Download Biodiversity on Oceanic Islands: Its Origin and

AMER. ZOOL., 34:134-144 (1994)
Biodiversity on Oceanic Islands: Its Origin and Extinction1
GUSTAV PAULAY
Marine Laboratory, University of Guam, Mangilao, Guam 96923
SYNOPSIS. The isolation and small size of oceanic islands make them
attractive models for studies of diversification; the sensitivity of their
biota makes them important subjects for studies of extinction. I explore
the origin of island biotas through dispersal and in situ diversification,
and examine the fate of these biotas since human contact. Island biotas
start out depauperate and disharmonic, facilitating the survival of relict
taxa and stimulating adaptive radiations. The often highly restricted range
and small population size of insular species, together with their limited
diversity of defenses, make island biotas particularly vulnerable to extinction, largely through habitat loss or interactions with introduced species.
INTRODUCTION
Islands, due to their discrete, isolated
nature, provide excellent opportunities for
understanding the origin, diversification and
extinction of terrestrial biotas. Here I examine these processes on oceanic islands,
focusing on those remote from continental
source areas. Such islands can serve as model
systems for addressing fundamental questions about biodiversity and conservation:
what areas are most likely to develop high
species diversity and endemicity, and what
makes particular species or biotas vulnerable to extinction?
In part, the biotas of oceanic islands
diversify in situ, yielding insight into the
evolutionary origin of diversity. Their biotas are usually endemic and among the most
vulnerable in the world, and are currently
undergoing unprecedented rates of extinction. An understanding of the processes
leading to these extinctions is important in
itself, and also serves as a model for conservation problems worldwide.
ORIGIN OF ISLANDS
Any isolated habitat can be considered
insular. Because of their simplicity, oceanic
islands are especially illuminating and are
the focus of this review. By definition, oceanic islands never had connections to continental land masses. They are the products
of volcanism or tectonic uplift, or the result
of organic reef growth upon foundations
formed by the first two processes. In contrast, most continental islands were joined
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to other continental land masses in the past,
having since become separated due to tectonics or sea level rise.
Many oceanic islands are the products of
volcanism from stationary, "hotspot,"
magma sources. As a tectonic plate drifts
over such a source, volcanoes incorporated
into the plate are carried away from the hotspot, and a linear chain of islands or seamounts results, with the youngest ones lying
nearest the hotspot {e.g., Hawaiian, Society
Islands) (see Menard, 1986 and Williamson,
1981 for reviews of island geology). Such
hotspot islands usually have a well defined
origin, and undergo predictable development with age. They change from high,
undissected, shield volcanoes to smaller,
highly eroded and topographically complex
islands before final submergence of their
volcanic portions, usually at 3-15 million
years of age. Reef buildup around tropical
islands may ultimately convert high islands
into atolls, with low lying coral sand cays
projecting a few meters above sea level.
Although atolls may persist for tens of millions of years, they have a propensity for
repeated submergence as sea levels rise and
fall; thus their terrestrial biotas are ephemeral (Paulay, 1991a). Continued activity of
a hotspot can yield an archipelago that is
persistent through time, with new islands
arising as older ones subside. The positions
of islands on a single tectonic plate do not
change relative to each other, although some
islands may drown while others arise.
Other oceanic islands are the result of
island arc volcanism, formed where one tec-
BIODIVERSITY ON ISLANDS
135
tonic plate is subducting under another (e.g., butes, and this is reflected in the composiSolomon, Tongan Islands). Such islands tion of island faunas. Thus land snail faunas
generally undergo a variety of changes due on remote oceanic islands are generally limto recurrent volcanism, subsidence, uplift ited to minute species, a legacy of their origand erosion in these tectonically complex inating largely through aerial dispersal
regions. In the Pacific, they often attain larger (Vagvolgyi, 1975). Of those species which
sizes than hotspot islands. The age(s) of can disperse to an island, a subset will be
emergence of an arc island is often difficult able to establish and maintain populations,
to define, and the size and degree of emer- depending on attributes of the species and
gence can vary greatly through time. Fur- the island. An island's size, its habitats, and
thermore, because such islands lie in tec- its preexisting biota are all important in
tonically active areas, their positions are determining the success of potential colocontinually changing relative to other land nists; preexisting species can place impormasses. Thus while the age, position, and tant limits on colonization via interactions
physiographic evolution of hotspot islands such as competition, predation, and symare simple and readily predictable, those of biosis.
arc islands are complex. This makes hotspot
While the biodiversities of solitary, young
islands more attractive as model systems for islands near continents can be solely attribthe study of diversification.
uted to colonization events (e.g., Krakatau
Island-Thornton et al, 1990), those of
ORIGIN OF BIOTA—COLONIZATION
remote, older archipelagoes often result from
An island's biota is the product of oversea extensive radiations of a handful of original
colonizations and local diversifications. colonists. Thus the estimated 10,000
Oceanic islands receive their biotas solely Hawaiian insect species evolved from ca.
through dispersal. The probability of colo- 350-400 successful colonists (Howarth,
nization is influenced by the island's geo- 1990), and the 1,138 indigenous Hawaiian
graphical and environmental settings as well vascular plants are derived from 387-396
as by organismal attributes (MacArthur and founders (Wagner, 1991).
