Metapopulations, fragmentation, corridors

Environment 121Conservation of BiodiversityVictoria Sork, 16 April 2009

I. MetapopulationsGeneral definition:

Population of populations in discrete patches linked by migration and extinction.

Examples of natural metapopulations• Islands (e.g. Caribbean Islands)• Habitat islands: areas which are not islands

– Lakes– Mountain tops (e.g. Sky Islands of Arizona)– Unusual geology (serpentine outcrops)– Desert oases

Examples of anthropogenic metapopulations– Remnant fragments after deforestation

Types of Metapopulations

Source: Harrison and Taylor 1997.

Elements of metapopulation dynamics1. Composed of discrete subpopulations2. Some degree of migration3. If migration is high, the set of

populations will be one large population4. Physical separation of patches5. Models are based on persistence and

existence of patches, not numbers of individuals within patches.

6. Regional or landscape scale

Dynamics of Sources and SinksDefinition: Metapopulation in which some patches have

negative population growth rate at low density and in the absence of immigration (sinks) and other patches have positive growth rate at low density (sources).

Patches vary in good and poor habitat qualitySource populations

– good habitats – population size is growing – emigrants colonize other sites

Sink populations – poor habitats– local populations are headed toward extinction– immigrants may constantly colonize site

California Gnatcatcher: a metapopulation case study

• Federally threatened subspecies inhabiting the coastal sage scrub community in southern California.

• The coastal sage scrub is a distinctive plant community that has declined due to extensive agricultural and urban development in this area.

• Analysis of the dynamics of the California Gnatcatcher in central and coastal Orange County, California.

• Used this habitat model as a basis of a metapopulation model, which included demographic data such as fecundity, survival, as well as variability in these demographic rates.

Example: Habitat suitability model• Patches are

connected• Legend indicates

suitability of habitats for Gnatcatchers

• Does the survival of the species rely on multiple patches?

Answer: They needed to preserve large patch and medium-sized patches OR species is at risk of extinction.

II. Habitat fragmentation• the loss and isolation of natural

habitats.• reduction of the total amount of habitat

type• apportionment of the remaining habitat

into smaller, more isolated patches.• Results in insularization • Often times creates a "shredded"

landscape

Fragmentation & Diversity

1. Initial exclusion 2. Isolation 3. Species-area

effects 4. Edge effects 5. Disruption of

natural processes

Biological consequences of fragmentation

 A. Initial exclusion

Important species may not be in preservesB. Barriers and isolation

1. scale of movement patterns2. Roads3. inbreeding and genetic drift4. landscape matrix can increase isolation

C. Crowding effects2. population sinks.

D. Local and regional extinctions1. Metapopulation dynamic

Example of fragmentation: Colombian oak forests

0 500 m

59

42 15

9 21

Source: Fernandez-M, J. F. and V. L. Sork. 2007. Genetic variation in fragmented forest stands of the Andean oak Quercus humboldtii Bonpl. (Fagaceae). Biotropica 39:72-78.

Map of remnant forest patches remaining from once continuous forest.

Question: Does fragmentation affect genetic diversity?

Fragment seedling populations had reduced genetic diversity—Why?

Impact of Fragmentation on Species DiversitySpecies Area

Relationship:• The number of species

in an area is a function of the area.

• Usually, for single taxa or guild

• Uses – predict expected species – Size of a sampling plot

A big consequence of fragmentation is loss of space for species

The Theory of Island Biogeography

Robert H. MacArthur & Edward O. Wilson

“One of Princeton University Press's Notable Centenary Titles”

Components– colonization rate – extinction rate

Prediction of equilibrium number of species based on:• island size (small vs. big) • isolation (near vs. far)

Vulnerable species to fragmentation 

1.rare species 2.species with large home range 3.species with limited dispersal 4.species with low reproductive potential 5.species with short life cycles 6.species depending on resources that are

unpredictable in time or space 7.ground nesting birds 8.species of habitats interiors 9.species exploited or persecuted by people

III. Corridors and Connectivity Connectivity: the ease

with which organisms and materials can travel between two points.

Benefits: – Gene flow– Colonization of new

patches– Habitats

Drawbacks:– Edge effects– Disease/predator/

parasite spread– Invasive species

http://www.unl.edu/nac/bufferguidelines/guidelines/2_biodiversity/5.html

Examples of Corridor ProjectsCentral America: www.pasopacifico.org/ home_area_map.htmlGreater Mekong Subregion: www

.adb.org/projects/ gms-biodiversity/

Conservation buffer guidelines• Source:

http://www.unl.edu/nac/bufferguidelines/index.html

• A partnership of USDA and US Forest Service