Two features characterize all natural communities. First, they are dynamic systems. The densities and age-structures of populations change with time, as do the relative abundances of species; local extinctions are commonplace (37). For many communities, a self-reproducing climax state may only exist as an average condition on a relatively large spatial scale, and even that has yet to be rigorously demonstrated (36). The idea that equilibrium is rarely achieved on the local scale was expressed decades ago by a number of forest ecologists (e.g. 10 1, 168). One might even argue that continued application of the concept of climax to natural systems is simply an exercise in metaphysics (41). While this view may seem extreme, major climatic shifts often recur at time intervals shorter than that required for a community to reach competitive equilibrium or alter the geographical distributions of species (6, 21, 43, 76, 92). Climatic variation of this kind influences ecological patterns over large areas, sometimes encompassing entire continents. Other agents of temporal change in natural communities operate over a wide range of smaller spatial scales (47, 242). Second, natural communities are spatially heterogeneous. This statement is true at any scale of resolution (242), but it is especially apparent on what is commonly referred to as the regional scale. (By region I mean an area that potentially encompasses more than one colonizable patch.) Across any land or seascape, one observes a mosaic of patches identified by spatial discontinuities in the distributions of populations (153, 159, 161, 231, 239, 240). Closer examination often reveals a smaller-scale patchwork of same-aged individuals (e.g. 85-87, 101, 146, 199,204,217-220,235,246). Discrete patch boundaries sometimes reflect species-specific responses to

The Role of Disturbance in Natural Communities

Multiple regression analyses were performed on a worldwide 236—site data set compiled from studies that compared species composition, aboveground net primary production (ANPP), root biomass, and soil nutrients of grazed vs. protected, ungrazed sites. The objective was to quantitatively assess factors relating to differential sensitivities of ecosystems to grazing by large herbivores. A key question in this assessment was: Do empirically based, broad—scale relationships correspond to ecological theories of plant—animal interactions and conceptual frameworks for management of the world's grazing lands? Changes in species composition with grazing were primarily a function of ANPP and the evolutionary history of grazing of the site, with level of consumption third in importance. Changes in species composition increased with increasing productivity and with longer, more intense evolutionary histories of grazing. These three variables explained >50% of the variance in the species response of grasslands or grasslands—plus—shrublands to grazing, even though methods of measurement and grazing systems varied among studies. Years of protection from grazing was a significant variable only in the model for shrublands. Similar variables entered models of change in the dominant species with grazing. As with species composition, sensitivities of change in dominant species were greater to varying ecosystem—environmental variables than to varying grazing variables, from low to high values. Increase of the dominant species under grazing were predicted under some conditions, and decreases were more likely among bunch grasses than other life—forms and more likely among perennials than annuals. The response of shrublands was different from that of grasslands, both in terms of species composition and the dominant species. Our analyses support the perception of grazing as a factor in the conversion of grasslands to less desirable shrublands, but also suggest that we may be inadvertently grazing shrublands more intensively than grasslands. Percentage differences in ANPP between grazed and ungrazed sites decreased with increasingly long evolutionary histories of grazing and increased with increasing ANPP, levels of consumption, or years of treatment. Although most effects of grazing on ANPP were negative, some were not, and the statistical models predicted increases in ANPP with grazing under conditions of long evolutionary history, low consumption, few years of treatment, and low ANPP for grasslands—plus—shrublands. The data and the models support the controversial hypothesis that grazing can increase ANPP in some situations. Similar to species variables, percentage differences in ANPP between grazed and ungrazed treatments were more sensitive to varying ecosystem—environmental variables than to varying grazing variables. Within levels not considered to be abusive "overgrazing," the geographical location where grazing occurs may be more important than how many animals are grazed or how intensively an area is grazed. Counter to the commonly held view that grazing negatively impacts root systems, there was no relationship between difference in ANPP with grazing and difference in root mass; as many positive as negative differences occurred, even though most ANPP differences were negative. Further, there was a weak relationship between change in species composition and change in ANPP, and no relationship with root mass, soil organic matter, or soil nitrogen. All three belowground variables displayed both positive and negative values in response to grazing. Current management of much of the world's grazing lands based on species composition criteria may lead to erroneous conclusions concerning the long—term ability of a system to sustain productivity.