Wilson, 1967). Because the probability of
Although the immediate effects of disdispersal is inversely related to distance, an persal limitations are depauperate and disisland may receive more of its propagules harmonic biotas on remote islands, this
from relatively depauperate nearby islands biotic poverty can lead to the survival of
than from very diverse but remote conti- unusual, relict taxa, and stimulate the develnents.
opment of adaptive radiations in successful
Organisms vary greatly in dispersal abil- dispersers, so that high, remote oceanic
ity, both with regards to distance traveled islands can become fairly species rich
and method of transport (Carlquist, 1974). through time.
The geographic range of a taxon is often
RELICT TAXA
correlated with its dispersal ability (Kohn
and Perron, 1993), and the biotas of remote
The view that evolution is an arms race
islands are largely limited to species with among predators sensu lato (including parexcellent dispersal abilities and their de- asites, pathogens, herbivores) and their prey,
scendents. In the island-rich oceanic Pacific, or among competitors (Van Valen, 1973),
where the far majority of organisms origi- may explain the temporal or spatial restricnates from western (Austro-Asian) source tion of certain groups in the history of life
areas, fewer and fewer taxonomic groups are (Vermeij, 1987). Species that cannot cope
represented as one moves eastward through with escalating biotic pressures in most
the basin (e.g., Zimmerman 1942), with environments become globally extinct or
poorly dispersing taxa (e.g., mammals, may survive in unusual habitats or areas
amphibians, gymnosperms) being especially that their adversaries have not penetrated,
limited to western island groups.
such as the deep sea, caves, or Australia
Certain methods of dispersal are avail- (Vermeij, 1987). Islands frequently offer
able only to organisms with particular attri- such refuge for relicts, and may also pro-
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GUSTAV PAULAY
FIG. 1. Distributions of the land snail families Endodontidae (solid line, based on Solem, 1976), Partulidae
(dashed line, based on Cowie, 1992) and Amastridae (dotted line).
mote the secondary, in situ evolution of species with limited defensive or competitive
abilities (Solem, 1979; Carlquist, 1974).
Thus many land crab and seabird species
are widespread but strictly insular, restricted
largely by mammalian predators (Hartnoll,
1988; Steadman, 1989). Numerous benthic
marine species are restricted to, but widespread among, the small oceanic islands of
the Pacific tectonic plate (Springer, 1982),
an enigmatic distribution that similarly
could be forced by biotic exclusion from
more diverse western areas.
Relict taxa are well known on large, old,
continental islands, the lemurs of Madagascar and the monotremes and marsupials
of Australia being classic (if not indisputable) examples. Although short-lived oceanic islands appear at first ill-suited for the
long-term survival of relict lineages, groups
with good dispersal abilities can survive by
island hopping.
Three (Partulidae, Achatinellidae, Amastridae) of the six families that comprise the
most primitive order (Orthurethra) of pulmonate land snails, as well as the anatom-
ically most generalized family (Endodontidae—Solem, 1976) of higher landsnails
(Sigmurethra), are restricted to islands
(Solem, 1979). The fact that these land snails
have reached even the most remote Pacific
islands, yet are virtually unknown from
continents, strongly implicates biotic factors rather than dispersal limitations as setting distributional boundaries (Fig. 1). The
past continental occurrence of at least achatinellids (Solem, 1979) and the extreme vulnerability of all four families to certain
introduced predators (see below) support the
hypothesis that their relict distribution is
the result of escalation.
The primitive weevil family Aglycyderidae is similarly restricted to islands in the
Pacific and Atlantic oceans, with one dubious exception (Zimmerman, 1948).
Although aglycyderids occur on most central Pacific high islands, they are rare and
species poor except on the Hawaiian Islands,
where they have undergone an explosive
diversification, resulting in over 170
described species (Nishida, 1992). Zimmerman (1970) argued that this Hawaiian
BIODIVERSITY ON ISLANDS
radiation was possible because of the absence
of the very diverse and ecologically dominant Miocalles weevils that cooccur with the
family elsewhere.
The restriction to and survival on ephemeral islands by such primitive and presumably ancient groups depends upon their
ability to continually disperse to new islands
as older ones sink below sea level. For
example, endodontoid land snails are known
from Lower Miocene to Pleistocene deposits on three islands (Midway, Bikini, and
Enewetak atolls) that have since subsided
greatly and no longer support the snails
(Solem, 1982); they persist on other, mostly
younger islands throughout the Pacific.
Similar dispersal from drowning to new
islands has been demonstrated within drosophilids and honeycreepers in the Hawaiian island-chain (Sibley and Ahlquist, 1982;
Beverley and Wilson, 1985; see below).
Such relict taxa not only survive on Pacific
islands, but can form a dominant portion
of the indigenous fauna, especially among
mollusks. The strictly insular land snail
families discussed above constitute 79% of
the Hawaiian and 70% of the Rapan (SE
Polynesia) land snail fauna (Solem, 1982,
1990).
