Biology Reference
In-Depth Information
in Europe as in North America.Two of the most frequent species to dam-
age white campion in Europe were specialist organisms that are absent or
rare in North America. Further studies of plants from numerous localities
in Europe and North America have shown that the North American pop-
ulations have evolved a more vigorous growth and reproduction pattern
(Blair and Wolfe, forthcoming). North American plants germinated more
quickly, grew larger, and branched and flowered more than did European
plants. The rate of seed germination was greater in North American
plants, as was the probability of overwinter survival. For female plants, on
the other hand, the density of spiny epidermal hairs on the calyx, which
encloses the maturing fruits, was less for North American than European
plants.These observations strongly suggest that North American plants are
investing more in growth and reproduction and less in herbivore defense
than are European plants.
Scotch broom (
Cytisus scoparius
), native to the British Isles, has become
a highly invasive shrub in the North American Pacific Northwest. Broom
is attacked by many specialist herbivores in the British Isles, but few of
these are present in western North America (Waloff 1966). Some 23
species of pod- and seed-feeding insects attack Scotch broom in Britain,
none of which are present in the Pacific Northwest. Of the five species of
true bugs of the family Miridae that occur on broom in Britain, only
three occur in western North America. Release from herbivores is evi-
dent in late summer in the Pacific Northwest, where the foliage of broom
lacks the insect damage that it shows in Britain. Specialist herbivores are
also deficient on broom in Australia and New Zealand (Memmott et al.
2000).
The success of Australian eucalypts when introduced to other world
regions is certainly due in part to escape from specialist insects that exert
heavy herbivory in Australia. Although many native insects have begun to
feed on eucalypts in areas to which these trees have been introduced, eco-
nomically significant damage is rare (Ohmart and Edwards 1991). In Cal-
ifornia, for example, eucalyptus trees of many species have been intro-
duced, beginning in the mid-1800s. Until recently, these species have
been free of herbivorous insects that feed on them in Australia. Since
1984, however, about 15 species of herbivorous insects have been intro-
duced, with many trees showing heavy damage from their feeding (Paine
et al. 2000).
Release from herbivory is also responsible for the improved perform-
ance of a number of South African shrubs introduced to Australia and of
Australian plants introduced to South Africa (Weiss and Milton 1984). For
