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Threats to biological diversity 3: Exotic Species. Lecturer: James Reeler Material by: Sam Hopkins & Vanessa Couldridge. BCB 341: Principles of Conservation Biology. Introduction. Invasion by alien species can have a significant impact on biodiversity

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Threats to biological diversity 3 exotic species

Threats to biological diversity 3: Exotic Species

Lecturer: James Reeler

Material by: Sam Hopkins & Vanessa Couldridge

BCB 341: Principles of

Conservation Biology



  • Invasion by alien species can have a significant impact on biodiversity

  • Usually there are few predatory species/diseases for successful invaders (competitive advantage)

  • Exacerbated by habitat destruction/disturbance

  • Possibly exacerbated by climate shifts – expansion into new suitable ranges

  • Primary contemporary cause – humans

    • Deliberate (plants/animals with economic/aesthetic uses)

    • Accidental – “piggybacking” on other species

Invasion who

Invasion – who?

  • Not all species that are transported to new areas become invasive.

  • There are several characteristics of good invaders:

  • High reproductive rate (quickly build up a large population under favourable conditions)

  • Generalist species (variable diet, no strong habitat requirements)

  • Good dispersers (can rapidly spread to new areas & find suitable habitats)

Invasion where

Invasion – where?

  • Certain areas are more vulnerable to invasion

  • Disturbed areas/early succession

    • Tend to have unexploited resources/empty niches

    • Little competition

  • Remote islands with low diversity

    • Simple food webs/empty niche space

  • Remote islands/fragments with no predators

    • Often naive prey (included plants poorly adapted to herbivory

Invasion how

Invasion – how?

  • Generally follows three stages:

  • 1. Usually start with a few individuals

    • High initial likelihood of population extinction

    • Initial establishment phase – growing population, little size expansion

  • 2. Spreads from initial site and increases range (expansion phase)

  • 3. Fills all available habitat and enters saturation phase.

Case studies

Case studies

  • The following examples of invasive species have been selected for discussion:

    • Rinderpest

    • The black rat (Rattus rattus)

    • The toad/platanna – Xenopus laevis

    • Chestnut blight



  • Viral disease that affects primarily cattle (also known as cattle plague)

  • All cloven-hoofed wild and domestic are animals susceptible to the disease

  • Belongs to the genus Morbillivirus

  • Affects gastrointestinal and respiratory systems

  • Highly contagious and usually fatal; it can wipe out entire populations

  • Death occurs 6-12 days after the first symptoms

Rinderpest introduction to africa

Rinderpest: Introduction to Africa

  • Introduced to Africa from Asia in 1887

  • Disease was present in Indian cattle imported to the east coast of Africa to feed the Italian army, which was invading Ethiopia at the time

  • Quickly spread to local cattle and wildlife populations

  • From there the disease swept across eastern and southern Africa, with devastating consequences

  • Within 10 years it had reached South Africa

Rinderpest spread in africa

Rinderpest: Spread in Africa

  • This map shows the spread of the disease across the African continent

  • The fauna and flora of Africa south of the Sahara changed completely as a result

Rinderpest plague of 1890s

Rinderpest: Plague of 1890s

  • Millions of animals died, both wild and domestic

  • Reports indicate more than 90% of cattle and wildebeest were wiped out

Rinderpest devastation caused

Rinderpest: Devastation Caused

  • Wildlife killed by rinderpest included wildebeest, buffalo, giraffe, warthog, eland, kudu, and other buck species

  • Predators also suffered as their prey species disappeared; lions reportedly became man-eaters

  • Pastoralists depending on cattle for their livelihood faced severe hardship and death

  • Ox-wagon transport was brought to a standstill

  • Loss of grazers transformed the landscape

Rinderpest control

Rinderpest: Control

  • The disease was eventually brought under control through early attempts at vaccination and natural immunity among surviving animals

  • 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

  • This was largely successful – 15 out of 17 countries were freed of the disease

  • Outbreaks still occur from time to time, but none as severe as the original plague of the 1890s

Rinderpest recovery

Rinderpest: Recovery

  • Vaccination of cattle in the 1960s eliminated rinderpest from wildlife populations, as cattle could 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; Buffalo numbers also increased dramatically

