Patches Of Short Grass Research Articles

Small‐scale fires interact with herbivore feedbacks to create persistent grazing lawn environments

Abstract Fire‐herbivory feedbacks strongly influence the formation of grazing lawns in savanna ecosystems. Preliminary findings suggest that small‐scale (<25 ha) fires can engineer grazing lawns by concentrating herbivores on the post‐burn green flush; however, the persistence of such grazing lawns over the longer term and without repeated fire is unknown. We used high‐resolution Light Detection and Ranging (LiDAR) to investigate the long‐term effects of fire manipulation on short grass structure (height, cover, volume and spatial continuity) and grazing lawn establishment in Kruger National Park, South Africa. We analysed the effects of fire exclusion and experimental burns applied over a 7‐year period (2013–2019) followed by a 1‐year cessation of burning at varying spatial scales during the early and late dry seasons. Fires contributed a fourfold increase in short grass cover, regardless of fire season or size. The distribution of grass height differed significantly between fire‐induced grazing lawns and recently unburnt parts of the landscape where controlled fires were excluded over the experimental period. The volume (corresponding to bulk density) of short grass on the landscape responded strongly to fires, with grass volume <20 cm in height increasing with both early and late dry season fires. Early dry season fires caused larger and more homogeneous short grass patches. Furthermore, early dry season fires were more influential in increasing the cover of the shortest grass height class (1–5 cm). Synthesis and applications. Our results demonstrate that fire‐induced grazing lawns can persist over the longer term, even when fires are no longer applied, leading to the creation of vertical and horizontal heterogeneity in the grass layer. Small‐scale fires, therefore, represent a feasible management approach to expanding grazing lawn extent, potentially benefiting grazer coexistence and diversity.

When less is more: heterogeneity in grass patch height supports herbivores in counter-intuitive ways

Herbivores are an integral part of the African landscape and have evolved with the vegetation to create the savanna landscape. Managers of these landscapes can benefit from a better understanding of how indigenous herbivores use the landscape to which they are adapted. In this study we observed which patches were frequently utilised, by doing regular monthly road counts, grass height observations and dung counts on selected short grass patches in the Kruger National Park. Smaller-framed impala and blue wildebeest (meso-herbivores) were most regularly seen on these nutritious patches, while from dung deposits it was clear that the even larger-framed buffalo (mega-herbivores) spent time there. This preference can be explained by considering the nutritional needs and food intake of the herbivores. Smaller-framed herbivores seem to be able to satisfy their dietary requirements on the high-quality forage patches, while larger-framed herbivores seem to supplement the quality forage by also spending foraging time on areas of higher grass biomass. From this insight we propose that range management should take herbivore preferences into account and allow herbivores to select and concentrate their foraging on the most nutritious forage. This approach is likely to decrease inputs while allowing animals to maintain or increase production.

How does stocking rate influence horse behaviour, performances and pasture biodiversity in mesophile grasslands?

Reducing grazing intensity is widely recommended as a strategy to increase grassland biodiversity through greater sward heterogeneity. Horses are playing an increasing role in the management of permanent grasslands across Europe, but the effects of horse grazing under contrasting stocking rates have been poorly studied. Here we describe the effects of two contrasted stocking rates (“High” 1.8LUha−1vs. “Moderate” 1.1LUha−1) on sward structure, horse foraging behaviour and performances, and pasture biodiversity in a mesophile grassland of central France. Horses selectively grazed vegetative patches of high nutritive value, especially at the moderate stocking rate (p<0.01). This enabled them to maintain diet quality (diet dry matter digestibility: 59% DM) and performances (daily liveweight gain>270ganimal−1d−1) at the same level in highly and moderately grazed plots despite contrasted herbage biomass and quality. Horses, with their two sets of incisors, created and maintained patches of short grass in a matrix of tall vegetation in both treatments. Consequently, sward structural heterogeneity did not significantly differ between highly and moderately grazed plots, and there was no detectable effect of grazing intensity on floristic and arthropod diversity over the four-year study. The two stocking rates did however result in a divergent evolution of legumes for which abundance increased at the high stocking rate (p<0.05). Abundance of Carabidae and grasshoppers from tall grasslands was higher at the moderate stocking rate (p<0.05). We conclude that decreasing stocking rate would only have a marginal effect on grassland biodiversity, while farm performance will be strongly affected by the decrease in the number of horses per unit area.

