Robert McAllister
November 18, 2005
Literature Analysis on D. S. Simberloff and E. O. Wilson’s Article
“Experimental Zoogeography of Islands: The Colonization of Empty Islands
Daniel S. Simberloff and E. O. Wilson’s article on the colonization of islands
examines the immigration and extinction patterns on six different experimental islands in
the Florida Bay area. Removal of original fauna through use of methyl bromide
fumigation wiped out the species on the islands and the rate of colonization by different
species was recorded. Simberloff and Wilson found that seasonality had no impact on
the rate of dispersal or colonization. Wind was the main transport of species to the
islands (Simberloff and Wilson, 1969).
The authors distinguish between which species were used for the study and which
were not. Those which regularly traveled between the mainland and the islands were not
included, while those that were carried, for example, through wind movement were
included as they were not frequent visitors to the islands. Early colonists, in particular
psocopterans and spiders, rapidly built up populations that were numerically dominant.
These types of organisms were also the first to go extinct. Other insect colonizers were
wasps which colonized many islands for long periods of time. Ants had the most orderly
pattern of colonization due to their ability to amount large populations in undisturbed
mangrove communities (Simberloff and Wilson, 1969).
One of the most striking findings was that “the increase of species present on all
our islands to approximately the same number as before defaunation” (Simberloff and
Wilson 1969). This argument, called the equilibrium theory, is based on a balance
between immigration and extinction. High extinction rates counter the immigration onto
the island and create an equilibrium. The author explains that interactions between
species were not the cause of extinction, but rather extinction was caused by the inability
of most species to colonize the islands under any type of condition (Simberloff and
Wilson, 1969).
Simberloff and Wilson’s paper has since branched off into many different areas of
research. Many scientists have looked at the size and distance of islands and how that
plays a role in species richness. Others have examined the genetic differences of the
island species from the mainland species and whether or not this varies from island to
island or just from the mainland. Many of the authors discuss the difficulty in making the
necessary observations to provide data for their study. This has led many people that
study island biogeography to examine at the colonization of islands of a particular species
of bird or insect. This allows for better observations, which then provide better results.
Current areas of research include population densities in correlation with space,
landscape influence on island biogeography, and the affects natural disasters on islands
have on biodiversity.
Simberloff and Wilson’s equilibrium theory has undergone certain scrutiny
through the years. In particular, F. S. Gilbert of the University of Cambridge states that
their study of the mangrove islands is the only irrefutable evidence for equilibrium
theory. Gilbert discusses many of the people who have refuted the equilibrium theory
and how Simberloff and Wilson have reacted to them. One of the major concerns of his
study, Gilbert addresses, is that the islands were really small and were the same type of
habitat. The question remained if this non-interactive rise to equilibrium through random
immigration was true for larger islands. Gilbert acknowledges the work by Whiteman
and Jones that suggests small islands are bad examples of biogeographical laws because
islands with many habitats do not face the same laws. The equilibrium theory, according
to Gilbert, was accepted before it had conclusively been proven true (Gilbert, 1980).
The question of whether or not an island’s area has any relationship to the
colonization of islands or patches has since become an area of concern following
Simberloff and Wilson’s work on island biogeography. Coleman et al. acknowledge this
question and state that the relationship between species richness and area can explain
many ecological patterns, such as extinction and dispersal rates. Coleman et al. studied
breeding birds on islands in Pymatuning Lake on the Pennsylvania-Ohio border. What
they found is that the distribution of breeding birds on the islands occurs at random.
They then move on to form the hypothesis of random placement, which implies a lack of
correlation between the locations of individuals (Coleman et al, 1982).
Another area of the classic study is that of proximity to the mainland. Simberloff
and Wilson’s study was conducted on small islands that were relatively close to areas
which ecologists are interested in. Moulton and Pimm took the experiment in
biogeography to a larger scale – the introduction of birds to the Hawaiian Islands. They
found that even with continued introductions, species numbers on some islands tended to
remain constant due to high extinction rates. The Hawaiian Islands have been introduced
to more bird species than anywhere else and many of these species have since lost their
habitats. The introduction patterns of bird species over the last 100 years was examined
and the data provided backup the idea that rising extinction rates will eventually match
immigration rates. As a result, a dynamic equilibrium will be reached (Moulton and
Pimm, 1983). This is the closest comparison to Simberloff and Wilson’s original article
in which there is further evidence that equilibrium is ultimately reached. These findings
are very similar to his, in that the extinction rates have countered the immigration onto
the islands to make it appear as if the islands are remaining constant in species number.
