156
6.1 Introduction
The grass family ( Poaceae ) contains a number of species that are of exceptional
importance to humans, including wheat, rice, corn, barley, rye, millet, sugarcane,
and grasses that provide forage for domesticated animals. Unfortunately, some
grass species that have been introduced by humans into new regions have become
destructive both to natural environments and to human agricultural systems. Pyšek
( 1998 ) listed the Poaceae family in the top four families that provide the most spe-
cies listed as alien in world fl oras and in the top six families that have the highest
proportion of invasive species relative to total species. He suggested that highly
evolved infl orescences, successful seed dispersal mechanisms, and ability to grow
in and adapt to diverse habitats all contribute to invasiveness in grasses.
The grasses in genus Bromus include a number of weedy species that have
impacted croplands, rangelands, and natural habitats and that have spread far out-
side of their native ranges. There are also many Bromus grasses that are not consid-
ered weedy and that have limited distributions. Western North America has had
exceptionally severe problems with Bromus invasions. Two species that are highly
invasive in North America are Bromus tectorum L. (cheatgrass or downy brome)
and Bromus rubens L. (syn. Bromus madritensis ssp. rubens (L.) Husnot, red
brome). B. tectorum is a weed of winter cereal crops. It is also an aggressive invader
in natural habitats, especially in the Great Basin region where it is common in sage-
brush ecosystems (Concilio et al. 2013 ). B. rubens has become a serious invader in
the Mojave Desert and Sonoran region and is broadly distributed in lower elevations
in the Great Basin (Salo 2004 , 2005 ; Salo et al. 2005 ; Brooks and Berry 2006 ).
A somewhat elusive goal of invasion science is to fi nd a fast and easy way to
determine which species are likely to become invaders and when invasions are
likely to occur. Unfortunately, it is increasingly clear that there is no easy way to
identify species with a potential for invasiveness or to recognize the early stages of
invasions. Researchers have identifi ed a number of factors associated with invasive-
ness, but few of these are consistent across a broad range of plant taxa and types.
According to the National Research Council ( 2002 , quoted by Powell 2004 ), there
are “no known broad scientifi c principles or reliable procedures for identifying the
invasive potential of plants, plant pests, or biological control agents in new geo-
graphical ranges.” Kolar and Lodge ( 2002 ) commented that “characteristics that
apply generally to all taxonomic groups and in all ecosystems” do not exist and that
characteristics important in the early stages of an invasion may differ from those
that facilitate later invasion stages. Lodge ( 1993 ) suggested that prediction requires
“focused studies at more specifi c levels of taxonomy or habitats.” Information about
invasiveness in specifi c plant families, genera, and species could better identify fac-
tors that can help to predict whether a given species is likely to become invasive
(Perrins et al. 1992 ; Burns 2004 ; Simberloff 2005 ; Pyšek and Richardson 2007 ;
Rejmánek et al 2005 ). Data on plant behavior, growth requirements, and ecological
interactions at the genus and species levels can also help managers evaluate the
likelihood of invasion by a given type of plant in a specifi c area or habitat and to
better manage existing invasions.
S.Y. Atkinson and C.S. Brown