Zach F. Jones

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.

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.

THE BOTTERI'S SPARROW AND EXOTIC ARIZONA GRASSLANDS: AN ECOLOGICAL TRAP OR HABITAT REGAINED?

Abstract The Botteris Sparrow (Aimophila botterii) is a bird of tall grasslands that temporarily disappeared from Arizona following heavy livestock grazing in the 1890s. Its return was noted first in sacaton (Sporobolus wrightii), an uncommon native floodplain tallgrass often >2 m in height, and subsequently in stands of exotic lovegrasses (Eragrostis spp.) spreading into adjacent uplands that otherwise supported shorter native grasslands. We examined whether the exotic grasslands provided suitable breeding habitat for Botteris Sparrows, compared to native grasslands. We counted birds for three years on 18 plots, monitored 314 nests on 323 home ranges, banded 583 birds, and measured vegetation on plots and home ranges, and at nests and fledgling locations. Abundance and site fidelity were positively associated with grass height and cover, being greatest in sacaton, intermediate in exotics, and lowest in native upland grasslands. The three habitats did not differ in young fledged per capita. Vegetation cover on Botteris Sparrow home ranges did not differ from the surrounding available habitat, but locations of flightless recently fledged young included taller and more dense vegetation than either nest sites or random locations, especially in sparsely vegetated native grasslands. These results suggest that fledglings required substantial cover to avoid predation while they completed development. Far from being an ecological trap, the exotic lovegrasses apparently are providing essential cover for the Botteris Sparrow in Arizona, perhaps allowing it to regain an abundance similar to what existed regionally prior to overgrazing of the late 19th Century.

Rodent Communities in a Grazed and Ungrazed Arizona Grassland, and a Model of Habitat Relationships Among Rodents in Southwestern Grass/Shrublands

Abstract We live-trapped rodents in 2000–2001 at eight sites on a 3160 ha grassland and mesquite-oak savanna in southeastern Arizona that had been ungrazed since 1968, and on eight paired sites on adjacent cattle ranches. There were 917 captures of 14 species during 5760 trap-nights. Four species of Muridae (Sigmodon fulviventer, Baiomys taylori, Reithrodontomys megalotis and R. fulvescens) were significantly more common on ungrazed plots, while no species was more abundant on grazed plots. However, Heteromyidae as a group (especially Chaetodipus hispidus and Perognathus flavus) comprised a significantly higher proportion of total captures on grazed plots, and heteromyids as a percentage of total captures was positively correlated across all plots with amount of bare ground. One of the eight cross-fence sites also had been trapped in 1981–1983. In the 17 y between trapping events at this site: (1) the grass canopy on both grazed and ungrazed plots had become dominated by taller species, (2) a kangaroo rat (Dipodomys merriami) that had been the second most common species in grazed areas disappeared from both plots, (3) pocket mice increased on the grazed plot and declined on the ungrazed plot and (4) Muridae (excluding Peromyscus) as a percent of all captures increased by greater than 1.5-fold on both plots. Based on these results, and those from other field studies, we propose a model for the composition of rodent communities in grass/shrublands of the Southwest and Intermountain West, based on ground cover. Kangaroo rats (Dipodomys spp.) are abundant in areas with the most bare soil, Muridae (specifically, Sigmodon, Baiomys and Reithrodontomys) dominate areas with the most and tallest ground cover, and pocket mice (Chaetodipus and Perognathus) are common in areas of intermediate cover. In relatively mesic grasslands, livestock grazing and fire drive the rodent community toward one dominated by heteromyids instead of murids. In more arid landscapes, grazing and fire favor kangaroo rats over pocket mice.

