Cattle Grazing Impacts on Annual Forbs and Vegetation Composition of Mesic Grasslands in California

Abstract:   Livestock grazing represents a major human alteration of natural disturbance regimes in grasslands throughout the world, and its impacts on plant communities have been highly debated. We investigated the impact of cattle grazing on the California coastal prairie plant community with a focus on native annual forbs, a number of which are of conservation concern. In spring 2000 and 2001, we surveyed the vegetation community composition, vegetation structure, and soil chemical parameters at 25 paired grazed and ungrazed sites over a 670-km range of the ecosystem. Native annual forb species richness and cover were higher in grazed sites, and this effect was concomitant with decreased vegetation height and litter depth. Soil properties explained less of the variation. Exotic annual grass and forb cover were higher in grazed sites. Native grass cover and species richness did not differ in grazed and ungrazed sites, but cover and species richness of native perennial forbs were higher in ungrazed sites. Our results suggest that cattle grazing may be a valuable management tool with which to conserve native annual forbs in the ecosystem we studied but that grazing differentially affects the various life-history guilds. Therefore, land managers must focus on creating a matrix of disturbance regimes to maintain the suite of species native to these mesic grasslands. The results of this and other studies highlight the importance of considering the adaptation of vegetation communities to disturbance in making recommendations for grazing management.     

Evaluation of Central North American Prairie Management Based on Species Diversity, Life Form, and Individual Species Metrics

Abstract:  Reintroduction of fire and grazing, alone or in combination, has increasingly been recognized as central to the restoration of North American mixed-grass and tallgrass prairies. Although ecological studies of these systems are abundant, they have generally been observational, or if experimental, have focused on plant species diversity. Species diversity measures alone are not sufficient to inform management, which often has goals associated with life-form groups and individual species. We examined the effects of prescribed fire, light cattle grazing, and a combination of fire and grazing on three vegetation components: species diversity, groups of species categorized by life-form, and individual species. We evaluated how successful these three treatments were in achieving specific management goals for prairies in the Iowa Loess Hills (U.S.A.). The grazing treatment promoted the greatest overall species richness, whereas grazing and burning and grazing treatments resulted in the lowest cover by woody species. Burning alone best achieved the management goals of increasing the cover and diversity of native species and reducing exotic forb and (predominantly exotic) cool-season grass cover. Species-specific responses to treatments appeared idiosyncratic (i.e., within each treatment there existed a set of species attaining their highest frequency) and nearly half of uncommon species were present in only one treatment. Because all management goals were not achieved by any one treatment, we conclude that management in this region may need refining. We suggest that a mosaic of burning and grazing (alone and in combination) may provide the greatest landscape-level species richness; however, this strategy would also likely promote the persistence of exotic species. Our results support the need to consider multiple measures, including species-specific responses, when planning and evaluating management .     

Response of Avian Communities to Historic Habitat Change in the Northern Chihuahuan Desert

Abstract: Throughout much of the northern Chihuahuan Desert, the grasslands that were widespread at the time of European settlement have been replaced by desert shrublands. Little is known about the effects of this change on avian communities. We analyzed historic U.S. Government Land Office records to assess large-scale changes in vegetation cover from the 1880s to the present day. We studied vegetation and avian communities in one grassland habitat type and four desert shrubland habitat types to examine (1) how breeding-bird communities may have changed in response to habitat conversion from grassland to desert shrubland and (2) whether breeding-bird communities differ among the four desert shrubland habitat types that compose Chihuahuan Desert scrub in this region. To estimate the characteristics of 1880s black grama (    Bouteloua eriopoda ) grassland, we focused on plots located within extensive patches of present-day black grama and compared the avian communities found there with those in desert shrubland. Species richness was higher in desert shrubland than grassland. Among the desert shrubland habitat types, species richness was consistently highest in mesquite. Avian abundance patterns differed among the four desert shrubland habitat types. At least 30% of the avian community in each habitat pair was distinct. Conversion of grassland to shrubland in south-central New Mexico has likely been accompanied by a major turnover in the avian community. Remaining tracts of black grama provide habitat for species that may be uniquely adapted to the northern Chihuahuan Desert and should be protected.     

