Agriculture Reference
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Figure 10.1
(a) Number offirst records ofinvading species by decade in the five
northwestern states ofthe USA.Line is a negative exponential curve fitted to the
data.Data point for 1890 includes all species that had arrived by that date.Ten
species that were present in only one or two counties in 1980 were not included.
(b) Mean number ofcounties infested by the 188 invasive weeds that had
established in the same region by 1910.(Plotted from data ofF.Forcella & S.J.
Harvey; see Forcella & Harvey,1983.)
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