Metapopulations I.

Metapopulations I.

So far, we have looked at populations with the assumption of geographic closure:no immigration, no emigration

What if we relax that assumption?

What is a metapopulation?

“Set of local populations occupying anarray of habitat patches and linked bythe movements of individuals”

Movement: emigration and immigrationnot back-and-forth movementsof individuals

Example of metapopulation: spotted owls in southern CA

(Ricklefs and Miller 2000 p. 331)

Origin

The term was first used by RichardLevins in 1969 and 1970

Levins’ model looked at patch occupancy:colonization and extinction events

Also known as the “classical” model or“blinking light” model

What was the persistence time of themetapopulation overall?

Does the idea work?

Many species seem to have ametapopulation structure….

Butterflies

Pool Frogs

Furbish’s LousewortPikas

Bull Trout

Acorn Woodpeckers

Different kinds of patchy populations….

•Classic metapopulation (Levins)

•Mainland-island metapopulationmigrant flow from big patch to small

•Patchy population (too much migrationfor a metapopulation)

•Separate populations (no migration at all,or very little)

An illustration:

(from Harrison and Taylor 1997)

A closer look at the classical model

dP/dt = mP(1-P) – eP

P: number of occupied patches

e: extinction rate

m: colonization rate

The assumptions of the classicalmetapopulation model

•Growth, regulation, and other dynamicsof local populations ignored

•Colonization and extinction events areinstantaneous (no growth/decline phase)

•All patches equally likely to receive newimmigrants (“midfield assumption”)

•Patches are all the same size and quality

The classical model

Immigrants only matter when they colonize an empty patch

Is a patch occupied, or not?

How long will the metapopulation persist?

What if we add some realism?

Most of the time, the distance betweenpatches does matter

Size also matters- patch size and distancefrom another patch also interact

Isolated patches that are also large arerelatively more likely to be occupiedthan small isolated patches

Example of size/density interaction

Glanville fritillary butterfly

(Ricklefs and Miller 2000 p. 333)

Example of size/isolation interaction

Common shrew(sorex araneus)

(Ricklefs and Miller 2000, p. 335)

The Rescue Effect

Populations may be saved fromextinction by arrival of immigrants

Example: acorn woodpecker populationin New Mexico

Acorn Woodpeckers

Population persistencedepends on immigrantsfrom populations whose

dynamics vary independently

Source-sink dynamics

Some patches are inherently betterquality than others

These patches support growingpopulations that produce emigrants

Other patches are poorer quality,and populations can only bemaintained by immigration

Example: Eastern Kingbird

Metapopulation in Charlotte Valley, NY

Inhabited upland, floodplain, and riparianzone habitats

Demographic rates and movementsamong breeding populations werestudied for 10 years

M. Murphy. 2000. Cons. Biol. 15:737-748.

Example: Eastern Kingbirds

Survival was highestin floodplain, butproductivity waslowest there

Riparian subpopulation was stable, butother two were declining

(M. Murphy. 2001. Ecology 82:1304-1318)

Example: Eastern Kingbirds

Modeling suggestedthat metapopulationoverall is a sink.

If riparian survival Increased slightly, the

Entire metapopulation would be sustained.

(M. Murphy, 2001. Cons Biol. 15:737-748)

Example: Eastern Kingbirds

Murphy suggestedthat creek habitat actedas a “pseudosink”

A pseudosink is a patch with density-dependent effects on demographic ratesso it seems unsustainable; if densitydeclined, demographic rates would improve

Another Example

Demographic rates in a patch may varynot only with density, but with environmental conditions-

A patch may be a source one year,and a sink the next.

Burrowing owls at NAS Lemoore, CA

Burrowing owls (again)

Population Growth of Burrowing Owls

0

10

20

30

40

50

60

70

80

90

1997 1998 1999 2000

Year

Nu

mb

er o

f N

ests

0

0.3

0.6

0.9

1.2

1.5Number of Nests

Population growth rate

(Gervais 2002)