ISLAND BIOGEOGRAPHY

R. R. Thaman

GE301/407: Applied/Advanced Island Biogeography and Ethnobiology

ISLAND BIOGEOGRAPHYISLAND BIOGEOGRAPHY


• The Study of the distribution, composition and abundance (including absence) of island biodiversity

• The study of island ecosystems, species, taxonomic and genetic diversity.


*Attempts to explain the reasons/factors responsible for these patterns and the nature of island biotas (flora, fauna and microbiota).

*Attempts to explain Richness and Poverty of species.


– Also interested in the impact of human societies on island biogeography

– ETHNOBIODIVERSITY (the study of the knowledge, uses, management systems, beliefs and language that island cultures have for their biodiversity) . . .

*Because island cultures have co-evolved with their biodiversity.


• Islands as unique ecological laboratories, where simple ecosystems can be studied more easily than continental ecosystems.

• Islands as laboratories for the study of evolution and co-evolution (HIGH ENDEMISM).

• The theory of evolution was based on Charles Darwin’s study of the island biota of the Galapagos Islands and Wallace’s study of the islands of Indonesia (MALESIA) and the Indian Ocean.

““Island Arks “largely responsible for Darwin and Wallace Island Arks “largely responsible for Darwin and Wallace challenging the biblical ark of Christendom and the challenging the biblical ark of Christendom and the

formulation of their theory on the evolution of new life formsformulation of their theory on the evolution of new life forms

“. . . it is not too much to say that when we have mastered the difficulties presented by the peculiarities of island life we shall find it comparatively easy to deal with the more complex and less clearly defined problems of continental distribution . . .”

Alfred Russel Wallace Island Life

(1902: 242 in Whittaker 1998)

• Two eminent ecologists, the late Robert MacArthur of Princeton University and E. 0. Wilson of Harvard, developed a theory of "island biogeography" to explain uneven distributions in the RICHNESS (numbers) of species of a given taxa on islands.

Equilibrium Theory of IBGEquilibrium Theory of IBG

• They proposed that the maximum number of species on any island (EQUILIBRIAL NUMBER) reflects a balance between:

– IMMIGRATION: the rate at which new species colonize it (HIGH IN THE BEGINNING), and,

– EXTINCTION: the rate at which populations of established species become extinct (LOW IN THE BEGINNING).

Equilibrium Theory of IBGEquilibrium Theory of IBG

• EXAMPLE: If a new volcanic island were to rise out of the ocean off the coast of a mainland inhabited by 100 species of birds, some birds would begin to immigrate across the gap and establish populations on the empty island.

• After a while some would begin to become extinct

• REAL CASE: Catastrophic eruption of Krakatoa Is. between Java and Sumatra (Indonesia) in 1883.

Theory of IBGTheory of IBG

• The rate at which these immigrant species could become established, however, would decline over time because:

– Each species that successfully invades the island will reduce by one the pool of possible future invaders

– In other words, the same 100 species continue to live on the mainland, but those which have already become residents of the island can no longer be considered to be potential invaders.


• The rate at which additional species will establish populations will be high when the island is relatively empty, and the rate at which resident populations go extinct will be high when the island is relatively full.

• Thus, there must be a point between 0 and 100 species (the number on the mainland) where the two rates are equal -- where new input from immigration balances output from extinction.


• Equally, the extinction on the island would be related to the number that have become residents.

• When an island is nearly empty, the extinction rate is low because few species are available to compete with each other or be killed off by extreme events and become extinct.

• And since the resources of an island are limited, as the number of resident species increases, the smaller and more prone to extinction the individual populations of each species are likely to become.


• That equilibrium number of species would be expected to remain constant as long as the factors determining the two rates did not change.

• But the exact species present should change continuously as some species go extinct and others invade (including some that have previously gone extinct), so that there is a steady turnover (SPECIES TURNOVER) in the composition of the fauna.


• This is the essence of the MacArthur-Wilson equilibrium theory of island biogeography.

• How well does it explain what we actually observe in nature?


• One famous "test" of the theory was provided in 1883 by a catastrophic volcanic explosion that devastated the island of Krakatoa, located between the islands of Sumatra and Java.

• The flora and fauna of its remnant and of two adjacent islands were completely exterminated, yet within 25 years (1908) thirteen species of birds had recolonized what was left of the island.

• By 1919-21 twenty-eight bird species were present, and by 1932-34, twenty-nine.

• Between the explosion and 1934, thirty-four species actually became established, but five of them went extinct.

RECOLONIZATION OF KRAKATAU (Krakatoa)RECOLONIZATION OF KRAKATAU (Krakatoa)

• By 1951-52 thirty-three species were present, and by 1984-85, thirty-five species.

• During the half century (1934-1985), a further fourteen species had become established, and eight had become extinct.


