Exotic Brome-Grasses in Arid and Semiarid Ecosystems of the Western US

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The geographic center of evolution for the Bromus species that are invasive in
the Western USA is between the Middle East and eastern Europe (Sales 1994 ;
Atkinson and Brown 2015 ), the region of origins of plant and animal domestication
(about 10,000 years BP; Flannery 1969 ). Bromus seeds were associated with
humans from at least 5000 years BP and likely earlier (Sales 1994 ; Marinova 2003 ;
White et al. 2014 ). Thus, they had ample time to evolve with both crops and domes-
tic livestock grazing. The annual life cycle is one adaptation that imparts grazing
tolerance to these species. As populations of B. tectorum decrease in one generation,

Past Selection Pressure

Grazing history

Plant community before

each grazing episode

Soil texture, depth,

temperature & moisture

regime

Grazing Action

Injury to individual plants?

Plant community during

a grazing event

Grazing use/intensity

differs among life-forms in

community

Community Reaction

Death or Regrowth of

Injured individuals

Community after

grazing event

Response of community

composition & structure

Past Selection Pressure

Annual Bromus Native Herbaceous

Community

Environment Graze Environment Graze Reaction

Great

Plains -+++NH => AB

Cold

Desert +++--AB >> NH

Warm

Desert +++--AB >> NH

Med.

California +++--AB >> NH

Fig. 11.2 Plant community structure (above) as shaped by grazing during grazing events, by the
community’s response to the grazing, and by the past selection pressures including the environ-
ment and grazing among the grazed plants at the site of evolution (modifi ed from Milchunas et al.
1988 ). In the table (below), negative (−) and positive (+) signs indicate degree of adaptation for
annual Bromus species (AB) and native herbaceous species (NH) to environments of four ecore-
gions and to grazing during the active growing season. Under community reaction, equal than
and greater than signs (>) refl ect dominance strength of AB and NH based on these adaptations

D.A. Pyke et al.