The Invasion Biology course discusses exotic species in detail. Rather than repeat everything, the following examples of invasive species have been selected for discussion: rinderpest, the black rat, the toad - Xenopus laevis and chestnut blight.
Rinderpest, or cattle plague, is an example of an introduced disease that had devastating consequences in Africa. It is a viral disease that affects primarily cattle, but all cloven-hoofed wild and domestic animals are susceptible to the disease. The rinderpest virus belongs to the genus Morbillivirus, and is similar to the influenza virus. It affects the gastrointestinal and respiratory systems. Rinderpest is highly infectious and almost always fatal when contracted. It has the ability to wipe out entire populations. Death occurs within 6 to 12 days after the first symptoms appear.
Rinderpest originates in Asia and was first introduced to the continent of Africa in 1887. The disease was present in Indian cattle imported to the east coast of Africa by the Italians to feed their army, which was invading Ethiopia at the time. It did not take long before the disease had spread to local cattle and wildlife populations. From East Africa, rinderpest spread rapidly across to the Atlantic ocean, and within 10 years it had reached South Africa. The ox-wagon transport system of the time contributed to the rapid spread of the disease.
This map shows the spread of the rinderpest disease across the African continent, from its introduction in 1887 until it reached the cape in 1897. The fauna and flora of Africa south of the Sahara changed completely as a result.
The devastation was enormous. Millions of animals, both livestock and wildlife, succumbed to the disease. Records from the time suggest that 90% of the cattle died.
This slide lists some of the devastation caused by the outbreak of rinderpest. Wildlife species that were directly affected through contraction of the disease included wildebeest, buffalo, giraffe, warthog, eland, kudu, and other species of buck. In turn, predators suffered as their prey species dwindled in number. Lions apparently became man-eaters because there was no other food for them and terrorized villages. Rinderpest also brought hardship and death to many of the local people, who depended on cattle for their livelihood. Transport at the time was largely animal powered and was brought to a standstill. Finally, the loss of so many grazers transformed the landscape.
The disease was eventually brought under control through a combination of early attempts at vaccination and some natural immunity that existed among surviving animals, which would have been passed on to their offspring. In the early 1960s a more reliable vaccine was developed and between 1962 and 1976 there was a large-scale attempt to eradicate rinderpest entirely from Africa through mass vaccination of cattle. This eradication programme was largely successful - 15 out of 17 countries were reportedly freed of the disease. Outbreaks do still occur from time to time across Africa, but none have been as severe as the original plague of the 1890s.
Vaccination of cattle in the 1960s also led to a virtual elimination of rinderpest in wildlife populations. This is because cattle could now no longer act as a reservoir for the disease. Wildebeest numbers in the Serengeti increased by about six-fold over a period of 15 years and then stabilized at this elevated level; buffalo numbers likewise increased dramatically.
This huge increase in wildebeest and buffalo numbers had an impact on the environment. An increase in grazers changed the long grass of the plains to short grass, thus eliminating much of the fuel for fires that are necessary to control tree growth, and thereby changing grassland into woodland. These pictures show the same area of the Serengeti photographed in 1980 and again in 2003. Note the increase in tree growth over this time period.
Ironically, it has been suggested that the eradication of rinderpest has led to an increase in the disease canine distemper among lions. Lions feeding on wildebeest infected with rinderpest may have gained some immunity to canine distemper, since the two viruses are very similar to each other - they are both Morbilliviruses. We now move from a virus to a vector (though not always directly) of many diseases - the rat.
The black rat originated in Asia but by Roman times it had made its way to the Near East and was in Europe by the 8th century. At this time Europeans were just setting off about the world in ships and they took the rats with them. Rats are nocturnal, omnivorous and are good breeders. They can breed throughout the year in favorable conditions and can have 3-6 litters a year with 10 young per litter. (images from www.nhpa.co.uk, www.lib.utexas.edu, www.map_of_asia.us/asia-map.gif, www.davrodigital.co.uk, www.mytravelguide.com, www.ncmuseums.org)
When we talk about invasive species we tend to think of it as a modern thing but the rat has been getting about the world for centuries. One of the most noticeable effects of the rats dispersal is in cases throughout human history of the plague. The rat as well as a few other rodents is largely responsible for the out breaks of the plague that have occurred through time. It is not the actual rat that causes the transmission of the plague but a flea such as Xenopsylla cheopis which carries the Pasteurella pestis. This is a blood born infection that is passed from rat to rat via the flea and when rat numbers get low fleas leaving dead rats will inevitably find a warm human to take a drink from. I say largely responsible because the disease can only travel where rats go and humans allowed the introduction of rats to all corners of the globe. Often the introduced rats would pass their fleas onto the native rodents which would allow for the spread of the plague even further. (images from www.bergen.edu, www.co.collin.tx.us)
The plague of Justinian occurred in the 6th century during the rule of Emperor Justintian. The human population in the Mediterranean reached the lowest numbers since the rise of the Roman empire. It is speculated that this plague caused the shift of European power from the Mediterranean to the north. There are texts that talk of a plague ravaging Britain and Ireland. It is suggested in some texts that it entered Britain from Ireland which was well connected with Gaul and the Mediterranean where pottery and corn and cloth are though to have been exchanged. When the plague hit England in 664 it killed off a number of notables and caused a number of changes. Firstly it killed the Archbishop of Canterbury, and the King of Kent, causing all his subjects to revert to paganism. It killed the bishop of the East Saxons and wiped out all of the monks who traveled to prey at his grave. In the black death it is reported that one quarter to one third of the population of Europe died. Although the thinking is that the great plague was caused by plague there are theories that it was another type of disease. The cases started in April 1665 in St Giles in the Fields and spread across the country from there. When it reached London the rich all left for the countryside but the poor were left. There are stories of houses with a plague victim in them being locked up with all of the family inside and the door would have a big red cross painted on the outside to warn everyone. There were also said to be plague pits owing to the large number of bodies and few people to bury them. The cry "bring out your dead" was called at night by a man with a barrow who took the bodies away. In a moment of brilliance 40,000 dogs and 80,000 cats were slaughtered thinking that they were spreading the disease however the decline in numbers of dogs and cats allowed the rat numbers to increase and meant that the fleas had less choice of victims.
