mascarene islands, biology

apace. By the beginning of the twenty-first century, the
atolls were home to over 51,000 people.
NUCLEAR TESTING
Some of the northwestern atolls were used for the first
postwar nuclear testing ever conducted. From 1946 to
1958, 67 nuclear devices were detonated on Bikini and
Enewetak atolls, with radioactive material that spread
over the rest of the country with long-term ramifications
that are still being investigated and debated.
Such is only part of what remains to be discovered
about the Marshall Islands. The atolls’ overall charming
appearance and outwardly simple environment belies
their true diversity, complexity, and importance.
SEE ALSO THE FOLLOWING ARTICLES
Atolls / Introduced Species / Nuclear Bomb Testing / Pacific Region
FURTHER READING
AUQ2: Please
provide page
numbers.
Amerson, A. B., Jr. 1969. Ornithology of the Marshall and Gilbert Islands.
Atoll Research Bulletin 127: 1–216.
Crisostomo, Y. A. 2000. Initial communication under the United Nations
framework convention on climate change. Majuro: Republic of the
Marshall Islands Environmental Protection Authority.
Erdland, A. 1914. Die Marshall Insulanur. Leben und sitte, sinn und religion eines sudseevolkes. Antropos Bibliothek 2.
National Biodiversity Team of the Republic of the Marshall Islands. 2000.
The Marshall Islands: living atolls amidst the living sea. The National Biodiversity Report of the Marshall Islands. Majuro: RMI Biodiversity Project.
Neidenthal, J. 2001. For the good of mankind: a history of the people of Bikini
and their islands, 2nd ed. Majuro, Marshall Islands: Bravo Publishers.
Republic of the Marshall Islands Biodiversity Clearing House Mechanism. http://www.biormi.org/.
MASCARENE ISLANDS,
BIOLOGY
CHRISTOPHE THÉBAUD
Université Paul Sabatier, Toulouse, France
BEN H. WARREN AND DOMINIQUE
STRASBERG
Université de La Réunion, Saint-Denis, Réunion.
ANTHONY CHEKE
Oxford, United Kingdom
The Mascarenes are an island group lying near the Tropic
of Capricorn in the southwestern Indian Ocean ∼700 km
east of Madagascar. This archipelago comprises three high
volcanic islands (Réunion, Mauritius, Rodrigues), scat-
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tered along a ∼600 km west-east axis, and a group of small
coralline islands (Cargados Carajos Shoals) ∼400 km to
the north of Mauritius, which sit upon a submarine bank
of volcanic origin that extends a further 700 km or more
to the northeast. The Mascarene Islands have an extraordinary status among islands: Mauritius was the former
home of the dodo, the universal symbol of human-caused
species extinction on islands. Although their recent history, since the first permanent human settlements in the
seventeenth century, has been an endless series of ecological disasters and species extinctions, these islands still
harbor up to 25% of their original forest cover and are
extremely rich in species and habitats, with high degrees
of endemism. Consequently, they are listed among the
world’s top Biodiversity Hotspots.
THE GEOGRAPHICAL CONTEXT FOR THE
EVOLUTION OF BIODIVERSITY IN THE
MASCARENE ISLANDS
Although less well-studied than the Hawaiian Islands,
the Mascarene Islands, Rodrigues excluded, are generally
believed to result from the same process of plate movement over a stationary hotspot. Today the Réunion hotspot is the source of frequent volcanism on the island
of Réunion. The Réunion hotspot’s activity, however,
can be traced northeast along the Mascarene Plateau to
India, where massive Deccan volcanism coincided with
the Cretaceous–Tertiary (K/T) mass extinction event.
Rodrigues, which sits next to the Central Indian ridge,
is thought to have arisen in relation to the tectonic evolution of the Rodrigues triple junction, located 950 km to the
southeast of the island. Typical of such archipelagoes, the
Mascarene islands of today have have never been connected
to larger land masses. Thus the biogeography and endemic
biodiversity of these islands are the product of oceanic
dispersal alone. The three main islands of today, Réunion,
Mauritius, and Rodrigues, are very different in their size
and current topography but are united by their relative
geographic proximity and volcanic origin. Réunion, the
largest (2512 km2) and most southerly (21° S, 55.5° E), is
nearest to Madagascar (665 km), whereas Mauritius, next
in size (1865 km2) is 164 km east-northeast of Réunion.
Rodrigues, the smallest (104 km2) and currently the most
isolated, is located 574 km east of Mauritius.
The islands are separated from each other by fracture
zones, and each island has developed independently.
