Islands Islands can serve almost as a laboratory for the study of biogeography. The biota of an island is simpler than that of a continental area, and the interactions are easier to understand. Islands are often depauperate in species numbers relative to mainland areas.
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Only 28 land bird species are found on the Galapagos, the result of maybe 13 colonization events. Some undifferentiated, others apparently derived by speciation within the archipelago. Equivalent area in South America would have a bird fauna 10 to 20 times as rich.
23 species of forest birds have disappeared since the island was formed.
It would have changed, however, as the result of independent evolution and extinction. The biota of island arcs and hotspot island chains originally arrived by trans-ocean dispersal.
In both cases, several islands exist at one time, creating the possibility for inter-island dispersal and a more complex pattern of evolutionary change.
Jared Diamond showed that, on very remote islands, the number of species may be less than that predicted by equilibrium theory. This is because of the great difficulty in dispersing to these islands.
In addition, the long distance dispersal of a plant species can typically be accomplished by a single spore or seed, where in animals it typically requires a pair of organisms or a pregnant female.
All native land birds of the Hawaiian Islands are endemic. Over 40% of plants on isolated oceanic islands are endemic.
Flightlessness in birds
Probably a response to reduced predation.
Plants tend to have lost defenses against herbivory. Why?
This may lead to groups of organisms playing ecological roles different from those they might fill on the mainland.
42% of native plants
All of mammals and reptiles.
Human impact dates from 16th Century.
Four islands have been settled. Total human population is now 9,000.
In 1959, uninhabited portions were declared a national park.
Tourism now major industry – 60-70,000 visitors annually.
Feral cattle, donkeys, horses and pigs also a problem. Introduced rats have probably led to the extinction of native rice rats on seven islands.
Introduced plants also a problem. Guava now dominant plant in many areas. Lantana, quinine tree.
22-24 colonizations by land snails have led to over 1000 species. 47 species and subspecies of songbirds.
Biggest difference is that the Hawaiian Islands had been heavily impacted by man before Cook got there in 1778.
Banana poka – vine from SA. Sort of a Hawaiian kudzu.
Hawaiian Islands contain more than a quarter of the threatened and endangered species in the US.
For these, and other reasons, islands tend to support fewer species than mainland areas of similar size.
Consider two islands of similar sizes but different distances from the mainland pool. Since extinction rates are a function of the available resources and should be related to the size of the island, we would expect them to be similar on the two islands. Colonization rates, however, should be greater for the island near the mainland than for the more distant island.
They found that species increased for a while, then reached an asymptote approximately equal to the original number. But the makeup of the species had changed.
Oceanic islands confirm pretty closely to the patterns predicted by island biogeographic theory. Land bridge islands are a different story.
… or an oceanic island.
So we see a different pattern for the number of species as a function of time for a: