Many small tropical and subtropical islands are thought to have
impoverished faunas, due to extinctions of local animals caused by human
activity, notably habitat modification and the introduction of exotic species.
However few such islands possess extensive records of the fauna present prior
to the onset of such human activities, making it very hard to assess the nature
of their native faunas.
In a paper published in the journal Acta Herpetologica in July 2013,
Yasuyuki Nakamura of the Tropical Biosphere Research Center at the University of the Ryukyus, Akio Takahashi of the Department of Zoology at Okayama University of Science and Hidetoshi Ota of the Graduate School of Human Scienceand Environment at the University of Hyogo discuss the discovery of a number of
Snake and Lizard bones from a garbage dump on Yoronjima Island in the Central
Ryukyus.
Yoronjima Island is a small, flat island covering 20.5 km2
with a maximum elevation of 97 m, and comprising mostly Pleistocene limestone
terraces, located about 30 km to the northeast of Okinawajima Island and 40 km
to the southwest of Okinoerabujima Island. It has been occupied sporadically
since the Jomon Period (slightly over 2000 years ago), and continuously since
at least the early fifteenth century, when a medieval fort was constructed on
the island. The island currently retains about 4.94 km2 of native
vegetation cover, including forest, shrub, and grassland.
The island is currently home to three species of Frogs, four species
of Lizards and two species of Snakes; Hallowell’s Tree Frog, Hyla hallowellii, the Ryukyu Kajika Frog,
Buergeria japonica, the Okinawa
Narrow-mouth Toad, Microhyla okinavensis, the Hokou Gecko, Gekko hokouensis, the House Gecko, Hemidactylus frenatus (probably introduced), the Ryukyu Five-lined
Skink, Plestiodon marginatus, the
Okinawa Green Grass Lizard, Takydromus smaragdinus,
the Okinawa Green Snake, Cyclophiops semicarinatus,
and the Brahminy Blind Snake, Ramphotyphlops braminus (also probably
introduced).
Four other Reptile species have been historically reported from the
islands, the Okinawa Keelback Snake, Amphies mapryeri,
the Ryukyu Odd-tooth Snake, Dinodon semicarinatum,
the Okinawan Tree Lizard, Japalura polygonata polygonata,
and the Common Four-clawed Gecko, Gehyra mutilata,
however these reports have never been confirmed, and their reliability is
uncertain. However a recent study found the remains of the Rhacophorid Tree
Frog, Rhacophorus viridis, which is
not currently found on the island, at a garbage dump in central Yoronjima,
suggesting that local extinctions of small animals have occurred here.
The island is also home to the Ryukyu Flying Fox, Pteropus dasymallus, and Watase’s Shrew, Crocidura watasei, which are thought to
be indigenous to the island, and the Musk Shrew, Suncus murinus, Norway Rat, Rattus norvegicus,
Black Rat, Rattus rattus, House Mouse,
Mus musculus, and Japanese Weasel, Mustela itatsi, all of which are known to
have been introduced. The Oriental Free-tailed Bat, Tadarida insignis, has also been reported on the island, though the
status of this is unclear.
The study site is an abandoned garbage dump in the central part of
the island. This has not been precisely dated, but from the nature of the Human
refuse at the site is thought to have been in use from the late nineteenth
century to the mid twentieth century. Most of the species recorded here are
thought to be too small to have been targeted for Human consumption, suggesting
that they were living on the site, probably consuming small arthropods drawn to
the refuse. The specimens were obtained by sieving fine sands from the site,
and are therefore all disarticulated.
The first species documented from the site is the Okinawa Green
Snake, Cyclophiops semicarinatus. This
is represented by 24 vertebrae, from the middle and posterior trunk sections of
the spine, which appear to come from at least two individuals on the basis of
their size. This species is widespread in the Central Ryukyus and is known to
still be found on the island.
Trunk vertebra of snake Cyclophiops semicarinatus. Left to right: (top) anterior, lateral
(left), posterior, (bottom) dorsal, and ventral views. Abbreviations are: con,
condyle; cot, cotyle; di, diapophysis; ezs, epizygapophyseal spine; hk, hemal
keel; na, neural arch; nc, neural canal; ns, neural spine; pa, parapophysisprz,
prezygapophyseal accessory process; przf, prezygapophysealarticular facet;
pozf, postzygapophysealarticular facet; sr, subcentral ridge; zy, zygosphene;
zyf, zygosphenalarticular facet. Scale bar equals 1 mm. Nakamura et al. (2013).
The second species recorded is the Ryukyu Odd-tooth Snake, Dinodon semicarinatum. This species is
represented by 21 vertebrae from the middle and posterior trunk parts of the
spine thought to be from at least two individuals, a juvenile and an adult.
This species has previously been reported from the island, but the reliability
of these reports has been unclear until now.
Vertebra of Dinodon semicarinatum. Left
to right: (top) anterior, lateral (left), posterior, (bottom) dorsal, and
ventral views. Abbreviation: zygaf, zygantralarticular facet. Scale bar equals 1
mm. Nakamura et al. (2013).
The third species reported is the Okinawa Keelback Snake, Amphies mapryeri. This species is
represented by 39 precaudal trunk vertebrae. Like the Ryukyu Odd-tooth Snake,
this species has previously been reported from the island, but in the absence
of supporting evidence the reliability of these reports has been unverifiable.
