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Greenology:  Metapopulation Theory

 

January 29, 2008

 

This info is paraphrased from Corridor Ecology by Hilty, Lidicker, & Merenlender at Island Press 2006.

 

 

 

 

Figure 1 shows four types of metapopulations and their dispersal mechanisms from occupied to empty habitats.  Movement among habitats is depicted with arrows.  Dashed arrows depict rare migrations.  

 

 

A metapopulation is a group of populations (demes) connected by migrations of individuals across populations.  A good analogy for a metapopulation is a series of towns.  Towns contain their own populations and a group of towns represent a specific metapopulation, for instance the western suburbs.  The towns are connected and that helps facilitate movement by individuals.  As some towns grow into cities from immigration and favorable conditions, other towns decrease into villages due to emigration and unfavorable conditions. 

 

In an example from nature, a series of mountain valleys may contain a metapopulation of lilies.  Lilies may spread to new valleys through wind blown seed or flash floods.  Other valleys may lose all their lilies through herbivory, disease, stochastic events, etc.  If enough suitable habitat remains and colonization of new valleys continue, the combined populations (metapopulations) of lilies will be stable.

 

Metapopulation theory attempts to describe the movement of populations throughout suitable habitats. 

 

The four models for metapopulation movement are in the figure above were developed by Harrison in 1991.

Š      Patchy – Demic system with chunks of suitable habitat spaced relatively close together.  Individuals move freely to and from the patches, replenishing any areas that suffer extinction (rescue effect).  Patchy metapopulations are common and extinction resistant.

 

Š      Core-Satellite – Also called “mainland-island” or “source-sink” because a large stable habitat supports a large stable population.  Frequent migration occurs from the “mainland” to habitats nearby.  Core-satellite is also extinction resistant.

 

Š      Levins Classic – Populations disperse from one area to the next.  Habitats where populations go extinct may be re-colonized or not.  Changes in the metapopulation over time is represented by the formula:

dp/dt = mp(1 – p) – ep 

p = occupied habitats.  Full occupancy p=1.  Extinct p=0

m = migration rate

mp = total amount of successful dispersals

(1 – p) = unoccupied habitats

e = extinction rate

ep = total amount of patches going extinct

 

Š      Non-equilibrium – A small patch contains the majority of the population, because isolation or an unfavorable matrix prevents frequent migrations.  Non-equilibrium populations are extremely vulnerable to extinction. 

 

 

 

 

 

Fun Fact (at least as fun as it gets) – Rodents help in environmental science too.

 

Levins developed metapopulation theory in 1970 based on the work of Soviet ecologists concerned with rodent control during WWII.   Like the Soviets, Levins was studying epidemiology involving parasitic insects and their rodent hosts.  In Levin’s model case, p represented the hosts for the parasitic insect populations.

 

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