ECOLOGICAL RELEASE AND LOCAL
DIVERSIFICATION
Island biotas are famous for large evolutionary radiations such as that of many
hundreds of species of Hawaiian drosophilids (Carson, 1983; Nishida, 1992), and
bizarre adaptations, like the boid snakes with
jointed upper jaws in the Mascarenes (Frazetta, 1970). Studies of adaptive radiations
on islands indicate that both diversification
and adaptive evolution are favored by a low
diversity of initial colonists, that both can
be extremely rapid, and that diversification
is stimulated by opportunities for isolation
on topographically complex archipelagoes.
The potential for taxa to diversify, to
evolve unusual adaptations and to expand
into novel adaptive zones can be thwarted
in diverse biotas by the high degree of utilization of existing niche space and high levels of escalation that limit evolutionary
experimentation. It has long been recognized that in low diversity settings, as after
137
mass extinctions or in remote insular settings, the potential of organisms to diversify
and enter novel adaptive zones can be better
realized (Simpson, 1953). Radiations are
especially prevalent on remote islands lying
at the edge of a group's dispersal range, in
peripheral areas termed the "radiation zone"
by MacArthur and Wilson (1967), where the
low diversity of colonists facilitates in situ
diversification (Diamond, 1977).
The effects of certain "keystone" taxa in
limiting the evolutionary options of others
are especially striking, as demonstrated
where such taxa are absent. Ants, for example, dominate arthropod communities
through most of the world; their absence in
Hawaii and SE Polynesia appears to be
largely responsible for the great radiations
of carabid beetles and spiders, and the
exploitation of predatory niches by such
unusual organisms as caterpillars, in those
island groups (Wilson, 1990).
That diversification can be very rapid is
immediately suggested by the sheer diversity and high levels of single-island endemism exhibited by certain clades on islands
a few million years old. Thus at least 70
species of Mecyclothorax beetles are
endemic to 1 My old Tahiti (Perrault, 1987),
25 of the 26 species of picture-wing Drosophila on 600,000 yr old Hawaii are
restricted to that island (Carson, 1983), and
at least 11 lineages of insects and land snails
have radiated to over 100 species each in
the Hawaiian Islands. That morphological
differentiation as well as speciation can be
very rapid is further attested by the minimal
genetic differentiation found within some
striking adaptive radiations {e.g., Helenurm
and Ganders, 1985; Lowrey and Crawford,
1985). The 19 species of Hawaiian Bidens
exhibit much more morphological and ecological diversity than this diverse genus does
in the Americas, yet the genetic diversity of
this entire insular radiation is comparable
to that found among populations within
American species (Helenurm and Ganders,
1985).
Within an oceanic archipelago, local
diversification can occur by inter- or intra-island speciation. Only the former is
generally available to organisms with good
over-land dispersion abilities (especially
138
GUSTAV PAULAY
vertebrates and plants). Diversifications
among such organisms are relatively
uncommon, with only a handful of isolated
archipelagoes showing large endemic radiations of birds or plants. The number of
adjacent islands appears to be more important than island size in facilitating bird radiations: the Hawaiian and Galapagos Archipelagoes have significant radiations in birds,
while the larger and more ancient, continental islands of New Caledonia and New
Zealand show little in situ diversification
(Olson, 1991). Inter-island speciation can
nevertheless lead to large radiations, as
attested to by honeycreepers, several lineages of plants, and numerous lineages of
highly volant insects in the Hawaiian Islands
(Wagner, 1991; James and Olson, 1991;
Nishida, 1992).
Intra-island, or "continental," speciation
is restricted to islands that are large enough
to allow for effective isolation of populations. While intra-island speciation in birds
requires an island the size of Madagascar,
for small land snails and flightless insects a
few square kilometers can be sufficient (Diamond, 1977; Paulay, 1985). Extreme geographic localization is especially prevalent
on topographically complex, highly dissected volcanic islands, a reflection of the
very steep topographic, climatic, and environmental gradients occurring across islands
only a few kilometers in diameter yet hundreds or thousands of meters high. Such
geographic localization can lead to remarkable radiations through intra-island speciation. Thus on isolated Rapa, a 40 km2, 650
m high, 5 My old island in SE Polynesia,
local radiations resulted in 67 flightless
Miocalles weevil, and 45 achatinellid and
24 endodontid land snail species (Solem,
1982; Paulay, 1985). Many of these species
have restricted ranges on Rapa, some
remarkably so {i.e., <1 km2). In contrast,
islands with low topographic differentiation, such as the flat-topped, uplifted limestone islands of Henderson and Niue (S
Polynesia), offer few opportunities for geographic restriction and generally lack intraisland diversifications (Paulay, 19916). The
importance of topographic, climatic and
environmental variation in limiting species
ranges and facilitating diversification is also
apparent in continental areas with steep
topographic or environmental gradients
{e.g., Tweedie, 1961).
