Lecture 22 Flashcards
Dispersal
movement from one population to another
dispersal allows organisms to:
- colonise new areas
- escape competition
- avoid inbreeding depression
in animals, dispersal relies on
active movement - running, flying, etc
how are plants able to disperse?
they have evolved traits that aid dispersal:
- sweet, fleshy fruit is an adaptation that attracts animal seed disperses
- other seeds are dispersed by wind or water
Describe how dispersal is important for colonisation of new habitats
- postglacial colonisation depends on plant and animal dispersal
- most of Canada was under ice ~12,000 years ago
- range shifts in response to climate change depend on dispersal
- islands
Metapopulation
a population of populations - a collection of specially distinct populations that are connected via dispersal
how is dispersal involved in the formation of metapopulations?
- dispersal connects populations
- We call each spatially distinct population a patch
describe how metapopulation structure can allow population persistence even when individual populations are doomed
- local populations can be reestablished by colonists from other populations after going extinct
source-sink dynamics
Source-sink dynamics:
* ‘Sinks’ are populations in small habitat
patches that would go extinct, except …
* Migrants from ‘source’ populations ‘rescue’
these populations
Oceanic Island - single island level
- some prey colonize empty island
- prey quickly grow toward carrying capacity
- some predators arrive and reproduce rapidly
- predators drive prey to extinction
- predators starve, island is empty
single island system
At the island level, this
system is inherently
unstable: both species
go extinct
archipelago of many such islands, each at a different stage, with some dispersal possible
- a group of weakly coupled, locally unstable
systems can be globally stable - The coupling is provided by occasional dispersal between islands
patch dynamics
- akin to population dynamics, except:
- instead of individuals in a population, we track patch occupancy through time
colonisation of patches is affected by:
- the fraction of currently occupied patches, P
- higher P = more sources for colonisers
- the fraction of empty patches, 1-P
- as patches fill up, there are few patches available to colonise
give a simple case for patch dynamics
- a large number of identical patches
- ignore population size within patches
- populations within patches go extinct at some constant rate e
- colonisation of patches is affected by P and 1-P
- colonisation rate is thus cP(1-P), where c is a constant
Levin’s patch occupancy model
differential equations
equilibrium patch occupanc7
when overall colonisation rate and overall extinction rate intersect
what does meta population structure facilitate?
species persistence and coexistence
- of a single species (eg tiger salamanders)
- of predators and prey
- of competitors
describe how meta population structure facilitates coexistence of competitors
- say A always outcompetes B within a habitat patch, so local coexistence is impossible
- global coexistence requires:
- 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-colonisation trade-off
Pikas as an example of meta populations
- Bodie, California
- mining ghost town
- Andy smith’s pika research site
- 1972 - present
- tailing piles from hard-rock mining create many small replicated patches of pika habitat
general conclusions on species coexistence
- populations can be driven to extinction in several ways
- but these tendencies are countered and the paradox of the plankton is resolved
- populations can be driven to extinction in several ways
Stochasticity: chance fluctuations in population numbers
- Competitive exclusion
- Through predator-prey (or host-parasite, etc.) interactions
- Allee effects at low density
but these tendencies are countered and the paradox of the plankton is resolved
Predation keeping competitive exclusion from going to completion (as in Paine’s sea star removal experiment)
- Non-equilibrial conditions, habitat patchiness, rescue-
by-migration, variation in life-history strategy (as in a competition-colonization trade-off)
meta community
a set of local communities linked by the dispersal of one or more of their constituent species
What determines the number of
species on an island?
Three processes:
* 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 …
MacArthur and Wilson’s theory
of island biogeography
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
draw theory of island biogeography in graphs
why colonisation rate decreases as no of species increases
- the fewer the number of species, the more likely a migrant will be a new species
why extinction rate increases as no of species increases
more competition
near island vs far island
- near island has higher colonisation rate than far island as it is easier to get to
- so has more species at equilibrium
large island vs small island
- extinction rate higher at small islands
- have fewer resources and can support smaller population sizes
- so have less species at equilibrium
is this model effective for species richness?
- Mahler studied anolis lizards
- species richness of anoles increase with area and decrease with isolation
when else does island biodiversity apply?
habitat fragmentation creates ‘islands’ of suitable habitat within an inhospitable matrix