Indices of Grassland Biodiversity in the Chihuahuan Desert Ecoregion Derived from Remote Sensing

Abstract: We used a relatively simple and direct remote-sensing approach to determine biodiversity values in arid ecosystems and thus identify potential conservation sites. We developed indices based on regression models between grass, shrub, litter, exposed-soil groundcover components, and Landsat thematic mapper satellite imagery reflectance values over a reference site in the northern Chihuahuan Desert in New Mexico. This site supports low-disturbance desert grasslands that have been excluded from livestock grazing for 55 years and moderate-disturbance grasslands that have been under a continuous grazing regime for over 100 years. Greater richness and abundance of noninvasive and nonruderal plant species were associated with the low-disturbance grasslands that had lower shrub abundance, increased litter and grass cover, and lower exposed soil. Using the thematic mapper indices, we computed an additive grassland biodiversity index such that, as exposed soil and shrub values go down, litter and grass values go up, as does the biodiversity index. When the biodiversity index was applied to the reference-site landscape, grasslands previously identified for their high conservation value were detected. As a further test, we applied the indices to a site in Chihuahua, Mexico, that supports similar grasslands but for which there are few other data on condition and conservation values. The soil, grass, and shrub indices were moderately effective in describing the range of variation at the Mexico site, but the litter equation was not. Still, higher biodiversity value in terms of nonruderal plant diversity tended to correspond to higher grass cover and lower soil exposure and a higher overall biodiversity index. Some localized calibration with geologic substrate may be required along with an assessment of the temporal constraints, but generally the index shows promise for quickly and efficiently detecting desert grasslands of high biodiversity conservation value.     

Early development of vegetation in restored dune plant microhabitats on a nourished beach at Ocean City, New Jersey

Topography and vegetation of restored dunes on a developed barrier island were examined after a large-scale beach nourishment project. Restoration began in 1993 using sand-trapping fences andAmmophila breviligulata Fern. plantings. Subsequent growth of dunes was favored by installing new fences and suspending beach raking to accommodate nesting birds. Plant species richness, percent cover of vegetation, and height ofA. breviligulata were sampled in 1999 on seven shore perpendicular transects in six dune microhabitats (backdune, primary crest, mid-foredune, swale, seaward-most fenced ridge, incipient dune on the backbeach). A total of 26 plant taxa were found at all seven sites. Richness and percent cover were greatest in the backdune and crest, especially in locations that predated the 1992 nourishment. Richness was greater where fences enhanced stabilization. Fences initially compensate for time and space and allow vegetation to develop rapidly, but maintenance nourishment is required to protect against wave erosion and ensure long-term viability of habitat. An expanded environmental gradient is an option, where beach nourishment provides space for a species-rich crest and backdune to develop, while the incipient dune remains dynamic. Options where space is restricted include a dynamic, full-sized seaward section of a naturally functioning dune (truncated gradient) or a spatially restricted sampler of a wider natural dune (compressed gradient) maintained using fences. Expanded and truncated gradients may become self-sustaining and provide examples of natural cycles of change. Compressed gradients provide greater species richness and flood protection for the available space, but habitats are vulnerable to erosion, and resident views may be impaired.     

Impact of cattle grazing on the occupancy of a cryptic, threatened rail

Impacts of livestock grazing in arid and semiarid environments are often concentrated in and around wetlands where animals congregate for water, cooler temperatures, and green forage. We assessed the impacts of winter-spring (November-May) cattle grazing on marsh vegetation cover and occupancy of a highly secretive marsh bird that relies on dense vegetation cover, the California Black Rail (Laterallus jamaicensis coturniculus), in the northern Sierra Nevada foothills of California, U.S.A. Using detection-nondetection data collected during repeated call playback surveys at grazed vs. ungrazed marshes and a "random changes in occupancy" parameterization of a multi-season occupancy model, we examined relationships between occupancy and habitat covariates, while accounting for imperfect detection. Marsh vegetation cover was significantly lower at grazed marshes than at ungrazed marshes during the grazing season in 2007 but not in 2008. Winter-spring grazing had little effect on Black Rail occupancy at irrigated marshes. However, at nonirrigated marshes fed by natural springs and streams, grazed sites had lower occupancy than ungrazed sites. Black Rail occupancy was positively associated with marsh area, irrigation as a water source, and summer vegetation cover, and negatively associated with marsh isolation. Residual dry matter (RDM), a commonly used metric of grazing intensity, was significantly associated with summer marsh vegetation cover at grazed sites but not spring cover. Direct monitoring of marsh vegetation cover, particularly at natural spring- or stream-fed marshes, is recommended to prevent negative impacts to rails from overgrazing.     

