The Demography of Bromus Tectorum: Variation in Time and Space

Abstract

(1) Amplitude in the variation of recruitment, survivorship and fecundity was examined for the introduced annual grass Bromus tectorum in three habitat types in eastern Washington (U.S.A.) for three consecutive generations. A total of 18 143 individuals in populations varying from 364 to 5322 members per site were mapped repeatedly from emergence to death with sufficient frequency to detect multiple constituent cohorts varying in age from fewer than 16 to more than 200 days. (2) Recruitment was usually concentrated in late summer and autumn, but occurred at any time until mid-May of the following year. (3) Most of any population experienced low death risk until June, although some cohorts emerging in late summer were devastated (Deevey Type III curve) during periods of drought or extended snow cover. (4) Most plants survived to produce seed. Loss of seed production from devastated autumn-emergent plants was off-set by the reproduction of late winter-spring recruits. Even individuals less than 45 days old often produced at least one viable seed by June. (5) B. tectorum persists under the vagaries of steppe environments by its ability to behave simultaneously on the same site as an ephemeral monocarpic, annual monocarpic and winter annual monocarpic species. (6) Year-to-year variation in environment (weather, predator activity) overrode the intrinsic differences among the three habitat types along a 200 km transect of varying moisture availability often producing considerable amplitude in population attributes (recruitment, survivorship and fecundity). Characterization of any species as a colonizer, etc. on the basis of life history traits alone may be erroneous; knowledge of the variation in such population attributes is also necessary.