Island biogeography I: the idea Bio 415/615

Questions 1. What are the opposing forces of island biogeography? 2. Why do islands have fewer species than continental areas of the same size? 3. How has IBT been tested? 4. What is the rescue effect?

Habitat diversity Population size Bigger ranges Recall species-area relationships Log Area Log Species Richness

Consider islands Bigger islands should have more species than smaller islands. Why? –More habitats (or env circumstances) –Support larger populations –Can hold larger animals (e.g., big ranges) What else determines how many species are on islands?

Richness as a f(area, distance)

Island Observations Islands have fewer species than samples within contiguous continental areas as a function of size Islands have ever fewer species as they get smaller (z is higher) Isolated islands have fewer species than less isolated islands of the same size

Beyond Islands: Habitat Loss & Fragmentation Area decreases (Grain decreases)Area decreases (Grain decreases) Isolation increases (Distance increases)Isolation increases (Distance increases) Fewer species expectedFewer species expected InsularizationInsularization

Occam’s principle of parsimony with a warning from Einstein One should not increase beyond what is necessary the number of entities required to explain anything William of Ockham Everything should be made as simple as possible but not simpler Albert Einstein

IBT

The Theory Immigration, extinctionImmigration, extinction –Straight to concave TurnoverTurnover Near, FarNear, Far Large, SmallLarge, Small

Immigration Rate Rate of New Species Immigration Number of Resident Species 0 Many Low High

Extinction Rate Rate of Extinction Number of Resident Species 0 Many Low High Rate of New Species Immigration

Equilibrium Number of Resident Species 0 Many Low High Equilibrium Rate of New Species Immigration Rate of Extinction Turnover

Isolation Number of Species Far from Mainland Rate of Extinction Rate of New Species Immigration

Area Number of Species Large Island Rate of Extinction Rate of New Species Immigration

Equilibrium Number of Species Small, Far Island Rate of Extinction Rate of New Species Immigration

Equilibrium Number of Species Small, Close Island Rate of Extinction Rate of New Species Immigration

Equilibrium Number of Species Large, Close Island Rate of Extinction Rate of New Species Immigration

Equilibrium Number of Species Large, Far Island Rate of Extinction Rate of New Species Immigration

IBT

Extensions of Theory Target effectTarget effect Rescue effectRescue effect IE DistanceMWRescue AreaTargetMW

Rescue effect Brown & Kodric-Brown 1977 Isolation influences extinction rates of extant species too, by ‘rescuing’ them from extinction through continuous supply of more individuals

Area and Isolation Number of Species Rescue Effect Target Effect Rate of Extinction Rate of New Species Immigration Bigger islands are bigger ‘targets’ for colonization

Extensions of Theory Target effectTarget effect Rescue effectRescue effect Landscape ecology: matrix, patch quality, corridorLandscape ecology: matrix, patch quality, corridor

Simberloff: Experimental Test

Simberloff’s mangrove islands

Land bridge islands Barro Colorado Island, Panama 1. Hilltop = 15.7 km2 of lowland tropical forest. 2. Isolated in 1914 when Lake Gatun was formed by construction of the Panama Canal. 3. Knowing area and period of isolation, can model extinction species of breeding birds in Terborgh used land bridge model to predict 17 would be lost in 50 years; really 13 = 12% of 108.

So why does insularization lead to species loss?

The 3 Step Process of Species Loss and Extinction Debt

InstantaneousFastSlow SamplingIsolationArea

Instantaneous FastFaster & Greater Loss as Area Slow

The 3 Step Process of Species Loss and Extinction Debt Instantaneous FastFaster & Greater Loss as Area Slow

The 3 Step Process of Species Loss and Extinction Debt Instantaneous FastFaster & Greater Loss as Area Slow Extinction debt Steeper z

Next: beyond islands