Biology Reference
In-Depth Information
mast-producing canopy tree has altered the ecology of much of the east-
ern deciduous forest of North America.
As we saw in the introductory example, sudden oak death fungus
could cause equal or greater ecological damage than chestnut blight.
Other kinds of disease agents can also cause serious impacts on dominant
plants. The pine wood nematode ( Bursaphelenchus xylophilus ), introduced
to Japan, has become a much more serious pathogen for endemic Japan-
ese pines than for pines from North America, for example (Kiyohara and
Bolla 1990).
Many animal diseases have behaved in a similar fashion. A number of
forms of the avian malaria pathogen (e.g., Plasmodium relictum ), for exam-
ple, infect many species of birds in continental regions of the tropics.
These infections are largely benign. In Hawaii, however, avian malaria is
highly virulent for native birds and is one of the principal reasons for the
disappearance of natives in lowland areas (Van Riper et al. 1986).Whirling
disease of salmonid fish is caused by an amoeboid protozoan, Myxobolus
cerebralis , that probably evolved in Europe (Markiw 1992). The European
brown trout ( Salmo trutta ) is relatively resistant to Myxobolus but is a car-
rier that can produce spores that infect tubificid worms, the secondary
host of the parasite. Myxobolus was introduced to North America in the
late 1950s and has had severe impacts on many native species of trout and
salmon.
Alien Plant Disease Agents and Their Evolution
Fungi associated with cultivated cereals and other crop plants are notori-
ous both for their ability to track their hosts to new geographical areas
and to evolve races that overcome resistant plant varieties. One of the
most dramatic examples was the ability of a strain of the blight fungus
Helminthosporium maydis to infect hybrid maize ( Zea mays ) varieties carry-
ing a particular gene for male sterility. In 1970, this fungal strain caused
production losses of maize valued at about $1 billion. Fungi and other
pathogens associated with woody plants tend to follow their hosts to new
world regions, where they are subject to new evolutionary pressures
(Wingfield et al. 2001).
As we saw in chapter 5, strains of individual fungi from different
regions have become mixed in areas to which the fungi and their hosts
have been introduced. Hybridization of these strains often creates
increased genetic diversity and new invasive forms in the fungus popula-
tion (Brasier 2001). In the case of potato late blight ( Phytophthora infestans ),
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