Dispersal, metapopulations, and island biogeography

Question 1

Dispersal allows organisms to

Answer 1

Colonize new areas
* Escape competition
* Avoid inbreeding depression

Question 2

Dispersal is important for
colonization of new habitats

Answer 2

Postglacial colonization
depends on plant and
animal dispersal
* Most of Canada was under
ice ~12,000 years ago
* Range shifts in response
to climate change
* Islands
* Etc.

Question 3

Metapopulations: “populations of
populations”

Answer 3

Dispersal connects
populations
* A metapopulation is
a collection of
spatially distinct
populations that are
connected via
dispersal
* We call each
spatially distinct
population a patch

Question 4

Source-sink dynamics

Answer 4

‘Sinks’ are populations in small habitat
patches that would go extinct, except …
* Migrants from ‘source’ populations ‘rescue’
these populations

Question 5

On an oceanic island

Answer 5

1. some prey colonize empty island
2. Prey quickly grow toward carrying capacity
3. Predators drive prey to extinction
4. Some predators arrive and reproduce rapidly
5. Predators starve, island is empty

Question 6

Patch dynamics

Answer 6

Akin to population dynamics, except:
* Instead of individuals in a population, we track
patch occupancy through time
* Imagine the simple case of:
* A large number of identical patches
* Ignore population size within patches
* Populations within patches go extinct at some constant
rate e
* Colonization of patches is affected by:
* The fraction of currently occupied patches, ܲ
* Higher P = more ‘sources’ for colonizers
* The fraction of empty patches, (1 – ܲP)
* As patches fill up, there are few patches available to colonize
* Colonization rate is thus ܿ ܲ cP(1-P) where c is a
constant

Question 7

Say A always outcompetes B within a habitat patch, so
local coexistence is impossible
* Global coexistence requires:

Answer 7

A must sometimes go extinct in a patch OR new patches
must be created from time to time
* B must be a better disperser than A
* So B must be a “fugitive”, “tramp” ,”weedy”,
“opportunistic”, “transient” species
* A competition-colonization trade-off

Question 8

General conclusions on species
coexistence

Answer 8

Populations can be driven to extinction in several

Question 9

Stochasticity

Answer 9

chance fluctuations in population numbers
* Competitive exclusion
* Through predator-prey (or host-parasite, etc.)
interactions
* Allee effects at low density

Question 10

metacommunity

Answer 10

is a set of local
communities linked by the dispersal of one or
more of their constituent species

Question 11

What determines the number of
species on an island?

Answer 11

Colonization: a species can arrive on an island
from elsewhere
* Extinction: a species can go locally extinct on an
island
* In-situ speciation: a lineage can split in two on an
island, but this is a very slow process

Question 12

MacArthur and Wilson’s theory
of island biogeography

Answer 12

Goal: predict the number
of species on an island
from the island’s size and
isolation (distance from
mainland)
* Ignored in-situ speciation;
only considered
colonization and extinction