Biology Reference
In-Depth Information
woody plants (Niemalä and Mattson 1996).About three-quarters of these
herbivores are from Europe. Most of them have colonized the same gen-
era of woody plants that they feed on in Europe. Only 12 species of Euro-
pean herbivores have expanded their feeding to genera of North Ameri-
can woody plants that do not occur in Europe; six of these are species
with a diet including many genera in their native area.
Interestingly, the invasion of Europe by herbivorous insects from
North America has been much weaker (Niemalä and Mattson 1996).
Only 34 North American forest insects have become established in
Europe, and most of these are apparently successful because of the intro-
duction of their North American host plants. Several historical reasons
may account for this remarkable imbalance. North America has a richer
vascular plant flora than Europe, especially among gymnosperms. Many
genera of woody plants that became extinct in Europe during the Pleis-
tocene, in particular, have survived in North America.Thus, there may be
more potential hosts for European invaders. European trees also appear to
have a greater diversity of herbivorous insects than do North American
trees, a relationship that may have favored selection for strong competitive
ability. Physiologically, European insects from higher latitudes may also
have less difficulty in dealing with photoperiod patterns at lower latitudes
in North America than would species moving in the opposite direction.
The combined impacts of repeated glacial cycles on forest distribution
and the long history of human disturbance may also have selected for effi-
cient dispersal and colonizing ability. Parthenogenetic reproduction, for
example, is unusually common in European forest insects, many of which
are successful colonists of North America.These patterns, in general, sug-
gest that Europe may have been an evolutionary crucible for species
unusually well adapted for invading North American areas with extensive
forests, a diverse flora of woody plants, and low native herbivore diversity
(Niemalä and Mattson 1996).
A recent example of a European invader is the European pine shoot
beetle (
Tomicus piniperda
), which first appeared in North America in
1992. The first occurrences were at Christmas tree farms and pine tree
nurseries. The beetle prefers Scotch pine (
Pinus sylvestris
) and Austrian
pine (
P. nigra
), which are Eurasian natives. In North America, however, it
can carry out its life cycle on most pines, including widespread native
pine species such as red (
P. resinosa
), ponderosa (
P. ponderosa
), and jack (
P.
banksiana
).
Although Europe has been the most important source of North