Utilisation of a coastal grassland by geese after managed re-alignment

In this study we evaluate the effect of coastal re-alignment on the utilisation of coastal grasslands by staging geese. We assessed vegetation change and utilisation by geese using repeated mapping and regular dropping counts in both the restored marsh and adjacent reference sites. All measurements were started well before the actual re-alignment. In addition, we studied the effects of livestock grazing on vegetation and geese, using exclosures. The vegetation transformed from fresh grassland into salt-marsh vegetation. A relatively large proportion of the de-embanked area became covered with secondary pioneer vegetation, and the overall cover of potential food plants for geese declined. Goose utilisation had initially dropped to low levels, both in autumn and in spring, but it recovered to a level comparable to the reference marsh after ten years. Exclosure experiments revealed that livestock grazing prevented the establishment of closed swards of grass in the poorly drained lower area of the restored marsh, and thereby negatively affected goose utilisation of these areas during spring staging. Goose grazing in the restored marsh during spring showed a positive numerical response to grass cover found during the preceding growing season. (1) The value of restored salt marsh as foraging habitat for geese initially decreased after managed re-alignment but recovered after ten years. (2) Our findings support the idea that the value of foraging habitats depends largely on the cover of forage plants and that this can be manipulated by adjusting both grazing and drainage.     

河南省淮河流域伏牛山区主要生态恢复演替序列构建

为构建河南省淮河流域内伏牛山区的主要植被类型在生态恢复过程中的演替序列,选取了登封市、鲁山县、确山县和泌阳市4个典型地点,利用空间代替时间方法调查了不同演替系列的主要群落类型,采用排序并结合演替度、分层频度等分析方法构建了该区域的演替序列。结果表明：河南省淮河流域伏牛山区生态恢复过程中的植被群落可分为5种类型,即生态林封禁抚育群落、灌草坡封禁保护群落、梯田耕作经济林群落、梯田经济林弃耕群落、禁牧禁樵生态林群落;群落稳定性大小依次为栎类林、经济林与人工针叶林;植被演替可构建为自然恢复、人工种植后的自然恢复、人工经济林、进展与逆行等4个演替序列;其基本序列可识别为草丛、灌丛、人工林、经济林弃耕恢复林、栎类幼龄林、栎类中龄林等,演替度平均值分别为1.06、5.33、6.19、11.09、12.86、12.26;阔叶树种通常具有较大的分层频度,表明其良好的进展性与稳定性,以其为主要种类所构成的群落类型基本上均处于演替序列中的后期阶段;因此,可以将阔叶林以及特殊条件下的人工林设定为该区域生态恢复的目标植被类型。将几种演替分析方法综合运用,可以较全面地从区域微观尺度上揭示植被的演替过程,能较好地阐明植被演替过程;研究结果可以为生态恢复过程中植被动态与水文生态功能动态相关性、流域生态功能区划、生态恢复目标的制定与过程设计、以及流域生态保护策略制定等提供依据。     

Hierarchical analysis of vegetation dynamics over 71 years: soil-rainfall interactions in a Chihuahuan Desert ecosystem

