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Ecology (BIOL 250 )
Lecture 21 : Island Biogeography and Metapopulations
Announcements Logistic growth reconsidered Island Biogeography The Metapopulation Colonization and Extinction Patch size and isolation Habitat heterogeneity The rescue effect
!What may make population variability more
likely?
" Environmental stochasticity
" Small populations… but why?
!Less genetic diversity !Closer to “minimum” population where alee effect becomes important.
Logistic Population Growth (reconsidered)
!Island Biogeography
"The number of different species found on
different islands depend on the size of the
island.
Island Biogeography
! Islands are similar to isolated patches on the landscape. " Ecologists now apply this concept to terrestrial habitats. ! Refined Definition: " The number of species established on an island represents a dynamic equilibrium between the immigration of new colonizing species and the extinction of previously established ones.
Island Biogeography
! MacArthur and Wilson : Model explaining
patterns of species diversity on islands as
result of immigration and extinction rates.
"Reasoned rates of immigration would be highest on
new island with no organisms.
!As species began to accumulate, rate of immigration would decline since fewer arrivals would be new species.
Island Biogeography
! Predicted rate of extinction would rise with
increasing number of species on an island for three
reasons:
" Presence of more species creates a larger pool of potential extinctions. " As number of species increases, population size of each must diminish. " As number of species increases, potential for competitive interactions between species will increase.
Island Biogeography
!Point where two lines cross predicts the
number of species that will occur on an island.
!Proposed rates of extinction on islands would
be determined mainly by island size.
"LG near islands will support highest number.
"SM far islands will support lowest number.
"SM near and LG far will support
intermediate number.
Island Biogeography
! Hanski et. al. found butterfly density
significantly affected by size and isolation of
habitat patches.
"Population size within patch increased with patch
area.
"Population density decreased as patch area increased.
"Isolated patches had lower butterfly densities.
!Population partially maintained by immigration.
Island Biogeography
Butterfly Populations
!Ecologists have proposed landscape structure
can influence movement of organisms between
potentially suitable habitats.
"Metapopulations: Populations of many species occur
in spatially isolated patches, with significant
exchange of individuals.
!Rate of movement of individuals between subpopulations can affect species persistence in a landscape.
Metapopulations
! Four traits of metapopulations
- The suitable habitat occurs in discrete patches that may be occupied by local breeding populations.
- Even the largest populations have a significant risk of extinction.
- Habitat patches must not be too isolated to prevent recolonization after local extinction.
- The dynamics of local populations are not synchronized.
Metapopulations
General definition: Metapopulation : population divided into discrete subpopulations linked by movement of individuals. Subpopulation : because the landscape is heterogeneous, individuals can form smaller groups, with much less interaction between groups than if the habitat were homogeneous.
Metapopulations Metapopulation - checkerspot butterfly
Patch Area and Isolation
Skipper butterfly ( Hesperia comma) !Primarily found in large patches
"Habitat variability may be important in a changing
environment.
! Why might this be critical?
"e.g., An extremely dry or wet year may mean that
the ideal habitat of an organism changes.
"Bush crickets prefer grasslands during normal years
but prefer sandier areas in wet years.
!Sandy areas not always available in smaller patches
Habitat heterogeneity
Habitat heterogeneity
CV=St.dev/mean
! Rescue effect
" An increase in population size and a decrease in the risk of extinction brought about by an increase of immigration into a population.
! Mainland-island metapopulation structure
" Single habitat is the dominant source of immigrants. " Many non-island populations have a similar structure.
! Metapopulations similar to the mainland-island
idea but emphasizes quality rather than size of
habitat
The Rescue Effect
! Checkered white butterfly
"Source population lives in unique
habitat on vegetation that grows on
dredge tailings.
"this area is the only region of its
range where it can overwinter
successfully
Source and Sink habitats Metapopulation - a review
