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Threats to biological diversity 3: Exotic Species

Vanessa Couldridge & Sam Hopkins

Biodiversity & Conservation Biology Department

Matteo GarbelottoUniversity of California

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 (Rattus rattus)

The toad/platanna – Xenopus laevis

Chestnut blight

IntroductionIntroduction

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

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RinderpestRinderpest

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: Introduction to AfricaRinderpest: Introduction to 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: Spread in AfricaRinderpest: Spread in Africa

Millions of animals died, both wild and domestic

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

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Rinderpest: Plague of 1890sRinderpest: Plague of 1890s

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: Devastation CausedRinderpest: Devastation Caused

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: ControlRinderpest: Control

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

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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

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Rinderpest: Landscape ChangeRinderpest: Landscape Change

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)

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Rinderpest and Canine DistemperRinderpest and Canine Distemper

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 RatThe Rat11

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

The Rat and the plagueThe Rat and the plague22

An early example is the plague of Justintian 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

History of the PlagueHistory of the Plague

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 – Other effects of rat invasion – Lundy Puffins Lundy Puffins 10, 1110, 11

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

Other effects of rat invasion – Other effects of rat invasion – Pacific Islands Pacific Islands 12,1312,13

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 –The Toad –Xenopus laevis Xenopus laevis 1414

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


Xenopus laevis are a problem because they

Predate upon and compete with native species

Are toxic to predators

Make water turbid


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 –The Toad –Xenopus laevisXenopus laevis

In South Africa X. laevis is an invasive

Animals are moved out of their natural range by fisherman

Animals make use of the habitat disturbed by humans

Have hybridized with Xenopus gilli


Chestnut Blight (Cryphonectria parasitica)

The American Chestnut(Castanea dentata)

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

“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

Wildlife depended on the abundant crop of chestnuts

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

American Chestnut: Economic Importance

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

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…”

Posts & railroad ties

Telephone poles (65 feet)

Construction

Fuel

Fine furniture & musical instruments

American Chestnut: Economic Importance

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

Near densely populated areas Chestnut often formed nearly complete stands

due to rapid growth from stump sprouts

repeated coppicing for fuelwood

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

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

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

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


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


American Chestnut produces a sweet but small nut

Chinese chestnut produces a large but generally tasteless nut


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 here from Ichang in 1900. to hybridize for ornamentals and nut production

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

Dispersal

Animals and insects

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

Rain (conidia)

Infects trunk and branches Only above ground parts of trees

active growth & sporulation

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.

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


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


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

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 Spread 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

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

“…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 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

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

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

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

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)

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: Europe 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

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 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

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

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 Status

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

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

Where are we now? Upper slopes – scarlet oak,

hickory, black gum

Mid slopes – red and white oak, red maple, & hickory

Coves – Poplar, hard maple, beech

Understory - American chestnut sprouts still persist, however they become infected between 1-12 yrs of age.

Approaches:

Hypovirulent strains

Asian blight resistance

Natural resistance Forest management practices

Blight Control and Restoration

Hypovirulent Strains Italian and French scientists observed non-lethal

cankers growing on trees in Italy (1960’s) Found that strains of the fungus associated with

the blight produced colonies of abnormal shape and pigment

Demonstrated that these strains contained some “contagious factor” responsible for the inability to produce lethal infections (i.e., Hypoviruses)

In North America, hypovirus-infected strains have been found in stands in Michigan.

Hypovirulent Strains In the last two decades, scientists have attempted

to debilitate the fungus by infecting it with a virus, a process called hypovirulence.

Hypovirulence gives chestnut trees a much less potent form of the disease and gives chestnuts a fighting chance for survival (i.e., fungus is restricted to the outer bark).

Once introduced into a few trees, hopes are that hypovirulence will spread throughout the forest, offering hope to surrounding trees as well.