By 1899 the Plague had made it to South Africa on the ships, it was introduced by rats and then passed on to humans and wild
Rodents. Don't go away thinking that this is a disease of the past, it is still occurring and outbreaks in human populations still make the news. In February of 2005 there was an outbreak in the Democratic Republic of Congo, it focused around a diamond mining community called Zobia and killed at least 60 and infected at least 350 people, it then moved on to Buta as people fled Zobia. In June 2006 100 cases of plague were found in the Ituri district and the DRC often report 1000 cases a year (images from www.historyforkids.org, www.bbc.co.uk/britian/o_late_med.shtml, www.insecta-inspecta.com/fleas/bdeath/black.html, en.wiki.org/wiki/black_death)
Plague is not the only problem with rat invasion. Lundy Island lies off the North coast of Devon in the Bristol Channel. The island itself is a site of special scientific interest and the waters around it are a marine nature reserve. The rats reached the island 200 years ago and rat numbers reached 40,000 at their highest. The rat extermination started in 2003 and cost 50,000 pounds but it seems to have been a success. The reason for the cull was largely down to two rare bird species. The first was the Puffin and the second the Manx Shearwater. These birds numbers had been declining. In fact in 1939 there were 3,500 nesting pairs of puffins on the island and by 2000 there were only 10 pairs. The rats were found to eat puffin eggs and chicks. Now that the rats have gone, hopefully the bird numbers will recover although this will take a number of years as puffins are long lived and slow breeding birds.
(Images from www.lundy island .co.uk )
The rats reached the Pacific islands in the 17th century. They are now established on 28 groups of islands. The rats eat a huge number of different things which makes them a threat to island wildlife. One of the well known cases is the Rarotonga flycatcher on the cook islands. In 1885 the formally abundant flycatcher was nearly extinct. In a bird survey in 1972 two were seen and three were heard giving an estimated population of 24- 48. In 1983 there were reported to be 21 birds and 2 nests. When the recovery program was set up it was noted that the eggs and chicks were being eaten by the ship rat. A poisoning rats and nest protecting program was started and by 1996 there were 132 birds.
(Images from homepage.ntlworld.com, www.odolep.com, www.fws.gov/midway/wildlife/bope, www.montereybay.com/creagrus/DRPE_id.html, www.audubon.org/bird/BOA/f41.c6a.html)
And now on to the toad. This frog a large species with female adults reaching 130 mm. It is mostly aquatic. The adult will either move to another pond or bury into the mud when the pond is dry. The frog eats insects, small fish, and young and larvae of its own species . The adults can breed more than once per season (Measey 2004).
(Image from lapecoranera.splinder.com)
Xenopus laevis is now found throughout the world owing to the usefulness of these animals in the lab and the pet trade and also their ability to detect if a woman is pregnant. These frogs either escape from the labs and tanks or are released when they no longer are wanted as a pet and have set up many viable populations all over the world. These frogs make great invaders because they do well in disturbed environments, will eat a varied diet, have a high reproductive rate, have a high salt tolerance for an amphibian, are relatively disease resistant and can move over land and through water systems.
These frogs are a problem because of their success. They predate upon and compete with native species. These animals may also be toxic to local predators because they use a toxic secretion as a defence against predation. They also make the water in which they live turbid.
(Image from www.sammakkolampi.net)
In southern California X. laevis has been present since the 1960s. They have been found to prey on the endangered tide water goby Eucyclobius newberryi. The frogs caught in California were found to have a tapeworm that has a secondary copepod host. This suggests that these tapeworms had to find an alternative host in the new country. To try and control the frogs rotenone has been used but this is no good because frogs breathe air and introduced predatory fish have been used along with other chemicals. In South Wales a feral population was studied for its gut contents and it was found that the diet was highly variable. They found zooplankton, bank voles, bits of rabbits and birds and Xenopus eggs. (Image from www.ucreserve.uscs.edu/youngerLagoon/ylr_fish.htm)
The invasion of this frog is also occurring in South Africa. It is moved out of its range by fishermen using the frogs as bait. The frog is also good at utilising disturbed habitat and makes use of human altered areas. It is common in farmers damns. In the Cape Flats it has invaded the range of X. gilli and hybridised with it. This is a problem because X. gilli is endangered due to habitat destruction and is now genetically threatened by the swamping of X. laevis genes. (Image from www.puk.ac.za) The examples started with an animal disease, they end with a plant disease - Chestnut Blight.
The selected examples demostrate the damage that invasive species can do to both the natural environment and human interests. For more information on invasive species see the Invasion Biology course. The image of the rat comes from http://www.the-piedpiper.co.uk/graphics1/blackrat.jpg