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Published: 9 July 2012

Rats in the ranks: the introduced black rat in Australia

Peter B. Banks and Nelika K. Hughes

Black rats (Rattus rattus) would have to be one of the most loathed animals on the planet. They spread disease, damage crops and infrastructure, and are a major threat to wildlife globally. But they are also one of the world’s most successful species, and have followed humans to almost every environment on Earth.

Black rats carry disease and pose significant threats to native wildlife including through predation and competition for resources.

For such a widespread and damaging pest, surprisingly little is known about black rat ecology and its impact, especially in Australian ecosystems. We recently published a review of what is known about black rat ecology in order to highlight the potential for impacts on humans and wildlife in Australia.

Black rats are widely distributed across coastal environments in Australia. They are perhaps most common in and around urban areas but they also occur in a range of forest habitats. It’s not clear when Australia was first invaded by black rats, but they almost certainly first came on ships from Europe. Black rat remains have been found in the gun barrels of sunken Dutch ships off the Western Australian coast, dating from the 1600s.

The oldest intact specimen lies in the Natural History Museum of London, where it was deposited by nineteenth-century naturalist, John Gould. In the introduction to his 1863 book, Mammals of Australia, Gould first described these specimens as a native species, Hapalotis arboricola.

Later, zoologist and paleontologist Gerard Krefft (1830-1881) drew the species climbing a banksia tree, and noted its presence in arboreal and suburban habits around Elizabeth Bay in Sydney. By the 1920s however, the hapless Hapalotis was no more and the species was recognised as the black rat.

The first described black rat in Australia, drawn by Gerard Krefft (1830-1881), one of Australia’s first zoologists and palaeontologists.
Credit: Mitchell Library, State Library of NSW, PXD 9/5

The black rat – also known as the ship rat or roof rat – is actually a species-complex of up to five genetically distinct forms.

The two most globally common forms are recorded in Australia: the Oceanic form of Rattus rattus and an Asian form, Rattus tanezumi. Research is under way at the South Australian Museum to identify where these different types occur in Australia and where they came from.

We found that, globally, black rats are major vectors of a suite of diseases that threaten humans, livestock and wildlife. Plague is the most infamous rat-borne disease, having devastated the world’s population though three global pandemics. Around 180 people died from plague in Australia in the 1900s, although the disease is now absent here.

Black rats spread a suite of other bacterial diseases in Australia, such as Leptospira, Listeria and Salmonella. They also carry the Toxoplasma protozoan parasite, which uses both rats and cats as hosts, but can be fatal to wildlife that ingest it; it can also cause pregnant women to miscarry.

The rat lungworm, Angiostrongylus cantonenis, is another parasite carried by black rats, as well as slugs and snails. Eating an infected snail can cause meningitis, leading to brain damage and even death.

Black rats also brought novel trypanosomes to Christmas Island, which has been linked to the rapid extinction of two species of rodents endemic to the island.

As predators, black rats seem to eat almost anything, and are linked to the extinction of island species across the Pacific. While brown rats and Polynesian rats have also invaded Pacific islands, black rats are considered the worst of the three because of the range of species they affect – from insects, geckos and skinks to bats and seabirds – and the severity of their impact.

Most of the recent extinctions of native small mammals on islands and the demise of many seabirds have been attributed to black rats. These situations involve three possible ecological mechanisms:

  1. direct predation of a species by rats

  2. competition from rats for resources

  3. hyper-predation effects, whereby rats support high numbers of other introduced invasive predators, such as cats, which go on to kill the native species.

Black rats have been linked to decline of birds, skinks and invertebrates on Lord Howe Island, and compete with native bush rats in natural bushland on mainland Australia.

Among the seabirds threatened by black rats, which invade nests and eat eggs, is the little tern (Sternula albifrons), which is listed as endangered in Queensland, New South Wales, Tasmania and Victoria.

Despite all this circumstantial evidence, black rats have largely been overlooked as a focus for research, compared with other introduced mammals in Australia. This may be due to the perception that this pest has a low environmental impact, or that it is less economically damaging or conspicuous than others.

We feel the lack of research into black rats in the Australian environment represents a significant knowledge gap that needs to be addressed if we are fully to understand the conservation and health threats posed by one of the world’s most successful invaders.

Peter Banks is Associate Professor in Conservation Biology at the School of Biological Sciences, The University of Sydney. Nelika Hughes is a wildlife ecologist affiliated with the University of New South Wales and the Australian Wildlife Conservancy, and is currently at the University of Antwerp, Belgium.

More information

Banks PB & NK Hughes (2012) A review of the evidence for potential impacts of black rats (Rattus rattus) on wildlife and humans in Australia, Wildlife Research.

How to tell a black rat from a bush rat







Published: 25 November 2014

Things warm up as the East Australian Current heads south

Jaci Brown

Occasional erratic bursts southward of the East Australian Current (EAC) are thought to have moderated the weather of south-east Australia this autumn and winter and they continue to introduce tropical and sub-tropical marine species to Tasmanian waters.

Tasmania’s east coast: tropical and sub-tropical marine species normally found off NSW are finding their way further south, thanks to changes in the East Australian Current.
Tasmania’s east coast: tropical and sub-tropical marine species normally found off NSW are finding their way further south, thanks to changes in the East Australian Current.