The most ancient dated lavas from Réunion, Mauritius,
and Rodrigues are dated at 2.1, 7.8, and 1.5 million years
ago, respectively. However, many exposed lavas in the
Mascarenes are the result of recent reactivation, and new
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data suggests that Rodrigues, instead of being the youngest, is at least as old as Mauritius. Thus Mauritius and
Rodrigues have been available for colonization by diverse
biota for about 8–15 million years, while Réunion became
habitable much later, about 2–3 million years ago. The
extent to which Mauritius and Rodrigues have been isolated from larger land masses over the course of their history has been influenced by Pliocene and Quaternary sea
level changes. Information from past sea level curves and
current ocean floor bathymetry supports the existence of
several large islands, as recently as 18,000–10,000 years ago,
along the 115,000-km2 Mascarene Plateau (currently under
water with depths ranging from 8 to 150 m) between the
granitic Seychelles and the Mascarenes. Drilling projects
establish a volcanic origin for (or volcanic contribution to)
these islands, with erosion and subsidence thereafter. It is
likely that these islands, and also the Chagos and Maldives
when fully above water, have played a role as a source of
colonists for the present islands. In addition, chains of
smaller islands would have reduced the distance for oceanic dispersal and could have served as steppingstones for
dispersal between India and the Mascarenes.
As a consequence of erosion and subsidence on older
islands and volcanic activity on younger islands, the greatest
elevations above sea level are currently found on Réunion,
with two main summits: Piton des Neiges (3070 m), which
is the highest peak in the Indian Ocean, and Piton de La
Fournaise (2631 m ), one of the most active volcanoes in the
world. The highest points of Mauritius (Black River Peak,
828 m) and Rodrigues (Mt. Limon, 398 m) are low in comparison. Réunion, like other young volcanic islands, has a
very dramatic topography, being highly dissected into huge
caldera-like valleys (cirques) caused by erosion under very
high rainfall, with very narrow outlets to the sea through
deep gorges. Mauritius, in spite of being an old island,
has undergone dramatic geological transformation until
recently. Volcanic eruptions have reshaped the island into
a series of small, eroded, “geological” islands (age 7.5–5.1
million years) embedded in a matrix of recent lava flows
(0.7–0.025 million years old). Thus, both Mauritius and
Réunion show considerable spatial heterogeneity in their
topography. While the significance of such heterogeneity for the evolution of colonist lineages is evident in the
case of Réunion, the biological implications of the “islands
within the island” structure of Mauritius, though obvious,
have been overlooked by most biologists until very recently.
As on the other islands, the main relief of Rodrigues is
composed of basaltic lava, but Rodrigues also has an area
of limestone plateau made of consolidated coral sands and
punctuated with caves.
Owing to their geological history, geographic isolation, and current climate, the Mascarene Islands show
more similarities to the Hawaiian Islands than to any
other archipelago, even though these two island systems
differ greatly in the numbers of islands currently present
and the degree of isolation from the nearest other masses.
Colonization of the Mascarene Islands by immigrating
lineages has occurred relatively recently, but in spite of the
simplicity of the present geographic setting, evolutionary
diversification in the archipelago has been strongly influenced by a rather complex volcanic evolution combined
with a regional geographic configuration that has greatly
changed since the first island was formed.
THE ECOLOGICAL THEATER
The Mascarene Islands have a tropical climate; that is,
temperatures are warm and show little seasonal variation.
The climate is strongly influenced by the humid prevailing winds blowing from the southeast, with annual rainfall varying from 500 mm in the driest leeward areas to
about 12 m in the wettest areas on the windward slopes of
Réunion. Such climate generally promotes the development of forests. From early reports and ecological inference from what is left of the original vegetation, all three
main islands were completely forested when discovered.
Exceptions are the high-elevation environments above
1900 m on Réunion, where the forests give way to a subalpine scrub.
The Mascarene Islands share with other oceanic
islands the habitat destruction and transformation associated with human activity. Low-altitude areas have been
subject to a much higher impact than high-altitude areas.
Although native vegetation remains, all the original forest
covering Rodrigues has been destroyed, and a mere 2% of
the original cover has been left in Mauritius. In contrast,
about 25% of the estimated original extent of Réunion’s
habitats are still in a good state. As a result of deforestation
and rugged topography, Réunion’s forest remnants are
severely fragmented, with large tracts found only above
500 m elevation, and with no more than 1% of lowland
forest remaining. Lowland forest remnants are mostly
located on the slopes of the active volcano, where they
often take the form of forest islands embedded in a matrix
of lava flows of various ages. Unfortunately, many of these
forest islands were wiped out by a massive volcanic eruption in 2007. Descriptions of the vegetation zones, using
historical accounts and subfossil record when necessary,
have emphasized five natural plant formations arranged in
broad moisture and altitudinal zones. Dry lowland forests
dominated by palms (Latania spp., Dictyosperma album),
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screw-pines (Pandanus spp), and trees such as Terminalia bentzoe (Combretaceae) were present from sea level to
200 m elevation in areas with less than 1000 mm average
annual rainfall. This ecosystem was probably the habitat
of some of the most spectacular endemic animals, in particular the now extinct giant tortoises (Cylindraspis spp.,
Testudinidae), but it no longer exists on the main islands.