Both island size and age may limit diversification. The frequent correlation between
species richness and island size may be the
result of increasing habitat heterogeneity
with island size, or an ecological or evolutionary equilibrium between origination
(colonization and local speciation) and
extinction rates (MacArthur and Wilson,
1967; Williamson, 1981). In remote islands,
where much of the diversity is the result of
local diversification, island age (the time
available for diversification) may be an
important control (Wagner, 1991). Because
hotspot islands subside and shrink with age,
older islands within an archipelago tend to
be smaller. This may give rise to opposite
diversity patterns, depending on whether
time or area is limiting diversification.
Groups capable of rapid rates of speciation (relative to island age and available
niche space) may so rapidly saturate even
young islands with species that they yield
the expected positive correlation between
species richness and island area. Such is the
case for the highly volant, Hawaiian Plagithmysus beetles, which exhibit high rates
of inter-island speciation (Fig. 2). In contrast, if diversification is relatively slow, then
diversity may strongly correlate with island
age and therefore be negatively correlated
with island area. Such is the case for the
flightless Hawaiian Rhyncogonus weevils,
which appear to diversify slowly via intraisland speciation (Fig. 2).
Age is important from both an evolutionary and conservation perspective, as old
islands often have the most divergent species and for some lineages, the most diverse
fauna. The largest land snail radiations
among SE Polynesian and Micronesian hotspot islands, both in numbers of species and
the development of endemic genera, occur
on the relatively old islands of Gambier,
Rapa and Pohnpei (all 5-7.5 My old) (Solem,
1982). The most interesting and divergent
insect fauna among the major islands of the
Hawaiian archipelago is on Kauai, the oldest island in the chain (Zimmerman, 1948).
The time available on an archipelago for
evolutionary divergence and diversification
of a lineage can be greater than the age of
the oldest emergent island. Thus the Hawai-
139
BIODIVERSITY ON ISLANDS
O Plagithmysus
• Rhyncogonus
O
50
to
O
o
<D
en
40 -
-
Q.
in
30
-
0)
-Q
E
o
a>
-Q
20
E
3
10
n
O
•
i
i
1
1
1
10°
Island age (My)
10"
Island area (km2)
FIG. 2. Diversification of Plagithmysus (Cerambycidae) and Rhyncogonus (Curculionidae) among the major
Hawaiian Islands. Species richness of Plagithmysus is strongly correlated with log island size (r = 0.93), but not
island age (r = -0.29); that of Rhyncogonus with island age (r = 0.96) but not log island size (r = -0.19). Data
from Nishida (1992) and Howarth and Mull (1992).
ian chain, in existence for at least 70 My,
was colonized by drosophilids and drepanidine honeycreepers well before the birth of
the present major high islands (Sibley and
Ahlquist, 1982; Beverley and Wilson, 1985).
VULNERABILITY, EXTINCTION,
CONSERVATION
The biodiversity crisis is nowhere more
apparent and in need of urgent attention
than on islands. Approximately 90% of all
bird extinctions during historic times have
occurred on islands (Vitousek, 1988). More
species of Polynesian landbirds appear to
have gone extinct due to human agencies
than survive today, and the survivors have
greatly reduced ranges (Olson, 1989; Steadman, 1989). Among the historically known,
indigenous plants of Hawaii, 10% are extinct
and almost 40% threatened (Wagner, 1991).
Comparable quantitative data on the
extinction rates of terrestrial arthropods are
not available, but indications such as the
virtual absence of native insects below 600
m altitudes and the extinction of land crabs
in Hawaii indicate similar tragedies (Zimmerman, 1948; Howarth, 1990).
Land snails are perhaps the most vulnerable members of insular biotas, and the four
relict families discussed above are at the
greatest risk. Most of the 331 described spe-
cies of Amastridae, a family endemic to the
Hawaiian group, are extinct, with only a
handful of arboreal species of this primarily
ground-dwelling family surviving (M. G.
Hadfield, personal communication, 1992).
The spectacular radiations of endodontoid
land snails are known to be largely or completely extinct in the Hawaiian Islands
(195+ species), the Gambiers (25 species),
Rarotonga (11 species), and St. Helena (12
species), these being the islands which have
been recently searched for the snails (Solem,
1976, 1977, 1982). Achatinellid and partulid land snails in the Pacific are losing
ground at a similarly alarming rate (Hadfield et al, 1993; Murray et al, 1988; Hopper and Smith, 1992).
Direct habitat destruction
The impact of humans on island habitats
is usually devastating, with the most important causes of human-induced extinction on
islands being habitat destruction and species introductions.
All Polynesian islands were entirely or
largely forested before man's arrival; fossil
pollen and mollusks provide evidence for
this even on islands or areas where no indigenous forests remain (e.g., Zimmerman,
1948;Paulay, 1985; Bahn and Flenley, 1992;
Kirch et al, 1992). Humans have been
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GUSTAV PAULAY
responsible for the partial or entire loss of
native forests from virtually all of these
islands (Kirch, 1984; Kirch et al., 1992).