Rinderpest landscape change

Rinderpest: Landscape Change



  • This had an impact on the environment by changing grassland into woodland – an increase in grazers eliminated the fuel for fires that control tree growth. Fires are now less frequent and do not burn as hot!=/


Rinderpest and canine distemper

Rinderpest and Canine Distemper

  • Ironically, it has been suggested that eradication of rinderpest has led to an increase in canine distemper among lions

  • Lions feeding on wildebeest infected with rinderpest may have gained immunity to canine distemper, since the two viruses are very similar to each other (both Morbilliviruses)

The rat 1

The Rat1

  • The Black Rat (Rattus rattus) was originally from Asia

  • It made its way to the near East in Roman time

  • It was in Europe in the 8th century

  • From Europe it had a boat ticket to the rest of the world

  • Rats are nocturnal

  • Rats are omnivorous

  • They are good breeders

The rat and the plague 2

The Rat and the plague2

  • The rat and a number of other rodents are largely responsible for out breaks of plague through history

  • Humans as carriers of rats also participated in the spread of the disease

  • Often the rats would then infect native rodents with the disease

History of the plague

History of the Plague

  • An early example is the plague of Justinian 3

  • 544, The first great plague 4

  • 1348, Black Death 5

  • 1665, Great Plague 6

  • 1899, Plague in South Africa 7

  • Recent plague – 2005/ 2006 DRC 8,9

Other effects of rat invasion lundy puffins 10 11

Other effects of rat invasion – Lundy Puffins 10, 11

  • Lundy island is off the coast of North Devon, UK

  • Rats reached the island 200 years ago

  • Rat numbers reached 40,000

  • Extermination started in 2003

  • Puffin and Manx Shearwater numbers had declined

  • Now rats gone, hopefully bird numbers will increase

Other effects of rat invasion pacific islands 12 13

Other effects of rat invasion – Pacific Islands 12,13

  • Reached Pacific Islands in the 17th century

  • Now established on 28 groups of islands

  • Eat native snails, beetles, spiders, moths, stick insects, and fruit, eggs and young of birds

  • Largest threat to the Rarotonga flycatcher

  • Other Island birds affected

    • Sooty terns, Seychelles

    • Bonin Petrels, Hawaii

    • Galapagos dark-rumped petrels Galapagos islands

    • White tailed tropic birds Bermuda

The toad xenopus laevis 14

The Toad –Xenopus laevis 14

  • Xenopus laevis is the common platanna in Southern Africa

  • It is mainly aquatic

  • Females reach 130 mm

  • Eats insects, small fish, young and larvae of its own species or other species of frogs

  • Adults can breed more than once per season

The toad xenopus laevis 141

The Toad –Xenopus laevis 14

  • Xenopus laevis is found about the world owing to

    • Lab animals

    • Pet trade

    • Pregnancy tests

  • These animals escape and can form viable populations

  • Now found in USA, Chile, Mexico, France, Indonesia and the UK

  • These frogs are a great invader owing to

    • Good in disturbed environments

    • Has a varied diet

    • High reproductive rate

    • High salt tolerance

    • Disease resistant

    • Can move overland or through rivers and streams

The toad xenopus laevis 142

The Toad –Xenopus laevis 14

  • Xenopus laevis are a problem because they

    • Predate upon and compete with native species

    • Are toxic to predators

    • Make water turbid

The toad xenopus laevis

The Toad –Xenopus laevis

  • Seen in Southern California

  • X. laevis has been present since the 1960s

  • Preys on the Tide Water Goby

  • Preys on the Endangered Red-legged frog

  • Also managed to establish parasites that need alternate hosts 15

  • In South Wales, Xenopus were found to have a very varied diet ranging from zooplankton to bank voles to Xenopus eggs 16

The toad xenopus laevis 17

The Toad –Xenopus laevis 17

  • In South Africa X. laevis is an invasive

  • Animals are moved out of their natural range by fisherman

  • Animals make use of habitats disturbed by humans

  • Have hybridized with Xenopus gilli

Chestnut blight cryphonectria parasitica

Chestnut Blight (Cryphonectriaparasitica)

The american chestnut castanea dentata

The American Chestnut(Castanea dentata)

American chestnut range

American Chestnut: Range

  • Maine to Georgia and west to Ohio and Tennessee. (Braun, 1950)

  • Commonly made up 25% or more of mixed stands

  • Formed pure stands on many dry Appalachian ridgetops and near densely populated areas.

