But is it True?

On June 14, 2016 the University of Queensland and Queensland Government researchers declared that the Bramble Cay melomys – the only mammal species endemic to the Great Barrier Reef – was the first mammal to go extinct due to anthropogenic human-induced climate change.  But no evidence for this as the cause was provided.

Melomys rubicola was only known to live on this small (8.3 acres) coral cay located in the Torres Strait, between Queensland in Australia and Papua New Guinea. A limited survey in March 2014 failed to detect the rodents, and a more thorough survey in August 2014 was conducted by Dr Luke Leung of University of Queensland and found not a single School of Agriculture and not a single melomys was captured or observed.  It was and is indeed extinct. Anecdotal information obtained from a professional fisherman who visited Bramble Cay annually since 2004 suggested that the last known sighting of the melomys was in late 2009.

Dr Leung said the key factor responsible for the destruction of this population was “almost certainly ocean inundation of the low-lying cay, very likely on multiple occasions, causing dramatic habitat loss and likely also direct mortality of individuals.”  Bramble Cay sits at most 3m above sea level. Extensive surveys did not find the melomys on any other Torres Strait or Great Barrier Reef Island. However, the Melomys rubicola species had never been found on any other cay or island in the Torres Strait. Other Melomys species, closely related to it are found on other cays and islands.

That last statement though is not supported by actual data which shows that severe and non-severe tropical cyclones in the area have actually decreased significantly since 1971.  Due to the fragile structure of the cay, any single cyclone could have totally destroyed the island.  The sea level rise on the island since 1971 is approximately 10 inches.  The lack of cyclonic activity may have actually contributed to the loss of area and sediment over this span and increased the infiltration of the salt water into the shallow soil, thus damaging and limiting the vegetation.

Erosion and loss of vegetation on the cay is the major threat to the species survival. The cay is in a constant state of flux with its movements strongly influenced by the prevailing weather patterns. While there appears to have been a net loss of high tide surface area in the cay in recorded history, recent measurements suggest the cay might be in a depositional phase. Erosion may be compounded by high winds, wave action and storm surges associated with cyclones. Though erosion may be reduced with fewer cyclones, deposition of new sediment is also reduced negatively affecting vegetation. 

Annual rainfall in the Torres Strait has remained relatively stable during the period over which the extent of vegetation on Bramble Cay has diminished, while annual mean maximum temperatures show only a slight overall increase from 1998 to 2014. With no trend in the annual rainfall data apparent over recent decades, this climate variable is unlikely to be a causative factor in the decline in vegetation cover on Bramble Cay during the decade prior to 2014.

The small, gradual increase in air temperature evident in the climatic record for Torres Strait could conceivably have affected plant health on Bramble Cay negatively over the past ten years, perhaps contributing to a reduction in the area of vegetation on the island. However, the relatively small change in average temperatures is unlikely to account for the large magnitude of change in vegetation extent that has taken place over such a relatively short time span. Significant changes in cays and their surrounding ecosystems result from natural phenomena such as severe El Niño–Southern Oscillation (ENSO) cycles. Also, tropical cyclones can either help build up or tear down these islands. 

A more likely cause of the marked decline in vegetation cover is ocean inundation. With a maximum elevation of only 3 m, Bramble Cay is particularly vulnerable to sea-level rise and the impacts of high-water events. Seawater inundation as a result of storms and high tides has previously been identified as a threat to the Bramble Cay melomys because it kills vegetation and reduces the area of habitat present.

Bramble Cay is located in the northeastern portion of the Torres Strait, approximately 50 kilometers (31 mi) from the mouth of the Fly River in Papua New Guinea. The rodent only occupied the vegetated portion of the island. The cay averages less than 3 (9¾ ft) meters above sea level at its peak. The habitalbe area of the cay varied over time and could change rapidly.  The two maps below of the cay were made in March 2014 (left) and August-September 2014 (right).  The dark shaded area outlines the boundary of the island that was above sea level at high tide and the lighter shade outlines the area exposed at low tide.  In just this 6-month period the amount of high tide land exposed had decreased by 40%.  It subsequently recovered.  The reconfiguration is primarily driven by changes in the shoreline sands which shift with storm activity and sea currents. 

Bramble Cay is an island formed when ocean currents transport loose sediment across the surface of a reef to where the current slows or converges with another current, releasing its sediment load. Gradually, layers of deposited sediment build up on the reef surface – a depositional node. Such nodes occur in windward or leeward areas of reefs (such as the one where Bramble Cay is located), where flat surfaces sometimes rise around an emergent outcrop of old reef or beach rock. The entire island encompasses 8.3 acres (measuring 340 by 150 meters or 1,120 by 490 ft), of which less than 5 acres is habitable.