The conversion of land for agriculture, construction, etc., and perturbations accompanying this process (pollution, erosion,
etc.), directly destroy forests, while species
introductions are further, indirect causes of
habitat destruction (see below). An especially
striking example of the environmental havoc
created by complete deforestation is Easter
Island, where destruction of the forests not
only caused untold extinctions, but was
apparently responsible for the collapse of
the island's megalithic culture (Bahn and
Flenley, 1992).
The vulnerability of habitats is largely
determined by their accessibility and suitability to humans, and by their resilience to
disturbance. Dry, gentle sloping, lowland
areas on volcanic terrain are among the most
vulnerable, while steep, wet, mountainous,
karstic areas are the least susceptible to
human disturbance. In the Hawaiian Islands,
humans had extensively altered 80% of all
native habitats below 450 m in elevation
by 1600 A.D. (Simon et al, 1984). Dry forests, which may host an even more diverse
and endemic biota than rain forests on
islands, are especially vulnerable, with but
fractions of their former cover remaining
on all but the most pristine islands (Olson,
1989). Olson's (1989) discovery, based on
subfossil remains, that bird faunas of dry
forests were much more diverse before
human occupation, led him to question
whether we are similarly underestimating
the diversity of mesic and arid habitats on
continents, an idea which has gained recent
support in the Americas (Mares, 1992).
As a result of millions of years of erosion
and subsidence, the oldest oceanic hotspot
islands tend to have low relief, elevation
and (therefore) precipitation, traits which
make them especially vulnerable to human
disturbance. Thus these islands, which likely
held the most interesting products of evolution, are also among the most disturbed
(e.g., Eiao, Chuuk, Tubuai, Gambier).
Among Pacific hotspot islands > 5 My old,
only Pohnpei (Carolines), Rapa (Australs),
Kauai (Hawaii) and the small rock-like, leeward Hawaiian high islands retain a reasonable cover of native vegetation; these
islands, because of their spectacular biotas,
should be among the highest conservation
priorities.
Introduced species
Introduced species threaten island biodiversity both indirectly through habitat
alteration and directly through interactions
with native species. Their effects can be even
greater than those of anthropogenic habitat
destruction; in this islands differ from continental environments, where exotics appear
to be much less often the cause of extinction
(Vitousek, 1988).
Introduced species, especially certain herbivorous mammals and plants, are among
the worst culprits in causing large-scale habitat destruction and alteration. Insular floras
have few defenses against herbivorous
mammals which, unchecked by predators,
can destroy most of the vegetation and lead
to massive erosion. Paradoxically, uninhabited islands can be among the most affected
by introduced feral mammals, due to lack
of human predation. The large-scale devegetation by mammalian herbivores of uninhabited Laysan (Hawaiian Is.) and Eiao
(Marquesas), and of St. Helena prior to
human settlement, led to the loss of much
oftheir endemic biotas (Ely and Clapp, 1973;
Montgomery et al., 1979; Cronk, 1989).
Another serious threat to native vegetation
is certain introduced plants, such as Miconia
in Tahiti, Psidium in Tubuai, Leucaena in
the Marquesas, which have the ability to
crowd out native vegetation with their
monospecific stands.
In addition to destroying habitat, introduced species can wreak havoc in direct
interactions (competition, predation) with
native species. Low diversity biotas such as
those on islands, whose species tend to have
less highly evolved defensive, competitive
and reproductive capabilities, are especially
vulnerable to biological invasion (Vermeij,
1991). As they evolved, insular organisms
were exposed to but a fraction of the diversity of predators sensu lato and competitors
experienced by mainland species; thus the
modern arrival of certain exotics, against
which the indigenous biota lack defenses,
can be devastating. As closely related species can all be at risk from a single exotic,
considerable portions of the biota can
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BIODIVERSITY ON ISLANDS
become rapidly endangered on islands where
large radiations occurred (Simon et ai,
1984).
The accidental introduction of the snake
Bioga irregularis to Guam has led to the
extermination of practically the entire native
bird fauna (Savidge, 1987); avian malaria
and avian pox have decimated Hawaiian
birds (Riper, 1991). Among insects, social
Hymenoptera are an especially serious
threat, and certain introduced ant species
have been implicated in the extinction of
large portions of the invertebrate faunas of
several Pacific island groups, especially on
some remote islands (see above) which
lacked indigenous ants {e.g., Cole et ai,
1992; Lubin, 1984; Howarth, 1990).
Humans themselves are a predatory
threat. Recent work on Holocene extinctions clearly implicates Homo sapiens in the
demise of insular birds. The arrival of
humans to Polynesia led to the extinction
of generally more than half, and in some
cases up to 80%, of each island's avifauna,
with large, predatory and flightless birds
being most vulnerable (Olson, 1989; Steadman, 1989). Similarities between insular
bird extinctions and the late Quaternary
demise of continental vertebrate megafaunas lends strong support to the interpretation that the latter were also human-caused
{e.g., Olson, 1989; Martin, 1990).