Vertebra of Amphies mapryeri.Left
to right: (top) anterior, lateral (left), posterior, (bottom) dorsal, and
ventral views. Abbreviations are:hy, hypapophysis; pap, parapophyseal process;
Scale bar equals 1 mm. Nakamura et al. (2013).
The fourth species reported is an unidentified Sea Snake, Hydrophiinae
sp., recorded from a single trunk vertebra. Unlike all the other specimens in
the study, this species is highly unlikely to have been living at the study
site, and almost certainly got there as a result of Human consumption (Sea
Snakes are eaten in the Ryukyus).
Vertebra of Hydrophiinae sp.. Left to right: (top)
anterior, lateral (left), posterior, (bottom) dorsal, and ventral
views. Abbreviation: di, diapophysis. Scale bar equals 1 mm. Nakamura et al. (2013).
The fifth species identified from the site is the Okinawan Tree
Lizard, Japalura polygonata polygonata,
identified from six right and four left maxillae, three right and three left
dentaries, a proximal part of the right humerus, and a proximal part of the
left femur. Like the Ryukyu Odd-tooth Snake and Okinawa Keelback Snake, this
species has previously been reported from the island, bt without supporting
evidence.
(A)
Right maxilla and (B) left dentary of the Okinawan
Tree Lizard, Japalura polygonata polygonata,
both in median view. Abbreviation: mg, Meckelian groove. Scale bars equal 1 mm.
Nakamura et al. (2013).
The sixth species reported is the Ryukyu Short-legged Skink, Ateuchosaurus pellopleurus, represented
by a single frontal bone (part of the skull). This species has never been
reported from Yoronjima Island, though it is found in the Central Ryukyus.
Ryukyu Short-legged Skink, Ateuchosaurus pellopleurus, frontal in dorsal view. Abbreviation: pf,
parietal foramen. Scale bar equals 1 mm. Nakamura et al. (2013).
The seventh species recorded from the site is the Ryukyu Five-lined
Skink, Plestiodon marginatus, which is
represented by one right dentary, two right and one left posterior mandibles,
five right and one left humeri, one right femur, and one right and one left
pelvic girdle. This species is still present on the island.
Right dentary of a Ryukyu Five-lined Skink, Plestiodon marginatus, in medial view.Abbreviation:
mg, Meckelian groove. Scale bar equals 1 mm. Nakamura et al. (2013).
The eighth species recorded is the Okinawa Green Grass Lizard, Takydromuss maragdinus, which is
represented by one left dentary and one right and one left pelvic girdle. This
species is still present on the island.
Left dentary of aOkinawa Green Grass Lizard, Takydromuss maragdinus, in medial view. Abbreviation:
mg, Meckelian groove. Scale bar equals 1 mm. Nakamura et al. (2013).
The ninth species recorded from the site is the Hokou Gecko, Gekko hokouensis, which is represented by
specimens—three right and three left maxillae, two right and three left
dentaries and five frontals. This species is still found on the island.
Lizard remains referred to Gekko hokouensis. (A) Right maxilla in medial view, (B) right dentaryin
medial view, (C, D) frontal in dorsal (C) and ventral (D) views. Abbreviations
are: amp, anteromesial process; pml, posterior maxillary lamina; pmp, posterior
maxillary process. Arrows in (C) indicate the posterior extents of the
prefrontal sutures. Scale bars equal 1 mm. Nakamura et al. (2013).
Next a number of bones belonging to one or
more unknown species of large Gekko.
These include three right maxillae, two right and one left dentaries, three
frontals, and three left pterygoids. Some of these bones appear very similar to
those of the Hokou Gecko, but are too large to be assigned to this species.
Bones assigned to an unknown Gekko species: (E) right maxilla in medial view, (F) right dentary
in medial view, (G, H) frontal in dorsal (G) and ventral (H) views. Abbreviations
are: amp, anteromesial process; pml, posterior maxillary lamina; pmp, posterior
maxillary process. Arrows in (G) indicate the posterior extents of the
prefrontal sutures. Scale bars equal 1 mm. Nakamura et al. (2013).
Finally one right and one left maxillae and four right and three
left dentaries are assigned to Kuroiwa’s Leopard Gecko, Goniurosaurus kuroiwae, a species not hitherto recorded from the
island.
Bones assigned to Kuroiwa’s Leopard Gecko, Goniurosaurus kuroiwae: left maxilla in
medial view, and (J) right dentaryin medial view. Abbreviation: amp,
anteromesial process. Scale bars equal 1 mm. Nakamura et al. (2013).
While it is possible that some of these species still remain on
Yoronjima Island undiscovered, the nature of the island, small and flat, makes
it particularly amenable to biological surveys, making this highly unlikely,
strongly suggesting that many species present at the dump have become extinct
in recent decades. Many of these species are considered to be forest loving,
and the island has lost much of its native forest cover, but several other
islands in the Central Ryukyus are known to host more diverse Reptile
assemblages with less woodland, making this an unlikely explanation in itself.
Instead Nakamura suspect that the decline in Reptile and Amphibian
species seen on the island may be linked to the introduction of the Japanese
Weasel, Mustela itatsi, in the 1950s
in an attempt to control the populations of introduced Rodents. This species
has been linked to the demise of other island dwelling Reptiles and Amphibians
in the Ryukyu and Izu Islands, where naïve prey species unused to Mammalian
predators have quickly succumbed to this aggressive nocturnal predator.
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