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Land-use and grassland community ecology

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Devan Allen McGranahan
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Many grassland ecosystems are disturbance-dependent, having evolved under the pressures of fire and grazing. Restoring these disturbances can be controversial, particularly when valued resources are thought to be disturbance-sensitive. We tested the effects of fire and grazing on butterfly species richness and population density in an economically productive grassland landscape of the central U.S. Three management treatments were applied: (1) patch-burn graze—rotational burning of three spatially distinct patches within a pasture, and moderately-stocked cattle grazing (N = 5); (2) graze-and-burn—burning entire pasture every 3 years, and moderately-stocked cattle grazing (N = 4); and (3) burn-only—burning entire pasture every 3 years, but no cattle grazing (N = 4). Butterfly abundance was sampled using line transect distance sampling in 2008 and 2009, with six 100-m transects per pasture. Butterfly species richness did not respond to management treatment, but was positively associated with pre-treatment proportion of native plant cover. Population density of two prairie specialists (Cercyonis pegala and Speyeria idalia) and one habitat generalist (Danaus plexippus) was highest in the burn-only treatment, whereas density of one habitat generalist (Cupido comyntas) was highest in the patch-burn graze treatment. Treatment application affected habitat structural characteristics including vegetation height and cover of bare ground. Historic land uses have reduced native plant cover and permitted exotic plant invasion; for some butterfly species, these legacies had a greater influence than management treatments on butterfly density. Conservation of native insect communities in altered grasslands might require native plant restoration in addition to restoration of disturbance processes.
Agricultural expansion has eliminated a high proportion of native land cover and severely degraded remaining native vegetation. Managers must determine where degradation is severe enough to merit restoration action, and what action, if any, is necessary. We report on grassland degraded by multiple factors, including grazing, soil disturbance, and exotic plant species introduced in response to agriculture management. We use a multivariate method to categorize plant communities by degradation state based on floristic and biophysical degradation associated with historical land use. The variables we associate with degradation include abundance of the invasive cool-season grass, tall fescue (Schedonorus phoenix (Scop.) Holub); soil organic carbon (SOC); and heavy livestock grazing. Using a series of multivariate analyses (ordination, hierarchical clustering, and multiple regression), we identify patterns in plant community composition and describe floristic degradation states. We found vegetation states to be described largely by vegetation composition associated primarily with tall fescue and secondarily by severe grazing, but not soil organic carbon. Categorizing grasslands by vegetation states helps managers efficiently apply restoration inputs that optimize ecosystem response, so we discuss potential restoration pathways in a state-and-transition model. Reducing stocking rate on grassland where grazing is actively practiced is an important first step that might be sufficient for restoring grassland with high native species richness and minimal degradation from invasive plants. More severe degradation likely requires multiple approaches to reverse degradation. Of these, we recommend restoration of ecological processes and disturbance regimes such as fire and grazing. We suggest old-field grasslands in North America, which are similar to European semi-natural grassland in composition and function, deserve more attention by conservation biologists.
Tallgrass prairies of central North America have experienced disturbances including fire and grazing for millennia. Little is known about the effects of these disturbances on prairie ants, even though ants are thought to play major roles in ecosystem maintenance. We implemented three management treatments on remnant and restored grassland tracts in the central U.S., and compared the effects of treatment on abundance of ant functional groups. Management treatments were: (1) patch-burn graze—rotational burning of three spatially distinct patches within a fenced tract, and growing-season cattle grazing; (2) graze-and-burn—burning entire tract every 3 years, and growing-season cattle grazing, and (3) burn-only—burning entire tract every 3 years, but no cattle grazing. Ant species were classified into one of four functional groups. Opportunist ants and the dominant ant species, Formica montana, were more abundant in burn-only tracts than tracts managed with either of the grazing treatments. Generalists were more abundant in graze-and-burn tracts than in burn-only tracts. Abundance of F. montana was negatively associated with pre-treatment time since fire, whereas generalist ant abundance was positively associated. F. montana were more abundant in restored tracts than remnants, whereas the opposite was true for subdominants and opportunists. In summary, abundance of the dominant F. montana increased in response to intense disturbances that were followed by quick recovery of plant biomass. Generalist ant abundance decreased in response to those disturbances, which we attribute to the effects of competitive dominance of F. montana upon the generalists.