Map of Historical Range of Castanea dentata (Saucier, 1973)

American chestnut habitat

  • Common on midslopes and other moderately dry soils

  • Shared moist meso-phytic soils with many other species

  • Tap root 4 to 5 ft down

American Chestnut: Habitat

Redwoods of the east

“Redwoods of the East”

  • Mature chestnuts could be 600 years old and average up to five feet in diameter and 100 feet tall

  • Many specimens of 8 to 10 feet in diameter were recorded

American chestnut ecological importance

American Chestnut: Ecological Importance

  • Wildlife depended on the abundant crop of chestnuts

  • Many species of insects fed on the leaves, flowers, and nuts

American chestnut economic importance

American Chestnut: Economic Importance

  • Throughout much of the range chestnut had the most timber volume of any species

  • It was half the standing timber volume of CT

  • Was the major source of tannin for leather pro-duction (6-11 % tannin content)

  • Chestnuts

From cradle to casket

  • Fast growing -reached half ultimate height by 20th year

  • Resistant to decay

  • Straight and tall - often branch free for 50 feet

  • Only white pine & tulip poplar could grow taller

“From cradle to casket…”

From cradle to casket1

“From cradle to casket…”

  • Posts & railroad ties

  • Telephone poles (65 feet)

  • Construction

  • Fuel

  • Fine furniture & musical instruments

American chestnut economic importance1

  • Scientific forest management in the US was just getting started when the country lost its most important hard wood species (Smith, 2000)

  • Foresters had begun to develop comprehensive plans for intensive management

American Chestnut: Economic Importance

Threats to biological diversity 3 exotic species

  • Near densely populated areas Chestnut often formed nearly complete stands

    • due to rapid growth from stump sprouts

    • repeated coppicing for fuelwood

Pure stand of chestnut in ct 90 years after clear cutting 1905

Pure stand of Chestnut in CT 90 years after clear-cutting, 1905.

  • Experts estimate that American Chestnut represented half the commercial value of all Eastern North American hardwoods

Threats to biological diversity 3 exotic species

“… the most valuable and usable tree that ever grew in the Eastern United States.”

Introduction of cryphonectria parasitica

Introduction of Cryphonectria parasitica

  • In 1904, Herman Merkel, a forester at the New York Zoological Garden, found odd cankers on American chestnut trees in the park

Introduction of cryphonectria parasitica1

Introduction of Cryphonectria parasitica

  • "rapid & sudden death of many branches stems & trees"

Introduction of cryphonectria parasitica2

Introduction of Cryphonectria parasitica

  • American Chestnut produces a sweet but small nut 

  • Chinese chestnut produces a large but generally tasteless nut

Introduction of cryphonectria parasitica3

Introduction of Cryphonectria parasitica

  • Thomas Jefferson

    • imported European or Spanish chestnut (Castanea sativa)

    • grafted it onto native root stocks at Monticello.

  • In 1876, a nurseryman in Flushing, NY, imported the Japanese chestnut (C. crenata).

    • More were brought over in 1882 and 1886.

  • Chinese chestnut (C. Molissima) was brought across from Ichang in 1900.

    • to hybridize for ornamentals and nut production

Cryphonectria parasitica

Cryphonectria parasitica

  • Ascomycete

  • Produces both conidia & ascospores

  • Pycnidia stromata break through the lenticels and produce conidia and perithecia producing ascospores are formed

Cryphonectria parasitica life cycle

Cryphonectria parasitica: Life Cycle


  • Animals and insects

  • Ascospores are shot into the air after rain storms in the fall

  • Rain (conidia)


Threats to biological diversity 3 exotic species

  • Infects trunk and branches

    • Only above ground parts of trees

active growth & sporulation

How does it kill the tree

How does it kill the tree?

  • Enters through fissures or wounds in the bark

  • Grows in and under the bark, girdling the cambium.

  • Kill the tree above the point of infection.

Threats to biological diversity 3 exotic species

  • Causes swollen or sunken orange-colored cankers on the limbs and trunks of the chestnut trees.

