1. What determines the number of species in a habitat?
Start by observing the processes affecting the number of species on islands
This subject is called
ISLAND BIOGEOGRAPHY
First described by MacArthur and Wilson in 1967

2. Bird Species on the Channel Islands, California (Diamond 1969)
Species Total
Extinct
Artificial
Natural
Turnover
1917
1968
Introduced
Immigration
San Nicholas 11 6 2 4
50%
San Clemente 28 24 9 1
25%
Santa Catalina 30 34
1. No island has the same number of species as a similar area on the mainland with
similar habitats.
2. Large number of species have gone extinct in 50 years.
3. There has been a large number of immigrant species.
4. The total number of species has remained the same.
So an 'equilibrium' number has resulted although there is a high turnover of species.

3. MANGROVE ISLAND E9
Simberloff & Wilson

4. OTHER MANGROVE ISLANDS
Simberloff & Wilson

5. ISLAND BIOGEOGRAPHY
1. Data from the Channel Islands (Diamond) off California
show that some species populations go extinct,
while other species immigrate,
and this results in a turn-over of species
(i.e. not the same species immigrate as those going extinct).
The total number of species in that community remains
roughly constant, suggesting an equilibrium number.
This agrees with the hypothesis of MacArthur and Wilson.
2.These points are also illustrated in the removal
experiments by Simberloff on small mangrove islands.

6. CAUSES OF EXTINCTION 3. Causes of extinction are:
i) chance extinction in very small populations
by random events, e.g. weather,
ii) new habitat from arrival new plants
iii) competition from arrival of later competitors
on islands,
iv) predation from arrival of predators.
Islands have small populations because of
few arrivals, small area and few habitats.

7. ISLAND BIOGEOGRAPHY
4. The study of islands allows us to understand the
dynamic equilibrium hypothesis of MacArthur and
Wilson 1967.
This predicts the number of species in a
community based on the balance between
rates of immigration to a community and
rates of extinction within that community.

8. RATES OF IMMIGRATION AND EXTINCTION
ON ISLANDS
MacArthur
& Wilson
1967

9. RATES OF IMMIGRATION AND EXTINCTION
5.Immigration rate declines with more species
arriving from mainland.
The rate is lower the further the island is from
the source of species.
6. Extinction rate increases as more species arrive.
This rate is higher on smaller islands than on larger.

11. EFFECTS OF
DISTANCE
-Islands off
New Guinea

13. EFFECTS OF
AREA

14. PLANT DIVERSITY v AREA
MEDITERRANEAN
BIOME
SW Australia
Cape S. Africa
Medit sea region
Chile
California

15. LANDBRIDGE ISLANDS
7. Landbridge islands are those cut off from the mainland
following the ice-age.
The decline of species number on these islands is
related to both the size of island and the length of
time since isolation.
These observations conform with the theory.

16. SUNDA ISLANDS Malaysia and Indonesia were once joined during the
Ice ages as one
mainland.
These islands are
Landbridge islands

17. Oceanic and small
Landbridge (LB) islands
reach equilibrium (S)
by the present time,
since being cut off
in the ice ages.
Large LB are still above
equilibrium.

18. No islands have the same number of species as the Malay mainland
SUNDA ISLANDS
No islands have the same number of
species as the Malay mainland

19. The older the island the more species have been lost
showing decline after detachment from mainland

20. LANDBRIDGE
ISLANDS OFF
NEW GUINEA
COMPARED WITH
OCEANIC ISLANDS

21. LANDBRIDGE ISLANDS 8. These points are relevant to habitat islands
on the mainland,
and the formation of protected areas for Conservation.