Legacy Effects of Different Land-Use Histories Interact with Current Grazing Patterns to Determine Grazing Lawn Soil Properties

Pastoralism and agriculture have affected rangeland ecosystems over the past millennia, including many ecosystems that are currently protected as reserves. However, the legacy of these land-use practices on current ecosystem functioning remains unclear. We studied legacy effects of former human land use on soil physical and chemical properties in a South African savanna. We did this by comparing soil properties in grazing lawns (patches of short grass maintained by the positive feedback between grazing intensity and forage quality) with the surrounding less grazed bunch grasslands within three different human land-use history contexts: (i) Abandoned bomas: permanent stone enclosures where livestock were kept overnight, and dung and urine accumulated for several years or decades. (ii) Old fields: areas where vegetation was cleared, soil tilled, and cultivated, but received little or no fertilization. (iii) Natural grasslands: not cultivated but grazed by livestock before the establishment of the reserve and wildlife thereafter. Former human land use rather than soil texture was the main determinant of grazing lawn location. Moreover, lawn soil properties also varied among land-use histories. In all grazing lawns, soil nutrient concentrations were higher than in adjacent grasslands but abandoned bomas contained three times more phosphorus, and twice as much nitrogen and carbon than old fields and natural grassland lawns. In addition to past land use, soil texture influenced lawn soil nutrients: Concentrations of phosphorus, potassium, calcium, magnesium, total nitrogen, and carbon in lawns were higher on clayey soils than sandy soils, whereas phosphorus, C:N ratio, and pH did not change with soil texture. Our study confirms previous findings on the effect of human land use on savanna heterogeneity, but also highlights how legacy effects may vary among different historic land-use practices.

Comparing drag partition schemes over a herbaceous Sahelian rangeland

AbstractModeling of dust emissions from the surface remains complex, especially in semiarid regions where vegetation must be accounted for because of its potentially important protective effect. Protection is directly linked to the fraction of the soil surface covered by vegetation, but it is also driven by the interaction of vegetation elements with the wind field. The sensitivity of simulated dust emissions to various drag partition schemes—mainly those proposed by Raupach (1992), Marticorena and Bergametti (1995), and Okin (2008)—is evaluated for a typical Sahelian rangeland, covered by a seasonal grass layer, over a complete vegetation cycle. The application of these schemes requires a fine characterization of the vegetation cover; field measurements from an ecological survey are used to derive the geometric dimensions of the grass patches. Models are run with meteorological forcing from automatic weather stations. As a result, the impacts of soil moisture and grass cover are estimated over April to September. Soil moisture inhibits dust emissions by 27% in mass. The different drag partition schemes exhibit distinctive limitations, mostly due to the properties of the Sahelian grass cover, which is composed of large and low patches of short grass, with a strong seasonal dynamics. However, the drag partition schemes result in remarkably coherent estimations of dust emissions. When soil moisture is taken into account, vegetation reduces the total vertical mass fluxes by 6 to 26% of the emissions of a bare soil, depending on the drag partition scheme.

N and P Cycling in Tanzanian Humid Savanna: Influence of Herbivores, Fire, and N2-Fixation

Availabilities of nitrogen (N) and phosphorus (P) have a strong influence on plant growth and the species composition of savannas, but it is not clear how these availabilities depend on factors such as fire, N2-fixation, and activities of wild herbivores and cattle. We quantified soil N and P availabilities in various ways (extractable pools, mineralization, resin adsorption) along vegetation gradients within a recently abandoned cattle ranch and a former game reserve in Tanzania (both areas now part of the Saadani National Park). We also assessed annual N and P balances to evaluate how long-term availabilities of N and P are affected by large herbivores, symbiotic N2-fixation, and fire. The results show that cattle ranching led to a spatial re-distribution of nutrients, with the local accumulation of P being stronger and more persistent than that of N. In the former game reserve, intensively grazed patches of short grass tended to have elevated soil N and P availabilities; however, because quantities of nutrients removed through grazing exceeded returns in dung and urine, the nutrient balances of these patches were negative. In dense Acacia stands, N2-fixation increased N availability and caused a net annual N input. Fire was the major cause for nutrient losses from tallgrass savanna, and estimated N inputs from the atmosphere and symbiotic N2-fixation were insufficient to compensate for these losses. Our results call into question the common assumption that N budgets in annually burned savanna are balanced; rather, these ecosystems are a mosaic of patches with both N enrichment and impoverishment, which vary according to the vegetation type.