Proliferation of woody plants in grasslands and savannas is a persistent problem globally. This widely observed shift from grass to shrub dominance in rangelands worldwide has been heterogeneous in space and time largely due to cross-scale interactions among soils, climate, and land-use history. Our objective was to use a hierarchical framework to evaluate the relationship between spatial patterns in soil properties and long-term shrub dynamics in the northern Chihuahuan Desert of New Mexico, USA. To meet this objective, shrub patch dynamics from 1937 to 2008 were characterized at patch and landscape scales using historical imagery and a recent digital soils map. Effects of annual precipitation on patch dynamics on two soils revealed strong correlations between shrub growth on deep sandy soils and above-average rainfall years (r = 0.671, P = 0.034) and shrub colonization and below-average rainfall years on shallow sandy soils (r = 0.705, P = 0.023). Patch-level analysis of demographic patterns revealed significant differences between shrub patches on deep and shallow sandy soils during periods of above- and below-average rainfall. Both deep and shallow sandy soils exhibited low shrub cover in 1937 (1.0% +/- 2.3% and 0.3% +/- 1.3%, respectively [mean +/- SD]) and were characterized by colonization or appearance of new patches until 1960. However, different demographic responses to the cessation of severe drought on the two soils and increased frequency of wet years after 1960 have resulted in very different endpoints. In 2008 a shrubland occupied the deep sandy soils with cover at 19.8% +/- 9.1%, while a shrub-dominated grassland occurred on the shallow sandy soils with cover at 9.3% +/- 7.2%. Present-day shrub vegetation constitutes a shifting mosaic marked by the coexistence of patches at different stages of development. Management implications of this long-term multi-scale assessment of vegetation dynamics support the notion that soil properties may constrain grassland remediation. Such efforts on sandy soils should be focused on sites characterized by near-surface water-holding capacity, as those lacking available water-holding capacity in the shallow root zone pose challenges to grass recovery and survival.     

Recovery of Endemic Dragonflies after Removal of Invasive Alien Trees

Abstract:  Because dragonflies are very sensitive to alien trees , we assessed their response to large-scale restoration of riparian corridors. We compared three types of disturbance regime—alien invaded , cleared of alien vegetation , and natural vegetation (control)—and recorded data on 22 environmental variables. The most significant variables in determining dragonfly assemblages were percentage of bank cover and tree canopy cover , which indicates the importance of vegetation architecture for these dragonflies. This finding suggests that it is important to restore appropriate marginal vegetation and sunlight conditions. Recovery of dragonfly assemblages after the clearing of alien trees was substantial. Species richness and abundance at restored sites matched those at control sites. Dragonfly assemblage patterns reflected vegetation succession. Thus , initially eurytopic , widespread species were the main beneficiaries of the removal of alien trees , and stenotopic , endemic species appeared after indigenous vegetation recovered over time. Important indicator species were the two national endemics ( Allocnemis leucosticta and Pseudagrion furcigerum ) , which , along with vegetation type , can be used to monitor return of overall integrity of riparian ecology and to make management decisions. Endemic species as a whole responded positively to restoration , which suggests that indigenous vegetation recovery has major benefits for irreplaceable and widespread generalist species .     

草灌混播在边坡绿化防护中的应用

边坡绿化防护是对人工开挖建设形成的各种坡面进行生态修复以及防止水土流失的一项重要措施。草、灌木混播应用于边坡防护与修复,较之于单一的草被措施,更有利于形成立体的、多样的植被结构,对边坡的稳定和美化有更好的效果。     

Controls on vegetation structure in Southwestern ponderosa pine forests, 1941 and 2004