Cyclones and storms reshape cays over time, both destroying and eroding them as well as building them up with new sediment. The storms smash corals, aggregate it, and then throw the coral rubble up into depositions beyond the highest tides. Cyclones are also necessary for the long-term replenishment of sediments along the shorelines. This is especially the case for coral cays as sea levels slowly rise. Without the cyclones the small islands would quickly erode into the ocean and disappear.  Many have done so.

The island resulting from sediment accumulation is made up almost entirely of the skeletal remains of plants and animals – biogenic sediment – from the surrounding reef ecosystems. If the accumulated sediments are predominantly sand, then the island is called a cay; if they are predominantly gravel, the island is called a motu.

Cay sediments are largely composed of calcium carbonate (CaCO₃), primarily of aragonite, calcite, and high-magnesium calcite. They are produced by myriad plants (e.g., coralline algae, species of the green algae Halimeda) and animals (e.g., coral, mollusks, foraminifera). Small amounts of silicate sediment are also contributed by sponges and other creatures. Over time, soil and vegetation may crop up on a cay surface, assisted by the deposition of bird guano. The cay is surrounded by a relatively small coral reef and is relatively isolated from other reefs in the Torres Strait. This provides some protection from sea erosion.

The northern side of Bramble Cay contains a huge deposit of guano and a large breeding site for several seabird species. The crested tern is the most common. Other species include the sooty tern, common noddy, brown booby, and seven other species. In 1862, a mining lease was granted to the Anglo-Australian Guano Company and occasionally boats would come to mine the low grade phosphatic rock but due to its low quality, the company did not create a permanent base there.  No mining is currently in progress.  The area covered in guano and near the nesting grounds was not a suitable habitat for melomys.

The rat’s extinction has been blamed specifically on inundation from storm surges, and also rising sea level. The state Government of Queensland in a report stated that the likely cause of extinction was inundation of the island multiple times during the last decade, leading to habitat loss for the species and also direct mortality. The sea level had been estimated to have risen by 0.6 centimeters (¼ in) every year between 1993 and 2010, while the incidence of cyclonic storms, also increased.

That last statement though is not supported by actual data which shows that severe and non-severe tropical cyclones in the area have actually decreased significantly since 1971.  Due to the fragile structure of the cay, any single cyclone could have totally destroyed the island.  The sea level rise on the island since 1971 is approximately 10 inches.  The lack of cyclonic activity may have actually contributed to the loss of area and sediment over this span and increased the infiltration of the salt water into the shallow soil, thus damaging and limiting the vegetation.

Erosion and loss of vegetation on the cay is the major threat to the species survival. The cay is in a constant state of flux with its movements strongly influenced by the prevailing weather patterns. While there appears to have been a net loss of high tide surface area in the cay in recorded history, recent measurements suggest the cay might be in a depositional phase. Erosion may be compounded by high winds, wave action and storm surges associated with cyclones. Though erosion may be reduced with fewer cyclones, deposition of new sediment is also reduced negatively affecting vegetation. 

Annual rainfall in the Torres Strait has remained relatively stable during the period over which the extent of vegetation on Bramble Cay has diminished, while annual mean maximum temperatures show only a slight overall increase from 1998 to 2014. With no trend in the annual rainfall data apparent over recent decades, this climate variable is unlikely to be a causative factor in the decline in vegetation cover on Bramble Cay during the decade prior to 2014.

The small, gradual increase in air temperature evident in the climatic record for Torres Strait could conceivably have affected plant health on Bramble Cay negatively over the past ten years, perhaps contributing to a reduction in the area of vegetation on the island. However, the relatively small change in average temperatures is unlikely to account for the large magnitude of change in vegetation extent that has taken place over such a relatively short time span. Significant changes in cays and their surrounding ecosystems result from natural phenomena such as severe El Niño–Southern Oscillation (ENSO) cycles. Also, tropical cyclones can either help build up or tear down these islands. 

A more likely cause of the marked decline in vegetation cover is ocean inundation. With a maximum elevation of only 3 m, Bramble Cay is particularly vulnerable to sea-level rise and the impacts of high-water events. Seawater inundation as a result of storms and high tides has previously been identified as a threat to the Bramble Cay melomys because it kills vegetation and reduces the area of habitat present.

Bramble Cay is located in the northeastern portion of the Torres Strait, approximately 50 kilometers (31 mi) from the mouth of the Fly River in Papua New Guinea. The rodent only occupied the vegetated portion of the island. The cay averages less than 3 (9¾ ft) meters above sea level at its peak. The habitalbe area of the cay varied over time and could change rapidly.  The two maps below of the cay were made in March 2014 (left) and August-September 2014 (right).  The dark shaded area outlines the boundary of the island that was above sea level at high tide and the lighter shade outlines the area exposed at low tide.  In just this 6-month period the amount of high tide land exposed had decreased by 40%.  It subsequently recovered.  The reconfiguration is primarily driven by changes in the shoreline sands which shift with storm activity and sea currents. 