How does it kill the tree1

How does it kill the tree?

  • The leaves above the point of infection die, followed by the limbs.

  • Within two to ten years the entire tree is dead.

  • Not uncommon to find many cankers on one tree

How does it kill the tree2

How does it kill the tree?

  • The fungus has girdled the tree and is producing yellow conidia asexual spores

Host range

  • Like most cankers - fairly specific host range

  • Serious pathogen: American & European (infects Japanese and Chinese much less)

  • Moderate pathogen: Chinquapin & Live Oak 

  • Can also be found infecting/living on numerous oak species in the US

Host Range

Rate of spread

Rate of Spread

  • Aggressive attempts to halt the spread of the blight were made by PA and NY

    • removed chestnut over a large area to halt southward spread

  • In 1911-1913, the U.S. Congress appropriated special funds to enable foresters to study and control the blight

Rate of spread1

  • Horticulturalists, found a blight-free area in Pennsylvania and quickly imported trees to form an experiment station

    • transported the blight and created a new epicenter

    • Accelerated spread in PA

  • Cuts in funding for Chestnut blight research:

    • With the onset of World War I in 1914

    • The evident futility of control efforts

Rate of Spread

Threats to biological diversity 3 exotic species

  • By 1926, fungus reported throughout native range

  • By 1940, virtually all (an estimated 4 billion) were dead or infected with the blight

  • Chestnut was the dominant wood processed at PA sawmills in the early 1920s,

    • salvage logging to make use of the dead and dying trees

Threats to biological diversity 3 exotic species

  • “…a tragic loss, one of the worst natural calamities ever experienced by this nation”

Cumulative impacts

  • Chestnut in Southern range was first affected by Phytophtera cinnamomum

  • Now affecting hybrids

Cumulative Impacts

Cumulative impacts1

Cumulative Impacts

  • In 1974, the Oriental Chestnut Gall Wasp (Dryocosmus kuriphilus) was brought to the US

    • Female lays eggs in chestnut vegetative buds

    • Galls suppress shoot elongation and reduce fruiting

    • Heavy infestations can kill the trees (afflicts both American and Chinese chestnuts at the southern end of their ranges)

  • Threatening complete extinction

    (Anagnostakis, 1994)

Varying outcomes europe

Varying Outcomes: Europe

  • The fungus was later introduced into Europe (for tree breeding) from America

  • Moved through Europe killing European Chestnut

  • However, it was observed that many trees, while infected and full of cankers, did not die

Threats to biological diversity 3 exotic species

Instead of sunken diffuse cankers, surviving European chestnuts had swollen cankers with evidence of "healing" along the margins.

Threats to biological diversity 3 exotic species

  • Many forest pathologists began working on this healing canker

  • Speculation that:

    • European Chestnut was less susceptible

    • That the fungus had mutated

    • That it was a different fungus altogether 

Threats to biological diversity 3 exotic species

  • Noticed that a different colored fungus was recovered from "healing cankers"

  • Instead of the typical orange colored Cryphonectria parasitica fungus, a white-colored fungus was found. 

    • White fungus was slower growing and produced fewer spores

  • When you "sprayed" the white fungus on a "killing canker" the "killing canker"  became a "healing canker" (Europe)

Threats to biological diversity 3 exotic species

  • Determined that the white hypovirulent strains had become infected with a simple dsRNA  virus

  • This virus was making the fungus "sick“

  • A slower fungus allowed the tree to respond to a point where the tree could survive infection

Varying outcomes europe1

  • Grente reported in 1965 that ‘hypovirulent’ strains from Italy did not kill chestnut trees

  • Began a program of active intervention when blight was found in France

    • blight strains with dsRNA passed hypovirulence to lethal strains

  • Treatment of new cankers as they formed resulted in a successful ‘biological therapy’ of the disease.

