Large mammalian herbivores not only depend on plant communities for their existence but cause major changes in plant community composition and structure. These changes have direct consequences for ecosystem processes, but recent studies of ungulate-ecosystem relations show widely divergent ungulate effects in different ecosystems. We reviewed studies of ungulate effects on plant community composition to gain insight into potential mechanisms of ungulate-induced changes in both community composition and ecosystem processes. Our analysis of these studies is based on the premise that the effect ungulates exert on plant communities depends on the balance between (1) feeding selectivity of herbivores (i.e., degree to which dif- ferent plant species or ecotypes experience different levels of tissue loss), and (2) differences among plant species in their ability to recover from tissue loss. A large number of studies clearly show that selective ungulate herbivory leads to the dominance of unpalatable, chemically defended plant species in communities. However, many studies have also demonstrated that intensive long-term herbivory does not lead to the invasion of unpalatable species into the community, and can even increase the dominance of highly palatable species. Our review indicates that high levels of nutrient inputs or recycling and an intermittent temporal pattern of her- bivory (often due to migration) are key factors increasing the regrowth capacity of palatable species and hence maintaining their dominance in plant communities supporting abundant herbivores. Key factors limiting un- gulate foraging selectivity, again limiting herbivore-induced dominance of slow-growing, unpalatable species, include herding behavior, early growing season and postfire herbivory, asynchronous phenology of palatable versus unpalatable species, and low relative abundance of unpalatable species. Our review indicates differences among ecosystems in the role played by ungulate herbivory result from the relative strength of these factors enhancing plant tolerance to herbivory and limiting foraging selectivity. Anthropogenic changes in these factors (e.g., alteration of migration patterns) therefore have the potential to significantly alter the effects of ungulates on plant communities and ecosystem processes.
Fire and grazing are interactive disturbance processes that are important to the structure and function of grassland ecosystems. Studies of nitrogen (N) availability report different effects following grazing and fire. However, these studies have largely neglected the spatially controlled interaction between fire and grazing. The objective of our work was to evaluate an application of the fire-grazing interaction model on N availability in a tallgrass prairie. We compared patches within a shifting mosaic landscape where each patch varied in time since focal disturbance (fire and intense grazing disturbance). We also evaluated N availability on a burned and grazed landscape where fires and moderate grazing occurred annually and uniformly across the entire landscape. These treatments were both burned and grazed where the only difference was spatial and temporal variability in fire application and grazing disturbance. Samples were collected from upland sites in May of 2003 and 2004. Total soil inorganic N (NH4+-N + NO3--N) and a growth chamber experiment with hard red winter wheat (Triticum aestivum L. cv. Jagger) were used to evaluate potential N availability. Our study produced patterns of N availability that are more similar to studies of grazing lawns where N availability is enhanced by focal grazing than from studies of fire without grazing. Overall, our study demonstrates that fire and grazing are interactive. Unburned patches have minimal grazing pressure and low N availability. Fire-grazing interaction may provide a management alternative that enables sustainable livestock production, through increased carrying capacity in focally disturbed patches, concomitant with biological diversity in tallgrass prairie.
In many rangeland ecosystems, the role of fire on vegetation dynamics has been the object of detailed studies. In Argentina, and especially in Patagonia, the knowledge of how fire changes vegetation is scarce. In 3 areas affected by wildfires on different dates (in 1988, 1994, and 1998), we determined the structure of the vegetation (plant cover, density, and biomass) and compared it with that of nearby unburned areas. Based upon these data, we present a qualitative state-and-transition model of this rangeland. For the sites burned in 1988 and 1994, aerial biomass, density, and cover of perennial grasses were significantly greater (P < 0.05) for burned than for unburned areas. For the site burned in 1998, although there were no significant differences in perennial grass biomass and cover, density was significantly greater compared to the unburned area. Total shrub cover was significantly lower (P < 0.05) in burned than in the unburned areas, attaining 49.7%, 15.0%, and 5.5% of that of the unburned areas for the sites burned in 1988, 1994, and 1998, respectively. Similar to cover, density for most shrubby species was significantly greater in the unburned than in the burned areas for the 3 sites. Grazing and fires of different intensities, combined with variable rainfall, makes the prediction of postfire vegetation changes difficult. However, it can be generalized that fire changed the vegetation from shrub-dominated steppes, a persistent state in northeastern Patagonia, into a grass-dominated transient state. This change has persisted for more than 10 years after a fire event. From the perspective of sheep raising, the significant postfire increase in perennial grass biomass represents a substantial improvement in the condition of these rangelands. However, repeated fires would be necessary to control the sprouting shrubs and maintain the grass-dominated state.
Although remote sensing has many potential applications for range management, its use by range managers thus far has been limited. To investigate the factors that encourage use of remote sensing and to examine its influence on decision making by individuals who manage privately owned rangeland, we evaluated the decision-making processes of 3 ranch owners and 1 professional ranch manager who were introduced to remote sensing while collaborating with us in a rangeland stewardship program in California. Two of the participants had extensive ranching experience (11 to > 20 years) and managed large cattle ranches (1 000 to > 2 000 ha), and 2 had less experience and managed smaller sheep ranches (< 200 ha). During the 5-year program, the participants implemented a series of new management practices, including prescribed burning, rotational grazing, and seeding of native grasses, with the aim of reducing noxious weeds and increasing productivity. We used remote sensing to quantify the effect of these practices and provided ranch-wide remote sensing analyses to each manager on a password-protected Web site. Using case study methodologies, we found that managers of larger, commercially active ranches found the experimental use of remote sensing to be a highly positive experience that convinced them that this technology could help address difficult management situations and increase ranch profitability. This suggests that the broad use of remote sensing by managers of privately held, commercial rangelands may be limited in part by the simple lack of opportunity to test these technologies. Programs that assist ranchers in obtaining appropriate remote sensing products thus may be a cost-effective way to enhance conservation on private rangelands. Our findings suggest that voluntary self-analysis by ranchers of the landscape dynamics of their own properties is likely to lead to more engaged conservation efforts than will top- down prescriptions.
Flea beetles (Aphthona spp.) are biological control agents introduced from Eurasia to reduce the cover and density of leafy spurge (Euphorbia esula L.). The potential for the introduced beetles to use alternate hosts for feeding and development in North America is slight; however, it is possible. Species at highest risk are native species closely related, ecologically and taxonomically, to leafy spurge. A native spurge, Euphorbia brachycera Engelm. is consubgeneric and sympatric with leafy spurge throughout the northern Great Plains of the United States, and was not included in prerelease host-specificity testing for Aphthona nigriscutis or Aphthona lacertosa. The objective of this study was to evaluate the actual and potential ecological overlap among leafy spurge, flea beetles, and E. brachycera. Wide-ranging and intensive field surveys indicate that E. brachycera is found well within the range of leafy spurge and flea beetles. E. brachycera occurs infrequently, in low densities, in areas with a high percentage of bare ground, and with a root system dissimilar to leafy spurge. Flea beetles released directly into populations of E. brachycera failed to persist beyond a single field season and plants showed no evidence of feeding by beetles. Our results suggest that the potential for flea beetles to host-shift is low due to differences in growth habit and root morphology between E. brachycera and leafy spurge.
