Vegetation changes after livestock grazing exclusion and shrub control in the southern Chihuahuan Desert

Vegetation cover and production were evaluated after nearly 7 years of livestock grazing exclusion and shrub control in an area with a long history of heavy livestock grazing in the southern Chihuahuan Desert, Mexico. An exclosure was established to prevent livestock grazing. In half of the excluded area, the main shrub, Larrea tridentata , was mechanically controlled. Outside the exclosure, heavy livestock grazing occurred as customary and shrubs were not controlled. Absence of grazing resulted in 50% higher grass cover and 35% higher total biomass. Larrea tridentata cover was twice as high on the grazed area as on the ungrazed area. Vegetation cover was dominated by grasses (42%) in the ungrazed area, whereas in the grazed area, cover was equally divided between grasses (28%) and shrubs (27%). Shrub control did not affect vegetation cover or herbage production. Multivariate analysis confirmed that inside the excluded area, shrub control had little impact on the plant community. The effect of grazing, however, clearly distinguished the community outside the exclosure from that inside the exclosure.     

Rabbit abundance relative to rainfall and plant production in northern Chihuahuan Desert grassland and shrubland habitats

Rabbit abundance patterns in the North American Southwest are poorly known, particularly relative to the potentially important influences of rainfall and plant production. We conducted a 10-year study on the comparative densities of black-tailed jackrabbits ( Lepus californicus ) and desert cottontail rabbits ( Sylvilagus audubonii ) in relation to annual rainfall and plant production in both black grama ( Bouteloua eriopoda ) grassland and creosotebush ( Larrea tridentata ) shrubland habitats in the northern Chihuahuan Desert. We found that both species of rabbits were more abundant in black grama grassland than in creosotebush shrubland and that plant production was positively correlated with rainfall at both habitats. However, rabbit densities were not positively correlated with rainfall or plant production in either habitat. In fact, both species of rabbits exhibited gradual, multiyear changes in abundance rather than the patterns of high annual variation observed in desert rodent communities. Our findings of no significant relationship between plant production and rabbit densities indicate that rabbit abundance in the northern Chihuahuan Desert may not be strongly regulated by short-term, bottom-up effects of annual variation in plant production. Long-term trends indicate that other factors such as disease or predation may be equally or more important. We suggest that future research evaluate both top-down and bottom-up influences to elucidate the temporal dynamics of Chihuahuan Desert rabbit abundance. Se conoce poco sobre los patrones de abundancia de conejos desérticos en el suroeste de Norte America, particularmente en relación con las influencias potencialmente importantes de la precipitación y la producción de plantas. Llevamos a cabo una investigación de diez años sobre la densidad relativa de la liebre de California ( Lepus californicus ) y el conejo cola de algodón ( Sylvilagus audubonii ) en los hábitats tanto de pradera de grama negra ( Bouteloua eriopoda ) como de matorral de gobernadora ( Larrea tridentata ) en el norte del Desierto Chihuahuense, en relación con la precipitación y la producción anuales. Encontramos que ambas especies de conejo fueron más abundantes en la pradera de grama negra que en el matorral de gobernadora, y que la producción de plantas estuvo correlacionada positivamente con la precipitación en ambos hábitats. Sin embargo, las densidades de conejos no estuvieron correlacionadas positivamente con la precipitación o con la producción de plantas en ninguno de los dos hábitats. De hecho, en cuanto a la fluctuación de su abundancia, las dos especies de conejo exhibieron tendencias graduales a lo largo de varios años, en comparación con los patrones de gran variación anual que se han observado en comunidades de roedores desérticos. La falta de una relación significativa entre la producción de plantas y la abundancia de conejos, indica que la abundancia de conejos en el norte del Desierto Chihuahuense no está fuertemente regulada por los efectos ascendentes a corto plazo de la variación anual en la producción de plantas. Los cambios reflejados en las tendencias a largo plazo indican que otros factores como las enfermedades o la depredación pueden ser de igual o mayor importancia, y sugerimos que investigaciones futuras deberán evaluar tanto las influencias ascendentes como las descendentes para entender las dinámicas temporales en la abundancia de conejos del Desierto Chihuahuense.     

