TheEquilibriumTheoryof Island$Biogeography · The$Importance$of$Islands$–$...

The Equilibrium Theory of Island Biogeography

Reading: MacArthur & Wilson

DiversificaCon – or – Why are there so many species?

Lecture 17 Recap

Jürgen Haffer


•  “Any organism which lives in a temperate or a cold climate is exposed at different periods of its life cycle or in different genera:ons to sharply different environments. The evolu:onary implica:ons of nature's annually recurrent drama of life, death, and resurrec:on have not been sufficiently appreciated. In order to survive and reproduce, any species must be at least tolerably well adapted to every one of the environments which it regularly meets. No maDer how favored a strain may be in summer, it will be eliminated if it is unable to survive winters, and vice versa. Faced with the need of being adapted to diverse environments, the organism may be unable to aDain maximum efficiency in any one of them. Changeable environments put the highest premium on versa:lity rather than on perfec:on in adapta:on.”

-‐Dobzhansky

Theodosius Dobzhansky


•  All of these quotes come from a great paper called Evolu:on in the Tropics (posted on the website)

•  Dobzhansky viewed the stability of the tropical climate as one of the main drivers of diversificaCon

•  Under a stable climate, there is Cme for mutualism, parasiCsm and other complex coevolved relaConships to emerge (also sympatric speciaCon)

Theodosius Dobzhansky


1.  Known climate fluctua:ons during the Pleistocene

2.  Reconstruc:on of rainforest refugia -‐  Rainfall paDerns -‐ Current bird distribu:ons (endemic & close habitat associa:ons)

3.  Bird specia:on processes can be quick 20-‐30 k years

Jürgen Haffer


Jürgen Haffer


•  SCmulated a number of studies debaCng the number, locaCon, and extent of rainforest refugia

•  Current evidence shows most of the speciaCon events occurred in the late Pliocene, before the climaCc shiXs of the Pleistocene

•  However, we know far more abut the speciaCon in the tropics thanks to Haffer’s hypothesis

Jürgen Haffer

The Importance of Islands – Islands as Natural Laboratories

•  Natural Laboratories

•  Natural systems where key factors vary so that their effects can be isolated



Sea Surface Temperature Wave Height



Tempe

rature

Wave Height



Tempe

rature

Wave Height

High Temp/Low Waves

Low Temp/ Low Waves

Low Temp/ High Waves

High Temp/ High Waves


•  Islands serve as perfect natural laboratories for biogeography

1.  Numerous

2.  Varied geographical circumstances -‐  Distance from mainland -‐  Age -‐ Size

3.  Tractable biotas


•  Theories obtained from these island systems are then applied to the conCnents

•  SCmulated the field of conservaCon biology, especially as it relates to habitat loss and fragmentaCon

•  Processes and mechanisms are not unique to islands, it is just that islands have provided the context for their isolaCon and analysis


•  Biogeographers have gained important insights from island systems throughout the history of the field (Galapagos, Malay Archipelago)

•  In the 1950s through the mid-‐1970s, modern work on marine (and conCnental) islands led to theories on the dynamics of species richness


•  Species vs. Area Rela:onship

•  Looked at plant communiCes on islands off the coast of Sweden

•  Found that as area increased, there tended to be more species

•  First to describe this relaConship mathemaCcally

Olof Arrhenius



Olof Arrhenius



•  Log-‐log plots are called Arrhenius plots

•  Also noCced that the slope of the line appeared to differ systemaCcally between islands and non-‐isolated areas on conCnents

Olof Arrhenius


•  In oceanic islands, species richness declines with distance from a mainland source

•  TradiConal explanaCon was impoverishment with distance, which held that Cme had been insufficient for remote islands to fill up

•  ImplicaCon that over Cme the species richness of these islands would increase further

•  This is a non-‐equilibrium explanaCon


•  An Equilibrium Theory of Insular Zoogeography

•  The authors argued for an equilibrium model of species richness along a distance gradient from the mainland sources, and that this was also a funcCon of island size

•  This theory represents a dynamic steady state due to the offseing effects of immigraCon (influenced by distance) and exCncCon (influenced by area)

Wilson & MacArthur


Wilson & MacArthur

Mainland


•  Much less likely that an organism from the mainland source populaCon is going to reach a remote island relaCve to a near island

•  Also the relaConship between the number of species and new immigrant species – the more species an island has, the less likely a new immigrant is going to represent a novel species that does not already exist on that island

Wilson & MacArthur


Wilson & MacArthur

ImmigraCo

n Ra

te

Number of Species Present

The relaConship between the number of species and new immigrant species – the more species an island has, the less likely a new immigrant is going to represent a novel species that does not already exist on that island


Wilson & MacArthur

ImmigraCo

n Ra

te


Near Island

Far Island

Much less likely that an organism is going to reach a remote island relaCve to a close island from the mainland with the source populaCon


•  As the number of species on an island increases, the rate of exCncCon will also increase, as there are more species to possibly go exCnct (given that all species are equally likely to die out), there is also less space overall for each species which means smaller populaCons

•  A smaller island will have greater exCncCon rates than a larger island for the same number of species as there is less space and can support a small populaCon sizes

Wilson & MacArthur


Wilson & MacArthur

ExCn

cCon

Rate


As the number of species on an island increases, the rate of exCncCon will also increase, as there are more species to possibly go exCnct (given that all species are equally likely to die out), there is also less space overall for each species which means smaller populaCons


Wilson & MacArthur

ExCn

cCon

Rate


A smaller island will have greater exCncCon rates than a larger island for the same number of species as there is less space and can support a small populaCon sizes

Small Island

Large Island


Wilson & MacArthur

ExCn

cCon

Rate


Small Island

Large Island

Near Island

Far Island

ImmigraCo

n Ra

te


1.  An island which is farther away from the source of colonizaCon will have fewer species, because the immigraCon curve will be lower and hence intersect the exCncCon curve farther to the leX

2.  ReducCon of the species pool of immigrants will reduce the number of species on the island

3.  If an island has a smaller area, or more severe climate, the exCncCon curve will rise and the number of species will decrease

Wilson & MacArthur


4.  If you have two islands with the same immigraCon curve but different exCncCon curves, any given species on the island with the higher exCncCon curve is more likely to die out

5.  The # of species on an island far from the source will grow more rapidly with island area than will near islands

6.  The # of species on large islands decreases with distance form the source of colonizaCon faster than does the number of species on small islands

Wilson & MacArthur


Wilson & MacArthur

ExCn

cCon

Rate


Small Island

Large Island

Near Island

Far Island

ImmigraCo

n Ra

te


•  Islands represent natural laboratories to study biogeographic panerns

•  The impoverishment with distance explanaCon is a non-‐equilibrium theory

•  The equilibrium theory represents a dynamic steady state due to the offseing effects of immigraCon (influenced by distance) and exCncCon (influenced by area)

Wilson & MacArthur Main Points

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