Great areas of our rubbish are known to form in parts of the Pacific and Atlantic oceans. But no such “garbage patch” has been found in the Southern Indian Ocean.
Our research – published recently in Journal of Geophysical Research: Oceans – looked at why that’s the case, and what happens to the rubbish that gets dumped in this particular area.
Every year, up to 15 million tonnes of plastic waste is estimated to make its way into the ocean through coastlines (about 12.5 million tonnes) and rivers (about 2.5 million tonnes). This amount is expected to double by 2025.
Some of this waste sinks in the ocean, some is washed up on beaches, and some floats on the ocean surface, transported by currents.
As plastic materials are extremely durable, floating plastic waste can travel great distances in the ocean. Some floating plastics collect in the centre of subtropical circulating currents known as gyres, between 20 to 40 degrees north and south, to create these garbage patches.
Here, the ocean currents converge at the centre of the gyre and sink. But the floating plastic material remains at the surface, allowing it to concentrate in these regions.
The best known of these garbage patches is the Great Pacific Garbage Patch, which contains about 80,000 tonnes of plastic waste. As the National Oceanic and Atmospheric Administration points out, the “patches” are not actually clumped collections of easy-to-see debris, but concentrations of litter (mostly small pieces of floating plastic).
Similar, but smaller, patches exist in the North and South Atlantic Oceans and the South Pacific Ocean. In total, it is estimated that only 1% of all plastic waste that enters the ocean is trapped in the garbage patches. It is still a mystery what happens to the remaining 99% of plastic waste that has entered the ocean.
Even less is known about what happens to plastic in the Indian Ocean, although it receives the largest input of plastic material globally.
For example, it has been estimated that up to 90% of the global riverine input of plastic waste originates from Asia. The input of plastics to the Southern Indian Ocean is mainly through Indonesia. The Australian contribution is small.
The Indian Ocean has many unique characteristics compared with the other ocean basins. The most striking factor is the presence of the Asian continental landmass, which results in the absence of a northern ocean basin and generates monsoon winds.
As a result of the former, there is no gyre in the Northern Indian Ocean, and so there is no garbage patch. The latter results in reversing ocean surface currents.
The Indian and Pacific Oceans are connected through the Indonesian Archipelago, which allows for warmer, less salty water to be transported from the Pacific to the Indian via a phenomenon called the Indonesian Throughflow (see graphic, below).
This connection also results in the formation of the Leeuwin Current, a poleward (towards the South Pole) current that flows alongside Australia’s west coast.
As a result, the Southern Indian Ocean has poleward currents on both eastern and western margins of the ocean basin.
Also, the South Indian Counter Current flows eastwards across the entire width of the Southern Indian Ocean, through the centre of the subtropical gyre, from the southern tip of Madagascar to Australia.
The African continent ends at around 35 degrees south, which provides a connection between the southern Indian and Atlantic Oceans.
In contrast to other ocean basins, the Indian Ocean is under-sampled, with only a few measurements of plastic material available. As technology to remotely track plastics does not yet exist, we need to use indirect ways to determine the fate of plastic in the Indian Ocean.
We used information from more than 22,000 satellite-tracked surface drifting buoys that have been released all over the world’s oceans since 1979. This allowed us to simulate pathways of plastic waste globally, with an emphasis on the Indian Ocean.
We found that unique characteristics of the Southern Indian Ocean transport floating plastics towards the ocean’s western side, where it leaks past South Africa into the South Atlantic Ocean.
Because of the Asian monsoon system, the southeast trade winds in the Southern Indian Ocean are stronger than the trade winds in the Pacific and Atlantic Oceans. These strong winds push floating plastic material further to the west in the Southern Indian Ocean than they do in the other oceans.
This allows the floating plastic to leak more readily from the Southern Indian Ocean into the South Atlantic Ocean. All these factors contribute to an ill-defined garbage patch in the Southern Indian Ocean.
In the Northern Indian Ocean our simulations showed there may be an accumulation of waste in the Bay of Bengal.
It is also likely that floating plastics will ultimately end up on beaches all around the Indian Ocean, transported by the reversing monsoon winds and currents. Which beaches will be most heavily affected is still unclear, and will probably depend on the monsoon season.
Our study shows that the atmospheric and oceanic attributes of the Indian Ocean are different to other ocean basins and that there may not be a concentrated garbage patch. Therefore the mystery of all the missing plastic is even greater in the Indian Ocean.
Mirjam van der Mheen, PhD Candidate in Oceanography, University of Western Australia; Charitha Pattiaratchi, Professor of Coastal Oceanography, University of Western Australia, and Erik van Sebille, Associate Professor in Oceanography and Climate Change, Utrecht University
A couple of days ago I published an article with Peter Green about the imminent release of a tiny wasp that will be used for biological control of a bug that feeds the crazy ants that kill red crabs on Christmas Island.
