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 an array of habitat patches and linked by the movements of individuals” Movement: emigration and immigration not back-and-forth movements of individuals
Example of metapopulation: spotted owls in southern CA (Ricklefs and Miller 2000 p. 331)
Origin The term was first used by Richard Levins 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 the metapopulation overall?
Does the idea work? Many species seem to have a metapopulation structure…. Bull Trout Butterflies Pool Frogs Acorn Woodpeckers Pikas Furbish’s Lousewort
Different kinds of patchy populations…. • Classic metapopulation (Levins) • Mainland-island metapopulation • migrant flow from big patch to small • Patchy population (too much migration • for 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 dynamics • of local populations ignored • Colonization and extinction events are • instantaneous (no growth/decline phase) • All patches equally likely to receive new • immigrants (“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 between patches does matter Size also matters- patch size and distance from another patch also interact Isolated patches that are also large are relatively more likely to be occupied than 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 from extinction by arrival of immigrants Example: acorn woodpecker population in New Mexico
Acorn Woodpeckers Population persistence depends on immigrants from populations whose dynamics vary independently
Source-sink dynamics Some patches are inherently better quality than others These patches support growing populations that produce emigrants Other patches are poorer quality, and populations can only be maintained by immigration
Example: Eastern Kingbird Metapopulation in Charlotte Valley, NY Inhabited upland, floodplain, and riparian zone habitats Demographic rates and movements among breeding populations were studied for 10 years M. Murphy. 2000. Cons. Biol. 15:737-748.
Example: Eastern Kingbirds Survival was highest in floodplain, but productivity was lowest there Riparian subpopulation was stable, but other two were declining (M. Murphy. 2001. Ecology 82:1304-1318)
If riparian survival Increased slightly, the Entire metapopulation would be sustained. Example: Eastern Kingbirds Modeling suggested that metapopulation overall is a sink. (M. Murphy, 2001. Cons Biol. 15:737-748)
Example: Eastern Kingbirds Murphy suggested that creek habitat acted as a “pseudosink” A pseudosink is a patch with density- dependent effects on demographic rates so it seems unsustainable; if density declined, demographic rates would improve
Another Example Demographic rates in a patch may vary not 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) (Gervais 2002)