    • treat every canker for several years

Varying Outcomes: Europe

Threats to biological diversity 3 exotic species

  • For a number of reasons biological control of chestnut blight does not work as well in the US

    • Different mating types of the fungus

    • Lack of chestnut to support conversion of the fungus by the virus

    • The many different types of virus in the United States

Varying outcomes michigan

Varying Outcomes: Michigan

  • Hypovirulent strains were found in the United States

    • Most notably in Michigan

  • Successful because:

    • Few mating types

    • High number of Chestnut

    • Isolated from the native range   

  • Less diversity of pathogen in MI so that hypovirulence can transfer more readily

Threats to biological diversity 3 exotic species

  • The transmission of hypovirulence from strain to strain of the fungus is restricted by a genetic system of vegetative incompatibility

  • Six loci, each with two alleles in a system of heterogenic incompatibility which keep the strains of the fungus from fusing and passing hypovirulence (Huber and Milgroom)

  • Virus transfer is restricted when there are different alleles at the vegetative incompatibility loci

Current status

Current Status

  • Reduced to a short lived sprouting understory tree

  • Fungus can not survive below the ground. 

    • roots continue to live and they send up stump sprouts.   

Current status1

  • Stump sprouts grow until infected

    • the stump re-sprouts again

  • Little chance for resistance to evolve

    • sprouts typically killed before they become sexually mature

    • sexual reproduction rare

Current Status

Last remaining stand of american chestnut

Last remaining stand of American Chestnut

  • Largest living (>3 ft dbh) about 20 miles east of La Crosse, WI.

  • 10 chestnuts planted in 1885

  • Seeds propagated around 50 acres and more than 3000 trees

  • Trees were blight free due to isolation until a canker was found in 1986

  • Now over 1600 cankers are present on 530 trees.

  • Virus was introduced in 1992 – not successful

Blight control and restoration

Blight Control and Restoration

  • Approaches:

    • Hypovirulent strains

    • Asian blight resistance

    • Natural resistance

    • Forest management practices


  • Combination of the four approaches can bring the chestnut back

    • Individual or group selection openings- an integrated management system using grafted trees, inoculating them with hypovirulent strains, and controlling hardwood competition

    • Timber production- backcross approaches


Concluding remarks

Concluding Remarks

  • 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.

References for the rat and the toad

References for the Rat and the Toad

  • Wikipedia contributors, Black Rat, [accessed 2006 July 30] Wikipedia, The free Encyclopaedia, Available Rat

  • Gross, L. (1995). How the plague bacillus and its transmission through fleas were discovered: Reminiscences from my years at the Pasteur Institute in Paris. Proceedings of the National Academy of Science 92: 7690-7611.

  • Dols, M. (1974) Plague in Early Islamic History Journal of the American Oriental Society 94:371-383

  • Maddicott, J. R.(1997) Plague in Seventh -Century England. The past and present society

  • Davis, D. The Scarcity of rats and the black death: an ecological history, Journal of Interdisciplinary history 16: 455-470

  • Storey of the Plague,(2006) channel 4 [accessed July 30th 2006]. Found at

  • May, J. M. (1952). Map of the world distribution of plague. Geographical review 42:628-630.

  • BBC news (2005)DR Congo plague outbreak spreads [accessed July 30th 2006]

  • the world health organization (2006). Plague in the DRC. [accessed July 30th 2006]

  • Lundy shore office [accessed July 30th 2006] http://www.lundy_

  • BBC news (2005) Lundy Rats. [accessed 30th July 2006]

  • Atkinson, I. A. E and Atkinson, T. J. (2000) Land vertebrates as invasive species on islands served by the south pacific regional ENVIRONMENT PROGRAMME. In: Invasive species in the pacific: A Technical review and draft regional strategy. South Pacific regional environment program Samoa: 19-84

  • G. McCormack. (2005). The Status of Cook Island Birds 1996, Cook Island Biodiversity and natural heritage

  • Measy, J. (2004). Global Invasive species database Xenopus laevis. [accessed 30th July 2006] Found at

  • Lafferty, K. and Page, C. (1997) Predation on the endangered Tide Water Goby, Eucyclobius newberryi, by the introduced African clawed frog I, Xenopus laevis, with notes on the frogs parasites. Copeia 1997: 589-592.

  • Measey, G. J. (1998) Diet of feral Xenopus laevis (Daudin) in South Wales, UK. Jnl Zool. 246:287-298

  • Measy G. J. (2004) Xenopus laevis. In Atlas and red data book of the frogs of South Africa, Lesotho and Swaziland, eds Minter, L. et al.

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