Article: Should Heterogeneity Be The Basis For Conservation? Grassland Bird Response To Fire And Grazing
Authors: Fuhlendorf, S.D. Harrell, W. Engle, D.M. Hamilton, R.G. Davis, C.A. Leslie, D.M.
Published: Ecological Applications 16(5), 2006, pp. 1096-1716
Management strategies on grazing lands can often reduce spatial heterogeneity. Sometimes this is by design and others it is a natural consequence of reducing complex systems to simple rules of thumb. As the article states, “a fundamental principle of rangeland management is to maintain uniform and moderate grazing across the entire landscape.” The consequence of this can lead to declines in overall biodiversity.
This paper quantitatively compares patch-level heterogeneity to uniform traditional management in the Tallgrass Prairie Preserve in north-central Oklahoma. The establishment of patch-level heterogeneity is described:
…each patch in the patch treatments was burned once (in either spring or autumn) every three years, and at any given point in time one-third of the pasture had been burned within the past year, one-third was burned one to two years prior, and one-third had not been burned for at least two years. Cattle were stocked in patch treatment pastures from mid-April through mid-July at a rate of 1.2 ha/270 kg steer. Cattle had free access to patches within each pasture, so cattle could choose between burned and unburned patches in the patch treatment. Recently burned patches were preferentially grazed by cattle, thus creating patches with focal disturbances within the patch treatment.
A traditional treatment was also established in which all paddocks were burned uniformly every year. These paddocks were grazed with similar parameters as described above, and were designed to mimic traditional management recommendations for the area.
The two treatments were compared on the basis of vegetation composition and structure and the abundance of grassland birds. Vegetation parameters included the percent cover of tallgrass, forbs, litter, and bare ground, vegetation height, and angle of obstruction. For patches, data was summarized in categories from month of disturbance.
Data for vegetation composition and structure revealed interesting patterns:
Differences between treatment averages were minimal, with the exception of cover of litter and bare ground at the pasture scale, but highly significant for some variables at the patch level. Analyses of variance indicated significant differences across patches for tallgrass, shortgrass, grass-like, litter, and bare ground. Within the patch treatment, bare ground increased, and tallgrass and litter cover decreased in patches with recent focal disturbances. Patches with >13 months since focal disturbance increased in cover of tallgrasses and litter, and decreased in bare ground with increasing time since focal disturbance.
This spatial heterogeneity created a patch-work of bare ground and covered ground, high grasses and low grasses, high areas of animal impact/grazing and areas of low animal impact/grazing. Similar patterns emerged from abundance counts of grassland birds:
Abundance of Upland Sandpipers was five times greater in patches with recent focal disturbance compared with the other patches in the patch treatments, and 2.5 times greater in patches with recent focal disturbance in the patch treatment than the traditional treatment. Lark Sparrows were not encountered in patches with 25-36 months of recovery or patches with >36 months since disturbance in the patch treatment. [Increasing time since focal disturbance] indicated an increasing abundance of Henslow’s Sparrow and Dickcissel….Henslow’s Sparrows were not encountered in patches with recent focal disturbance…Eastern Meadowlark reached peak abundance in patches in the intermediate recovering stages (13-36 months) in the patch treatment and reached the lowest abundance in patches with recent focal disturbance…
And “increases in litter cover over time in the patch treatment were highly correlated with changes in the grassland bird community.”
The data presented above reveals that managing for biodiversity in avian species requires heterogeneous patches of vegetative structure. In this case, this is achieved by the combined use of fire, grazing, and animal impact, which the authors argue mimic the ecological processes of the Great Plains before European settlement.
Wildlife biologists and ornithologists are well aware of the differing habitat requirements of birds, mammals, and even terrestrial insects. The authors elaborate: “So, the patch treatment included species that require vegetation structure associated with undisturbed habitats and species that require vegetation structure associated with heavily disturbed habitats occurring within the same pasture.” Species’ reliance on landscape diversity is a direct reflection of differing eating and reproductive strategies. It is therefore incumbent upon land managers who seek to maximize biodiversity to do so, in part, through strategies that provide diverse habitats and ecosystem structure.
One way of doing this, as the authors demonstrate, is through the combined use of fire and grazing. Holistic Grazing Planning and Holistic Land Planning are well-suited to this type of strategy. Controlled burns can be incorporated at the paddock level to attract animals to certain areas. Stock density can also be varied across the landscape through the use of attractants, herding, and fencing. Grazing planning can and should reflect these strategies, and also allows for the manager to control for other critical variables like calving, animal performance, and variability in animal behavior. As we collect and assimilate this type of research, we increase our capacity to apply these principles in a concrete fashion through active management within the Holistic Management community.