Spatial Ecology: Metapopulations

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Spatial Ecology Metapopulations

• Peter B. McEvoy
• Oregon State University

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Outline

• History of metapopulation theory in the past 50
  years
• Basic model persistence via a balance in
  colonization and extinction rates
• Case study of the Glanville fritillary butterfly
• Interaction web of the Glanville fritillary
• Metapopulation structure and dynamics
• Influence of area and isolation
• Influence of parasitoids
• Abiotic forcing by rainfall
• Principal messages

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The past fifty years(Hanski 1999 Metapopulation
Ecology)

• Local populations connected by migration
  colonization and extinction (Andrewartha and
  Birch 1954) local demes (Sewall Wright 1940)
• Bottle Experiments with blowflies (Nicholson
  1933) and mites (Huffaker (1958)
• Island biogeography the balance between
  immigration and extinction (MacArthur and Wilson
  1967)
• Evolution of dispersal in a metapopulation
  (Gadgil 1971) and spreading the risk (den Boer
  1968)
• Metapopulation persistence reflecting a balance
  between extinction and recolonization (Levins
  19969, 1970)
• Theoretical development takes off beginning in
  1990 (Hanski 1999)

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Patch occupancy and population dynamics covary in
a mite predator-prey system a bottle
experiment
Prey
Predator
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Application of the Theory of Island Biogeography
Distance Effect
Area Effect
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Experimental Island Biogeography distance effect
Equilibrium number of species higher on islands
closer to mainland source
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Model of Levins (1969, 1970)

• The rate of change in the fraction of occupied
  habitat (patches, p)
• dp/dt mp(1-p) µp
• where µ is the rate of local extinction of
  patches and m is the rate of recolonization of
  empty patches
• Structurally identical to the logistic (Hanski
  1994)
• dp/dt (m - µ)p 1- p/1- (u/m)
• The total metapopulation will reach a stable
  equilibrium with a fraction p 1- u/m patches
  occupied

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Alternative Stable Equilibriabiomodal
distribution of patch occupancy p

• Heterogeneity among habitat patches may give rise
  to a bimodal equilibrium distribution of the
  fraction of patches occupied in an assemblage of
  species (the core-satellite distribution).

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Central Result

• It is possible to have instability on local
  scales Even if none of the local populations is
  stable in its own right
• Yet have stability on more global scales a
  metapopulation can stably persist as a result of
  a balance between random extinctions and
  recolonizations
• Local extinction and colonization play organizing
  roles. Persistence in a metapopulation depends
  less on local rates of birth and death,
  immigration or emigration and more on the rates
  of extinction and colonization
• Adding Relevant Detail. Levins model does not
  take account of variation in potentially
  influential variables/processes such as size of
  patches, their spatial locations, nor the
  dynamics of populations within individual patches

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Persistence in Glanville fritillary butterfly is
lower for populations than metapopulations
Metapopulations
Populations
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A problem for empiricists When is a patchy
population a metapopulation?

• Subpopulations
• Frequent extinction and recolonization
• Sufficiently high colonization rate
• Sufficient asynchrony in local dynamics
• Concept applies easily to insects, but plants
  pose problems does recolonization following
  extinction apply to plants with a seed bank? To
  succession following disturbance? (Husband
  Barrett 1996 Bullock et al. 2002)
• In general, the processes influencing the
  dynamics of ecological systems are disturbance,
  colonization, and local interactions (e.g.
  competition, predation, mutualism) that set in
  motion a successional process. Spatial ecology is
  not tied to any particular model (e.g. a
  metapopulation is a special case of more general
  theory of spatio-temporal dynamics)

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Glanville Fritillary and Host Plants
Ilkka Hanski
Veronica spicata
Plantago lanceolata

• Melitaea cinxia
• Glanville fritillary (UK)

http//www.helsinki.fi/science/metapop/english/cin
xias.htm
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Interaction Web for Glanville Fritillary
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Primary parasitoids of the Glanville fritillary
A gregarious endoparasitoid, laying one to about
40 eggs inside a host larva, depending on the
size of the host. Multivoltine. Limited
dispersal ability, less vagile than host.
A solitary endoparasitoid, laying eggs in first
instar host larvae just before the larvae hatch
from the egg. Univoltine. High dispersal ability.

