Disturbance and Reserve Disturbance and Reserve DesignDesignNeville HandelNeville Handel

Fall 2005Fall 2005

Pickett and Thompson (1978)Pickett and Thompson (1978)

• Patch Dynamics and the Design of Nature Reserves

• Island biogeographic theory applied to reserve design• On habitat “islands”, extinction > immigration • “internal disturbance dynamics become the critical design feature of

reserves”

Minimum Dynamic Area (MDA)Minimum Dynamic Area (MDA)

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

MDA depends upon patch dynamicsMDA depends upon patch dynamics::

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

Reserve design characteristics: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 MDA’s of each habitat type

Mcarthy and Lindenmayer (1998)Mcarthy and Lindenmayer (1998)

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

• 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

The Ada Forest Block: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 (>5500 ha) resulting from fires and logging

• Multiple use forest (logging and conservation)

*** images taken from McCarthy and Lindenmayer 1997

Metapopulation model characteristicsMetapopulation 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

Sensitivity AnalysisSensitivity Analysis

What did they find? : single patchWhat did they find? : single patch

• As patch size increases:

• Risk of extinction decreases

• Time to extinction increases

• Heterogeneity lowers risk of extinction

What did they find? Area (Ada block)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

What did they find? Area and TimeWhat did they find? Area and Time

Again, more area (total) = less likely extinction

Single large reserve results in lower risk of extinction at small (<30 ha) sizes and long time spans

Over time, larger reserves are more likely to maintain populations

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

Heterogeneity and Correlation MatterHeterogeneity 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

ConclusionsConclusions

• 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 <10%

• 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

Generalizations?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