Although it may be impossible to arrest
the arrival of all introduced species to islands
with large human populations without causing extreme restrictions to travel and shipping, it is critical that we stem this flow as
much as possible. The scale of this ongoing
biotic interchange is astounding. Almost
3,000 species of terrestrial arthropods are
known to have been introduced to Hawaii,
the vast majority since European contact
(Nishida, 1992). Control can be more easily
implemented for islands with few or no
human inhabitants, by restricting and carefully screening access. Control of especially
harmful exotics should be a priority, and is
increasingly being pursued (Gillis, 1992).
One aspect of this biotic interchange that
can be controlled is the purposeful introduction of scores of species for biological
control. Many of these are fairly generalist
predators and parasites that can lead to the
extinction of numerous endemics (Howarth,
1991). The purposeful and thoughtless
introduction of the "cannibal snail," Euglandina rosea, has led to the extinction of
large suites of endemic land snails on one
island after another, providing some of the
most detailed case-studies of insular extinction (Hadfield et al, 1993; Murray et ah,
1988). Apparent relict groups, such as many
land snails, are among the most sensitive to
introduced predators, and the malacofauna
of Pacific islands is undergoing a veritable
mass extinction, with only an estimated 2535% of Hawaii's 1,461 described land snails
remaining extant (Solem, 1990, see also
above).
CONCLUSIONS
The isolation and small size of oceanic
islands yield differences in the dynamics of
diversification and extinction when compared to continents. These attributes, however, also allow a more intimate understanding of insular biotas: diversity on
islands is limited and the geographic and
ecologic ranges of species can be precisely
determined, providing excellent grounds for
studies on the distribution and origin of biological diversity. The extreme vulnerability
of insular biotas not only makes their study
and conservation ever more urgent, but the
mass extinctions occurring on many islands
facilitate analysis of numerous well defined
examples of human-caused extinctions,
studies which can lead to better conservation policies.
Low primary (founding) diversity promotes in situ diversification and the evolution of unusual adaptations. The most isolated islands, those with the lowest number
of original colonists, often have the largest
adaptive radiations and most peculiar species. Whether intra-island and inter-island
speciation is more important depends on
the dispersion ability of the taxon and
opportunities for isolation. Evidence from
islands shows the importance of topographic and environmental variability in
limiting species ranges and promoting in situ
diversification. Diversification can be very
rapid and result in large speciesflockswithin
the normally 3-15 My lifespan of oceanic
hotspot islands.
The small size and isolation of islands
result in relatively small and localized pop-
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GUSTAV PAULAY
ulations, and low primary diversity. Small
population size, especially prevalent among
species with large body size or those at high
trophic levels, makes many island species
especially vulnerable to extinction. The often
highly restricted ranges of flightless invertebrates exacerbate this problem, because
entire species can be vulnerable to even limited habitat destruction. Data from islands
also imply that mesic and xeric environments may have been equally or more
diverse, and certainly much more vulnerable, than moist habitats.
The relatively low levels of escalation
resulting from limited diversity of founders
allow for the survival of relicts and the
development of species with limited repertoires of defenses; thus island ecosystems
are especially vulnerable to invasion and
extinctions caused by introduced species.
Introduced species are much more devastating on islands than continents, and are
presently the greatest threat on many islands.
The most devastating exotics include plants
that can literally push out native forests with
their monospecific stands, social insects
(especially certain ants) and herbivorous
mammals. Studies of vertebrate extinctions
on islands strongly implicate humans as the
cause of late Quaternary extinctions worldwide. Purposeful introductions of generalist
predators for biological control have also
led to large scale extinction.
The 50-95 + % levels of extinctions
exhibited by taxa with a fossil record, like
birds and land snails, on oceanic islands,
are comparable to the most severe mass
extinctions in earth history (Jablonski,
1991). Arresting habitat destruction, reducing chances of future introductions, and
controlling harmful exotic species are
urgently needed to prevent the annihilation
of these most spectacular showcases of evolution.
ACKNOWLEDGMENTS
I thank Bern Holthuis, Frank Howarth,
Alex Kerr, Alan Kohn, Scott Miller, and
Barry Smith for comments on the manuscript and Trish Morse for organizing the
symposium. This is contribution 331 from
the University of Guam Marine Laboratory.
REFERENCES
Bahn, P. and J. Flenley. 1992. Easter Island, Earth
Island. Thames and Hudson, London.
Beverley, S. M. and C. A. Wilson. 1985. Ancient
origin for Hawaiian Drosophilinae inferred from
protein comparisons. Proc. Nat. Acad. Sci. U.S.A.
82:4753-4757.
Carlquist, S. 1974. Island biology. Columbia University Press, New York.
Carson, H. L. 1983. Chromosomal sequences and
interisland colonizations in Hawaiian Drosophila.
Genetics 103:465-482.
Cole, F. R., A. C. Medeiros, L. L. Loope, and W. W.
Zuehlke. 1992. Effects of the argentine ant on
arthropod fauna of Hawaiian high-elevation
shrubland. Ecology 73:1313-1322.
Cowie, R. H. 1992. Evolution and extinction of Partulidae, endemic Pacific island land snails. Phil.
Trans. Roy. Soc. Lond. B 335:167-191.