Some relicts may be found on a small islet (Round Island)
off the northern tip of Mauritius and in a few places on
Réunion.
Semi-dry sclerophyllous forests occurred between
coastal areas and 360 m on all sides of Mauritius and
Rodrigues, but were restricted to 200–750 m elevation on
the western slopes of Réunion, where they still exists in
small forest remnants. This ecosystem has an average annual
rainfall of 1000–1500 mm and is characterized by ebonies
(Diospyros spp., Ebenaceae) and other trees such as Pleurostylia spp. (Celastraceae), Foetidia spp. (Lecythidaceae),
Olea europea subsp. africana (Oleaceae), Cossinia pinnata
(Sapindaceae), Dombeya spp. (Sterculiaceae), and a variety
of Sapotaceae species (Sideroxylon boutonianum, Mimusops
spp.). The ecosystem is also home to several spectacular
endemic species of Hibiscus (Malvaceae). Many species of
this zone, such as Zanthoxylum spp. (Rutaceae), Obetia
ficifolia (Urticaceae), and Scolopia heterophylla (Flacourtiaceae), exhibit developmental heterophylly, with juvenile
leaves being more divided than those of adults. Such convergence may have evolved to deter herbivory by extinct
giant tortoises.
Lowland rainforests occur above 360 m (on Mauritius)
and all over the eastern lowlands from the coast to 800–
900 m and, on the western side, from 750 to 1100 m (on
Réunion) (average annual rainfall 1500–6000 mm). These
forests have a canopy of tall trees up to 30 m high and
represent the richest plant communities of the Mascarene
Islands. Characteristic plants include trees in the plant
family Sapotaceae (e.g. Mimusops spp., Labourdonnaisia
spp., Sideroxylon spp.), Hernandiaceae (Hernandia mascarenensis), Clusiaceae (Calophyllum spp.), and Myrtaceae
(Syzygium spp., Eugenia spp., Monimiastrum spp.); shrubs
in the plant family Rubiaceae (Gaertnera spp., Chassalia
spp., Bertiera spp., Coffea spp.); and numerous species of
orchids (e.g., Angraecum spp., Bulbophyllum spp.) and
ferns (e.g., Asplenium spp., Hymenophyllum spp., Trichomanes spp., Elaphoglossum spp.).
Dense cloud forests occur on Réunion between 800
and 1900 m on eastern slopes (average annual rainfall
2000–10,000 mm) and between 1100 to 2000 m on western slopes (average annual rainfall 2000–3000 mm) and
are also restricted to a small area of Mauritius around
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Montagne Cocotte above 750 m on Mauritius (average
annual rainfall 4500–5500 mm). On both islands these
low forests, with a canopy of 6 to 10 m high, are rich in
epiphytes (orchids, ferns, mosses, lichens), emergent tree
ferns (Cyathea spp.), and, originally, palms (Acanthophoenix rubra), but these now survive only in areas of Réunion
where poaching has not wiped them out. Untransformed
cloud forests still cover large areas on Réunion (44,000 ha
in 2005). These forests are characterized by trees such as
Dombeya spp. (on Réunion only) and species in the plant
family Monimiaceae (Monimia spp., Tambourissa spp.) as
canopy species, with small trees and shrubs such as Psiadia spp. (Asteraceae) and Melicope spp. (Rutaceae) in the
understory. They also include large areas of three monodominant plant communities, forests with Acacia heterophylla (Fabaceae) as canopy species that are very similar to
Acacia koa forests in Hawaii, thickets dominated by Erica
reunionensis (Ericaceae), or hyperhumid screw-pine forest
(Pandanus montanus).
Finally, above the tree line, at elevations where frosts
occur regularly in winter (1800–2000 m), is a unique
subalpine scrub dominated by shrubs in the plant families of Ericaceae (Erica spp.), Asteraceae (Hubertia spp.,
Psiadia spp., Stoebe passerinoides), and Rhamnaceae
(Phylica nitida), with some notable endemic species such
as Heterochaenia rivalsii (Campanulaceae), Eriotrix commersonii (Asteraceae), and Cynoglossum borbonicum (Boraginaceae) (average annual rainfall 2000–6000 mm).