Although 11 species of plants have been recorded on the island in the past, only three to five species have been recorded as present at the same time.  Three species were observed in 1994. A 1998 study showed significant loss of vegetation since 1924, mostly on the southern and northern shores of the island. The island was generally described as being vulnerable to severe weather and rising sea level, as a result of its low elevation. Herbaceous cover on the cay decreased from 5.3 acres in 2004 to .16 acres in March 2014 before recovering somewhat to .47 acres later. The vegetation decline was primarily due to ocean inundations from storms and also the gradual, natural erosion from shifting currents.

The Bramble Cay melomys, or Bramble Cay mosaic-tailed rat (Melomys rubicola) belonged to the genus Melomys, which includes approximately 20 species of rodents living in the wet habitats of northern Australia (Far North Queensland), New Guinea, other Torres Strait islands and islands of the Indonesian archipelago. The name, Bramble Cay, derives from European surveyors on HMS Bramble, who came upon the island in April 1845.  The following month, May 1845, visiting Bramble Cay aboard the HMS Fly, naturalist John MacGillivray and Joseph Jukes collected a holotype, stored today in the British Museum of Natural History. From the specimen, Oldfield Thomas formally described and named the species Melomys rubicola in 1924. The melomys was the only mammal observed living on the tiny, 8.3-acre island (measuring 340 by 150 meters or 1,120 by 490 ft), of which less than 5 acres was habitable.

 DNA obtained from the historic specimens indicates that its closest relative in Australia is the Cape York melomys (Melomys capensis), with the divergence between the two species being so small that it was barely above what would be expected for diversity within species. This would indicate that Melomys rubicola was a fairly recent arrival on the small cay. Genetic analysis of the species reveals a level of inbreeding which theoretically could lead to inbreeding depression and ultimately extinction.  There was no escape from inbreeding on the tiny cay as the population was always small and no external genetic input ever occurred. It was almost identical in appearance to the numerous Cape York melomys, to which it is closely related genetically.

The island is also home to large populations of nesting seabirds, as well as a breeding site for green sea turtles. As a result, the island’s vegetated areas are subjected to seasonal disturbance, particularly by adult green turtles that come ashore to lay eggs during the nesting season. The Bramble Cay melomys preferred the more densely vegetated areas and avoided those parts of the island that had high densities of seabirds. The melomys were observed to feed on Portulaca oleracea vegetation as well as on turtle eggs. By 2014 this vegetation was no longer observed on the island. The breeding season of the melomys was lengthy, and the sex ratio was skewed towards females.  The rodents were also prey for the seabirds and became more vulnerable as the vegetation decreased and as the seabirds nesting area encroached on the remaining vegetated area.

 Population estimates for the species varied widely. Observers in 1845 stated there were “hundreds” of the animal present, as did a survey from 1978. A 1998 survey caught 42 animals, and based on that, estimated the population size at approximately 90 individuals. A surveys in 2011 failed to find any animals. Due to its very limited size, the cay was never able to sustain a large population of mammals.  By 2014 the melomys was extinct.

The origins of the Bramble Cay melomys population on Bramble Cay are uncertain. There are two hypotheses to explain how the species came to have such a tiny, restricted distribution. The first is that it represents a relictual (remnant) population of a species that previously inhabited the low-lying Sahul Plain that once connected the land masses of Australia and New Guinea.  The melomys was left stranded on Bramble Cay as sea levels rose and submerged the land bridge.  The island itself has ties to a volcanic past. The coral reef originally formed around a basalt outcrop created by Pleistocene volcanic activity. It is composed of foraminiferal sand, compacted guano and, at its south-eastern end, a low phosphatic rock platform.

The present-day island was formed as sea-levels rose at the end of the last ice age. At some point, this trapped a small population on the island.  Global sea level has risen by more than 120 meters (390 ft) as the vast ice sheets of the last Ice Age melted back. This melt-back lasted from about 19,000 to about 6,000 years ago, meaning that the average rate of sea-level rise was roughly 1 meter per century over that time span. Current rates of sea level rise are approximately 3.3 mm/yr. Sea levels will continue to rise in the future until the next ice age cycle begins.

