Immersion in seawater kills sea turtle eggs, suggesting that sea turtles are increasingly at risk from rising seas, according to research published today in Royal Society Open Science.
In a laboratory experiment, researchers immersed green turtle eggs in seawater for varying lengths of time. The researchers tested eggs of various ages, and then counted the number of eggs that hatched. They found that immersion for six hours reduced survival by a third.
The study partly explains reduced numbers turtle of hatchlings recorded at Raine Island, home to the largest population of green sea turtles in the world.
David Pike, lecturer in tropical biology at James Cook University and lead author of the study, said turtle nests low down on beaches could be underwater for six hours during abnormally high “king” tides or storm surges.
Michele Thums, ecologist at the Australian Institute of Marine Science, said that given climate projections for increased severe weather events, this could mean fewer hatchlings survive in the future.
But every beach will see different impacts from rising seas, said Tim Dempster, senior lecturer in marine biology at University of Melbourne.
“You can’t just take [a…] scenario of a certain degree of warming, say that will lead to a certain amount of sea level rise, project how much land will be inundated and then project what proportion of nesting habitat will be affected,” he said.
Turtle embryos need oxygen to develop into baby turtles, and immersion in water prevents oxygen from the soil entering the eggs. The embryos effectively suffocate, a process known as “hypoxia”.
Thums said that while most turtles nest above the high tide line and are rarely immersed for six hours, “there are always inexperienced turtles that will lay further down the beach and also there is competition at high density nesting sites like Raine Island”.
Compared to the rest of the world, green sea turtles on Raine Island have a much lower level of breeding success, which could lead to a large decline in the number of breeding adults in the future.
Pike said the low level of success could be partly explained by inundation, but there were likely other factors at work.
“One possibility is that the sand is full of bacteria from all of the rotting eggs that are beneath the sand, and that any fresh eggs laid there may be exposed to bacteria that overgrow the egg and kill the embryo,” he said.
“Another possibility is that contaminants (heavy metals, pesticides) are being passed from the mother turtle to the eggs, and that may cause the embryos to die.”
The Queensland Department of Environmental Heritage and Protection is currently trying to raise low lying spots on Raine Island by moving sand. The island could lose between 7 and 27% of its area thanks to rising seas.
With Janelle Braithwaite, editor at The Conversation.
Changing wildlife: this article is part of a series looking at how key species such as bees, insects and fish respond to environmental change, and what this means for the rest of the planet.
As the world warms, animals and plants will shift their ranges to keep pace with their favoured climate. While the changing distributions of species can tell us how climate change is affecting the natural world, it may also have a direct impact on us.
One good example is the disease carried by insects.
Those small, familiar flies called mosquitoes are responsible for much human suffering around the globe because of their ability to transmit diseases.
Could climate change cause these diseases to spread? While this an extremely important health question, the answer is far from simple.
Complicated life cycle
The life cycle of mosquitoes and its viral parasites is particularly complicated.
Only adult females consume blood, and the immature stages (larvae) live in fresh or brackish water, filtering out small organic particles.
The virus undergoes certain parts of its lifecycle inside particular mosquito organs, but also requires other organs in the vertebrate host to complete its life cycle. And to get into a vertebrate, such as us, it relies on a hungry blood-sucking insect.
These viruses always have other hosts besides humans, which may include native and domestic animals. The pathway that these viruses take to infect humans is often via our domestic animals, which are also bitten by the same mosquitoes that feed on us.
In addition, rates of virus transmission to humans is also affected by the human built environment, and also human behaviour.
Because mosquitoes breed in water, changes in rainfall patterns are likely to change the distribution and abundance of mosquitoes, and therefore could affect disease transmission.
Kunjin virus is mainly transmitted by a small mosquito called Culex annulirostris, the common banded mosquito, in Australia. We are lucky because human infection rarely causes disease, even though Kunjin and the common-banded mosquito are widespread in Australia.
Kunjin’s close relative, the US strain of West Nile Virus is much more virulent, causing more human disease. These viruses are well known for their ability to mutate quickly, so they are always keeping medical authorities on their toes.
Higher than average rainfall and flooding in eastern Australia in the second half of 2010 and 2011 provided ideal conditions for breeding common banded mosquitoes, and in 2011 a dangerous strain of Kunjin appeared that caused acute encephalitis (swelling of the brain) in horses. This disease has only been detected in one human, however this mosquito feeds on both humans and horses.
This new virulent strain of Kunjin also appeared in new areas east of the Great Dividing Range, suggesting other unknown changes in transmission.
As temperatures increase, mosquito activity will begin earlier in the season and reach higher levels of abundance sooner, and maintain higher populations longer. These factors will all probably tend to increase the rate of transmission of Kunjin to both humans and animals.
While flooding may have helped spread Kunjin, drought may have helped another mosquito-borne virus.
It would be simple to assume that drought would reduce mosquito populations by reducing the larval habitat (water), and thereby reduce the incidence of mosquito-borne disease in Australia.
However, this is not necessarily the case. Another Australian mosquito, Aedes notoscriptus, the striped mosquito, is responsible for transmitting Ross River and Barmah Forest Virus in Australia.
The striped mosquito is unusual in comparison to its cousins because it breeds in small containers of water, such as tree holes in natural environments. The main carrier of Dengue in Australia, Aedes aegypti, shares this habit.
These small container habitats abound in Australia’s urban backyard, with water features, water and food bowls for pets, and various toys providing such breeding places.
With the drought, Australians became much more water wise, and installed various water storage devices in their gardens, ranging from buckets left out in a storm, to professionally installed rain tanks. All these are potential habitat for the striped mosquito to breed.
In this case drought has caused an increase in the abundance of a mosquito virus carrier because of a change in human behaviour.
The return of Dengue?
Dengue fever is transmitted in Australia by the mosquito Aedes aegypti. The mosquito is restricted to Queensland, and Dengue fever transmission is restricted to coastal northern Queensland.
Recent modelling predicts that moderate climate change would extend the Dengue risk zone to Brisbane, exposing much larger human populations to risk.
However, before the 1930s, Dengue fever transmission was known south almost to Sydney, and Aedes aegypti was known throughout mainland Australia except the deserts.
Both the mosquito, and the disease, have retreated to Queensland since then, and we don’t know why. What is clear is that we don’t really understand what controls the distribution of Aedes aegypti or Dengue in Australia, but given the contraction of the disease in historical time, it is unlikely that a warming climate will produce a simple response in the insect or the disease.
Australian insects will be affected by climate change, but simple predictions based on increasing average temperatures and changing rainfall patterns miss the important effects of complex biological interactions.
In addition, we are only just beginning to use models that are sophisticated enough to consider how insects might evolve under changing climate.
Investing in a deeper understanding of these complex biological webs, and their outcomes for human society, will result in great returns. Our predictions of the future state of Australian plants and animals will become more accurate and we will also improve human health and manage our biodiversity more sustainably into the future.