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. 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.
Barro Colorado Island in Panama was formed in 1913 by the damming of the Chagres River. 23 species of forest birds have disappeared since the island was formed.
There are three types of islands: • Islands that were originally part of a nearby continent, but were separated by rising sea levels (land-bridge islands). • Islands that are part of a volcanic island arc. • Seamount chains which formed over geological “hotspots”.
The Hawaiian Islands have formed as a plate passed over a geological hot spot.
The types of islands have different characteristic flora and fauna. Islands formed by isolation from continents would have a biota which would be a subset of that on the continent. 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.
There is no doubt that the degree of isolation of an island or island group is a factor in determining the biota that it will support.
The ratio of observed species to the expected number declines with distance from New Guinea. 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.
For conifers and flowering plants in the Pacific, diversity is much lower in the more isolated island groups of the central and eastern Pacific.
If we plot the number of genera vs. island area, it becomes clear that the two are related. The more isolated islands (represented by ) have fewer genera that less isolated islands of the same size.
Some flying animals, such as birds and bats, are capable of reaching even very distant islands.
However, most land animals must rely on dispersal mechanisms like drifting on masses of debris. Although this process is likely rare, it certainly happens and has been documented for organisms like iguanas.
Dispersal to islands is typically by a “sweepstakes” route,. The dispersing organisms share adapations that allow them to reach the island, rather than adaptations allowing them to live there once they reach it. This is one factor that restricts the diversity of life on islands.
Long distance dispersal in plants is much more likely. A great many plants are adapted for such dispersal. 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.
Islands also show high endemicity. All native land birds of the Hawaiian Islands are endemic. Over 40% of plants on isolated oceanic islands are endemic.
We may often see adaptive radiation in island populations Hawaiian honeycreepers
Certain ecological groups are underrepresented. Large predators. Flightless mammals tend to be absent. Certain types of plants adapted for disturbed sites tend to be absent.
There is a tendency toward certain modified behaviors. Flightlessness in birds Fearlessness Probably a response to reduced predation. Plants tend to have lost defenses against herbivory. Why?
Another characteristic that may be seen on islands is ecological release, leading to niche expansion. This may lead to groups of organisms playing ecological roles different from those they might fill on the mainland.
Major human impacts on islands • Hunting • Destruction of native vegetation • Introduced species
Hunting has impacted several native species. Galapagos tortoises. Ships might take hundreds at a time. At least three races hunted to extinction. Galapagos tortoise
High degree of endemicity 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.
On island of Pinta, one male and two female goats were introduced in 1959. In 1973 the populaton was estimated to be about 30,000 (almost 200/square km). 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.
Over 1200 species of flowering plants – 95% endemic. 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.
Island life is probably more hazardous than that on the mainland. For one thing, catastrophic events have more severe effects. There is typically no place to hide.
Also, when a species is lost by extinction, it is more difficult to replace it be immigration than in a mainland situation. For these, and other reasons, islands tend to support fewer species than mainland areas of similar size. Lizards Ants
Island populations are more likely to go extinct than those on mainlands, for several reasons: • Populations are typically smaller. • They have less genetic diversity. • They were not originally adapted to the island habitat.
How do we explain the fact that islands are typically depauperate in species richness relative to mainland areas of comparable size. Originally, this was explained by a nonequilibrium theory of island biogeography which stated that islands are depauperate because they have not had sufficient time to accumulate species by immigration.
In 1963, Robert MacArthur and E.O. Wilson presented a new hypothesis to explain patterns of species richness on islands. Their equilibrium theory of island biogeography proposed that the lower number of species on islands was not the result of insufficient time, but rather the result of an equilibrium process peculiar to all islands.
The theory is based on the idea that, at any given time, the number of species on an island is the result of a balance between two processes: extinction and colonization.
When a new island forms, species begin to colonize. As more and more species accumulate, the colonization rate begins to decline. The extinction rate, on the other hand, begins to increase with increasing diversity.
At some point, the two processes balance each other, and the number of species on the island should stabilize. This equilibrium number is known as S
The equilibrium theory can also be used to explain the effect of size and distance on the number of species found on islands. 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.
This should result in a difference in the equilibrium number of species, with Nnear > Nfar
A similar argument can be used to explain the effect of island size. If two islands are of relatively equal distance from the mainland, we can expect colonization rates to be similar. Extinction rates, however, should be greater on the smaller island. Therefore, we expect a higher equilibrium number of species on the large island.
So, the two approaches (nonequilibrium and equilibrium) make very different predictions about the nature of island species. • The equilibrium theory predicts that the number of species will not change over time. The nonequilibrium theory predicts that the number of species should increase with time. • The equilibrium species predicts that, although the number of species will remain relatively constant, the actual makeup of those species will change.
Several datasets have been developed that support the equilibrium theory. Jared Diamond looked at bird species on the Channel Islands off the California coast.
In 1969, E.O. Wilson and Daniel Simberloff conducted an experiment employing mangrove islets in the Florida Keys.
They surveyed a series of islands of differing sizes and distances from shore, concentrating on the arthropod fauna found on the islands.
Then, they defaunated the islands by enclosing them in plastic and pumping in methyl bromide to kill all the arthropods. 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.
Following the publication of the theory, a number of other studies were conducted to examine its validity. A study on plant species on a group of islands off Britain showed that, in that case, the effect of size was indirect. Large islands had a greater degree of habitat heterogeneity, and therefore greater diversity.