Archer, Steve. 1982. Plant Community Structure, Competitive Interactions and Water Relations as Influenced by Herbivores. Colorado State University, Ft. Collins, CO. 114 p.
The rate and extent of change caused by prairie doy (Cynomys ludovicianus) activity in a mixed-grass prairie plant community in southwestern South Dakota is documented. Direct and indirect (correspondence analysis) ordinations showed that distinct changes in plant species composition and diversity occurred as a function of colonization history. Uncolonized stands were dominated by the perennial grasses Buchlöe dactyloides, Stipa comata and Poa pratensis. Stands colonized for 2 yr were dominated by B. dactyloides and Agropyron smithii. After 3 yr of colonization the site was codominated by B. dactyloides and Dyssodia papposa. Total species diversity increased by 63 and 126%, respectively, during the first 2 and 3 yr of colonization. However, after 4-6 y of colonization plant diversity had decreased to 84% that of uncolonized stands. These changes in species composition and diversity were accompanied by an increase in bare soil, decreased litter accumulation and reduced plant canopy stature. Subsequent field experiments suggested that alterations in plant competive interactions, which played a key role in plant responses to defoliation, are likey an important factor governing plant fitness in grazing systems. Negative effects of defoliation, are likey an important factor governing plant fitness in grazing systems. Negative effects of defoliation were found to decrease with reductions in competition. Plants defoliated under reduced competition had increased leaf production, a shift toward the production of younger leaves and reduced mortality compared with clipped plants under full competition. Because of the potentially important effects of competition in determining the outcome of clipping treatments care must be taken when designign defoliation experiments and interpreting and extrapolating their results.
Although prairie dog grazing activities did result in the creation of warmer soil microenvironments on an intensively studied colony, soil moisture content to 20 cm on the colony was compared to or slightly higher than that off the colony. Seasonal plant water potentials of three grass species were comparable on and off the colony, suggesting that grazing induction of water stress was not a factor leading to the elimination of the graminoids from the community. Artemisia frigida plants were, however, generally under less water stress on the colonly. This may partially account for its increase in abundance under heavy grazing pressure.