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

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.

MANGROVE ISLAND E9 Simberloff & Wilson

OTHER MANGROVE ISLANDS Simberloff & Wilson

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.

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.

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.

RATES OF IMMIGRATION AND EXTINCTION ON ISLANDS MacArthur & Wilson 1967

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.

EFFECTS OF DISTANCE -Islands off New Guinea

EFFECTS OF AREA

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

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.

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

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.

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

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

LANDBRIDGE ISLANDS OFF NEW GUINEA COMPARED WITH OCEANIC ISLANDS

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