The summits of the volcanoes are covered by large mineral areas with sparse grasslands rich in endemic grasses
(Poaceae, e.g., Festuca borbonica, Agrostis salaziensis, Pennisetum caffrum) and orchids (Orchidaceae, e.g., Disa
borbonica), ericoid thickets, or thickets of the small tree
Sophora denudata (Fabaceae), depending on substrate
texture and age.
The Mascarene Islands are surrounded by approximately 750 km2 of coral reef. Rodrigues has nearly continuous fringing reefs bounding an extensive lagoon with
deep channels, whereas Mauritius is surrounded by a discontinuous fringing reef and a small barrier reef. In contrast, Réunion has very short stretches of narrow fringing
reefs along the western and southwestern coasts only. The
islets of the Cargados Carajos Shoals, which have a very
depauperate terrestrial biota owing to being so low-lying
and swamped during cyclones, are bound to the east by
an extensive arc of fringing reef, which accounts for ∼30%
of the reefs of the Mascarene Islands. Lagoon reefs and
reef flats are dominated by scleractinian corals such as
branching and tabular Acropora, Porites massives, foliaceous Montipora and Pavona, and sand consolidated with
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beds of seagrass such Halophila spp. (Hydrocharitaceae).
Among coral reef fishes, wrasses (Labridae), damselfish
(Pomacentridae), carnivorous groupers (Serranidae),
and surgeonfishes (Acanthuridae) are particularly well
represented.
BIODIVERSITY AND ENDEMISM
Identifying species that are unique (endemic) to a group
of island or individual islands relies upon extensive biological inventories, including in-depth systematic investigations, and the ability to recognize cryptic species in
lineages that lack subtstantial morphological differentiation across their range. Although there is currently much
effort to fill the gaps, there are still many taxonomic
groups in the Mascarene Islands that have not been thoroughly investigated and for which rigorous figures for
endemism are not yet available. The Mascarenes therefore present an exciting relatively unchartered study system for island biologists. However, the Mascarene biota,
like most other oceanic island biotas, has suffered many
recent extinctions that are a source of information bias,
particularly in groups of organisms that leave no subfossil
materials or that were not recorded and described by the
early travelers.
The Mascarene biota exhibits high levels of endemism
in many groups of related taxa: about three-quarters
of the approximately 960 native flowering plant species, ∼65% of the Coleoptera (∼1550 species), and 90%
of the nonmarine molluscs (∼200 species) are endemic
(Table 1). These degrees of endemism are very close to
those observed in similar groups in the Hawaiian Islands
or New Caledonia. The Mascarene Islands had the richest
oceanic island reptile fauna before the arrival of people.
For nonmarine reptiles, the percentage of endemic species
TABLE 1
A Summary of Mascarene Biodiversity and Endemism for Different
Taxonomic Groups with Adequate Systematic Knowledge
Taxonomic Group
Flowering plants
Ferns and allies
Mosses and allies
Nonmarine mammals
Reef fishes
Landbirds
Seabirds
Nonmarine reptiles
Nonmarine molluscs
Coleoptera
Number of
Percent of
Number of
Endemic
Endemic
Species
Species
Species
959
265
∼800
7
923
60
21
32
200
1538
691
58
40–80
4
42
51
3
30
180
979
72
22
5–10
57
5
85
14
94
90
64
has been estimated to be 94%, but more than half of the
30 endemic species known to have occurred in the Mascarene Islands have gone extinct in the last four centuries,
including five species of Indian Ocean giant tortoises.
Of three endemic snakes, only one boa still exists (Casarea dussumieri). Apart from bats there are no terrestrial
mammals, but all three species of fruit bats (Pteropus
spp.; 1 extinct) and at least two (two newly recognized
species of Mormopterus) of the four species of microbats
are endemic. The Mascarene Islands once had a very rich
avifauna, with an estimated 81 native species, 54 of which
(67%) were endemic to the archipelago. Apart from three
seabirds (Pterodroma baraui, Pseudobulweria aterrima,
and an undescribed Pterodroma known only from subfossil bones), most endemic species were landbirds. A large
fraction of these endemic birds, especially the larger ones,
have now gone extinct, among which were found the legendary dodo (Raphus cucullatus, Columbidae, formerly
Rhaphidae), its flightless relative the Rodrigues solitaire
(Pezophaps solitarius), and the Réunion solitaire (Threskiornis solitarius, which was not related to the dodo but was
an ibis, Threskiornithidae).
