
  climatic, geometric features
• Preserved by evolutionary factors
• Ecotones are evolutionarily active zonessee
  Enserink 1992
• and by underlying processes
• Fire or another ecological gradient
• Butconsidered only a single scale

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Todays papers
Condit, R., Pitman, N., Leigh, E. G., Chave,
J., Terborgh, J., Foster, R. B., Nunez, P.,
Aguilar, S., Valencia, R., Villa, G.,
Muller-Landau, H. C., Losos, E., and S.P Hubbell
2002. Beta-Diversity in Tropical Forest Trees.
Science 295(5555) 666-669. Spector, S. 2002.
Biogeographic Crossroads as Priority Areas for
Biodiversity Conservation. Conservation Biology.
16(6) 1480-1487.
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How do you measure ecological distance between 2
sites?
A
B
Distance metrics Similarity between
plots Sorenson 2w / AB Shared abundance
over total abundance Jaccard w / AB-w
Proportion of combined abundance not shared Other
distance measures they could have used
Euclidean, Correlation distance, Mahalenobis,
Chi-square Probability metric using null
model Probability FSfi1 f j2 The sum of the
abundance of a spp at site 1 times abundance at
site 2 for all species. Probability that two
trees chosen randomly from 2 plots are of the
same species. Takes into account relative
abundance of species.
w
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Condit et al. Figure 1
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• In Panama, plots within 1 km very similar, but
  very different from plots 50 km away
• In Peru and Ecuador, plots within 1 km nearly as
  similar as plots 50 km away

• Panama Species turnover due to environmental
  factors climate, geology, soil
• Peru and Ecuador Species turnover due to
  dispersal limitation

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Null model based on a Bessel function of decay

• Null hypothesis Only dispersal and speciation
  affect spp distributions.
• Used Hubbells null model all spp identical in
  death, reproduction, dispersal,
  speciationextinction, etc.
• Probability that 2 individuals r kilometers apart
  are the same species is a function of distance,
  speciation, dispersal distance and tree density.

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Condit et al Figure 2

• Graphs showing experimental (points) vs.
  theoretical decay of species similarity with
  distance
• - Variance is higher for Panama
• Decay in similarity from 0-100 m not well
  simulated by model

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Condit et al Figure 2

• Graphs showing experimental (points) vs.
  theoretical decay of species similarity with
  distance
• - Variance is higher for Panama
• Decay in similarity from 0-100 m not well
  simulated by model

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Condit et al Figure 2

• Graphs showing experimental (points) vs.
  theoretical decay of species similarity with
  distance
• - Variance is higher for Panama
• Decay in similarity from 0-100 m not well
  simulated by model

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Conclusions from Condit paper

• Species more aggregated than dispersal theory
  predicts
• Abundant species not well modeled with dispersal
  theory (theyre just everywhere, regardless of
  distance)
• Null model of dispersal works well for distances
  .2-50 km, but other factors influence beta
  diversity at different scales

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Duivenvoorden et al Variance partitioning

• Using Condits data, found that (linear)
  distance, elevation, precipitation, stand age and
  bedrock type were all significant predictors of
  species similarity.
• Still, distance and environment together
  explained left 59 of floristic variation
  unexplained
• A re-reanalysis using presence-absence data
  instead of abundances, and log distances, was
  able to leave only 41 of data unexplained
  (Ruoklainen and Tuomisto, 2002).

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Spector paper

• We should conserve biogeographic crossroads
  where there is high species turnover
• Areas meet goals of representativeness and
  complementarity
• Conserve evolutionary processes speciation and
  coevolution
• Congruence assumed to be high
• Environmental factors act on most taxa to create
  zones of high turnover

Scarab beetle
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How have others considered biogeographic
crossroads?

• Clements 1905 Tension zones where principle
  species from adjacent communities meet
• Delcourts (numerous) shifting ecoclines

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Sky Islands (stacked biotic communities)
Marshall 1957 cited by Warshall 2003
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Spectors Caveats

• Ecotones harbor species at the edges of their
  ranges, which are often more fragmented, more
  extinction prone (is this true or not?)
• Park size should equal the size of the ecological
  gradient and processes maintaining the ecotone.
  How does one quantify this?

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Discussion questions

• Do biogeographic crossroads exist? At what
  scale?
• Given that ecotones often mark the edge of a
  given species range, should we mark areas of
  high species turnover for conservation? Consider
  also that ecotones are predicted to be highly
  sensitive to climate change (Turner, Gardner, and
  ONeill 1991).
• What caused the differences in diversity patterns
  between Panama and the two South American sites?
  
• Which variables predicted beta diversity in the
  Tropics? How transferable are those results?

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Are these biogeographic crossroads?