Importance of species replication in understanding plant invasions into North American grasslands - Invasive Plants: Ecological and Agricultural Aspects - page 49

was not measured by Daehler [24], but that is important to grazed grasslands:
tolerance to either real or simulated defoliation by grazers. There are few stud-
ies with replicated species responses to grazing or clipping, so this variable
was analyzed with the combined data set (pooling replicated and non-replicat-
ed studies).
A total of 61 studies were analyzed. Of those 61 studies, only 10 replicated
both native and exotic species (16%). Seven studies replicated natives only and
3 replicated exotics only, for another 16%. Most studies (41 out of 61, 67%)
did not replicate either species type, i.e., they compared a single exotic species
to a single native species (Tab. 1).
I found that results differed between studies that did and did not replicate
native and exotic species (Fig. 1). I used a vote count technique [24] and clas-
sified studies into categories of 1) exotic mean > native mean, 2) no signifi-
cant difference between exotic and native species means and 3) native mean >
exotic mean. In contrast to Daehler [24], who classified studies as exotic =
native if any of the natives equaled the exotic (which biases the results towards
Figure 1. Proportion of studies cited by Daehler [24] that found higher growth rates in exotics than
natives (E > N), no significant difference (n.s.), or higher rates in natives than exotics (N > E) among
studies that replicated either native or exotic species identity, or both (a) or among studies that did not
replicate species (b).