For the marine biota, there is a dearth of comprehensive systematic investigation at the scale of the archipelago. The average percentage of marine species in the
Mascarene Islands that are endemic is apparently lower
(2–15%) than in the terrestrial biota. However, the degree
of endemism may vary considerably among taxonomic
groups, and it is likely that cryptic species, having virtually indistinguishable morphologies, are more widespread
in the sea than previously thought. Future broad-scale
examination of groups with many wide-ranging species,
such as marine molluscs (with at least 3000 species of gastropods occurring in the western Indian Ocean region),
crustaceans (with a minimum total of 780 species for the
western Indian Ocean), or bryozoans (with at least 500
species in the western Indian Ocean), using molecular
taxonomy and new morphometric approaches, may reveal
similar levels of endemism to those observed in some terrestrial groups.
There are many endemic genera of plants and animals
in the terrestrial biota. For example, 32 genera of flowering plants (11% of the total number of genera) and 89
genera of Coleoptera (14% of the total number of genera)
are restricted to the Mascarene Islands. Some of these genera provide spectacular examples of diversification within
the archipelago, such as weevils (Cratopus, 86 species), leaf
beetles (Trichostola, >25 species) and several shrubs (e.g.,
Badula [14 species], Heterochaenia [3 species], Trochetia
[6 species]) (Fig. 1). However, the highest numbers of
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FIGURE 2 Forgesia racemosa, or bois de Laurent-Martin. This enigFIGURE 1 Badula borbonica, or bois de savon (Myrsinaceae). Badula is
matic and beautiful species, common in cloud forests, is in the Escal-
a species-rich genus, endemic to the Mascarene Islands . Here is shown
loniaceae family. Its closest known relatives live in the Andes, South
a large-leaved species that forms a medium-sized unbranched shrub
America. Photograph by Christophe Thébaud.
in the dense cloud forests of Réunion. Photograph by Christophe
Thébaud.
endemics are often found in nonendemic genera (numbers of species in parenthesis), for example, Gonospira
landsnails (28), Phelsuma geckos (9), Diospyros trees (14),
Dombeya trees (13), Gaertnera shrubs (14), Pandanus screw
pines (22), or daisy trees Psiadia (26). Such a pattern
suggests that a number of very recent species radiations
have been a significant factor in the buildup of endemic
biodiversity in the Mascarene Islands.
PHYLOGEOGRAPHY, PROCESS OF SPECIES
FORMATION, AND ADAPTIVE RADIATION
An important question for understanding endemic biodiversity is the geographical origin of colonizing lineages.
As expected from current geography, Mascarene biota
has close affinities with Madagascar and Africa. However, unexpectedly, many elements are related to more
remote regions, notably Asia and the Indo-Pacific region.
In flowering plants, about two-thirds of the genera are
shared between the Mascarene Islands and Madagascar
and Africa (e.g., Angraecum, Diospyros, Dombeya, Psiadia)
whereas at least 20% are shared with Asia and the IndoPacific region (e.g., Astelia, Ochrosia, Terminalia) (Fig. 2).
This pattern has been suggested for many other groups,
including birds, insects, and even reptiles, including the
endemic Mauritian boa family Bolyeridae. Recent phylogenetic analyses using DNA markers have confirmed
either the western (e.g., the fruit fly Drosophila mauritiana, Falco kestrels, Cylindrapsis tortoises, Phelsuma
geckos, Angraecum orchids, Gaertnera shrubs, Phylica
shrubs, Polyscias trees, Psiadia daisy trees) or the eastern
(e.g., Mormopterus free-tailed bats, Leiolopisma skinks,
Nactus geckos, Hypsipetes bulbuls, Aerodramus swiftlets,
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Gillespie08_M.indd 616
Psittacula parakeets, Alectroenas pigeons, the climbing
shrub Roussea simplex) origins of some groups (Fig. 3).
They have also revealed the Asian origins in groups with
poorly understood evolutionary history. The dodo and
the Rodrigues solitaire, whose geographic origins have
long been mysterious, appear to have dispersed from
southeast Asia to the Mascarene Islands at some point
in the past. The phylogenetic relationships of Indian
Ocean white-eyes (Zosterops) point to an Asian origin for
Mascarene species, contrary to intuition (Fig. 4). That a
significant portion of colonist lineages comes from the
east emphasizes the likely role played by now-submerged
land masses between the Mascarene Islands and India and
also sea currents and winds in drawing high numbers of
colonists from Asia and the Indo-Pacific region into the
southwestern Indian Ocean region.
FIGURE 3 The day gecko Phelsuma cepediana, one of the seven sur-
viving Mascarene species, is currently the sole pollinator and seed disperser of Roussea simplex, a climbing shrub endemic to the mountains
of Mauritius that was named after Jean-Jacques Rousseau, the Swiss
philosopher of the Enlightenment. Photograph by Dennis Hansen.
