Transcript metapopulations

Ch. 12 Metapopulations
• Several local populations interacting
• Models: assume no immigration and emigration
• Many species show metapopulation structure
• Subpopulations differ in growth dynamics
• Local extinctions, re-establishments possible
Metapopulation examples
-Aquatic organisms in lakes
-Caribou occur in 32 herds based on
calving grounds, herds mix on
wintering grounds
4 conditions define a metapopulation (Hanski)
• 1.suitable habitat only found in discrete patches
• 2.all subpopulations have a risk of extinction
• 3.patches close enough to be recolonized
• 4. local populations growth dynamics not synchronized
• Others: presence of core populations and satellite poplns sufficient criteria
Edith checkerspot butterfly metapopulations and dynamics
Fig c. population changes of local populations in 3 areas
Fig d. population changes of Jasper Ridge data pooled
Two scales operate in metapopulations
• 1. local, within patch
– individuals move
– reproduce
– population changes and regulation
• 2. metapopulation, between patch
– interactions between patches determine dynamics
– dispersal from and colonization of patches
• If no dispersal, patches independent
• If lots of dispersal, patch boundaries disappear
• If intermediate, local extinctions & colonizations may balance
Extinction & recolonization rates drive metapopulation
dynamics
e = probability a patch will go extinct
m = probability a patch will be colonized
Equilibrium metapopulation size
P = 1-e/m
Skipper butterflies:
Occupied habitat patches most likely to be larger patches, closer to
other patches
Unoccupied habitat patches most likely to be smaller, more
distant from other patches
Habitat heterogeneity (variety) increases
local patch persistence
 patch size   local populations  population persistence
Larger patches  patch heterogeneity
If environment is variable,
If species is sensitive to changing environment,
If a patch has greater variety of habitats for it to use,
-expect less extinction in that patch
Kindvall studied effect of patch habitat variety on fluctuations in the population
sizes for bush crickets.
Bush cricket,
population variability
smaller when more
habitat heterogeneity
Population variability
coefficient of variation of
population sizes over time
Habitat heterogeneity
index of the vegetation types
(low grassland, high grassland,
sparse grassland)
Habitat heterogeneity
decreased population
variability and prob. of local
extinction
Some habitat patches “re-seed”
patches that go extinct
James Brown coined term “rescue effect”
Bay checkerspot butterfly
Needs serpentine soils for plants its
caterpillars eat
Morgan Hill patch is a large “source” patch,
smaller patches are “sinks”
Only smaller patches near Morgan Hill patch
are regularly colonized
Frequent extinctions and recolonizations
“Mainland-island” metapopulation structure
Current presence and size of a
patch population may be very
misleading….
The patch may be populated by
immigrants, may not be able to
sustain reproduction, then
becomes extinct.
If dynamics of local populations are
synchronized, then large-scale declines
(or increases) may occur
Bay checkerspot, regional drought in
1975-1977, caused synchronized
extinction of local populations (its food
plant declined)
Skipper butterfly in England
<1920, most chalklands (brown)
1960, reduced in range (green)
1982, reduced further (black)
Heavily grazed calcareous soils needed,
these disappeared in 1900s
Species characteristics influence rates of colonization and
extinction
• r-selected species - good reproduction, good dispersal, aiding colonization –
result of natural selection
• Flight is an adaptation for dispersal, flightlessness associated with increased
environmental homogeneity and habitat persistence
• Asexually reproducing plants usually have lower rates of extinction, but
dispersal may be reduced
Species characteristics influence rates of colonization and
extinction -2
• smaller animals support larger densities per unit area than larger animals
• smaller endothermic animals have higher metabolic costs, may be hard to
fuel if food supply declines
• Of 3 shrews studied on islands, the smallest species with highest metabolic
rate had highest extinction rate
Population - a hierarchical context
• 1. Local population, can reasonably determine birth, death, immigration,
emigration rates
• 2. Metapopulation, collection of local populations close enough to exchange
members and colonize empty patches
• 3. Subspecies, collections of metapopulations in a geographic area; enough
isolation for genetic variation to accumulate
• 4. Species itself, collection of all subspecies, the whole range of species
included
Iberian lynx
<1,000 survive
9 partly isolated populations
(genetically, spatially)
Habitat fragmentation created
metapopulations
Crucial for lynx survival is…?
Genetic exchange between
populations, what is needed
for this…?
Suitable habitat corridors