Ocean monitoring by Australia’s Integrated Marine Observing System is providing scientists with significant new insights into the changing structure of the EAC. Over the past 50 years sporadic warm bursts have become more common as the EAC moves further south. With global warming, the warm burst we’ve seen this year may also become the norm.

Had our little friend Nemo the clownfish been riding the EAC this year he might have found himself holidaying in Tasmania rather than admiring the Sydney Opera House. He wouldn’t have been on the trip alone, though. Sea nettles (Chrysaora spp.) have headed from their usual home in Sydney to be found for the first time ever in Tasmania and the Gippsland Lakes.

<i>Chrysaora woodbridge</i>, or sea nettle, was found in surprising numbers in Tasmania this year.
Chrysaora woodbridge, or sea nettle, was found in surprising numbers in Tasmania this year.
Credit: copyright Lisa-ann Gershwin

Waters in the EAC travel southward along the east coast of Australia, with most of it splitting from the coast near Sydney and heading for New Zealand. A small part of the current, known as the EAC Extension, works its way southward past Victoria and Tasmania.

A typical signature in this region are the large eddies, around 200 kilometres across and hundreds of metres deep. Some of the warm water is trapped here along with marine life.

The EAC starts at the Great Barrier Reef and travels south to Sydney before turning eastward to New Zealand. Some of the water can still push southward via a series of strong eddies.
The EAC starts at the Great Barrier Reef and travels south to Sydney before turning eastward to New Zealand. Some of the water can still push southward via a series of strong eddies.
Credit: Eric Oliver

This year a larger proportion of the EAC was sent southward instead of breaking away to the east. Winter ocean temperatures off Bass Strait were around 19°C, an increase of 4°C. This impacted local fishing, beach conditions and the weather.

In the video (above) the animation on the left shows the actual sea surface temperature and speed of the ocean currents. The animation on the right shows the difference in the temperature from average conditions.

Through autumn and winter, you can see two interesting changes occur. A strong warm current heads down the coast from Sydney to the coast of Victoria. At the same time, warm water peels off from the EAC and swirls around in large eddies as it meanders toward Tasmania.

An unusual catch down south

One advantage of warm eddies is the refuge they provide for tuna. They congregate in the centre of the eddy where the waters are warm and dine at the nutrient-rich edges.

Local fishers in north-east Tasmania report a remarkable year that allowed them to fish longer than usual, providing game fishers with more opportunities to catch tuna.

Last summer’s (2013–2014) warmth provided an abundance of skipjack and striped marlin, while winter brought a run of bluefin tuna.

Redmap is a website where locals can report sightings of marine species that are unusual for a given area.

Last summer a manta ray, a tropical cartilaginous fish (in a group including rays and skates), was sighted off the north-eastern coast of Tasmania. Previously the southern-most sighting of a manta ray was just south of Sydney.

<i>Manta birostris</i> spotted off north-east Tasmania on Australia Day 2014.
Manta birostris spotted off north-east Tasmania on Australia Day 2014.
Credit: Redmap/Leo Miller

It’s not just new species visiting Tassie either. Local jellyfish such as the Lion’s Mane (Cyanea) – more commonly known as ‘snotty’ – are usually quite elusive, but turned up in unprecedented numbers last summer in Tasmania.

But there’s a catch

This movement south of the EAC may have an impact on other systems, including our health. We rely on fish such as those from the Tasman Sea as a source of omega-3 fatty acids for our brain health. But the concentration of omega-3 fatty acids in the fish is likely to decrease with global warming.

Algae are the original source of fatty acids. As our waters warm, we will see more of the algae from the tropics take up residence in the south-east.

But the algae from the tropics are much smaller, which means more steps in the food chain from the algae to the fish we eat. The more steps in the food chain, the more the omega-3 fatty acids in the fish are replaced by fatty acids that are less favourable to brain health.

The warmer coastal waters also contributed to the balmy autumn and winter in south-eastern Australia this year. Afternoon sea breezes cool coastal temperatures by drawing cool oceanic air onto the coast.

Sydney’s heat wave in May this year had 19 consecutive days of 22°C or more – this is partly due to the sea breezes failing to bring in the usual cooling air.

What’s causing the EAC to move south?

Over the past 50 years the EAC Extension has stretched about 350 km further south. This extension doesn’t happen smoothly but in erratic bursts.

The southward extent of the EAC is controlled by the collective behaviour of the winds between Australia and South America. Over that same 50-year period these winds changed their pattern due to a strengthening of a climate system known as the Southern Annular Mode.

The changes to this mode have been attributed to a combination of ozone depletion and increasing atmospheric CO2.

One of the most robust and consistent responses of the climate system to increasing CO2 is a further strengthening of the Southern Annular Mode.

So the result will likely be a further enhancement of the EAC extension southward and even warmer waters in the Tasman Sea.

Dr Jaci Brown is a senior research scientist with the Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology. Her research focuses on the El Nino Southern Oscillation (ENSO) and climate change. This article was originally published on The Conversation. Read the original article.






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