Disturbance and Reserve Design

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Disturbance and Reserve Design

• Neville Handel
• Fall 2005

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Pickett and Thompson (1978)

• Patch Dynamics and the Design of Nature Reserves
• Island biogeographic theory applied to reserve
  design
• On habitat islands, extinction gt immigration
• internal disturbance dynamics become the
  critical design feature of reserves

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Minimum Dynamic Area (MDA)

• the smallest area with a natural disturbance
  regime which maintains internal recolonisation
  sources and hence minimizes extinctions

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MDA depends upon patch dynamics

• Disturbance/patch size
• Frequency and longevity
• Mobility/dispersal/biology of taxa
• Community composition
• Climate and geomorphology

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Reserve design characteristics

• Much larger than largest disturbance patch size
• Contain recolonisation sources
• Contain mosaic of different age patches
• Sufficient area for large consumer population
  post-disturbance
• Contain separate MDAs of each habitat type

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Mcarthy and Lindenmayer (1998)

• Incorporating Metapopulation Dynamics of Greater
  Gliders Into Reserve Design in Disturbed
  Landscapes

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• P. volans
• Folivorous arborial marsupial
• Habitat specialist of old-growth mountain ash (E.
  regnans) trees in eastern Australia needs old
  hollow trees for nesting and foliage for feeding
• Small home range and limited dispersal
  territorial

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The Ada Forest Block

• 6700 ha in the central highlands of Victoria
• Previous studies on P. volans conducted there
• Contains multiple patches (160 ha total) of old
  growth E. regnans and various stages of re-growth
  (gt5500 ha) resulting from fires and logging
• Multiple use forest (logging and conservation)
• images taken from McCarthy and Lindenmayer
  1997

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Metapopulation model characteristics

• Single vs. multipatch scenarios
• Spatial correlation of fire regime and patches
  (based on 1983 fire)
• Patch heterogeneity/homogeneity
• Habitat quality a function of time since and age
  at last fire
• Only females simulated
• Mean dispersal distance of 2 km
• Demographic and environmental stochasticity
• Variation in reserve size/location
• Mortality proportional to disturbance size
• Sensitivity analysis (fire interval, spatial
  correlation, fecundity/survivorship/ carrying
  capacity, dispersal) to determine optimum reserve
  configuration

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Sensitivity Analysis
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What did they find? single patch

• As patch size increases
• Risk of extinction decreases
• Time to extinction increases
• Heterogeneity lowers risk of extinction

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What did they find? Area (Ada block)

• As area increases, risk of extinction decreases
• Decrease in extinction risk depends on where land
  is added to the reserve
• The addition of independent areas of old growth
  (replication of reserves) decreases extinction
  risk more than adding re-growth to existing old
  growth

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What did they find? Area and Time

• Again, more area (total) less likely extinction
• Single large reserve results in lower risk of
  extinction at small (lt30 ha) sizes and long time
  spans
• Over time, larger reserves are more likely to
  maintain populations

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• In reality, the risk of extinction is likely
  greater because of demographic stochasticity

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Heterogeneity and Correlation Matter

• Heterogeneity decreases the likelihood of
  extinction in large patches
• High quality habitat patches continuous with
  heterogeneity
• Increased spatial correlation increases
  extinction risk

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Conclusions

• At current reserve and old growth status,
  extinction risk in the Ada forest block over 300
  year period 44
• Logging and burning will decrease amount of old
  growth present, increasing the risk of extinction
• Risk of extinction decreases as re-growth forest
  is added to old growth, especially the larger
  patches addition of 600 ha of re-growth will
  reduce extinction to lt10
• SLOSS??? Depends on spatial and temporal scale,
  but one large old growth reserve maximizes time
  to extinction in this case
• Sensitivity analysis changes risk of extinction,
  but did not greatly change optimum reserve design

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Generalizations?

• Reserve design depends greatly on target species
• SLOSS??? It depends (still)
• Multiple replications of habitat are better than
  single reserves in lowering risk of extinction
• Disturbance is natural (and sometimes not)
  reserves must be designed accordingly and provide
  MDA
• Reserves must be self-sufficient in the face of
  intensifying land use and decreased outside
  sources of propagules
• Realistic models can help improve reserve design