Home ranges of, and habitat use by, the grassland earless dragon (Tympanocryptis pinguicolla) in remnant native grasslands near Canberra

The grassland earless dragon (Tympanocryptis pinguicolla) is a habitat specialist restricted to the highly fragmented native temperate grasslands of the Australian Capital Territory (ACT) and nearby New South Wales (NSW). These grasslands, which are listed as an endangered ecological community, continue to be affected by a range of processes including development and overgrazing. Although changes to its habitat present a significant threat to the grassland earless dragon, little is known about its spatial and habitat requirements. We used radio-tracking to study the home-range areas and habitat requirements of 10 adult lizards in native grassland adjacent to Canberra, ACT. We found that burrows excavated by arthropods are an important resource for grassland earless dragons, with individuals having one or two home burrows around which they maintained home ranges of between 925 m2 and 4768 m2. Fidelity to these burrows increased with the onset of winter, indicating their importance as over-winter refuge sites. Within the native grasslands, grassland earless dragons were found to use a broad range of grassland structure as habitat. This result contrasts with the prevailing view that these dragons are confined to well drained, minimally disturbed areas, which include large patches of short grass dominated by Austrodanthonia. We conclude that habitat management (i.e. grazing, mowing, burning) for this species should aim to retain structural heterogeneity of native grasslands rather than impose a uniform structure.

Population distribution and conservation of the four-horned antelope (Tetracerus quadricornis) in the tropical forest of Southern India

Population density, distribution patterns and conservation problems of the four-horned antelope (Tetracerus quadricornis) were assessed in the Mudumalai Wild Life Sanctuary, Southern India. The antelopes were found to be restricted to dry deciduous and dry thorn forests in the northern and eastern areas of the sanctuary in an area of approximately 79 km 2 . The ecological density of the antelope was 0.88 individual/km 2 and the crude density was ~0.22 individual/km 2 . Within the dry deciduous forest, the antelopes were further restricted to flat short grass patches and hilly areas with open canopy patches of tall grass. The antelopes were relatively more abundant in the dry deciduous short grass habitat than in the dry deciduous tall grass and dry thorn forests. They also seemed to have higher preference for hill slopes with semi-open canopy (>70%). Antelopes and evidence of their presence have not been seen in recent years in some eastern areas of the sanctuary where antelopes were seen until 1990. We attribute the range decline to the overgrazing by cattle and the resultant weed invasion in the recent years.

Selection of feeding sites by horses at pasture: Testing the anti-parasite theory

Management of grazed grasslands for production and/or conservation objectives requires a thorough understanding of the choices of feeding sites by herbivores, and of the biological processes involved. Most models of the feeding strategies of herbivores are based on the principle that optimising the intake of energy (or some nutrient) is the primary goal of foragers but other selective forces, such as parasitism, could be important. Gastrointestinal parasites (including cyathostome nematodes) have powerful effects on the fitness of herbivores and may act as a major selection pressure favouring host behaviour that reduces the risk of encountering parasites. Among large herbivores, horses have perhaps the most marked tendency to select particular feeding sites within grasslands. We test here: (1) whether horses select feeding patches with relatively low parasite densities and (2) if their choice is affected by their parasite load. We used 10 two-year old saddle-horses and three periods. In the first period, the horses were under natural parasitism which varied strongly among individuals; in the second period they were all dewormed, and in the third, a sub-set of the horses was experimentally infected with cyathostome larvae. Ninety-eight percent of the infective larvae in the pasture were found <1 m from faeces. The main determinant of the choice of feeding patch by horses was the availability of patches of different parasite risk and grass height. Controlling for availability, the horses used tall grasses (>16 cm) less than expected, whether the grass was contaminated or not, and they selected for short patches >1 m from faeces, where the risk of encountering parasites was low. These results suggest that selection of feeding sites by horses is driven by an interaction between their nutritional and anti-parasite strategies: the horses avoid the patches of tall grass which are generally of low quality and areas contaminated by parasite larvae which leads them to prefer the patches of short grass far from faeces. The parasite status of the horses at the time of the experiment had no effect on their feeding choices. However, before concluding that the challenge by cyathostomes has no effect on the selection of feeding sites in horses, it will be necessary to test whether the history of parasitism of the individuals, rather than the current status, is important.