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FIGURE 4 Zosterops mauritianus belongs to Mascarene grey white-
eyes, an anomalous group of warbler-like white-eyes with no “whiteeye” with Asian affinities that appears to have undergone a cryptic
adaptive radiation in the Mascarenes. Photograph by Charlie Moores.
How biodiversity builds up in an archipelago like the
Mascarene Islands after the first island has appeared above
sea level depends on patterns of dispersal and subsequent
diversification of founding lineages within the nascent
archipelago, including within-island speciation. Some
taxa were never successful in colonizing the archipelago.
For example, amphibians were absent from the original
fauna. Among lineages that colonized the archipelago,
some may have repeatedly colonized the archipelago or
diversified more than others. Data on the diversification
of the Mascarene biota are still too scanty to draw any
generalization, but recent molecular studies provide good
illustrations of the processes that have led to species diversity in this region. An example in which species diversity
within the archipelago reflects multiple successful colonizations from source areas comes from fig trees (Ficus),
fruit bats (Pteropus), and orchids (Angraecum). Recent
phylogenetic hypotheses imply that the five species of figs
found in the Mascarenes, three of them being endemic,
have arisen from five separate colonization events. The
three endemic fruit bat species apparently originated
from three distinct colonizations. Concerning Angraecum, a genus represented by approximately 30 species in
the Mascarene Islands, 21 of which are endemic, phylogenetic data implies at least 20 independent colonization
events. In contrast, biodiversity in other groups appears
to result from single or a few colonization events followed
by the evolution of species radiations. The daisy trees
(Psiadia), the second most species-rich genus of flowering
plant in the Mascarene Islands (Angraecum is the first),
display phylogenetic relationships that are consistent with
a double archipelago colonization, followed by extensive
species radiations within both Mauritius and Réunion.
The phylogenetic hypothesis for day geckos (Phelsuma)
is concordant with a single colonization of the archipelago, something that is also true for other endemic reptile
taxa in the Mascarene Islands (e.g., Cylindrapsis tortoises,
Nactus geckos). The nine Mascarene species of day geckos
are best explained by a combination of inter-island dispersal and intra-island speciation events. Understanding
how speciation proceeds within small islands in groups as
diverse as flowering plants, reptiles, insects, and even birds
is not well understood yet in the Mascarenes. Considerable morphological variation is found in diverse organisms on both Mauritius and Réunion. Many species of
streptaxid land snails and several species of day geckos display high among-population morphological and/or genetic
variation within the islands. The Mascarene grey white-eye
(Zosterops borbonicus), a bird endemic to Réunion, shows
spectacular variation in plumage traits that coincide with
differences in the habitats occupied within this topographically and climatologically diverse island. Thus, it
seems likely that natural selection is the key to diversification both among and within islands, although the
detailed processes involved remain to be studied.
CONSERVATION ISSUES
Before the arrival of the Europeans in the sixteenth
century, the Mascarene Islands had evaded discovery
by seafarers. The early visitors released ungulates and,
on Mauritius, rats and monkeys, but the islands were
settled only in the the mid-seventeenth century, when
commercial rivalry induced European trading nations to
annex and settle the islands. The Mascarene Islands had
not experienced major perturbations of the biota when
early visitors started to describe what they found. Hence,
nowhere in the world has the tragic loss of species and
the alteration of pristine tropical island ecosystems been
documented as thoroughly as in the Mascarenes.
Historical records demonstrate unambiguously that
forest clearance, human hunting, and the introduction of nonindigenous predators have been the primary
causes of species extinctions in these islands. In total, the
Mascarenes have lost about 40% of their native vertebrate
species, and most of these were already gone by the middle of the nineteenth century. Some iconic species such as
the dodo, the Rodrigues solitaire, and the Réunion solitaire even disappeared from Mascarene landscapes by the
late seventeenth or early to mid-eighteenth century. The
last giant tortoises were seen in Rodrigues around 1800,
and when Charles Darwin visited Mauritius aboard the
Beagle in 1836, there had been no record of any tortoise on
this island for more than a century. These extinctions and
several others mostly predate the first spells of extensive
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human-caused habitat destruction. This fact implicates
human hunting and predation by rats, cats, and pigs,
rather than forest clearance, as the primary cause of vertebrate extinctions in the Mascarenes. Patterns of vertebrate
species loss also show clearly that the impacts of human
settlement and introductions of predators occurred very
rapidly. They may have been exacerbated by the fact that
many species possessed characteristics that increased their
susceptibility to human hunting (large body size) or the
impact of nonindigenous predators (lack of mammalian
predator escape response, including, e.g., flightlessness,
tameness).