Varied success increase in stem size and stem number

(Anagnostakis 2001) strains do not persist (Peever et al. 1997)

Virulent strains

Hypovirulent strains

Graph of Numbers of Living Stems of Intensively treated American Chestnuts

Intensively treated - cankers sampled, paired with hypovirulent strains, and reintroduced into the canker.

Graph of Numbers of Living Stems of American Chestnuts with Limited Treatment

Limited treatment - cankers were sprayed with a mixture of conidia from Hypovirus-infected strains that had been used for treatment of the intensively treated plot.

Graph of Numbers of Living Stems of American Chestnuts with the Control Treatment

Control Treatment

Factors contributing to failure

High blight susceptibility

Abundance of virulent inoculum

Restricted movement of the hypovirulence viruses

among the many strains

Europe and Michigan strains

Factors contributing to failure The transmission of hypovirulence from strain to

strain of the fungus is restricted by a genetic

system of vegetative incompatibility.

Genetic studies found that there are six loci, each

with two alleles in a system of heterogenic

incompatibility which keep the strains of the

fungus from fusing and passing hypovirulence

(Milgroom and Cortesi).

More factors contributing to failure

Environmental stress

Superficial canker instability (i.e., hypovirulent

cankers produced change back into a killing

canker after one or more winters)

Asian blight resistance Early breeding efforts unsuccessful:

Poor form

< 50% AC parentage

Poor survival

1981 backcross breeding method proposed (Burnham)

Better form

Field blight resistance

Resistant Asian X Susceptible American

Partially resistant X American again

1 out of the 4 will have 1 copy of both resistant genes

Process repeated until a final cross of 2 trees with partial resistance yields 1 having 2 copies of both resistant genes making it fully resistant

American chestnut resistance

Breeding programs

Scions were grafted into chestnut rootstocks to

establish seed orchards

Seeds and seedlings have been distributed

that have low levels of blight resistance by

artificial inoculation with a standard virulent

strain

Site Factors High vs. low elevation

High elevation sites contain the highest density of

chestnut sprouts

Studies found the superficiality rating of cankers to

decrease greatly (disease developed at the vascular

cambium) after several winters at high elevation sites

May be a result of physiological stress from low

temperatures in mid- to late winter which may decrease

host defense mechanisms in chestnut towards weak

pathogens, such as hypovirulent strains

Site Factors

Xeric vs. mesic sites

Blight control greatest on mesic sites

Competition- high levels of hardwood competition,

especially on mesic sites

Browse damage

Restoration

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

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.

Concluding RemarksConcluding Remarks

1. Wikipedia contributors, Black Rat, [accessed 2006 July 30] Wikipedia, The free Encyclopaedia, Available from:En.wikipedia.org/wiki/Black Rat

2. 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.

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

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

5. Davis, D. The Scarcity of rats and the black death: an ecological history, Journal of Interdisciplinary history 16: 455-4706. Storey of the Plague,(2006) channel 4 [accessed July 30th 2006]. Found at

http://www.channel4.com/history/microsites/H/history/plague/experts.html

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

8. BBC news (2005) DR Congo plague outbreak spreads [accessed July 30th 2006] news.bbc.co.uk/1/hi/world/africa/4290783.stm9. the world health organization (2006). Plague in the DRC. [accessed July 30th 2006]

www.who.int/csr/don/2006_06_14/en/index.html10. Lundy shore office [accessed July 30th 2006] http://www.lundy_ island.co.uk

11. BBC news (2005) Lundy Rats. [accessed 30th July 2006] www.bbc.co.uk/devon/content/articles/2005/06/08/lundy_rats_feature.shtml)

12. 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

13. G. McCormack. (2005). The Status of Cook Island Birds 1996, Cook Island Biodiversity and natural heritage http://cookislands.bishopmuseum.org/showarcticle.asp?id=7

14. Measy, J. (2004). Global Invasive species database Xenopus laevis. [accessed 30th July 2006] Found at http://www.issg.org/database/species/ecology.asp?si=ISO&fr=1&sts=sss

15. 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.

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

17. 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.

References for the Rat and the ToadReferences for the Rat and the Toad

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