A Metapopulation Approach to Farmer Seed Systems

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A Metapopulation Approach to Farmer Seed Systems

• M. Eric Van Dusen
• Ciriacy-Wantrup Post-Doctoral Fellow
• UC Berkeley

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• Metapopulation
• a set of local populations which interact via
  individuals moving among populations -Hanski and
  Gilpin (1991)
• Farmer Seed System
• a set of farmers whose crop varieties are
  related through the exchange of seeds

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Seed Systems In Situ Conservation

• Move from targeting individual farmers to larger
  spatial scales of communities and regions
• Environmental heterogeneity limits the extent of
  genetic erosion
• landraces survive in niches
• In Situ conservation is dynamic, encompassing
  evolutionary processes

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Zoatecpan, Puebla infra-subsistence production,
small landholdings,
contiguous maize plots
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Seed Systems Biosafety

• Escape of transgenes in Mexico
• How did it get there?
• Possible Impacts and Containment
• Selection Pressures, Selection Practices
• Mixing with Local materials
• Biosafety for future releases
• How far does material travel?
• Document farmer practices

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Government maize supplies hundreds of tons per
week
Private Traders 40 tons of maize direct from
the US border at Laredo
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Seed Systems dissemination of improved
materials

• Green Revolution gains limited by low varietal
  turnover rates
• Farmer-to-Farmer exchange fundamental to
  dissemination in many areas
• Move towards participatory approaches, especially
  to reach marginal environments
• Robust approaches to disaster seed relief

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Participatory Breeding new emphasis on
techniques to integrate with local practices,
focus on local selection behavior, target
marginal conditions and marginal farmers
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Evolution of Meta-population theory
MacArthur and Wilson (1967) Mainland - Island
Bio-geographic Model
Levins (1969) Meta-Population Ecological Model
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Metapopulation characteristics

• Patchiness of the environment
• Heterogeneity of landscape creates ecological
  niches, where certain species dominate
• Local extinction possible
• As long as there is some degree of migration,
  local extinction in any given patch is possible
• Extinction Debt present but declining
• Genetic Rescue add enough variability to make
  patch viable
• Colonization of empty patches
• Distance and distribution of patches matters
• Successful establishment can depend on other
  factors

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Crop meta-populations

• Individual farmers manage local populations, and
  are linked through seed exchange and gene flow
• Seeds are adapted to local agro-ecological
  conditions (patchiness)
• Farmers experience loss of seed (local
  extinction), but this is mitigated through
  seed exchange (migration)
• Varieties may compete for the same land area for
  in situ conservation (habitat fragmentation)

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Biology matters

• Self pollinated - Wheat, Rice
• Exchange seed without loss of quality
• Seed remains relatively pure
• Geneflow less common
• Open Pollinated Maize
• adapt to local conditions
• high diversity within one seed lot
• Geneflow through pollen

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Case Study Mexican milpa system

• Survey Sample
• 280 HH
• 24 villages
• 2 ecological zones
• Social Economic module
• Seed System module

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Extinction parameters(i.e. my dissertation)

• Household-Farm model of activity choice
• Link diversity outcomes to economic forces
• Nest household, agro-ecological and market models
• Major versus Minor Crops
• Varieties blue and yellow maize
• Species intercropped beans and squash
• Land area, agro-ecological conditions drive maize
  diversity
• Household characteristics, market integration,
  labor intensity impact secondary crop diversity

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Migration parameters to derive from household
data

• Geneflow
• Pollen
• Seed sample size drift, inbreeding, mutations
• Turnover Rate
• Age of Seed Lots
• Loss, Change, Replacement
• Exchange
• Within community
• Within ecological region

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Geneflow - Pollen Drift

• Contamination decreases with distance
• Field size determines level of exposure to pollen
  drift

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Geneflow pollen drift
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Effective Population SizeSelection Behavior
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Effective Population SizeMinimum number of ears
selected
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Turnover Rate Age of Maize Seed Lots by type
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Source of seed by type
Crosstabulation Source versus Age
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Turnover Rate
Age and Origin of Bean Seed
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How old is a seed lot, really?
Q1- How long have you had the seed you are
currently planting? Q2 When is the last time
you renewed your seed?
Crosstab Age vs Renewal
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Econometrics

• Link seed age to socio-economic factors
• Tobit age of seed lots (censored at gt25)
• Nest household, farm, market conditions
• Other specifications on
• Logit who holds seed forever, who replaces
  frequently
• Duration Model Semiparametric specs

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Summary Statistics
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Tobit Regresion Age of Seed Lots
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Tobit RegresssionTotal Varieties Planted
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Directions for Future Research

• Build simulation model with empirical parameters
• Compare across crops and regions
• Build different scenarios for diffusion,
  conservation, genetic escape
• Incorporate genetic data

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Cases

• Contamination - Spread of Gene into local
  population
• Solve for Rate under a) selection b) no selection
• Drift Accumulation of Mutations -
• Solve for Effective Population size / Renewal
  Rate
• Spread of Improved Materials
• Solve for rate of adoption/ diffusion

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Three scales of analysisand parameters for model

• 1) Farmer and Field
• Contamination Rate
• Field Size 0.1-2 ha
• Inflow Rate 0.001 0.005
• Shape of Field Square, Rectangle
• Rate of Deleterious mutations 0.001 0.01?
• 2) Group of Farmers in Village
• Field Size 0.1-2 ha
• New Seed renewal Rate 1-2 Farmers/Village/Yr
• Seed Age Classes 0-5 yrs, 5-25 yrs, gt25 yrs
• Spatial Configuration lattice, hub-spoke,
  non-scaling
• 3) Group of Villages
• Rate of Exchange between villages 1-5
• Spatial Configuration lattice, hub-spoke,
  non-scaling

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How many populations can you see in this picture?