Africa Invaded: Rats

Rats are undoubtedly the world’s most widespread invasive alien mammals, with the greatest economic impact. The costs associated with the approximately 250 million rats in the United States, for example, have been estimated at $19 billion per year. However, rats also cause significant environmental impacts, and have contributed to the extinction of many species of wildlife.

Black and brown rats

The two most common invasive alien rats worldwide are the black rat Rattus rattus and the brown rat Rattus norvegicus. The black rat is also known as the house rat or ship rat. It is native to south and east Asia, but has spread around the world on sailing ships. Widely distributed in Africa, it is evident in the archaeological record from before 800 AD.
The brown rat, also called the Norwegian rat, was a later introduction. It is believed to have originated in northern China and spread to Europe by the early 1700s, after which it was probably transported on ships to Africa. Today its African distribution remains limited to ports and large cities, where it thrives in sewers and buildings. It tends to inhabit the basements and cellars of buildings, while the black rat prefers the upper stories and
ceilings.

Indiscriminate feeders

Both rat species are omnivorous, and eat a wide range of foodstuffs, including seeds and seedlings, fruits and berries, eggs and small animals. By preying on other species or competing with them for food, they have caused the decline of many small mammals, birds, reptiles and invertebrates. Their effect has been particularly severe on islands. In the Seychelles, for example, rats have had more impact on endemic biodiversity than any other factor.

In Mauritius, they are believed to have caused the extinction of a number of snakes and lizards, and contributed to the threatened status of many birds. The black rat is able to climb trees to prey on forest birds, while the brown rat targets ground-nesting species.
Rats also cause a variety of socio-economic impacts by eating crops and stored grain, contaminating food stocks with their waste, chewing through power cables and spreading diseases.

Control

Most successful control programmes have made use of poisoned bait, usually containing brodifacoum as the active ingredient. In the past, cats were sometimes released on islands to control rat populations, with devastating consequences for birds and other small animals. On the Seychelles, feral cats have been blamed for the extinction of magpie robins on Aride and Alphonse islands, and have even been observed preying on turtle hatchlings. Likewise, the Indian mongoose – introduced to Fiji, the West Indies, Mauritius
and Hawaii to control rats – caused serious problems in these locations, including the extinction of several endemic species of birds, reptiles and amphibians.

Rats and 'Black Death'

Rat-borne diseases have claimed more human lives than all the wars in history combined! As a reservoir for the bubonic plague bacterium Yersinia pestis, the black rat is held accountable for 200 million deaths in medieval times alone.
Bubonic plague is transmitted by fleas from rats to people, but then spreads rapidly as it is highly infectious. An outbreak occurred in China in the early 1330s, but the disease was not introduced to Europe until 1374, when several Italian merchant vessels re t u rned from a trip to the Black Sea – a key trade link with China. Many of those onboard were already dying when the ships docked in Sicily, and the disease quickly spread through the surrounding countryside. The following year it reached England, where it was known as Black Death because its symptoms included black spots on the skin. The disease ultimately killed almost a third of Europe’s people.

Outbreaks of bubonic plague continue to occur – mostly in rural areas – with the World Health Organisation reporting 1000 to 3000 cases globally every year. Fortunately, the disease can nowadays be treated with antibiotics.

Reference: Matthews S. & Brandt K.   Africa Invaded: The growing danger of invasive alien species  Global Invasive Species Programme 2004