Cronk, Q. C. B. 1989. The past and present vegetation of St. Helena. J. Biogeogr. 16:47-64.
Diamond, J. M. 1977. Continental and insular speciation in Pacific land birds. Syst. Zool. 26:263268.
Ely, C. A. and R. B. Clapp. 1973. The natural history
of Laysan Island, Northwestern Hawaiian Islands.
Atoll Res. Bull. 171:1-361.
Frazetta, T. H. 1970. From hopeful monsters to
bolyerine snakes? Am. Nat. 104:55-72.
Gillis, A. M. 1992. Keeping aliens out of paradise.
BioScience 42:482-485.
Hadfield, M. G., S. E. Miller, and A. H. Carwisle.
1993. The decimation of endemic Hawaiian tree
snails by alien predators. Am. Zool. 33:610-622.
Hartnoll, R. G. 1988. Evolution, systematics, and
geographical distribution. In W. W. Burggren and
B. R. McMahon (eds.), Biology of the land crabs,
pp. 6-54. Cambridge University Press, Cambridge.
Helenurm, K. and F. R. Ganders. 1985. Adaptive
radiation and genetic differentiation in Hawaiian
Bidens. Evolution 39:753-765.
Hopper, D. R. and B. D. Smith. 1992. Status of tree
snails (Gastropoda: Partulidae) on Guam, with a
resurvey of sites studied by H. E. Crampton in
1920. Pacific Sci. 46:77-85.
Howarth, F. G. 1990. Hawaiian terrestrial arthropods: An overview. Bishop Museum Occ. Pap. 30:
4-26.
Howarth, F. G. 1991. Environmental impacts of classical biological control. Ann. Rev. Entomol. 36:
485-509.
Howarth, F. G. and W. P. Mull. 1992. Hawaiian
insects and their kin. University of Hawaii Press,
Honolulu.
Jablonski, D. 1991. Extinctions: A paleontological
perspective. Science 253:754-757.
James, H. F. and S. L. Olson. 1991. Descriptions of
thirty-two new species of birds from the Hawaiian
Islands: Part II. Passeriformes. Ornithological
Monographs 46:1-88.
Kirch, P. V. 1984. The evolution of Polynesian chiefdoms. Cambridge Univ. Press, Cambridge.
Kirch, P. V., J. R. Flenley, D. W. Steadman, F. Lamont,
BIODIVERSITY ON ISLANDS
and S. Dawson. 1992. Ancient environmental
degradation. Nat. Geogr. Res. Explor. 8:166-179.
Kohn.A. J. and F.E. Perron. 1993. Development and
distribution: Comparative life history and biogeography ofIndo-Pacific Conus. Oxford Univ. Press,
Oxford.
Lowrey, T. K. and D. J. Crawford. 1985. Allozyme
divergence and evolution in Tetramolopium
(Compositae: Astereae) on the Hawaiian Islands.
Syst. Bot. 10:64-72.
Lubin, Y. D. 1984. Changes in the native fauna of
the Galapagos Islands following invasion by the
little red fire ant, Wassmannia auropunctata. Biol.
J. Linn. Sci. 21:229-242.
MacArthur, R. H. andE. O. Wilson. 1967. Thetheory
of island biogeography. Princeton University Press,
Princeton.
Mares, M. A. 1992. Neotropical mammals and the
myth of Amazonian biodiversity. Science 255:976979.
Martin, P. S. 1990. 40,000 years of extinctions on
the "planet of doom." Palaeogeogr. Palaeoclimatol. Palaeoecol. 82:187-201.
Menard, H. W. 1986. Islands. W. H. Freeman, New
York.
Montgomery, S. L., W. C. Gagne, and B. H. Gagne.
1979. Notes on birdlife and nature conservation
in the Marquesas and Society Islands. 'Elepaio 40:
152-155.
Murray, J., E. Murray, M. S. Johnson, and B. Clarke.
1988. The extinction ofPartula on Moorea. Pacific
Sci. 42:150-153.
Nishida, G. M. (ed.) 1992. Hawaiian terrestrial
arthropod checklist. Bishop Museum Tech. Rep.
l:viii + 262.
Olson, S. L. 1989. Extinction on islands: Man as a
catastrophe. In D. Western and M. C. Pearl (eds.),
Conservation for the twenty-first century, pp. 5053. Oxford Univ. Press, New York.
Olson, S. L. 1991. Patterns of avian diversity and
radiation in the Pacific as seen through the fossil
record. In E. C. Dudley (ed.), The unity of evolutionary biology, pp. 314-318. The Proceedings of
the Fourth International Congress of Systematic
and Evolutionary Biology. Dioscorides Press,
Portland, Oregon.
Paulay, G. 1985. Adaptive radiation on an isolated
oceanic island: The Cryptorhynchinae of Rapa
revisited. Biol. J. Linn. Soc. 26:95-187.
Paulay, G. 1991a. Late Cenozoic sea level fluctuations and the diversity and species composition of
insular shallow water marine faunas. In E. C. Dudley (ed.), The unity of evolutionary biology. The
Proceedings of the Fourth International Congress
of Systematic and Evolutionary Biology, Vol. 1,
pp. 184-193. Dioscorides Press, Portland, Oregon.
