1656.3 Results and Discussion
6.3.1 Summary of Results
Based on the data we collected in the database, we categorized 24 species in the
genus Bromus as widely introduced, 35 species as crop weeds, 37 species as ruderal
weeds, and 23 species as environmental weeds. These categories are not exclusive;
15 species were categorized as widely introduced and were in all three weed catego-
ries and 16 species were categorized as both widely introduced and environmental
weeds, meeting the criteria used here to be considered invasive.
Table 6.2 includes a summary of the analyses results for each hypothesis.
Tables 6.3 and 6.4 show the number of species used, Wald Chi-Square values,
p -values, and odds ratios generated by SAS for the logistic regressions. The uni-
variate logistic regressions showed most of the independent variables with the
exception of mean seed mass to be signifi cantly correlated with wide introduc-
tion and with all three categories of weediness. Polyploidy was signifi cantly cor-
related with wide introduction and weediness for crop and ruderal weeds, but not
for environmental weeds.
The species used for the logistic multiple regression were those species for which
data for multiple independent variables were available. This species set is more
biased toward species that are invasive, weedy, or cultivated by humans. These
results showed signifi cant correlations between early year and weediness. Long
maximum awn length and low minimum precipitation were signifi cantly correlated
with weediness for crop and ruderal weeds, and wide temperature range was signifi -
cantly correlated with weediness for ruderal weeds. Also, in this data set, low mean
seed mass was weakly correlated with weediness for crop weeds.
6.3.2 Taxonomy and Life Span (Hypotheses 1 and 2)
We hypothesized that the short-lived species in sections Genea and Bromus would
be more widely introduced and weedy than long-lived species (Hypotheses 1 and 2,
Table 6.2 ). The results of the univariate analysis supported these hypotheses
(Table 6.3 ). In the genus Bromus , taxonomy and life span are closely related.
Species in sections Bromus , Genea , and Nevskiella are mostly annuals. A few are
biennials. All species in section Bromopsis are perennials with the single exception
of Bromus texensis (Shear) Hitchc. (Texas brome). The species in sections
Ceratochloa and Neobromus tend to fall somewhere in between, with life spans
ranging from annual to perennial (Fig. 6.1 ).
Stebbins ( 1981 ) believed that the genus Bromus evolved in Eurasia, along with
grassland ungulates including wild cattle, bison, and sheep and that sections
Bromopsis (subgenus Festucaria ), Ceratochloa , and Neobromus differentiated dur-
ing the Pliocene. He suggested that Bromus grasses in sections Ceratochloa and
6 Attributes That Confer Invasiveness and Impacts Across the Large Genus Bromus...