Importance of species replication in understanding plant invasions into North American grasslands - Invasive Plants: Ecological and Agricultural Aspects - page 48

comes among studies, suggests that differences in sampling design (e.g., vote
counting versus meta-analysis) might have affected the conclusions reached.
Problems with generalizing from studies of exotic-native pairs
Most land managers deal with large multi-species communities that contain
multiple exotic species (salt marsh dominated by Spartina spp is the exception,
e.g., [28]). They are commonly interested in knowing, in general, how all of
the exotic species in their area are affecting community and ecosystem
processes. The literature that compares exotic and native species growth char-
acteristics (e.g., [24, 25]) is highly relevant to these management issues.
However, because species are not replicated in most comparisons of native and
exotic species within a given site, results may not provide the best information
for management. Results and conclusions will be as highly variable as the vari-
ation among species. As any basic statistics topic will explain, a greater num-
ber of replicates will lead to more precise estimates that approach the actual
mean. Using few or no replicates may be leading to the widely varying results,
with some results falling well above the actual mean and some falling well
below the actual mean. Among-species variation in morphological or physio-
logical traits is enormous among both native and exotic species. Picking one
species out of a large distribution will give much greater weight to outlier spe-
cies and lead to widely varying conclusions among studies. Thus, I hypothe-
size that conclusions about exotic-native differences by Daehler [24] may have
been different if species had been replicated within sites.
Another, but less common problem with exotic-native species comparisons
is that growth form (e.g., annual versus perennial) is sometimes confounded
with native-exotic status [29, 30]. In many cases, the objective of the
researcher is to test hypotheses associated with how to restore native species
dominance [29, 30]. In these cases, researchers sometimes choose a perennial
native species to compare to an annual exotic. In this case, annual-perennial
(i.e., successional stage) and native-exotic status are confounded. This makes
sense in the context of restoration ecology, but it makes less sense in compar-
isons of natives to exotics (Tab. 1).
Literature review of native and exotic species comparisons
Here, I analyze data from papers cited in the review by Daehler [24] as well as
a few more recent studies, and break down the analysis into two data sets:
those that compare a single pair of native and exotic species and those that
replicate either native or exotic species, or both (Tab. 1). My prediction is that
studies that replicate species within their study areas will give a more accurate
estimate of the overall effect of exotic species, and will be less variable than
studies that use single pairs within sites. Furthermore, I analyze a variable that