Quantitative effects of grazing on vegetation and soils over a global range of environments

The evolutionary shift from xenogamy (outcrossing) to autogamy (selfing) has been mediated through decreased flower size and alterations in floral morphology (Ornduff, 1969) which reduce the energetic cost per flower (Cruden, unpubl.) and facilitate self-pollination, respectively. Recently, a number of workers have shown that the flowers of self-incompatible and other xenogamous taxa produce more pollen grains than closely related self-compatible and/or autogamous taxa (Arroyo, 1973; Baker, 1967; Cruden, 1973; Gibbs et al., 1975; Lloyd, 1965; Vries, 1974). In contrast to other floral characteristics the number of ovules per ovary in the taxa studied did not decrease. In these taxa the evolutionary shift from xenogamy to autogamy has been accompanied by a decrease in the ratio of pollen grains to ovules (PIO). It is generally accepted that differences in breeding systems tend to be correlated with successional stages. Colonizers of disturbed habitats and plants of early successional stages tend to be self-compatible and/or autogamous (Baker, 1955, 1959, 1965; Stebbins, 1958) whereas plants of later successional stages, including tropical forest trees, tend to be xenogamous (Baker, 1959; Bawa, 1974; Bawa and Opler, 1975; Stebbins, 1950, 1958). Since, as I will show, PIO's are an integral part of a plant's breeding system, they also should be correlated with habitat or successional stage. My overall objective is to show that PIO's reflect the likelihood of sufficient pollen grains reaching each stigma to result in maximum seed set. The more efficient the transfer of pollen, the lower the PIO should be. It follows logically that cleistogamous flowers should have the lowest PIO's, and that autogamous flowers will have lower PIO's than xenogamous flowers, i.e., that PIO's are correlated with breeding systems. My data show that this is the case and suggest that PI O's are integrated with other facets of a plant's breeding system. Second, I show that there is a strong correlation between the breeding system, hence PIO's, and successional stage. In addition, I briefly circumscribe what constitutes a sufficient number of pollen grains to assure maximum seed set and discuss evidence that supports the notion that some plants tend to minimize pollen production. Finally, I discuss deviations from the general pattern to show that PIO's are a better predictor of a plant's breeding system than other morphological characteristics.

Pollen-ovule ratios: a conservative indicator of breeding systems in flowering plants

Key risks associated with projected climate trends for the 21st century include the prospects of future climate states with no current analog and the disappearance of some extant climates. Because climate is a primary control on species distributions and ecosystem processes, novel 21st-century climates may promote formation of novel species associations and other ecological surprises, whereas the disappearance of some extant climates increases risk of extinction for species with narrow geographic or climatic distributions and disruption of existing communities. Here we analyze multimodel ensembles for the A2 and B1 emission scenarios produced for the fourth assessment report of the Intergovernmental Panel on Climate Change, with the goal of identifying regions projected to experience (i) high magnitudes of local climate change, (ii) development of novel 21st-century climates, and/or (iii) the disappearance of extant climates. Novel climates are projected to develop primarily in the tropics and subtropics, whereas disappearing climates are concentrated in tropical montane regions and the poleward portions of continents. Under the high-end A2 scenario, 12-39% and 10-48% of the Earth's terrestrial surface may respectively experience novel and disappearing climates by 2100 AD. Corresponding projections for the low-end B1 scenario are 4-20% and 4-20%. Dispersal limitations increase the risk that species will experience the loss of extant climates or the occurrence of novel climates. There is a close correspondence between regions with globally disappearing climates and previously identified biodiversity hotspots; for these regions, standard conservation solutions (e.g., assisted migration and networked reserves) may be insufficient to preserve biodiversity.

https://www.pnas.org/content/pnas/104/14/5738.full.pdf

Projected distributions of novel and disappearing climates by 2100 AD

Ecosystem services are natural processes that benefit humans. Birds contribute the four types of services recognized by the UN Millennium Ecosystem Assessment-provisioning, regulating, cultural, and supporting services. In this review, we concentrate primarily on supporting services, and to a lesser extent, provisioning and regulating services. As members of ecosystems, birds play many roles, including as predators, pollinators, scavengers, seed dispersers, seed predators, and ecosystem engineers. These ecosystem services fall into two subcategories: those that arise via behavior (like consumption of agricultural pests) and those that arise via bird products (like nests and guano). Characteristics of most birds make them quite special from the perspective of ecosystem services. Because most birds fly, they can respond to irruptive or pulsed resources in ways generally not possible for other vertebrates. Migratory species link ecosystem processes and fluxes that are separated by great distances and times. Although the economic value to humans contributed by most, if not all, of the supporting services has yet to be quantified, we believe they are important to humans. Our goals for this review are 1) to lay the groundwork on these services to facilitate future efforts to estimate their economic value, 2) to highlight gaps in our knowledge, and 3) to point to future directions for additional research.