It is understandable that people are nervous about the introduction of exotic species to manage wildlife in a natural setting. It turns out that ecologists are even more nervous than the public about this, so if they have decided to do it anyway, then there is a remarkably good reason.
Parasitoid wasps use scale insects
The release of the wasp has concerned some readers because they imagine swarms of biting insects setting up their nests in the back garden. The truth is that the wasps that will be released are tiny and unlikely to be noticed at all.
First of all, Tachardiaephagus somervillei are only 2 mm long and cannot sting humans or other animals. They do not form colonies, they do not swarm, and they do not build nests. In fact, they won’t be at all interested in hanging around human habitations unless there is a tree nearby containing a colony of the yellow lac scale insect (Tachardina aurantiaca).
This is because these wasps are parasitoids – a type of parasitic organism that kills its host species. They don’t need a nest or a colony because the scale insects they target are both their food source and their home.
The specificity of the wasp for this particular type of scale insect can be seen in the first part of their Latin names: Tachardiaephagus literally means “eater of Tachardina”.
Scale insects use ants
Scale insects are a type of true bug (in the Order Hemiptera) that line up along tree branches like barnacles, sucking sap from the tree and in their mature form, releasing a sweet liquid known as honeydew from their backsides for the benefit of ants. They don’t do this for nothing. Their strategy is to use the ants as body guards.
In a situation where scale insects are relatively rare this increases the number of the ants who will in turn protect the scale insects. On Christmas Island, where the introduced yellow lac scale insects have become common because they do not have any natural predators, the invasive crazy ants have access to large quantities of honeydew. In this case, the crazy ants are using the yellow lac scale insects as a super abundant food source.
The super colonies that have formed as a result have instigated an environmental disaster. The crazy ants kill red crabs and other species mostly due to their extremely high densities driven by the abundance of honeydew.
Any detractors concerned about the dangers of yet another invasive species have not fully grasped the consequences of doing nothing. Chemical baiting of the ants is ongoing but has consequences for other animals and is not environmentally desirable or sustainable.
People using wasps
If the scale insects can use the ants as bodyguards and the ants can use the scale insects as a free food source, why can’t we use a tiny wasp as a biological control?
Unlike birds, lizards or other predators that may be deterred by ants crawling all over the scale insects, the tiny parasitoid wasps can slip through and lay their eggs in a scale insect without being noticed by the ants. Their eggs hatch and develop inside the scale insect, emerging as adult wasps that are ready to lay their eggs in another scale insect nearby.
In essence, the wasp uses the scale insect as a one-stop nursery, food source and conveniently located launching pad for the next generation. Inside a scale insect colony, they are likely to find another scale insect less than a centimetre from where they were born.
Consider how this will allow the wasp population to quickly grow and, perhaps, reduce the scale insect colony density so that the wasps will eventually have to fly further and further to find another scale insect. At some point the effort to find more scale insects will balance the benefit of finding an insect, and the two populations (wasp and scale insect) will reach a new equilibrium at a lower density.
How will the crazy ants respond?
The wasp will not run out of food, nor will the scale insects become extinct, but the ants will find themselves deprived of excess honeydew and will have to adjust their populations accordingly.
How do you empirically test the response of the ants to the removal of excess honeydew from their environment? Well, you can’t remove the scale insects but you can prevent the ants from getting into the trees where the scale insects live, even though it wasn’t easy. Apparently, doing this involves Glad wrap, Mr Sheen furniture polish, and daily vigilance by a research student.
The result was a 95% decrease in crazy ant activity in a few weeks, an outcome that suggests this approach has every chance of reducing the impacts of crazy ants on Christmas Island.
What happens next?
I understand that the team is gathering in Malaysia today to pack up some wasps and fly them to Christmas Island. The release will not happen right away, as the wasps will be acclimatised and grown up in large numbers in a dedicated facility. Monitoring programs are planned to observe the impacts, both short and long term, on the scale insects, the ants, the crabs and the forest structure.
The research to understand the ecology of Christmas Island sufficiently to identify a biological control agent started decades ago, and many scientists were involved along the way. It is not possible to provide links to all the research articles produced thus far, but here is a link to the final risk report.
I am not involved with the research but am familiar with it and in my view there are two things that could happen next. Either the wasp will fail to reduce the scale insect populations and nothing changes, or they will reduce the scale insect populations which could kick start a cascade of beneficial environmental outcomes for Christmas Island.
We are all really hoping that it is the latter.
The link below is to an article reporting on the decline of Dugongs in the Indian Ocean.
For more visit:
Tiny Native Bat Now Feared Extinct
The following link is to an article reporting on the feared extinction of the Christmas Island Pipistrelle.