Biology Reference
In-Depth Information
The model (Fig. 17.6) corresponded closely to observed values. It predicted that
92% of existing area had yellow starthistle infestation and located another 39% of
the area that was susceptible to invasion by the species. The likelihood of yellow
starthistle occurrence was affected by both slope (31 ± 8°) and aspect (237 ± 89°),
which indirectly provided information on the plant's ability to reproduce and dis-
perse seed. The approach also was used to develop a likelihood of survival function
for plant communities subject to invasion by the two invasive plant species and to
predict the movement of either species across a landscape over time (Fig. 17.6).
17.4.3 ThresholdsandSuccession
Thresholds are fundamental to integrated pest management (IPM) in agriculture
and other natural resource production systems because cost-effective weed man-
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Fig. 17.6
Projections of range expansion of yellow starthistle. Background vegetation index
image shows percent green midsummer vegetation. Areas with annual grass are red to brown,
yellow areas have a mix of grass and shrubs, and green areas have trees and shrubs. Each contour
line represents movement of yellow starthistle over 20 years assuming a 15-m maximum spread
each year (from Lass and Prather 2007)