Ecosystem Services Provided by Birds

Large vertebrates are strong interactors in food webs, yet they were lost from most ecosystems after the dispersal of modern humans from Africa and Eurasia. We call for restoration of missing ecological functions and evolutionary potential of lost North American megafauna using extant conspecifics and related taxa. We refer to this restoration as Pleistocene rewilding; it is conceived as carefully managed ecosystem manipulations whereby costs and benefits are objectively addressed on a case‐by‐case and locality‐by‐locality basis. Pleistocene rewilding would deliberately promote large, long‐lived species over pest and weed assemblages, facilitate the persistence and ecological effectiveness of megafauna on a global scale, and broaden the underlying premise of conservation from managing extinction to encompass restoring ecological and evolutionary processes. Pleistocene rewilding can begin immediately with species such as Bolson tortoises and feral horses and continue through the coming decades wi...

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Pleistocene rewilding: an optimistic agenda for twenty-first century conservation.

Livestock have been excluded from a 3,160-ha range in southeastern Arizona since 1968. Compared to an adjacent continuously grazed area, in 1981-82 a protected upland site supported 45% more grass cover, a comparatively heterogeneous grass community, and 4 times as many shrubs. Grama grasses (Bouteloua spp.) were equally common in and outside the exclosure, while a variety of other species, especially plains lovegrass (Eragrostis intermedia) and Arizona cottontop (Trichachne cal~ornicum) were much more abundant on the protected site. The grazed area supported significantly higher numbers of birds In summer, while densities did not differ in winter. Rodents were significantly more abundant inside the protected area. Species of birds and rodents more common in the grazed area included those typical of more xeric lowland habitats and those preferring open ground for feeding. Species more common on the protected site were those which characterize semidesert or plains grasslands, and which prefer substantial grass or shrub cover. Grazing appeared to favor birds as a class over rodents. Livestock exclosures have potential value in assessing the effects of grazing upon vegetation and wildlife (e.g., Turner et al. 1980). However, most protected sites are too small for meaningful evalution of grazing impacts on birds and mammals. The AppletonWhittel Research Ranch is a large (3,160 ha) National Audubon Society preserve in southeastern Arizona, protected from livestock grazing since 1968. In 1981-82, we compared vegetation, bird, and rodent populations between the exclosure and a moderately grazed but otherwise similar and adjacent grama grass-shrubland site. Study Area and Methods The study area lies at about 1,500 m elevation in Santa Cruz County, Ariz. Temperatures range from a mean January minimum of -I .7O C to a mean June maximum of 32.4’ C. Average annual precipitation in this semidesert grassland is 43 cm, with about half occurring in July and August. The site sampled was a 300-ha mesa extending from the exclosure across a boundary fence onto an adjacent cattle ranch. This was the only topographically uniform area large enough to permit meaningful comparison of bird and rodent populations, where we could be confident that the differences across the fenceline were due entirely to effects of livestock grazing. Slopes were <5% throughout, and soils a uniform gravelly loam of the White House series (Richardson et al. 1979). Livestock density on the grazed pasture just prior to and during data collection was approximately I cow per 10 ha. Grazing levels doubtless were much higher historically, both on and off the exclosure (Bahre 1977), but stocking data are unavailable. Two grazed and two ungrazed bird census plots (50 X 500-m) Authors are at the Appleton-Whittell Research Ranch, P.O. Box 44, Elgi?. Ariz. 8561 I, and Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder 80309. This study was supported by Earthwatch and the Center for Field Research. by the University of Colorado Council on Researchand Cnative Work, and by the Naclo?al AudubonSociety. We thank D. Armstrong, M. Holmgren, B. Johnston, P. San+nl, B. Wcbb,,and 23 Earthwatch volunteers for advice and field assistance. Mr. W’dham Brophy Lmdly allowed access to the Babocomari Ranch. Manuscript received February 16, 1983. JOURNAL OF RANGE MANAGEMENT 37(3), May 1984 were established perpendicular to the boundary fence. These were located to avoid mesa edges and ravines, but to be at the maximum possible lateral distance from one another (about 400 m). Teams of 6-8 observers walked abreast through the plots, counting and identifying individual birds flushed from them. It is unlikely that many birds escaped detection, since inter-observer distance was only 7-8 m, and the habitat was open and low in profile. Plots were sampled no more than once per day, in early mornings of clear and relatively calm days. Each count required about 30 min. Counts were made from late July to early August of 1981 and 1982 (I 1 counts/plot = I1 hours total census time per treatment); winter counts were made in January of 1982 and 1983 (I 6 counts/ plot q I6 hours total census time per treatment). Total numbers of sightings were compared within-season between the grazed and ungrazed areas using the chi-square statistic. Two 600-m lines of Sherman live-traps were located centrally on the mesa and perpendicular to the fenceline, one in grazed and one Table 1. Ground cover, expressed as percent of points on line transects, in grazed vs. ungrazed semidesert grassland in southeastern Arizona (XI q 800 points per treatment). Chi-squere values were calculated using 8ctual numbers of points. Species shown are those comprising 21% cover on at least one site. Category Percent ground cover Grazed Ungrazed Chi-square 55.6 80.4** 36.03 Grasses total Andropogon barbinodis Aristida gkwca A. purpurea A. temipes Bouteloua chondrosioides B. curtipendula B. hirsutum B. gracilis Eragrostis intermedia Hilaria betkngeri Lycurus phleoides Panicum obtusum Trichachne califomicum 1.3 0.3 2.1 2.5 3.1+ 15.6 4.5 20.1 0 2.0* 2.4 0.6 0.1 1.3 0 1.0 3.60 7.42’ 23.21 4.6* 5.07 1.5 4.56 14.0 0.71 3.5 1.00 22.0 0.67 6.1** 49.00 0.4 8.89