As a consequence of growing human populations and
colonial policies, the rate of forest destruction peaked
during the nineteenth century. Forests were cleared for
agriculture development to produce sugar cane in both
Mauritius and Réunion and maize, coffee, and geranium oil in Réunion, and for slash-and-burn agriculture combined with free-range livestock production in
Rodrigues. Such forest destruction likely caused many
extinctions by wiping out entire habitats (e.g., semi-dry
sclerophyllous forests). Another, almost inevitable, consequence of large-scale forest destruction was increased
fragmentation of habitats coupled with invasion by
a wide range of introduced plants, additional animals
(e.g., Herpestes mongooses, Calotes agamid lizard, and
Lycodon wolf snakes), and pathogens. Notable extinctions that occurred during this period include the hoopoe starling (Fregilupus varius, a bird species that was
still common in forested areas of Réunion into the early
1850s but had vanished before 1860), the pigeon hollandais (Alectroenas nitidissima), the Mauritius lizard-owl
(Mascarenotus sauzieri, last seen in the 1820s or 1830s),
the slit-eared skinks (Gongolymorphus spp., which disappeared with the arrival of the wolf snake and survive
only on islets offshore), and the anomalous hole-roosting flying-fox (Pteropus subniger, which vanished on
Réunion around 1840).
During the twentieth century, destruction of forests
continued (often for short-lived agricultural initiatives
or even make-work programs), fragmentation of forest
remnants increased, nonindigenous species continued
to arrive in the Mascarenes, and the number of species
on the verge of extinction rose steadily. Today, Mauritius
and Rodrigues are so extensively altered that most of their
native biotas are already extinct or severely threatened.
In addition, Mauritius forest remnants are permeated
with hordes of invasive mammal species such as monkeys, deers, pigs, and mongooses. By contrast, Réunion
is relatively free of these animals. Hence, the survival of
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relatively intact Mascarene ecosystems largely depends
on adequate conservation on the island of Réunion,
although even here the best surviving lowland forest was
mostly lost to ill-conceived forestry in the 1970s. From a
network of reserves finally set up in the 1980s onwards,
approximately 1000 km2 of Réunion (40% of the island
area) were designated as a national park in 2007, with a
further 35 km2 of marine nature reserves. In Mauritius,
nominally protected areas are much longer established
(dating from 1951), and a national park covering 66
km2 and much of the best remaining habitat was established up in 1994. Total reserves cover a mere 75 km2,
but considerable effort has been devoted to conservation
management work. Pioneering habitat restoration programs have been underway since the late 1960s, but until
recently the programs have been focused on a handful of
endangered bird species. Through captive-breeding and
release programs, including eradication of nonindigenous predators, the Mauritius kestrel (Falco punctatus),
pink pigeon (Nesoenas mayeri), and echo parakeet (Psittacula eques) were rescued from imminent extinction,
while the future of Mauritius fody (Foudia rubra) and
Mauritius olive-white-eye (Zosterops chloronothus) now
also looks more secure. An extraordinary result is that
the population of Mauritius kestrels recovered from a
single wild breeding pair in 1974, when its prospects were
considered to be hopeless, to over 900 individuals in the
wild today. The populations of echo parakeets and pink
pigeons have also bounced back from 10 (early 1980s) to
300 and 10 (1991) to 360 individuals in the wild today,
respectively.
In the Mascarene Islands, eradication of nonindigenous predators has become a high conservation priority
to prevent further extinctions and is often a prerequisite to ecosystem restoration work. On islets around
Mauritius and Rodrigues such as Round Island, Ile aux
Aigrettes, or Gunner’s Quoin, eradication programs
have succeeded in clearing these islands from feral goats,
rabbits, rats, cats, and mice, although house shrews,
agamid lizards, and wolf snakes have proved harder to
remove. Removal of these predators has led to increase
in numbers of native plant and animal species living on
the islets, and the now stabilized conditions on Round
Island have allowed the palm forest to recover and the
unique reptiles there to thrive, and one species, Telfair’s
skink (Leiolopisma telfairii), has been reintroduced onto
islands it formerly inhabited, now again rat-free. However, complete eradication of all predators is not always
possible, and eradication programs easily become a difficult conundrum for conservation. In Réunion, where
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cats and rats threaten endemic petrels that breed on
mountain tops, conservation managers have to take into
account possible mesopredator effects. While intuition is
that cats should be eliminated first, this may not be the
best strategy if a population explosion of rats might follow and hit the petrel populations even harder. However,
cats mostly eat adult petrels, while rats only attack eggs
and chicks. Thus, even if the eradication of cats leads to
an increase in rat density, this might not necessarily mean
a decline in the petrel populations. Removal of cats and
rats in nesting areas is under way, with long-term monitoring projects to ensure that conservation action leads
to increased population sizes, not to unwanted decreases
due to mesopredator effects.
