The Theory of Island Biogeography is a 1967 book by Robert MacArthur and Edward O. Wilson.
It is widely regarded as a seminal piece in island biogeography and ecology. The Princeton
University Press reprinted the book in 2001 as a part of the "Princeton Landmarks in Biology"
series. The book popularized the theory that insular biota maintain a dynamic equilibrium
between immigration and extinction rates. The book also popularized the concepts and
terminology of r/K selection theory.
== Background == The Theory of Island Biogeography has its
roots in Wilson's work on the ants of Melanesia. MacArthur synthesized Wilson's ideas about
competition, colonization and equilibrium into a simple graphical representation of
immigration and extinction curves, from which one can determine the equilibrial species
number on an island. MacArthur and Wilson's ideas were first presented in a paper published
in 1963, and were further developed into a book.
== Summary ==
=== Introduction ===
In the introductory chapter, MacArthur and Wilson highlight the significance of studying
island biogeography. Since islands are less complex and more numerous than larger ecosystems,
islands provide better opportunities to develop insights and perform replicable field research.
Given that insular microcosms are common to all ecosystems, principles from island biogeography
can be applied generally.
=== Area-diversity pattern === In Chapters 2 and 3, MacArthur and Wilson
postulate that insular species richness depends on island size and isolation from source regions.
The authors present an equilibrium model that is based on the following concept: when there
is an addition of the number of species on an island, the island's immigration rate of
new species will decrease while the extinction rate of resident species will increase. MacArthur
and Wilson thus assume that there will be an equilibrial point where the immigration
rate equals the extinction rate. They further hypothesize that an increase in island size
will lower extinction curves while a decrease in distance between the island and the source
region will raise immigration curves. Since the intersection of immigration and extinction
rate curves determines the species number, the authors predict that larger islands will
have more species than smaller islands (assuming these islands are comparably isolated) and
isolated islands will have fewer species than islands more proximal to source regions (assuming
these islands are equally large). There is additional discourse on how insular clusters
and stepping stones affect this model.
=== Modeling colonization and dispersion === Chapter 4 discusses survivorship theory. The
authors describe a model which states that the probability for successful colonization
is dependent on birth rate, death rate, and carrying capacity of the environment. From
this model, conclusions are made on the average survival time for a propagule's offspring,
the average survival time of a saturated population, and characteristics of successful propagules.
In Chapter 5, MacArthur and Wilson examine why species can be excluded from insular environments
and how the niche of a species changes after introduction. The authors surmise that pioneering
species can be excluded for the following reasons: the island has saturated levels of
pre-existing competition, the pioneering species cannot maintain a population large enough
to avoid extinction, and the island hosts too many or too few natural predators. When
a species colonizes a new area, the authors state that the species will either shift,
expand or contract its realized niche. Chapter 6 is a theoretical exploration of
dispersal models. The authors consider how insular stepping stones affect the dispersion
of species—particularly, the effects that size and isolation of stepping stones have
on dispersion. Further consideration is given to how dispersal curves and average distance
travelled by pioneers impacts this study. In Chapter 7, the authors state that there
are generally 3 consecutive phases to the evolution of populations after colonization.
Initially, there is a trend for colonizers to evolve from r-strategists into K-strategists.
The founder effect may also influence colonizing populations during this first phase. The second
phase is marked by long term adaptations to the local environment. In this period, abilities
for dispersal are commonly reduced, and colonizers will either differentiate or assimilate with
competing species. In the third phase, the evolution of colonizing populations may result
in speciation and/or adaptive radiation.
== Field testing == The insular biota equilibrium theory was experimentally
tested by E. O. Wilson and his then-graduate student Daniel Simberloff in six small mangrove
islands in the Florida Keys. The islands were fumigated to clear the arthropod populations.
Immigration of species onto the island was then observed in a first and second year census.
Wilson and Simberloff confirmed that there was an inverse relationship between the number
of species on an island and the distance to the source region as predicted in The Theory
of Island Biogeography.
== Impact ==
=== Applications === MacArthur and Wilson's theory of island biogeography
has been widely applied outside of island ecosystems. For microbiota, the theory has
been applied to the distribution of ectomycorrihzal fungi on trees, the distribution of bacteria
in water-filled treeholes, and the distribution of fungi among shrubs. While for flora and
fauna, the theory's predictions have been realized with the species richness of plants
on mountains and with the species richness of aquatic snails in bodies of water. Novel
applications looked at plants as islands for insect species and the dependence of the species
richness of mites on the areas of the host ranges of rodent species. MacArthur and Wilson's
work has been used as a basis in other ecological theories, notably the unified neutral theory
of biodiversity, and has been foundational for the fields of landscape ecology, invasion
biology, and conservation biology.
=== Criticism === Several studies have disputed the underlying
assumptions in MacArthur and Wilson's theory of island biogeography: specifically, the
interchangeability of species and islands, the independence between immigration and extinction,
and the insignificance of non-equilibrial processes. In the 2001 preface, Wilson stated
that "the flaws of the book lie in its oversimplification and incompleteness".
=== Legacy === In 2007, a symposium was held at Harvard University
honoring the fortieth anniversary of The Theory of Island Biogeography. Following this conference,
a collection of papers was published in the book The Theory of Island Biogeography Revisited.
== See also == Landscape ecology
R/K selection theory Conservation biology
Unified neutral theory of biodiversity