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Responses of birds, rodents, and vegetation to livestock exclosure in a semidesert grassland site.

In a 4-yr field experiment, we tested the hypotheses that insectivorous birds (1) controlled densities of herbivorous grasshoppers in an ungrazed semiarid grassland in southeastern Arizona, and (2) functioned as keystone predators, by limiting abundances of grasshoppers that otherwise might change vegetation cover and species composition, and/or by mediating the effects of otherwise competitively superior members of the grass- hopper assemblage. We measured grasshopper densities and vegetation on 32 464-m2 grassland plots for 1 yr, then enclosed 16 of these plots with bird exclosures and continued data collection for 3 yr. Eight of the 16 experimental plots were further modified in the last 2 yr of the study by installing fine-mesh 1 m high barriers designed to retard grasshopper dispersal. Micro- climates of caged plots differed only slightly from open plots. Lizards and rodents increased inside the exclosures, but they were removed and released elsewhere such that their average abundances did not differ among treatments. By the final year of the study, mean annual adult grasshopper density was > 2.2 times higher on plots from which birds were excluded, and where grasshoppers were enclosed by dispersal barriers, than on unmanipulated control plots. Mean nymph density was > 3.0 times higher in the same comparison. Grasshoppers were significantly more abundant in bird enclosures with insect dispersal barriers, indicating that experimental plots were dis- persal sources rather than sinks. Seven of 12 common grasshopper species were more abundant inside the bird exclosures, while none was less abundant. Among the more abundant taxa, those responding most positively were grass feeders: Eritettix simplex, Opeia obsciira, Paropomala Tyomingensis, and Phoetaliotes nebrascensis. We found no evidence that grasshoppers competed with one another under increased densities inside the bird exclosures. Although the amount of insect herbivory was somewhat higher inside the bird exclosures, and was positively correlated with grasshopper density across all 32 plots (r = 0.87), overall vegetation cover and species composition did not differ among treatments by the end of the study. Dactylotum variegatut, an aposematic species apparently immune to avian predation, showed no significant responses to the experiment. Birds clearly limited grasshoppers in this grassland ecosystem, but they failed to qualify as keystone predators, at least in the short term, for two reasons: (1) in their absence, increased grasshopper densities had no appreciable impact on vegetation cover or species composition; and (2) there was no evidence that birds mediated competition among grass- hoppers.

Effects of bird predation on grasshopper densities in an arizona grassland

Species richness and evenness are components of biological diversity that may or may not be correlated with one another and with patterns of species abundance. We compared these attributes among flowering plants, grasshoppers, butterflies, lizards, summer birds, winter birds, and rodents across 48 plots in the grasslands and mesquite-oak savannas of southeastern Arizona. Species richness and evenness were uncorrelated or weakly negatively correlated for each taxonomic group, supporting the conclusion that richness alone is an incomplete measure of diversity. In each case, richness was positively correlated with one or more measures of abundance. By contrast, evenness usually was negatively correlated with the abundance variables, reflecting the fact that plots with high evenness generally were those where all species present were about equally uncommon. Therefore richness, but not evenness, usually was a positive predictor of places of conservation value, if these are defined as places where species of interest are especially abundant. Species diversity was more positively correlated with evenness than with richness among grasshoppers and flowering plants, in contrast to the other taxonomic groups, and the positive correlations between richness and abundance were comparatively weak for grasshoppers and plants as well. Both of these differences can be attributed to the fact that assemblages of plants and grasshoppers were numerically dominated by small subsets of common species (grasses and certain spur-throated grasshoppers) whose abundances differed greatly among plots in ways unrelated to species richness of the groups as a whole.

Jane H. Bock

Papers

Pleistocene rewilding: an optimistic agenda for twenty-first century conservation.

Responses of birds, rodents, and vegetation to livestock exclosure in a semidesert grassland site.

Effects of bird predation on grasshopper densities in an arizona grassland

Relationships between species richness, evenness, and abundance in a southwestern savanna

Responses of Birds and Deer Mice to Prescribed Burning in Ponderosa Pine