Agriculture Reference
In-Depth Information
Several comprehensive reviews exist on invasive plant species [15-21].
Rather than providing another overview of the topic, I will focus on a smaller
aspect of the issue, i.e., whether invasive-species conclusions based on single
species pairs would differ from conclusions based on means from multiple
invasive and native species. I test this hypothesis with a literature review and
with data from a common garden experiment.
Are there differences in growth characteristics between natives and exotics?
Two important predictions have been made about exotic species: 1) introduced
species have higher growth rates than do natives and 2) introduced species are
more tolerant to grazing than are native species, at least in regions where plants
evolved with low intensities of grazing. Introduced species are predicted to
have higher growth rates because they 1) have been 'released' from their nat-
ural enemies (pathogens or herbivores), 2) are able to shift allocation of
resources from secondary compounds to growth (reviewed in [22]), or 3) have
undergone rapid evolution for high growth rate and increased competitive abil-
ity (e.g., [23]). A factor that is seldom mentioned is the possibility that people
consciously selected fast growing species (out of a wide distribution of possi-
ble species and genotypes) to introduce. Whatever the mechanism, exotics are
predicted to have a higher rate of above-ground growth than natives when
grown under common conditions.
Several studies have reviewed papers that compared growth rates and com-
petitive abilities between native and exotic plant species (e.g., [24, 25]).
Daehler [24], in a literature review, compared natives and exotics for 9 growth
related traits, 4 spread related traits, and 3 “composite” traits. He found few
differences between natives and exotics. Among the growth related traits, only
leaf construction costs and leaf area were significantly different in exotic spe-
cies. Spread related traits were not significantly different between natives and
exotics, except for survival rate, which was higher in natives. Phenotypic plas-
ticity was significantly greater in exotics, and growth generally responded
more to nutrients in exotic than in native species. This provides an important
link to theory developed by Burke and Grime [26] and Davis et al. [27] that
predicts that invasions will be most likely to occur when unused resource puls-
es occur. Daehler [24] did not analyze tolerance to grazing or clipping,
although many of the cited papers contained data on this variable. Daehler con-
cluded that the major difference between exotics and natives is that the former
can more rapidly adjust to the fluctuating conditions of disturbed sites.
However, Vilá and Weiner [25] found in a meta-analysis that exotics had a
higher overall growth rate than natives. They went on to note that differences
between natives and exotics may have been influenced by biased choices of
investigators, who may have compared highly invasive exotics with “well
behaved” native species in at least some cases [25]. The differing conclusions
between Daehler [24] and Vilá and Weiner [25], and the large variation in out-
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