Lecture 4- Island Biogeography

theory of island biogeography overvie

mathematical theory that seems complicated at first glance

actually super useful in understanding complex probs in biogeography

applications to land management decisions i.e. where to designate new wilderness/protected areas

theory of land biogeography (actual theory)

(1) larger islands will host more species than smaller islands

• extinction rates will be lower on larger islands than on small islands

(2) islands close to mainland will be more diverse than more distant islands

• extinction rates will be lower on islands close to mainland

• colonization rates will be larger on islands close to mainland

the species-area effect

“larger islands will host more species than smaller islands”

the wider of an area we look at, the more species we find

this is a general principle in any region, not just islands

**applies across diff clades and diff continetns

why is there a species-area effect?

larger geographic region contains more diff ecoregions

• area from la to long beach = 1 single level IV ecoregion (1 level II, 1 level II, and one lvel IV)

• zooming out to socal there are 20 level IV ecoregions (4 level III, 2 level II, and 2 level I)

• zooming out to all of california, there are (100 level IV, 11 level II, 5 level II, 4 level I)

what does species area effect explain

species area effect explains # of species on diff caribbean islands

explains # of species on diff ocean islands globally

island

can be an actual island or something like a tree, pond, or mountain

isolated populations on them = metapopulations

metapopulation def

group of populations that occupy habitat patches and where individuals can move between patches at least a little bit

dispersal

movement betwen patches

if there’s dispersal, they have connectivity

metapopultaions (info)

immigration rate = amt of dispersal of individuals from one big dense pathc (the source) to another, smaller patch (the sink)

source

population is a patch where repro rate is high enough to sustain pop and keep it from going extinct

sink

patch where repro rate is insufficient on its own to keep pop from going extinct

only influx of individuals from a source keeps the species present in the patch

what is source-sink contrast also called

core-satellite dynamic

island diversity decreases w distance to the mainland

islands close to mainland will be more diverse than more distant islands

islands are a kind of sink and mainlands are their source for species immigraiton

it’s easier for species to travel to an island (“colonize it”) if the island the source of the species are closer together

• it’s easier for new species to reach the island for the first time

rescue effect

it’s easier for additional individuals to reach the island and boost their populations there, which prevents the species going extinct

ways to disperse to islands

• swim

• fly

• catch a ride over on an animal that swims or flies

• flaot

• raft on floating vegetation/logs

**no matter the means of dispersal, it’s easier if you’re not going as far

island immigration rate vs. species richness

the more species there are on an island, the less often another new species arrives

immigration rate = how many new species appear on island per year (y axis)

# of species on island = x axis

inverse relationship, as # of species increases, immigration rate decreases

if the island is closer to mainland, the same pattern holds but the rate of new species’ arrival is higher for every species richness (above the one for far)

island extinction rate vs. species richness

the more species there are on an island, the more competition between species for limited resources like food or nesting sites

competitie exclusion increases rate of local extinction on the island

species will go extinct on the island more often if th island is smaller

# of species on island = x axis, extinction rate = y axis

as # of species increases, extinction rate increases, slightly exponential looking, not entirely straight 

if the island is smaller, the line is straighter and the line is more curved if the island is bigger

how does island extinction rate vs. species richness relate to species area effect?

when there’s more area, there’s more ecoregions, which means more diff kinds of resources

species can avoid competing with each other by specializing on diff food, hiding places, etc.

equilibirum

where immigration and extinction rates are equal

whatever island size or distance to land, island biogeograpjy expects there is some species richness at which the avg # of new species that arrive each yr is the same as the avg number that go extinct on the island each year

**species count is called equilibrium species richness

insects

If researchers kill all the species of insects on a (very small) island—and people have

actually done this—the number of species (richness) increases steadily up to a point

where it stays about the same over future years. That point is the same richness as

before the major disturbance. This is an example of islands reaching equilibrium again.