RESOURCE PARTITIONING AMONG SAVANNA GRAZERS MEDIATED BY LOCAL HETEROGENEITY: AN EXPERIMENTAL APPROACH

Recent theoretical studies predict that body size-related interspecific differences in spatial scale of perception and resource use may contribute to coexistence of species that compete for the same class of resources. These studies provide a new theoretical framework for explaining resource partitioning patterns among African ungulates that coexist in spatially heterogeneous savanna grasslands. According to these studies, different-sized ungulates can coexist because larger species forage at a coarser scale but can tolerate lower quality food, whereas smaller species need higher quality food but forage at a finer scale. To test this hypothesis in an African savanna, we created an experimental mosaic with variation in grain (spatial detail) and quality of short-grass patches and directly observed the visitation of naturally occurring grazers to this mosaic over a two-year period (total of 903 observation hours). Of the seven species that visited our experiment, warthog, impala, zebra, and white rhino visited long enough to allow data analysis. We showed that warthog and impala avoided plots with a finer grain of short grass and that warthog preferred fertilized plots to unfertilized plots. Zebra and white rhino did not avoid the finer grain plots. Our results suggest that differences in grain and quality of a resource might indeed contribute to partitioning of this resource by savanna ungulates. Although four focal species is unusually high for an experimental study on resource partitioning among naturally occurring savanna ungulates, this number is too low to evaluate the allometric basis of our hypothesis. Our results, however, encourage wider experimental testing of the role of spatial heterogeneity in facilitating the coexistence of potentially competing savanna herbivores.

NEST SURVIVAL RELATIVE TO PATCH SIZE IN A HIGHLY FRAGMENTED SHORTGRASS PRAIRIE LANDSCAPE

Understanding the influences of habitat fragmentation on vertebrate populations is essential for the protection and ecological restoration of strategic sites for native species. We examined the effects of prairie fragmentation on avian reproductive success using artificial and natural nests on 26 randomly selected, privately owned patches of shortgrass prairie ranging in size from 7 to 454 ha within a cropland matrix in Washington County, Colorado, summer 2000. Survival trends of artificial and natural nests differed. Daily survival of artificial nests increased with patch size up to about 65 ha and differed little at larger patch sizes, whereas daily survival of Lark Bunting (Calamospiza melanocorys) and Horned Lark (Eremophila alpestris) nests decreased with increasing size of the grassland patch. We hypothesize that our unexpected findings of lower survival of natural nests with increasing patch sizes and different trends between artificial and natural nests are due to the particular structure of predator communities in our study area and the ways in which individual predators respond to artificial and natural nests. We recommend that the value of small habitat patches in highly fragmented landscapes not be overlooked.

PREDICTIVE MODELS OF MOVEMENT BY SERENGETI GRAZERS

Many animal species are unevenly distributed across the landscape, in spatial patterns that continually shift over time. Such a shifting mosaic is thought to have profound implications for the persistence and stability of ecosystems. Management and conservation of natural systems would be enhanced if we could accurately predict movement. Such prediction has not yet been possible. Here we use an extensive set of field data on food abundance and quality, combined with experimentally derived measures of nutritional value, to predict the spatial distribution of Thomson's gazelles (Gazella thomsoni thomsoni Gunter) on the Serengeti Plains of East Africa. Twelve plausible models, based on alternate foraging objectives or movement rules, were assessed against field data on food and grazer abundance gathered at biweekly intervals (every two weeks) over the course of the wet seasons in two different years. Nomadic movements of gazelles closely tracked changes in the spatial distribution of short grass swards. Gazelles left short grass patches when local daily energy intake dropped below the expected intake averaged across the landscape. Subsequent redistribution of gazelles among neighboring patches was proportional to daily rates of energy intake in each patch. Thus, nomadic movements by Thomson's gazelles were predictable on the basis of local energy gain. This suggests that adaptive behavioral models can provide useful predictive tools for understanding the dynamics of complex natural systems.

Do land-use patterns influence nest-site selection by burrowing owls (Athene cunicularia hypugaea) in northeastern Colorado?