CONSERVATION OF GRASSLAND BIRDS IN AN URBANIZING LANDSCAPE: A HISTORICAL PERSPECTIVE

Abstract Landscape alterations resulting from urban expansion are among those factors negatively impacting Great Plains grassland birds. The City and County of Boulder, Colorado, manage one of the largest grassland open-space systems in North America, but it is embedded in an area of rapid urban growth. We compared bird count data from the 1980s and 1990s with checklist information about the Boulder grassland avifauna between 1900 and 1937, with the goal of determining what proportion and which species of the historical grassland avifauna are being sustained on Boulder open space. Of 29 native species present in 1900, 22 (76%) appeared on our counts in the 1990s. Formerly common species that disappeared or declined mostly were birds associated with shortgrass prairie, including Burrowing Owl (Athene cunicularia), Common Nighthawk (Chordeiles minor), Loggerhead Shrike (Lanius ludovicianus), and Lark Bunting (Calamospiza melanocorys). By contrast, species associated with mixed and tallgrass habitats increased or held steady, including Vesper Sparrow (Pooecetes gramineus), Savannah Sparrow (Passerculus sandwichensis), Grasshopper Sparrow (Ammodramus savannarum), Bobolink (Dolichonyx oryzivorus), and Western Meadowlark (Sturnella neglecta). We conclude that municipal open spaces can support populations of many Great Plains grassland birds, but that sustaining species associated with the shortgrass prairie may prove especially challenging. Conservación de Aves de Pastizal en un Paisaje de Crecimiento Urbano: Una Perspectiva Histórica Resumen. La alteración del paisaje causada por la expansión urbana es uno de los factores que impacta negativamente a las aves de los pastizales de los “Great Plains.” La ciudad y el condado de Boulder, Colorado, maneja uno de los sistemas de espacios abiertos de pastizal más grande de Norteamérica, el cual se encuentra rodeado por un área de crecimiento urbano. Comparamos datos de conteo de aves de los 1980s y 1990s con informacíon de la avifauna de Boulder entre 1900 y 1937, con el propósito de determinar qué proporción y cuántas especies de las aves de pastizal registradas históricamente se mantuvieron en los espacios abiertos de Boulder. De 29 especies nativas presentes en 1900, 22 (76%) aparecen en nuestros conteos de los 1990s. Especies anteriormente comunes pero que desaparecieron o disminuyeron en cantidad, fueron aves relacionadas principalmente con praderas de pasto corto, incluyendo Athene cunicularia, Chordeiles minor, Lanius ludovicianus y especialmente Calomospiza melanocorys. Por el contrario, especies asociadas a hábitats mixtos o de pasto alto han aumentado o se han mantenido, incluyendo Pooecetes gramineus, Passerculus sandwichensis, Ammodramus savannarum, Dolichonyx oryzivorus y Sturnella neglecta. Concluimos que los espacios abiertos municipales pueden sostener poblaciones de varias especies de aves de pastizal. Sin embargo, será más difícil mantener aquellas especies que prefieren praderas de pasto corto, porque las áreas continuas y amplias que necesitan, ya no existen.

Grasshopper Abundance in an Arizona Rangeland Undergoing Exurban Development

Abstract Housing developments are replacing ranches in the southwestern United States, with potentially significant but little-studied ecological effects. We counted grasshoppers (Orthoptera: Acrididae) and measured vegetative cover for 2 years in a grassland and mesquite/oak savanna in southeastern Arizona, on 48 transects that were grazed by livestock, embedded in low-density housing developments, or both, or neither. Grasshopper species richness was unrelated to grazing or development, but grasshopper abundance was much higher on exurban transects where homeowners kept livestock than in the other areas. Forb canopy and basal area also were highest in grazed exurban areas, perhaps because exurban grazing was relatively patchy, frequently involved horses, and created disturbances more conducive to forb establishment than did relatively uniform grazing on nearby ranches. Abundance patterns of 3 grasshopper subfamilies were generally consistent with their known habitat preferences. Counts of grass-feeding Gomphocerinae were relatively high in ungrazed and unburned areas, and positively correlated with grass cover. Numbers of forb- and mixed-feeding Melanoplinae were positively correlated with forb cover across all transects, and melanoplines dominated counts on grazed exurban properties. Band-winged grasshoppers (Oedipodinae) prefer areas of sparse vegetation, and their numbers were negatively correlated with height of ground vegetation and positively associated with the presence of livestock, in both exurban and undeveloped landscapes. Overall, our results suggest that heterogeneous landscapes in exurban areas that included small livestock pastures had higher grasshopper densities than either ungrazed areas or large cattle ranches.

Lizard Abundance in an Exurban Southwestern Savanna, and the Possible Importance of Roadrunner Predation

Abstract Ranches are being converted to low density exurban housing developments in the southwestern United States, with potentially significant but little studied impacts on biological diversity. We counted lizards in a grassland and mesquite/oak savanna in southeastern Arizona, along 48 transects evenly divided among landscapes that were grazed by livestock, embedded in housing developments, both or neither. Terrestrial lizards as a group (mostly Aspidoscelis uniparens and Holbrookia maculata) were scarce in developed areas, especially where homeowners kept livestock. Combined counts of lizards (Sceloporus clarkii, Sceloporus undulatus and Urosaurus ornatus) associated with three dimensional substrates did not differ among landscape categories. Roadrunners (Geococcyx californianus), known predators of terrestrial lizards, were more common in developed areas, especially those that were grazed. These results are consistent with the hypothesis that the relative scarcity of terrestrial lizards in developed landscapes was due to increased predation by roadrunners, but predation by other species, especially domestic pets, may also have been important.