• As the theory predicted, the rate of increase declined as more and more species colonized the island.

• In addition, as equilibrium was approached there was some species turnover. The number of bird species remained roughly the same while the species COMPOSITION gradually changed.


• The theory predicts other things, too.

• For instance, everything else being equal, distant islands will have lower immigration rates than those close to a mainland, and equilibrium will occur with fewer species on distant islands.

• Close islands will have high immigration rates and support more species AT EQUILIBIRUM.


• By similar reasoning, large islands, with their lower extinction rates, will have more species than small ones -- again everything else being equal (which it frequently is not, for larger islands often have a greater variety of habitats and more species for that reason).


• Island biotas/ecosystems are often disharmonic (missing major types or groups of organisms/missing major taxa).– Large predators (carnivores)– Large herbivores– Ants (e.g., Hawai’i had no native ants)– Birds of prey/raptors (hawks, eagles, etc.)– Aggressive weeds– Disease organisms (e.g., influenza, whooping cough

and measles, which decimated Pacific Islands human populations and avian malaria that devastated Hawai’I’s endemic birds

Characteristics of Island Biotas/BiodiversityCharacteristics of Island Biotas/Biodiversity

• They have smaller population sizes than continental ecosystems

• Have limited genetic diversity within the population because they have descended from a single or limited number of “founder species”/the founder population (FOUNDER EFFECT)

• these small populations are more prone to extinction (total extinction) and extirpation (local extinction).


• There is a high degree of endemism (% of unique plants and animals that are found nowhere else), because of the process of adaptive radiation and ecological release into unexploited or inadequately filled HABITATS and NICHES.

• Many islands are considered to be global “BIODIVERSITY HOTSPOTS”


• There is also a very low level, or absence, of endemism (% of unique plants and animals that are found nowhere else) on atolls and small low-lying islands, which are usually populated with “ubiquitous” pantropical or pan-Pacific plants and animals.

• Many are considered to be global “BIODIVERSITY COOLSPOTS”


• Island ecosystems are usually very fragile because they have evolved in a state of less competition, usually because of “disharmony” and because they have small populations (total number of individuals of a given species) and limited genetic diversity within populations.


• Dispersability of plants, animals (vertebrates and invertebrates) and microorganisms.

• Distance of island (SPECIES-DISTANCE EFFECT)from source areas and centers of diversity

• Richness of the source area of colonizing organisms.

Factors Controlling Island BiodiversityFactors Controlling Island Biodiversity

• Size of island (SPECIES-AREA EFFECT) – leads to greater chance of colonization, greater habitat diversity, more space, greater protection from coastal marine influences, greater habitat and microhabitat diversity and higher population numbers

• Elevation/topography – greater habitat and climatic diversity


• Island type or substrate – continental, plate boundary volcanic, hotspot volcanic, raised limestone, low-lying atolls and limestone islands and sand cays

• Geologic age – Ancient continental and volcanic islands, ancient limestone islands and recent volcanic islands.


• Climate (moisture, temperature, latitude/day length, winds, etc.)

• Frequency and severity of extreme events (tropical cyclones, droughts, floods, tsunamis, volcanic eruptions, disease outbreaks)


• Nature of ocean currents (e.g., for dispersal of plankton and planktonic larval stages of marine organismsceanic

• Wind patterns (e.g., for the dispersal of birds, bats, insects and atmospheric plankton (very small insects and microorganisms).


IMPACT OF HUMANS

• A. overexploitation

• B. habitat destruction

• C. introductions of exotic alien plants and animals (deliberately and accidentally)

• D. pollution


• Western relationships (affinity) of PIBD (relationships to Asia, Malesia and the Indo-West Pacific)

• Diversity gradient form west to east (attenuation of species and taxa)

• Gradual elimination of major groups of plants and animals from west to east in both the marine and terrestrial environment (i.e., increasing disharmony).

Basic Patterns in the Biogeography of the Basic Patterns in the Biogeography of the Pacific Islands.Pacific Islands.

• Very high endemism on high isolated islands.

• Greater endemism among terrestrial and freshwater organisms than among marine organisms (gene flow and dispersal more restricted in the terrestrial and freshwater)

• Very low or no endemism on atolls and small low-lying islands


• Increasing taxonomic (compositional) and structural complexity on larger, older islands.

• Changes in sea level (EUSTATIC) have affected the genetic background and the nature of the biotas and biodiversity of terrestrial, freshwater and marine biodiversity in the greater Indo-Pacific Biogeographical


• Importance of successional change on islands due to natural and artificial disturbance.– Primary and secondary successions– r-adapted and K-adapted species

• The TAXON CYCLE ON islands and the gradual change for r-adapted species and evolution or change to specialized K-adapted endemics


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ISLAND BIOGEOGRAPHY

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