The interactive effects of grazing ungulates and aboveground production on grassland diversity
- 557 Downloads
The variable and nonlinear relationships between plant species richness (SR) and aboveground production (NAP) among terrestrial ecosystems indicate that the energetic capacity of ecosystems interacts with other environmental factors to control diversity. One contributing factor determining plant diversity is herbivory; but few studies have effectively examined the interaction of herbivores and NAP on SR. The objective of this study was to investigate how NAP and herds of native migrating ungulates determine plant SR in grasslands of Yellowstone National Park. Plant SR at peak aboveground biomass was compared inside and outside ungulate exclosures at two spatial scales, 1.0 m2 (“local”) and 100 m2 (“community”), in ten variable grasslands. NAP also was determined inside and outside exclosures. The relationship between SR and NAP was unimodal for grazed and ungrazed grassland at both spatial scales. Grazers increased local SR, independent of NAP. In contrast, herbivore effects on community SR ranged from no effect among low-productive grassland to an increasingly positive influence as NAP increased. In addition, ungulates reduced beta diversity (the contribution to community SR attributed to variability among local patches) at dry, low-productive and wet, high-productive sites. These results suggest that the size of the pool of species available to colonize grassland is an important factor controlling the response of grassland SR to herbivory, particularly from low- to intermediate-productive grassland.
KeywordsBiodiversity Grassland Ungulate Herbivory Yellowstone National Park
I thank J. Carlson, V. Kurth, and H. Loring for help in the field, Yellowstone National Park Center for Resources and the Ranger division for logistic support, and M. Anderson and S. McNaughton for comments on the manuscript. This study was supported by NSF grants DEB-972669 and DEB-0318716 and the InterAmerican Institute for Global Change Research grant CRN-012.
- Augustine, DJ, Frank DA (2001) Effects of migratory grazers on spatial heterogeneity of soil nitrogen properties in a grassland ecosystem. Ecology 82:3149–3162Google Scholar
- Fahnestock JT, Knapp AK (1994) Plant responses to selective grazing by bison: interactions between light, herbivory and water stress. Vegetation 115:123–131Google Scholar
- Gough L, Grace JB (1998) Predicting effects of environmental change on plant species density: expeimental evaluations in a coastal wetland. Ecology 80:882–890Google Scholar
- Grime JP (1979) Plant strategies and vegetation processes. Wiley, LondonGoogle Scholar
- MacArthur RH, Wilson EO (1967) The theory of island biogeography. Princeton University Press, PrincetonGoogle Scholar
- Mittlebach GG, Steiner CF, Scheiner SM, Gross KL, Reynolds HL, Waide RB, Willig, MR, Dobson SI, Gough L (2001) What is the observed relationship between species richness and productivity? Ecology 82:2381–2396Google Scholar
- Tilman D (1982) Resource competition and community structure. Princeton University Press, PrincetonGoogle Scholar