Paulay, G. 199It. Henderson Island: Biogeography
and evolution at the edge of the Pacific plate. In
E. C. Dudley (ed.), The unity of evolutionary biology. The Proceedings of the Fourth International
Congress of Systematic and Evolutionary Biology,
Vol. 1, pp. 304-313. Dioscorides Press, Portland,
Oregon.
Perrault, G. G. 1987. Microendemisme et speciation
de genre Mecyclothorax (Coleoptera—Carabidae
143
Psydrini) a Tahiti (Polynesie Francaise). Bulletin
de la Societe Zoologique de France 112:419-427.
Riper, C. van III. 1991. The impact of introduced
vectors and avian malaria on insular passeriform
bird populations in Hawaii. Bull. Soc. Vector Ecol.
16:59-83.
Savidge, J. A. 1987. Extinction of an island forest
avifauna by an introduced snake. Ecology 68:660668.
Sibley, C. G. and J. E. Ahlquist. 1982. The relationships of the Hawaiian honeycreepers (Drepaninini) as indicated by DNA-DNA hybridization.
Auk 99:130-140.
Simon, C. M., W. C. Gagne, F. G. Howarth, and F. J.
Radovsky. 1984. Hawai'i: A natural entomological laboratory. Bull. Entomol. Soc. Amer. 30(3):
8-17.
Simpson, G. G. 1953. The majorfeatures ofevolution.
Columbia University Press, New York.
Solem, A. 1976. Endodontoid land snailsfrom Pacific
Islands (Mollusca: Pulmonata: Sigmurethra). Part
I. Family Endodontidae. Field Museum of Natural
History, Chicago.
Solem, A. 1977. La faune terrestre de Tile de SainteHelene. Fam. Charopidae. Mus. Roy. l'Afrique
Centrale—Tervuren, Belgique Ann., Ser. 8, Sci.
Zool. 220:521-533.
Solem, A. 1979. Biogeographic significance of land
snails, Paleozoic to Recent. In J. Gray and A. J.
Boucot (eds.), Historical biogeography, plate tectonics, and the changing environment, pp. 277287. Oregon State University Press, Corvallis.
Solem, A. 1982. Endodontoid land snails from Pacific
Islands (Mollusca: Pulmonata: Sigmurethra). Part
II. Families Punctidae and Charopidae, zoogeography. Field Museum of Natural History, Chicago.
Solem, A. 1990. How many Hawaiian land snail species are left? and what we can do for them. Bishop
Museum Occ. Pap. 30:27-40.
Springer, V. G. 1982. Pacific plate biogeography, with
special reference to shore fishes. Smiths. Contr.
Zool. 367:1-182.
Steadman, D. W. 1989. Extinction of birds in eastern
Polynesia: A review of the record, and comparisons with other Pacific island groups. J. Archaeol.
Sci. 16:177-205.
Thornton, I. W. B., T. R. New, R. A. Zann, and P. A.
Rawlinson. 1990. Colonization of the Krakatau
Islands by animals: A perspective from the 1980's.
Phil. Trans. Roy. Soc, London. B 328:131-165.
Tweedie, M. W. F. 1961. On certain Mollusca of the
Malayan limestone hills. Bull. Raffles Mus., Singapore 26:49-65.
Vagvolgyi, J. 1975. Body size, aerial dispersal, and
origin of the Pacific land snail fauna. Syst. Zool.
24:465-488.
Van Valen, L. 1973. A new evolutionary law. Evol.
Theory 1:1-30.
Vermeij, G. J. 1987. Evolution and escalation, an
ecological history oflife. Princeton University Press,
Princeton, New Jersey.
Vermeij, G. J. 1991. When biotas meet: Understanding biotic interchange. Science 253:1099-1104.
Vitousek, P. M. 1988. Diversity and biological invasions of oceanic islands. In E. O. Wilson (ed.),
144
GUSTAV PAULAY
Biodiversity, pp. 181-189. National Academy
Press, Washington, D.C.
Wagner, W. L. 1991. Evolution of waiffloras:A comparison of the Hawaiian and Marquesan archipelagoes. In E. C. Dudley (ed.), The unity of evolutionary biology. The Proceedings of the Fourth
International Congress of Systematic and Evolutionary Biology, Vol. 1, pp. 267-284. Dioscorides
Press, Portland, Oregon.
Williamson, M. 1981. Island populations. Oxford
University Press, Oxford.
Wilson, E. O. 1990. Success and dominance in eco-
systems: The case of the social insects. Ecology
Institute, Oldendorf/Luhe, Germany.
Zimmerman, E. C. 1942. Distribution and origin of
some eastern oceanic insects. Am. Natur. 76:280307.
Zimmerman, E. C. 1948. Insects of Hawaii, Vol. 1.
Introduction. University of Hawaii Press, Honolulu.
Zimmerman, E. C. 1970. Adaptive radiation in
Hawaii with species reference to insects. Biotropica 2:32-38.