Hyposoter horticola  (Gravenhorst) (Ichneumonidae
Campoplaginae)
Cotesia melitaearum  (Wilkinson)
(IchneumonoidaeBraconidae)

• http//www.helsinki.fi/science/metapop/english/Spe
  cies/Hypsoter.htm

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Influences on local population size

• Competition for resources quantity and quality
  can be limiting on local scales drought reduces
  host plant quality
• Natural enemies
• Predators
• Parasitoids
• Hyposoter horticola (Ichneumonidae) solitary,
  univoltine, large dispersal range
• Cotesia melitaearum (Braconidae) gregarious,
  2-3 generations per host generation, shorter
  dispersal range than butterfly host
• Interactions and movements of adults males and
  females differ in emigration rates Allee
  effects migration distances generallylt500 m
  4000 habitat patches divide into tens of
  networks
• Abiotic conditions temperature influences
  development larvae exploit thermal heterogeneity
  in the environment wind reduces fight
  precipitation affects mortality
• Habitat Loss and Alteration natural and
  anthropogenic (e.g. grazing) disturbances

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Summary of DD process known to influence local
dynamics

• Food shortages at local scales
• Parasitism by primary parasitoid Cotesia
  melitaearum only significant in regions and years
  with large-well connected host populations
• Inversely DD emigration and immigration and
  difficulty of finding mates in low-density
  populations contribute to an Allee effect on
  local dynamics
• Inbreeding depression in small populations
  increases extinction risk

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Glanville Fritillary Map of a Metapopulation

• Occupied (filled) and empty (open) habitat
  patches suitable for the Glanville fritillary in
  the Åland Islands off west coast of Finland in
  the autumn 2005.

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A related butterfly with a different
metapopulation structureBay checkerspot
butterflies

• Euphydryas editha bayensis butterflies occur in
  discrete patches metapopulation south of San
  Francisco on serpentine grassland
• Patches occupied in 1987 indicated by arrows
• Occupancy of suitable environments changes from
  year to year
• Some populations are sources and others are
  sinks

Wilson 1992
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Ups (?) and Downs (?) in metapopulation size over
time in the Glanville fritillary
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Effect of Connectivity on Colonization
Related to Distance to nearest population)
Proportion of colonized patches
No. Empty
No. Colonized
Related to Connectivity S
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Effect of Patch Area Larger patches increase
population size and occupancy of M. cinxia
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Influences on colonization

• Connectivity increases with Increasing number,
  increasing sizes, and decreasing distance to
  local populations that occur within the migration
  distance from the focal habitat patch
• Apart from connectivity increasing propagule (
  found population) size, increasing size of
  habitat patch, increasing host plant quantity and
  quality, reduced grazing, increasing abundance of
  nectar plantsall increase the rate of successful
  colonization

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Influence of Abiotic Conditions Increasing July
precipitation yields large changes in Population
Size N and Occupancy O
Temperature
Precipitation
Change N and O
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Abiotic Forcing of Metapopulation
DynamicsPrecipitation in Space and TimeWeather
events can be spatially correlated
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Classic Metapopulation Structure and Dynamics
well represented by Glanville Fritillary

• Suitable habitat occurs in small discrete patches
• Local populations have a high risk of extinction
• Patches are not too isolated to prevent
  recolonization
• Local and regional dynamics are asynchronous
  enough to make simultaneous extinction of all
  local populations unlikely

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Is there a balance between Colonization and
Extinction?Plotting Data in Table 4.2 Nieminen,
Siljander, Hanski 2004
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Principal Messages of Metapopulation Ecology

• Population size is affected by migration
• Population density is affected by patch area and
  isolation
• Asynchronous local dynamics
• Population turnover, local extinctions and
  establishment of new populations
• Presence of empty habitat
• Metapopulations persist despite population
  turnover
• Extinction risk depends on patch area
• Spatially realistic models can be used to make
  predictions about metapopulation dynamics in
  particular fragmented landscapes (e.g. Glanville
  fritillary)
• Metapopulation coexistence of competitors (e.g.
  inferior competitor is superior colonizer)
• Metapopulation coexistence of predator and prey
  (e.g. two-spotted spider mite Tetranychus urticae
  and predatory mite Phytoseiulus persimilis in
  greenhouses Nachman 1988, 1991)