Long-term studies can broaden our ecological understanding and are particularly important when examining contingent effects that involve changes to dominance by long-lived species. Such a change occurred during the last century in Southwestern (USA) ponderosa pine (Pinus ponderosa) forests. We used five livestock grazing exclosures established in 1912 to quantify vegetation structure in 1941 and 2004. Our objectives were to (1) assess the effects of historical livestock grazing on overstory structure and age distribution, (2) assess the effects of recent livestock grazing and overstory on understory vegetation, and (3) quantify and explain changes in understory vegetation between 1941 and 2004. In 1941, canopy cover of tree regeneration was significantly higher inside exclosures. In 2004, total tree canopy cover was twice as high, density was three times higher, trees were smaller, and total basal area was 40% higher inside exclosures. Understory species density, herbaceous plant density, and herbaceous cover were negatively correlated with overstory vegetation in both years. Most understory variables did not differ between grazing treatments in 1941 but were lower inside exclosures in 2004. Differences between grazing treatments disappeared once overstory effects were accounted for, indicating that they were due to the differential overstory response to historical livestock grazing practices. Between 1941 and 2004, species density declined by 34%, herbaceous plant density by 37%, shrub cover by 69%, total herbaceous cover by 59%, graminoid cover by 39%, and forb cover by 82%. However, these variables did not differ between grazing treatments or years once overstory effects were accounted for, indicating that the declines were driven by the increased dominance of the overstory during this period. Our results demonstrate that historical livestock grazing practices are an aspect of land-use history that can affect ecosystem development. Grazing history must be considered when extrapolating results from one site to another. In addition, the understory vegetation was more strongly controlled by the ponderosa pine overstory than by recent livestock grazing or by temporal dynamics, indicating that overstory effects must be accounted for when examining understory responses in this ecosystem.     

Landscape Characteristics of Fragmented Shrubsteppe Habitats and Breeding Passerine Birds

We examined the influence of local and landscape-level attributes of fragmented habitats in shrubsteppe habitats on the breeding distributions of Sage ( Amphispiza belli ) and Brewer's ( Spizella breweri ) Sparrows, Sage Thrashers ( Oreoscoptes montanus ) Horned Larks ( Eremophila alpestris ), and Western Meadowlarks ( Sturnella neglecta ) in the Snake River Plains of southwestern Idaho. We developed habitat (resource) selection models for each species by combining bird counts conducted from 1991 through 1933 with local vegetation characteristics and landscape attributes derived from satellite imagery. Site selection by shrubsteppe species (Sage and Brewer's Sparrows, and Sage Thrashers) depended on local vegetation cover and landscape features, such as the patch size of shrub habitats or the spatial similarity of sites. Marginal sites for these species (with species present in one of three years) were intermediate between unoccupied (never present) and occupied sites along environmental gradients characterized by increasing size of shrub habitat patches and total shrub cover and by decreasing disturbance. Horned Larks and Western Meadowlarks, typical grassland species, were not sensitive to landscape features, and their occupancy depended on the amount of grassland or shrub cover. In contrast to shrubsteppe species, sites that varied by occupancy rates of Western Meadowlarks did not significantly differ in vegetation or landscape components. Our results demonstrate that fragmentation of shrubsteppe significantly influenced the presence of shrub-obligate species. Because of restoration difficulties, the disturbance of semiarid shrubsteppe may cause irreversible loss of habitat and significant long-term consequences for the conservation of shrub-obligate birds.     

The impact of grazing on spider communities in a mesophytic calcareous dune grassland

During 1994–1995 and 1997–1998 spiders were sampled with pitfall traps in a botanically rich, mesophytic, calcareous dune grassland in Belgium. As a consequence of intensive cattle grazing, vegetation variation in a large part of the area had diminished. The study area was also patchily grazed by rabbits. Community analysis with TWINSPAN revealed five distinct spider communities. Ecological differentiation was best explained by combination of the habitat variables: distance from grazed or non-grazed vegetation,Rosa pimpinellifolia cover and grass cover in both summer and winter. Species diversity was highest in the border zone between the cattle-grazed and non cattle-grazed sites. Correlation of the most abundant spider species with the vegetation determinants explains the ecological differentiation between the spider communities. Species were classified into seven major groups that reflect the species’ habitat preferences. The group showing clear association with non cattle-grazed, tall vegetation consists of common species. Characteristic species for the intensively cattle-grazed sites are common aeronauts and rare species such asWalckenaeria stylifrons, Mastigusa arietina, Ceratinopsis romana andPardosa monticola. The latter are shown to be dependent on ungrazed vegetation for juvenile development and overwintering. Intensive grazing results in homogeneous short vegetation, which can only be colonized by ‘open ground’ species with a well-developed dispersal capacity, or by species which are not dependent on litter-rich situations for juvenile development. An extensive cattle grazing regime results in a patchy mosaic grassland where, in addition to the above mentioned groups of species, other species survive by migrating between the buffered litter rich ungrazed vegetation and the short vegetation. Additionally, some typical and rare species prefer the transition zone between the grazed and the ungrazed vegetation because they are associated with specific habitat structures or inhabiting ant-species.     

