Community Diversity and Stability in a Grazed Riparian Habitat


by Barbara A. Frase
Bradley University and the Rocky Mountain Biological Laboratory

I first learned of the Colorado Riparian Association in the summer of 1991 when I was invited on a field tour of the upper East River in Gunnison County. I had been doing research at the Rocky Mountain Biological Laboratory in nearby Gothic since 1976, and jumped at the opportunity to learn more about the ecology of our valley’s grazed riparian habitats. It was on this field tour that I first heard of short-duration, high-intensity grazing and its benefits; other than the occasional nuisance of having to do field work in the midst of cows, I’d not given grazing much thought.

But what I saw and heard on that field tour continued to intrigue me in the ensuing months, and by mid-winter I’d decided to try to discover the answers to what I considered some truly interesting related questions.  Consequently, as part of a long-term study of the effects of cattle grazing in several high-altitude habitats, my students and I began looking at the riparian plant community in the upper East River Valley in 1992. The original objectives were to establish baseline data on species richness, and to monitor the effect of current grazing practices on the riparian vegetation. In 1997, the study was expanded to examine the relationship between levels of native and non-native plant diversity in this grazed riparian corridor.

Maintaining the ecological integrity of streamside habitats, hydrology, and productivity of the surrounding region is critical given the importance of such corridors. Furthermore, riparian habitats typically are rich in species diversity. For example, the diversity of ecological microsites characteristic of riparian zones (Gregory et al. 1991) creates in turn a patchy distribution of plant species; such microhabitat diversity leads to high plant diversity within the zone.

Riparian plant communities often are considered particularly vulnerable to damage from trampling and grazing by cattle. Free ranging cattle may spend a disproportionate amount of time in riparian habitats (Fleischner 1994), thus magnifying their effect on the vegetation. The upper East River Valley has been grazed by cattle for decades, and has experienced diverse management strategies. The current grazing permit holder is Trampe Ranches of Gunnison County. Manager Bill Trampe practices short-duration, high-intensity grazing throughout his USFS Gothic Allotment, including the riparian corridor.

Each summer from 1992 through 1996, we sampled a section of the East River corridor approximately every two weeks. At 5-m intervals, a 1 m square quadrat was placed along each of two 30-m transects paralleling the river. Percent herbaceous ground cover within each quadrat was estimated and the species present recorded. For willows in the quadrats, modal height was estimated for each and their areal cover calculated. These data were used as a measure of habitat stability.

These measures of community stability showed no trend over the five years of the study. The current short-duration, high-intensity grazing regimen does not cause degradation of the riparian habitat. The mean herbaceous ground cover was 68%, and there was considerable variability among years. In a given year, percent ground cover could not be coupled to grazing intensity (which varied from year to year); the timing and extent of spring flooding seemed to have a greater affect on the density of the herbaceous layer than did grazing. The willows showed little evidence of browsing, and none had the “mushroom” shape often associated with hard use by cattle. The mean modal height for all five years was 109 cm with numerous young shrubs being recruited into the population. The willow thickets are quite dense; in fact it was difficult in places to force a path through them during sampling.

This 60-m stretch of riparian corridor was indeed species-rich; we identified 85 plant species in the 210 quadrats examined. Of these 85, only eight are considered non-native to Colorado. Those data on alien plant species were the impetus for a more extensive look at invasive plants in the upper East River riparian corridor. Classic ecological theory predicts that higher biodiversity should be associated with lower rates of invasion by alien species (Elton 1958), but actual field data are scarce and contradictory. This East River riparian system seemed an ideal place to test the hypothesis that an inverse relationship exists between the number of native and the number of non-native species. Riparian ecosystems by virtue of their cycle of natural disturbance may be more prone to invasion by exotics (Naiman et al. 1993). Additionally, this valley has a long history of grazing, is visited by hundreds of recreationists each summer, and nonetheless is species-rich with a thriving native biota.

Therefore, we added three riparian sites to our study. The sites are maximally a little over 6 km apart, and include a tributary of the East River. The plant community was inventoried at the “patch scale,” encompassing 15,000 square meters at each site. The sites were inventoried by random walk at frequent intervals throughout two growing seasons. Diversity was high. Total species numbers at the four sites ranged from 146 to 161, the number of non-native species from 10 to 15. The mean percentage of non-natives was relatively low, only 8.3. Interestingly, there was no significant linear relationship between the total number of species and the number of non-native species. There were no differences in the proportion of invasives among the sites. Only four sites were examined and our research on invasives is ongoing, but the data so far indicate that higher native diversity did not confer a greater resistance to invasion by exotics in this grazed riparian corridor.

It of course does not follow that preserving natural species diversity is not worthwhile, even though the hypothesis linking diversity and lowered incidence of invasion was not supported by this research. What these data and the results of other studies do suggest is that in a complex and dynamic ecosystem, invasibility is too multifaceted to be explained by a simple linear relationship between it and native diversity levels.

Partial funding for this research has been provided by the Gunnison County Stockgrowers Association, Habitat Preservation Partners, and the US Forest Service.


Literature Cited:
Elton, C. 1958. The ecology of invasions by animals and plants. Methuen & Co., London.
Fleischner, T. 1994. Ecological costs of livestock grazing in western North America. Conservation Biology 8:629-644.
Gregory, S. et al. 1991. An ecosystem perspective of riparian zones. BioScience 41:540-551.
Naiman, R. et al. 1993. The role of riparian corridors in maintaining regional biodiversity. Ecological Applications 3:209-212.