Nonindigenous plant invasions are widespread in
the remnant native ecosystems of the Mascarenes. Most
invaders colonize human-disturbed sites most successfully, with sizable forest remnants being still dominated
by native species. In Mauritius and Rodrigues, and to
lesser extent in the lowlands of Réunion, forest remnants
are small, disturbed, and heavily invaded. To improve the
prospects of long-term survival of these fragments and
the species that inhabit them, pioneering restoration
programs began in the late 1960s in Mauritius, and have
been much extended, with Rodrigues added, from the
early 1980s. Conservation management areas were established in remnants of the major original habitat types and
have demonstrated that habitat restoration can effectively
reduce the rate of species loss if based on sound ecological knowledge. These areas were fenced to reduce access
by introduced herbivores and regularly weeded to control populations of nonindigenous plants. They usually
showed an improvement in natural regeneration of native
plants in less than ten years, including the legendary tambalacoque tree (Sideroxylon grandiflorum), popularly supposed to be dependent on the extinct dodo. However,
although some species that were not regenerating before
management were now thriving, nearly half of the species
were still not regenerating. This lack is likely related to
alterations of plant-pollinator and plant-disperser interactions as a result of extreme habitat fragmentation and
animal species extinctions. The loss of native mutualists
can limit natural regeneration of native plants that were
once dependent on them, through the shortage of pollinators or seed dispersers (Fig. 5). Pioneering work is currently under way to reconstruct missing elements of these
critically endangered ecosystems by filling the gaps left
by the lost species using analogues (e.g., related species
surviving in other parts of the Indian Ocean). However,
suggestions to reintroduce from Mauritius species lost
FIGURE 5 Giant Aldabra tortoises (Aldabrachelys gigantea), intro-
duced into Iles aux Aigrettes (Mauritius), can be used as ecological
analogue seed dispersers of Syzygium mamillatum, a rare endemic
tree. Photograph by Dennis Hansen.
in Réunion (several birds and a fruit bat), and vice versa
(Réunion harrier, Circus maillardi), have so far not been
acted on.
SEE ALSO THE FOLLOWING ARTICLES
Biological Control / Coral / Deforestation / Dodo /
Mascarene Islands, Geology
FURTHER READING
Atkinson, R., J. C. Sevathian, C. N. Kaiser, and D. M. Hansen. 2005. A
guide to the plants in Mauritius. Vacoas, Mauritius: Mauritius Wildlife
Foundation.
Austin, J. J., E. N. Arnold, and C. G. Jones. 2004. Reconstructing an
island radiation using ancient and recent DNA: the extinct and living
day geckos (Phelsuma) of the Mascarene islands. Molecular Phylogenetics and Evolution 31: 109–122.
Blanchard, F. 2000. Guide des milieux naturels: La Réunion–Maurice–
Rodrigues. Paris: Editions Eugen Ulmer.
Bosser J., T. Cadet, J. Guého, and W. Marais. 1976–2005. Flore des Mascareignes. Paris: Editions de l’Institut de Recherche pour le Développement (IRD).
Cheke, A., and J. Hume. 2008. Lost land of the Dodo: an ecological history
of the Mascarene Islands. London: T & AD Poyser.
Griffiths, O. L., and V. F. B. Florens. 2006. A field guide to the non-marine
molluscs of the Mascarene Islands (Mauritius, Rodrigues and Réunion)
and the northern Dependencies of Mauritius. Mauritius: Bioculture
Press.
Motala, S. M., F.-T. Krell, Y. Mungroo, and S. E. Donovan. 2007. The
terrestrial arthropods of Mauritius: a neglected conservation target.
Biodiversity and Conservation 16: 2867–2881.
Probst, J. M. 1997. Animaux de La Réunion: guide d’identification des
oiseaux, mammifères, reptiles, et amphibiens. Saint-Denis, Réunion:
Editions Azalées.
Turner, J., and R. Klaus. 2005. Coral reefs of the Mascarenes, Western
Indian Ocean. Philosophical Transactions of the Royal Society A 363:
229–250.
Warren, B. H., E. Bermingham, R. P. Prys-Jones, and C. Thébaud. 2006.
Immigration, species radiation, and extinction in a highly diverse songbird lineage: white-eyes on Indian Ocean islands. Molecular Ecology 15:
3769–3786.
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