Projected vegetation changes for the American Southwest: combined dynamic modeling and bioclimatic-envelope approach

This study focuses on potential impacts of 21st century climate change on vegetation in the Southwest United States, based on debiased and interpolated climate projections from 17 global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Among these models a warming trend is universal, but projected changes in precipitation vary in sign and magnitude. Two independent methods are applied: a dynamic global vegetation model to assess changes in plant functional types and bioclimatic envelope modeling to assess changes in individual tree and shrub species and biodiversity. The former approach investigates broad responses of plant functional types to climate change, while considering competition, disturbances, and carbon fertilization, while the latter approach focuses on the response of individual plant species, and net biodiversity, to climate change. The dynamic model simulates a region-wide reduction in vegetation cover during the 21st century, with a partial replacement of evergreen trees with grasses in the mountains of Colorado and Utah, except at the highest elevations, where tree cover increases. Across southern Arizona, central New Mexico, and eastern Colorado, grass cover declines, in some cases abruptly. Due to the prevalent warming trend among all 17 climate models, vegetation cover declines in the 21st century, with the greatest vegetation losses associated with models that project a drying trend. The inclusion of the carbon fertilization effect largely ameliorates the projected vegetation loss. Based on bioclimatic envelope modeling for the 21st century, the number of tree and shrub species that are expected to experience robust declines in range likely outweighs the number of species that are expected to expand in range. Dramatic shifts in plant species richness are projected, with declines in the high-elevation evergreen forests, increases in the eastern New Mexico prairies, and a northward shift of the Sonoran Desert biodiversity maximum.     

Rodent Communities in an Exurbanizing Southwestern Landscape (U.S.A.)

Abstract:  Ranches are being converted to exurban housing developments in the southwestern United States, with potentially significant but little-studied impacts on biological diversity. We captured rodents on 48 traplines in grasslands, mesquite savannas, and oak savannas in southeastern Arizona that were grazed by livestock, embedded in exurban housing developments, grazed and embedded in development, or neither grazed nor embedded in development. Independent of habitat or development, rodent species richness, mean rank abundance, and capture rates of all rodents combined were negatively related to presence of livestock grazing or to its effects on vegetative ground cover. Exurban development had no obvious effects on rodent variety or abundance. Results suggest southwestern exurban developments can sustain a rich assemblage of grassland and savanna rodents if housing densities are low and houses are embedded in a matrix of natural vegetation with little grazing.     

Fire Regime Changes in La Michilía Biosphere Reserve, Durango, Mexico

Abstract: The ability of reserves to maintain natural ecosystem processes such as fire disturbance regimes is central to long-term conservation. Fire-scarred tree samples were used to reconstruct fire regimes at five study sites totaling approximately 230 ha in pine (   Pinus spp.) and oak ( Quercus spp.) forests of La Michilía Biosphere Reserve on the dry east slope of the Sierra Madre Occidental, Durango, Mexico. Study sites covered a 20-km environmental gradient of elevation, topography, and human land uses. Plant communities ranged from oak-pine to mixed conifer forests. Fires were frequent at all sites prior to 1930, when large-scale grazing of domestic livestock was initiated. Widespread fires have been excluded from four of the five sites since 1945, with an essentially uninterrupted regime of frequent fires continuing only in the reserve core. Xeric sites had many, smaller fires, whereas mesic sites had fewer but larger fires. On a reserve-wide scale, a fire burned on at least one site nearly every year, usually in the dry spring or early summer season, but fire years were rarely synchronous among the sites. Fire occurrence was weakly related to the Southern Oscillation climate pattern; major reserve-wide fire years almost never coincided with wet Southern Oscillation extremes but only occasionally matched dry extremes. Maintenance of the long-term frequent-fire regime in the reserve core is one indicator that the biosphere reserve model has been successful in conserving natural processes, but the protected area is small ( 7000 ha). Because of the key role of frequent-fire regimes in regulating ecosystem structure and function, restoration of the ecological role of fire disturbance is a desirable conservation strategy.     

