Carl E. Bock

Avian habitat evaluation: should counting birds count?

There are times when birds reproduce at higher rates in places where they are less abundant, limiting the generally accepted value of bird counts as environmental indicators. But how often, and under what circumstances, does this happen? In 109 published cases involving 67 species across North America and Europe, higher density sites displayed greater recruitment per capita and per unit of land area in 72% and 85% of cases, respectively. The frequency of negative relationships between abundance and reproductive success did not differ between different kinds of birds or habitats. However, density was negatively related to reproductive success more often in areas of human disturbance than in relatively natural areas. Although further study is needed to confirm the generality of this pattern, especially in areas such as the tropics, results suggest that birds can fail to recognize ecological traps or opportunities in landscapes that differ from those in which they evolved.

Pleistocene Rewilding: An Optimistic Agenda for Twenty‐First Century Conservation

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 with elephants and Holarctic lions. Our exemplar taxa would contribute biological, economic, and cultural benefits to North America. Owners of large tracts of private land in the central and western United States could be the first to implement this restoration. Risks of Pleistocene rewilding include the possibility of altered disease ecology and associated human health implications, as well as unexpected ecological and sociopolitical consequences of reintroductions. Establishment of programs to monitor suites of species interactions and their consequences for biodiversity and ecosystem health will be a significant challenge. Secure fencing would be a major economic cost, and social challenges will include acceptance of predation as an overriding natural process and the incorporation of pre‐Columbian ecological frameworks into conservation strategies.

RESPONSES OF BIRDS, RODENTS, AND VEGETATION TO LIVESTOCK EXCLOSURE IN A SEMIDESERT GRASSLAND SITE

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

Effects of Bird Predation on Grasshopper Densities in an Arizona Grassland

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.

Synchronous Eruptions of Boreal Seed-Eating Birds

Analysis of Audubon Society Christmas-count data (1962-1971) revealed a generally synchronous pattern of winter eruptions between eight species of seed-eating birds whose winter ranges normally include boreal forests. Species and populations occupying montane conifer forests in the West did not fit this pattern as well. Literature analysis revealed a circumboreally synchronized pattern of seed crop fluctuations in certain high-latitude tree species and an apparently resulting pattern of southward eruptions of birds dependent upon these foods. Between 1948 and 1971 eruptions were relatively large in 1949, 1955, 1957, 1959, 1961, 1963, 1965, 1968, 1969, and 1971.

Distribution-Abundance Relationships of Some Arizona Landbirds: A Matter of Scale?

Distribution and local abundance of 62 landbird species were measured in winter across an elevational gradient in the Huachuca Mountains of southeastern Arizona. The number of 35-m radius plots and the number of habitats occupied by the species were positively correlated with their average abundances within occupied plots and habitats. Common species were no more conspicuous than rare ones, as measured by detectability on variable distance point counts. The same species that were most abundant locally also were most abundant on Christmas bird counts across Arizona and throughout the western United States. The positive correlation between distribution and abundance of winter landbirds appears to be neither an artifact of conspicuousness, nor a consequence of the geographic scale of comparison. Rather, it seems to be an intrinsic property of the species themselves, and one that has important ecological and evolutionary implications. An individualistic approach to avian ecology is indicated, emphasizing comparisons of rare and common species.

Abundance of diurnal raptors on open space grasslands in an urbanized landscape

We conducted point counts of diurnal raptors on Boulder, Colorado, grasslands for three winters and summers, and compared results to landscape features of the count areas. Four wintering species were scarce on plots that included significant amounts of urban habitat, with a critical landscape threshold at about 5-7% urbanization: Bald Eagle (Haliaeetus leucocephalus), Ferruginous Hawk (Buteo regalis), Rough-legged Hawk (B. lagopus), and Prairie Falcon (Falco mexicanus). Counts of the first three species also were positively correlated with proximity of the count plots to the nearest colony of black-tailed prairie dogs (Cynomys ludovicianus). Two breeding species, the Red-tailed Hawk (B. jamaicensis) and Swainsons Hawk (B. swainsoni), were more abundant on plots dominated by lowland hayfields and tallgrass prairies, as opposed to upland mixed and shortgrass prairies. They, along with the ubiquitous American Kestrel (Falco sparverius), were not sensitive to the amounts of urbanization (up to 30%) that occurred in the landscapes sampled. Results of this study suggest that urban open space grasslands can support sizable populations of most diurnal raptors, as long as prey populations persist, but that some species are highly sensitive to landscape urbanization.

Social plasticity in the acorn woodpecker.

Acorn woodpeckers (Melanerpes formicivorus) in southeastern Arizona exhibited two different types of social organization: one of highly cooperative and resident groups and another of birds that migrated and formed only temporary male-female pairs during reproduction. The occurrence of both patterns in the same population indicates a high degree of social flexibility in this species.

Bird species distribution patterns in riparian habitats in Southeastern Arizona

Bird species densities were determined for summer and winter on 132 study plots grouped into 25 riparian habitats in or near the Huachuca Mountains of southeastern Arizona. The habitats were defined based on the dominant riparian tree species, the size of the riparian stand, and the type of adjacent upland vegetation. Vegetation characteristics and physical environmental data were collected at each plot. The type of dominant riparian tree species influenced bird species richness and total density during the breeding season. Cottonwood habitats had the greatest richness, and both cottonwood and sycamore habitats had high densities. Upland vegetation was an important factor related to winter species richness and abundance, with plots in open grassland areas having greater richness and density. Riparian stand size was a relatively poor predictor of avian density or richness in either season. Groups of bird species that shared similar density distributions in the summer were associated with specific riparian habitats. The winter pattern of species groups was not as clear, and groups could not be assigned to riparian habitats, but they were related to either wooded or open upland vegetation. Riparian habitats were also clustered based on similar densities of birds. In summer, high-elevation habitats were distinct from low-elevation and foothill habitats. In winter, riparian habitats separated into categories of wooded vs. open adjacent vegetation.

RELATIONSHIPS BETWEEN SPECIES RICHNESS, EVENNESS, AND ABUNDANCE IN A SOUTHWESTERN SAVANNA

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.