In discussing species richness on islands in relation to theories of island
biogeography, Rydin and Borgegard (1988) looked at plant species richness on small
islands in Lake Hjalmaren. The water level of the lake dropped considerably in the late
1800s, creating new islands and the flora of the islands has since been studied. Their
findings are in accord to Simberloff and Wilson’s findings that the occupation of islands
approach constancy in regards to species richness. However, depending on the size of the
island, the time it takes to reach this equilibrium can vary. For smaller islands which may
be more dependent on soil formation and more prone to wave action, the process can take
much longer. According to the authors, size was determined to be the best predictor of
species richness. Again, the size of the islands comes into play when examining island
biogeography. Rydin and Borgegard acknowledge the criticism that the equilibrium
theory has received, especially that of Gilbert, but conclude that there does seem to be a
constancy to which the islands reach in terms of species richness (Rydin and Borgegard,
1988).
Due to the fact that the islands which Simberloff and Wilson studied were small, a
look at larger islands’ colonization rate could help to verify his theory. Wardle et al.
examined a boreal forest archipelago consisting of fifty islands in Sweden. Some of
these islands, much larger than those Simberloff and Wilson studied, were frequently
burned by lightning strikes. What they found was that there was a significant difference
in island area and plant species composition. In their experiment, the smaller islands had
much more plant diversity, indicating higher ecological stress. The larger islands were
dominated by one particular species. The findings on the small islands resemble those of
Simberloff and Wilson’s. However, the larger islands were absent from their study, and
Wardle et al’s findings on these islands indicate that ecological stress is low on larger
islands, allowing one species to dominate the area. The classic authors found the same
patterns for all islands as size was never a determining factor for how well an island was
colonized (Wardle et al., 1997).
Delving deeper into island biogeography, Johnson et al. use colonization and
extinction rates to test evolutionary predictions of island biogeography. Their model of
island biogeography includes several of the factors (colonization, migration, and
extinction) which shape an island’s species richness. Using population genetics data, the
model can estimate the rates of migration and extinction. Using allelic differences in
DNA, the authors found that species of larger and more distant islands are more
genetically diverse from the mainland than smaller and closer islands (Johnson et al,
2000).
R. P. Gillespie studied the immigration of spiders from a main land mass to three
remote archipelago hot spots in the Pacific Ocean for relative importance of species
accumulation. He found that each archipelago hot spot has its own monophyletic group
of spiders. Immigration on each hot spot occurred independently of the other and all
three archipelagoes seem to have migrated from a continental source. The species from
one archipelago are not related to those on the other. The sequences of mitochondrial
DNA from the spiders allowed Gillespie to come to this conclusion (Gillespie, 2002).
His findings introduce a new area of study in the colonization of islands: that of
relatedness between immigrants onto different islands. Previous studies have not touched
this topic and it introduces a new element to the study for ecologists. Also, Simberloff
and Wilson did not touch on the idea of islands being used as stepping stones, although
this does not appear to be the case.
It is clear through studying this topic that many scientists are trying to support
Simberloff and Wilson’s findings, especially that of equilibrium theory. A conclusive
study which analyzes all aspects of island biogeography needs to be performed. A study
similar to Simberloff and Wilson’s which takes into account the size of islands and
proximity to mainland could provide significant results. One area that would be
interesting to expand upon is that of competition on recently colonized islands. This
could be beneficial in determining how valid the equilibrium theory is as it has not taken
this into consideration. Simberloff and Wilson suggest that it plays no role, but it would
seem likely that colonists interact with each other and these interactions could be shaping
the extinction and colonization rates.
References
Coleman, B. D. et al. 1982. Randomness, area, and species richness. Ecology 63: 11211133.
Gilbert, F. S. 1980. The equilibrium theory of island biogeography: fact or fiction?
Journal of Biogeography 7: 209-235.
Gillespie, R. G. 2002. Biogeography of spiders on remote oceanic islands of the Pacific:
archipelagoes as stepping stones? Journal of Biogeography 29: 655-662.
Johnson, K. P. et al. 2000. Genetic and phylogenetic consequences of island
biogeography. Evolution 54: 387-396.
Moulton, M. P. and S. L. Pimm. 1983. The introduced Hawaiian avifauna –
biogeographic evidence for competition. American Naturalist 121: 669-690.
Rydin, H. and S. O. Borgegard. 1988. Plant-species richness on islands over a century of
primary succession - Lake Hjalmeren. Ecology 69: 916-927.
Simberloff, D.S. and E.O. Wilson. 1969. Experimental zoogeography of islands: The
colonization of empty islands. Ecology 50:278 - 296.
Wardle, D. A. et al. 1997. The influence of island area on ecosystem properties.
Science 277: 1296-1299.
Any research in the 1970s pertaining to this? Paper needs some editing.
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