Populations of western burrowing owls (Athene cunicularia hypugaea) are declining. In the Great Plains this decline maybe related to a decline in black-tailed prairie dog (Cynomys ludovicianus) towns that the owls use for nest sites. One potential cause of prairie dog decline is conversion of native prairie to agriculture. We predicted that owl-occupied prairie dog towns would be in less fragmented landscapes that contain more prairie then owl-unoccupied prairie dog towns. To test this prediction, we used a geographic information system and spatial analysis metrics to examine the landscape within 1000 and 2500 m radius circles surrounding prairie dog towns in the shortgrass prairie in northeastern Colorado. We compared landscape features of irrigated cropland, non-irrigated cropland, and shortgrass patches around owl-unoccupied (N = 7) and owl-occupied (N = 15) prairie dog towns at these two spatial scales. Contrary to our predictions, at the 2500-m scale burrowing owls prefer to nest in towns surrounded by a landscape in which shortgrass patches were a smaller percentage of the total landscape, relatively far apart from other shortgrass patches, and more numerous then crop patches. At the 1000-m scale owls nested in towns in which shortgrass patches were a smaller percentage of the total surrounding landscape. In Logan County owls may select nest sites in prairie dog towns in more fragmented landscapes because prey availability maybe higher than in less fragmented landscapes and (or) prairie dog control programs may be more active on shortgrass rangelands than in croplands. In addition, the level of fragmentation in our study area (62% of the study area is occupied by native grassland) may not be high enough to have negative consequences on nest occupancy of burrowing owls. However, it is difficult to generalize about the effects of landscape fragmentation on burrowing owls because any effects are probably a complex function of local habitat structure, landscape structure, and local prey and predator availability.

Grazing Management Strategies for Reseeded Rangelands in the East Kimberley Region of Western Australia.

Grazing management strategies involving continuous grazing, wet season rest, dry season rest and a range of stocking rates of steers were evaluated on reseeded rangeland at Ord Regeneration Research Station by the use of temporary exclosures within continuously grazed paddocks. The rangeland is a patchwork of plant communities in various stages of regeneration. Under continuous grazing, liveweight gain in three of the four years of the trial was more closely related to botanical differences between paddocks than to stocking rate although all paddocks were confined to the one land unit. Animals generally selected those parts of the pasture where regeneration of perennial grasses was least advanced and the vegetation was characterized by short annual and semi-perennial species. Differences in the yield of these species between paddocks accounted for much of the variation in liveweight gain. Perennial species contributed most to animal production when seasonal conditions were poor. Over the study as a whole, year-in-year-out stocking rate was much more important than management system (proportion of wet season grazing) in determining vegetation changes. Nevertheless, selective grazing of the short grass patches will prevent the use of a continuous grazing strategy in these pastures. However, continued regeneration should be feasible under a tactical management system in which grazing is managed to ensure that a minimum level of cover is maintained on the short grass phase and that the end of dry season utilization level for the key perennial species does not exceed an average (over years) of about 30 per cent.

Behavior of Whistling and Capped Herons in the Seasonal Savannas of Venezuela and Argentina

Behavior, habitat use, and foraging of the Whistling Heron (Syrigma sibilatrix) and Capped Heron (Pilherodius pileatus) were studied in Venezuela and Argentina. Both foraged primarily alone or in pairs, but Whistling Herons also fed and roosted socially. Whistling Herons nested solitarily, maintained post-fledging associations with young, and appeared to defend feeding territories. They foraged in patches of short grass in wet savannas, especially where water was a few centimeters deep. Capped Herons also foraged in wet savanna, but the edges of streams or ponds seemed to be their preferred habitat. Whistling Herons fed by standing and slow walking, usually in an upright posture. At times they waved their heads in an exaggerated manner. Capped Herons crouched for long periods and walked slowly. Although the basic foraging tactics of the two species were similar, differences support the suggestion that the Capped Heron is a bird of forested streams and ponds while the Whistling Heron is a bird of open savanna. In many ways, the Whistling Heron resembles paleotropical grassland herons. The Whistling Heron (Syrigma sibilatrix) and the Capped Heron (Philherodius pileatus) are among the least known ardeids in the Western Hemisphere. They are exclusively neotropical with overlapping ranges in south-central and northern South America (Blake 1971). Their systematic relationships have long been uncertain (Bock 1956, Humphrey and Parkes 1963, Payne and Risley 1976), and little is known about their behavior (Hancock and Elliott 1978). The Capped Heron is generally uncommon (e.g., Mitchell 1957, Haverschmidt 1968, Hancock and Elliott 1978, Thomas 1979) whereas the Whistling Heron is locally common but patchily distributed. In this paper we contrast aspects of habitat use, behavior, foraging, and general adaptive habits of these two herons in the seasonal savannas of Venezuela and Argentina.