The second hypothesis is that the species is of New Guinean origin, and that colonizing individuals rafted across to Bramble Cay on flood debris emanating from the mouth of the Fly River, located just over 50 km from the island. This idea has been proposed even though the Bramble Cay melomys is not known in New Guinea. Available genetic and biochemical evidence shows the species has closest affinity with Australia’s Cape York melomys Melomys Capensis from which it is estimated to have diverged around 900,000 years ago. This was also during a period when land bridges existed between Cape York and New Guinea, including the region in which Bramble Cay now lies. This evidence is used to provide support for the former hypothesis.  However, little fauna survey work and genetic sampling of melomys from the Fly River delta of Papua New Guinea have been undertaken. Consequently, the possibility exists that the Bramble Cay melomys (or a related species with even closer taxonomic affinity with the Bramble Cay melomys than the Cape York melomys) has been overlooked or remains to be discovered in this region of Papua New Guinea.

There are other factors favoring the second hypothesis of potential colonization of Bramble Cay by melomys individuals rafting across from Papua New Guinea on flood-borne debris. The shoreline of Bramble Cay is frequently littered with beach-washed nypa palms, logs of pandanus, mangrove plant parts and large pieces of other vegetation of New Guinean provenance, primarily originating from the Fly River. The last known Bramble Cay melomys individuals on the island around 2009 was found sheltering in a dug-out canoe washed ashore from New Guinea.

The observations supplied by David Carter, a turtle researcher who lived on Bramble Cay for six months during the 1979–80 wet season, are particularly pertinent. He wrote:

“One of my strongest recollections is dawn light revealing a sea dotted with flood debris right to the horizon in every direction: trees, great rafts of nipa [sic] palms and tangles of grass and reeds from the Fly River we presumed. Some of these washed up on the Cay or stranded in the reef lagoon. As with every new feature, there were always rats climbing around on these beach washed items even as they tossed in the surf.”

Anecdotal reports collected during the present fieldwork indicate flood debris, undoubtedly from the Fly River delta, may at times be large enough to support upright palm trees or entire human structures. In light of this and given the evident arboreal tendencies of the Bramble Cay melomys, trees and other large items of flood debris would clearly be sufficient to support the melomys on the relatively short ocean crossing from Papua New Guinea to Bramble Cay. Colonization of the island by just a single pregnant female or a female nursing young may have been all that was required to establish this population. Such a scenario would also concur with the results of genetic investigations demonstrating that the Bramble Cay melomys population possessed only one mtDNA (mitochondrial female DNA) genotype, suggesting that a single, small colonization event took place.  It also suggests the colonization was relatively recent in time.

It is possible that both hypotheses are true. Bramble Cay did not exist until volcanic activity around 19,000 years ago pushed the basaltic platform on which it formed to sea level.  While a land bridge existed to the Australian and New Guinea headlands there may have been multiple colonization of the area.  When sea levels rose sufficiently to create a basaltic island was formed. Whatever mammal species was on the island would have been trapped there or perished if sufficient habitat was not available.  For bird and sea life, it became a nesting shelter as they were not dependent on any food resources A coral reef built itself on and around the outcrop and the natural course of sedimentary deposit and erosion formed and reformed the small island over time.  The nearby Fly River, the largest river in the world by volume without any dams in its catchment, provides a steady flow of potential vegetation and animal input. It also shapes the cay, both in deposition and erosion.

During periods of flooding, rats may be washed from rainforest understory along the Fly River, arrive at Bramble Cay, multiple in numbers for a period (particularly when there is an abundance of turtle eggs), but the population will always be susceptible to extinction because the island is so small. When a habitat lacks complexity and refugees it is always more vulnerable to extinction from predation by birds and/or snakes that can be intense. It is possible, in fact highly likely, that there have been cycles of migration from the highlands of New Guinea, extinction due to predation and loss of habitat and then recolonization with the next significant flood event.  The climate in the Torres Strait has not changed significantly over the span of time the melomys was observed on Bramble Cay.  In fact, the prevalence and number of cyclonic events, which could totally inundate the cay and destroy the mammalian population, have decreased during that period.  The melomys became extinct because of the erosion of their habitat and predation from the increasing seabird population.  The Melomys rubicola was a unique species only barely.  This was caused by its forced inbreeding and isolation.

Significant changes in cays and their surrounding ecosystems result from natural phenomena such as severe El Niño–Southern Oscillation (ENSO) cycles. Also, tropical cyclones can either help build up or tear down these islands. There is much debate and concern over the future stability of cays in the face of growing human populations and pressures on reef ecosystems, and changes and sea level rise. There is also debate around whether these islands are relict features that effectively stopped expanding two thousand years ago during the late Holocene or, as recent research suggests, they are still growing, with significant new accumulation of reef sediments.

Given the combination of factors, namely the declining trend in abundance of the Bramble Cay melomys from a very low base since the 1970s, the very small size of the species’ location and the extremely small and diminishing extent of habitat at that location, as well as the likelihood of ongoing severe threats, possibly worsening with time, the species was in severe peril of extinction. The melomys on Bramble Cay was living in a fragile environment with no ability to adapt or escape.   Anthropogenic climate change had nothing to do with it.