Pyke investigated the influence of Montane meadow mouse (Microtus montanus) grazing on the population biology of the annual Agropyron spicatum and on seedlings of the perennial Bromus tectorum, using four grazing frequencies and ungrazed controls, initiated at four plant ages in an unheated glasshouse. The proportional survival of both species rose with decreased grazing frequency and with increased age at initial grazing. The age-by-species interaction showed that B. tectorum increased survival from 17% for plants grazed at 7 days old to 69% for plants grazed at 30 days old or older. For A. spicatum, survival remained at 5% when plants were grazed at 30 days or younger but increased to 57% as grazing was delayed until plants were older. For both species weekly grazing significantly reduced survival. For B. tectorum, the other three less frequent grazing classes did not significantly differ among themselves in survival whereas for A. spicatum as frequency of grazing decreased, survival increased. Frequency of grazing and age at initial grazing had no significant effects on seed production of Bromus tectorum. The concludes that in the steppe of eastern Washington, small mammals can play a major role in shaping the plant communities.
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