ABUNDANCE OF COTTONTAILS (SYLVILAGUS) IN AN EXURBANIZING SOUTHWESTERN SAVANNA

Abstract Ranches are being converted to low-density exurban housing developments in the Southwest, with potentially significant but little studied impacts on biological diversity. We counted cottontail rabbits (Sylvilagus audubonii and S. floridanus) in a grassland and in a mesquite-oak savanna in southeastern Arizona, on 48 plots evenly divided among landscapes that were grazed by livestock, or embedded in housing developments, or both, or neither. Cottontails were more abundant in exurban neighborhoods than in undeveloped areas, independent of grazing. They also were more common on ungrazed than on grazed lands, independent of development. Counts were positively correlated with the number of homes near plots. Counts also were positively correlated with height of ground cover and percent tree canopy, but only among plots in undeveloped landscapes. These results are consistent with the hypothesis that cottontails benefited from exurban development because of increased cover provided by structures and landscaping, especially in areas such as open grasslands with relatively little natural cover.

Effects of Fire Frequency and Intensity on Velvet Mesquite in an Arizona Grassland

Abstract Increases of velvet mesquite (Prosopis velutina Woot.) in southwestern grasslands might have been caused by livestock consumption of fuels that once burned with sufficient frequency and intensity to kill the trees. However, attempts to control mesquite with fire usually have failed. We measured fire damage and 5 years of postfire recovery for 225 mesquite trees > 1 m tall, following a 2002 wildfire that included grasslands differing in fire history, presence vs. 34-year livestock exclusion, and predominance of native vs. exotic grasses. The fire burned 100% of ground cover in ungrazed areas and 65% on grazed lands. Top-kill was 100% for trees in exotic ungrazed grasslands (the areas with highest fuel loads), 79% for trees in ungrazed native grasslands, and 28% for trees in grazed grasslands. Most top-killed trees produced ground sprouts, so that by 2006 the combined foliage volume from ground sprouts and surviving branches was 78% (± 3.2 SE) of preburn foliage volume in grazed areas, 66% (± 3.3) in ungrazed exotic grasslands, and 57% (± 4.0) in ungrazed native grasslands. Fire damage was greater among surviving trees in ungrazed areas that had burned twice (1987 and 2002) than among those that had burned only once since 1968 (in 2002), especially in native grasslands where postfire foliage recovery for twice-burned trees was only 47% (± 6.3) by 2006. Only 1 of 84 trees died in the area burned once, whereas 12 of 66 (18.2%) died in the area burned twice, including several individuals > 3 m tall. These results suggest that repeated fires likely could have prevented the historic spread of velvet mesquite into southwestern grasslands, but probably could be used to control mesquite today only in areas where abundant herbaceous growth provides sufficient fine fuels.

Response of Rodents to Wildfire and Livestock Grazing in an Arizona Desert Grassland

Abstract Livestock grazing and fire influence the composition of desert grassland communities, including their rodent populations. However, there have been few studies of the interactions between grazing and wildfire in arid grasslands of the southwestern United States. We trapped rodents and measured vegetation on grazed versus ungrazed plots before (2001) and following (2002–2007) an intense 15,000 ha wildfire in southeastern Arizona. The fire reduced grass canopy on grazed plots for 2 y and on ungrazed plots for 3 y. Some rodents in the family Cricetidae (genera Baiomys, Reithrodontomys and Sigmodon) were more abundant on ungrazed plots before the fire. Cricetidae as a whole declined following the fire and did not return to preburn levels until the sixth postfire year (2007). Nine of ten cricetid species contributed to this general pattern. By contrast, the abundant species of Heteromyidae (Chaetodipus hispidus, C. baileyi, Perognathus flavus) increased following the fire, especially on ungrazed plots. These results are consistent with a model predicting that fire-caused reductions in grass cover should favor Heteromyidae over Cricetidae. Fires elsewhere in the Southwest have had little impact on rodent populations, but these were smaller and cooler burns with relatively minor effects on vegetation. Future studies of large wildfires of varying intensities would further elucidate the generality of the model.