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Available at http://planet.uwc.ac.za/nisl
Threats to biological diversity 3: Exotic Species
Vanessa Couldridge & Sam Hopkins
Biodiversity & Conservation Biology Department
Matteo GarbelottoUniversity of California
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 –The Toad –Xenopus laevis Xenopus laevis 1414
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Xenopus laevis are a problem because they
Predate upon and compete with native species
Are toxic to predators
Make water turbid
The Toad –The Toad –Xenopus laevis Xenopus laevis 1414
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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
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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
The Toad –The Toad –Xenopus laevis Xenopus laevis 1717
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Chestnut Blight (Cryphonectria parasitica)
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The American Chestnut(Castanea dentata)
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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)
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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
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“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
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American Chestnut: Ecological Importance
Wildlife depended on the abundant crop of chestnuts
Many species of insects fed on the leaves, flowers, and nuts
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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
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“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
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“From cradle to casket…”
Posts & railroad ties
Telephone poles (65 feet)
Construction
Fuel
Fine furniture & musical instruments
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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
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Near densely populated areas Chestnut often formed nearly complete stands
due to rapid growth from stump sprouts
repeated coppicing for fuelwood
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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.
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“… “… 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.”
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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
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Introduction of Cryphonectria parasitica
"rapid & sudden death of many branches stems & trees"
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Introduction of Cryphonectria parasitica
American Chestnut produces a sweet but small nut
Chinese chestnut produces a large but generally tasteless nut
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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 here from Ichang in 1900. to hybridize for ornamentals and nut production
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Cryphonectria parasitica
Ascomycete
Produces both conidia & ascospores
Pycnidia stromata break through the lenticels and produce conidia and perithecia producing ascospores are formed
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Cryphonectria parasitica: Life Cycle
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Dispersal
Animals and insects
Ascospores are shot into the air after rain storms in the fall
Rain (conidia)
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Infects trunk and branches Only above ground parts of trees
active growth & sporulation
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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.
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Causes swollen or sunken orange-colored cankers on the limbs and trunks of the chestnut trees.
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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
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How does it kill the tree?
The fungus has girdled the tree and is producing yellow conidia asexual spores
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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
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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
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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
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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
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“…a tragic loss, one of the worst natural calamities ever experienced by this nation”
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Cumulative Impacts
Chestnut in Southern range was first affected by Phytophtera cinnamomum
Now affecting hybrids
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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)
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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
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Instead of sunken diffuse cankers, surviving European chestnuts had swollen cankers with evidence of "healing" along the margins.
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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
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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)
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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
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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
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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
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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
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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
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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.
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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
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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
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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.
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Approaches:
Hypovirulent strains
Asian blight resistance
Natural resistance Forest management practices
Blight Control and Restoration
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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.
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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.
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Varied success increase in stem size and stem number
(Anagnostakis 2001) strains do not persist (Peever et al. 1997)
Virulent strains
Hypovirulent strains
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Graph of Numbers of Living Stems of Intensively treated American Chestnuts
Intensively treated - cankers sampled, paired with hypovirulent strains, and reintroduced into the canker.
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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.
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Graph of Numbers of Living Stems of American Chestnuts with the Control Treatment
Control Treatment
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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
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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).
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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)
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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
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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
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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
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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
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Site Factors
Xeric vs. mesic sites
Blight control greatest on mesic sites
Competition- high levels of hardwood competition,
especially on mesic sites
Browse damage
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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
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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
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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
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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.
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References for the Rat and the ToadReferences for the Rat and the Toad