Cover of Perennial Grasses in Southeastern Arizona in Relation to Livestock Grazing

Tolerance of particular grasslands to the activities of domestic livestock may depend on their historic association with native grazing animals. Southwestern grama ( Bouteloua ) grasslands are floristically allied to the North American Central Plains but lie outside the historic range of the plains' principal ungulate grazer, alics bishop . We compared perennial grassland cover and species composition on eight sites transacted by the boundary fence of a 3160-ha, 22-year-old livestock exclosure in a grama grassland in southeastern Arizona. Total grass canopy cover was greatest on the ungrazed portion of each of the eight sites. Two short stoloniferous species ( Hilaria belangeri and Bouteloua eriopoda ) were the only taxa substantially more abundant on grazed quadrats overall. Among these and eight taller budgerigars, there was a strong positive correlation between potential height and response to release from grazing, with the three tallest species showing the greatest increases on ungraded treatments ( emization curtailment, Boilermaker barbarians , and emizations intermixed ). emization gracious , the most abundant grass in the region, showed an intermediate response to livestock exclusion, Gram grasslands at the Arizona site have changed more and in different ways following livestock exclusion than those on the Central Plains of Colorado. Contributing factors may include: (1) greater annual precipitation at the Arizona site, (2) the much larger size of the Arizona livestock exclosure, and (3) the absence of extensive grazing by native ungulates in the Southwest since the Pleistocene. Livestock grazing appears to be an exotic ecological force in these southwestern grasslands, and one destructive of certain components of the native flora and fauna.     

The oasis effect: response of birds to exurban development in a southwestern savanna

Ranches are being converted to exurban housing developments in the southwestern United States, with potentially significant but little-studied impacts on biological diversity. We counted birds in grasslands and savannas in southeastern Arizona that were grazed by livestock, embedded in low-density exurban housing developments, or both, or neither. Species richness and bird abundance were higher in exurban neighborhoods than in undeveloped landscapes, independent of livestock grazing. The positive response to development was particularly evident among doves, quail, hummingbirds, aerial insectivores, and some but not all ground-foraging sparrows. Effects of livestock grazing were comparatively minor and mostly involved birds with requirements for tall ground cover or the lack of it. The average rank correlation between counts of individual species and housing density was positive across all transects. However, this relationship disappeared among the exurban transects alone, and bird species richness on the exurban transects was negatively correlated with the number of homes nearby. These results suggest that the positive influence of exurban development on avian abundance and variety was greatest at the lowest housing densities. We attribute the attraction of many birds to exurban development to an oasis effect, in which resources otherwise scarce in arid southwestern environments (shade, nectar, nest sites, and especially water) are relatively abundant around exurban home sites. This finding is consistent with the hypothesis that exurban home sites represented resource supply points inside birds' home ranges otherwise consisting mostly of natural vegetation.     

Predicting Richness of Native, Rare, and Exotic Plants in Response to Habitat and Disturbance Variables across a Variegated Landscape

Species richness of native, rare native, and exotic understorey plants was recorded at 120 sites in temperate grassy vegetation in New South Wales. Linear models were used to predict the effects of environment and disturbance on the richness of each of these groups. Total native species and rare native species showed similar responses, with richness declining on sites of increasing natural fertility of parent material as well as declining under conditions of water enrichment (resulting from human-induced changes in drainage characteristics, leading to increased run-off), severe livestock grazing, and soil disturbance. The response of rare native species to water enrichment, however, was significantly greater than that of all native species. Exotic species richness varied in reverse to that of native species with positive responses to water enrichment and soil disturbance. The contrasting behaviors are attributed to differences in the evolutionary history of native and exotic assemblages and their resulting preadaptations to a landscape recently subjected to agricultural settlement. It would appear that for exogenous disturbances, the intermediate disturbance hypothesis is not supported by our data. In the sampled region, pastures represent the major land-use in terms of area, but have relatively low densities of native and rare species compared with more lightly grazed areas. However, their management is considered to be essential to the maintenance of diversity on a regional scale.