Biology Reference
In-Depth Information
a substantially reduced parasite load. In North America, house sparrows
have only 37 species of parasitic arthropods, compared to 60 species in
Europe (Brown and Wilson 1975). Similarly, in North America, European
starlings have only 22 species of parasitic worms, compared to the 70
species recorded in Europe (Hair and Forrester 1970). Mammals intro-
duced to oceanic islands also tend to show reduced parasite loads (Dob-
son 1988).
Enhanced Growth and Performance of Alien Species
Several investigators have suggested that plants typically show more lux-
uriant growth in regions to which they are alien than in their native
region. Crawley (1987), for example, compared published information on
plant height for species of European plants in Europe and in their intro-
duced range in California. For 228 species, he found that in California 99
species were taller, 66 shorter, and 63 about the same in height. He con-
cluded that escape from natural enemies might have contributed to the
significantly greater number of species that were taller. Blossey and Nöt-
zold (1995) likewise suggested that many alien invasive plants show
greater competitive ability, expressed as increased vegetative growth, in
areas to which they have been introduced, compared to their native
region. Increased growth capability was hypothesized to result from
reduced allocation to herbivore defense.Testing this hypothesis with pur-
ple loosestrife (
Lythrum salicaria
), a native of Europe, Blossey and Nötzold
found that plants from North America grew taller and produced more
biomass than did plants from Europe. Tests with native European root-
feeding weevils, natural herbivores of the plant in Europe, also showed
that larvae of these insects showed higher survival and reached greater
weights on North American plants than on European plants.Willis et al.
(1999) also conducted experiments on purple loosestrife growth and per-
formance of foliage-feeding insects. Shoot mass of North American
loosestrife did tend to be higher than that of European plants. The con-
centration of phenolic compounds, which often function as herbivore
repellents, in North American plants also tended to be lower than that in
European plants. Larval development and adult size of foliage-feeding
insects, however, did not differ for North American and European plants.
Other studies have failed to show consistent differences in plant
growth between European plants in native and alien regions (Willis et al.
2000). Thébaud and Simberloff (2001), for example, conducted a more
comprehensive analysis, comparing published information on native