Patterns of artificial avian nest predation by introduced rats in a fragmented laurel forest (Tenerife,Canary Islands)

The importance of introduced rats as damaging factors on island biotas is globally recognized. The predation of artificial avian nests by Ship rats (Rattus rattus) in fragmented laurel forests in Tenerife (Canary Islands) was explored. The influence of road edge, patch type, and position (ground versus tree) on nest survival in two forest remnants differing in conservation degree, were assessed using failure‐time analysis. Overall, nest predation was greater in the preserved than in the disturbed remnant. Nest predation also differed among and within patches, variation being greater within the disturbed remnant. The probability of nest failure was higher at the interior than along the road edge in both remnants, but the road edge effect on nest predation was more intense in the disturbed remnant. Predation pressure was higher in patches of mature, closed canopy or dense understorey with stump sprouting. Ground nests were predated at higher rates than tree nests at any location. These results contrast with other nest predation studies in fragmented landscapes where forest edges rather than the interior are more frequently used by predators. Predatory activity by rats seems negatively affected by forest disturbance and road edge effect. Overall, artificial nest predation patterns by rats confirm a potential predation risk for the avifauna of the Canarian laurel forest. This should be considered in implementing conservation management programmes.     

Characteristics of natal and non-natal kit fox dens in the northern Chihuahuan Desert

We studied kit fox den characteristics of the northern McGregor Range of Fort Bliss Military Reservation, New Mexico, during 1994 to 1995. Twenty radio-collared kit foxes used 132 different dens, including 16 natal dens. Kit fox dens were located primarily in creosote-dominated habitat found in relatively flat, well-drained terrain. Natal dens were virtually indistinguishable from non-natal dens; however, natal den entrances were taller than non-natal den entrances. Entrances found at all dens were oriented more frequently toward the northwest and southwest. Kit foxes used more dens during the breeding (January-February) and pup-rearing season (May-July) than during gestation.     

Influence of water size and type on bat captures in the lower Sonoran Desert

We compared bat use by mist-netting at 4 different types of wildlife water developments in southwestern Arizona during summer 2000 and 2001. Scaling our results by netting effort, we caught bats more frequently and observed higher species diversity at tinajas (modified natural rock pools) with larger open-water area compared with ""guzzler"" type water developments that had less open water and more obstacles to bat flight. We caught the fewest bats at guzzlers with buried concrete vault drinkers, which impede bat access and have the smallest areas of open water. Water development designs that minimize evaporative water loss by reducing the amount of open water apparently reduce use by bats in this area.     

Predation of artificial Sage Grouse nests in treated and untreated sagebrush

We measured predation on 120 artificial Sage Grouse ( Centrarcus urophasianus ) nests in montane sagebrush grassland in northern Utah. We examined nests in areas that had been chained and seeded 25 years previously (treated areas) and in areas that were untreated. Predation rates of artificial nests were higher in areas of untreated sagebrush, even though these areas had greater sagebrush cover, taller shrubs, and greater horizontal plant cover. These results differ from those previously hypothesized for treated sagebrush habitat and may reflect a greater abundance of other potential prey species, especially lagomorphs, in untreated areas that attracted greater densities of predators. In addition, over 80% of nests were depredated by mammals, which hunt using olfaction and are less likely than avian predators to be affected by nest cover. We conclude that, after treated sagebrush has recovered to some degree, predation rates of Sage Grouse nests may be lower in treated sagebrush. Consequently, factors other than nest predation (e.g., winter food, thermal cover, insects, perennial forb abundance) may be more important reasons for preserving mature sagebrush stands for Sage Grouse.     

Influence of fire and juniper encroachment on birds in high-elevation sagebrush steppe

We examined relationships between high-elevation sagebrush ( Artemisia spp.) steppe habitats altered by prescribed fire and western juniper ( Juniperus occidentalis ) encroachment on breeding distributions of Brewer's Sparrows ( Spizella breweri ), Vesper Sparrows ( Pooecetes gramineus ), Green-tailed Towhees ( Pipilo chlorurus ), and Sage Thrashers ( Oreoscoptes montanus ) on Steens Mountain in southeastern Oregon. In 2000 we conducted fixed-radius point count surveys at 172 sites encompassing burned and unburned sagebrush habitat and a range of juniper densities. For each bird species we developed habitat models using local variables measured in the field and landscape variables derived from remotely sensed data. Akaike's Information Criterion (AIC c ) was used to select the best-approximating model from a suite of a priori candidate models. Brewer's Sparrows, Sage Thrashers, and Green-tailed Towhees were positively related to increasing local sagebrush cover or percent sagebrush in the landscape, whereas Vesper Sparrows were negatively associated with sagebrush cover and positively related to increases in sagebrush fragmentation at local and landscape scales. Including a measure of juniper encroachment substantially improved models for all species in the analysis. Green-tailed Towhees showed a curvilinear response to the amount of juniper in the landscape. All other species showed a strong negative relationship with juniper. Our results indicate that, although changes in sagebrush habitat associated with fire had a negative influence on sagebrush birds, juniper encroachment due to fire suppression also impacted this high-elevation sagebrush bird community.     

Microhabitat-specific controls on soil respiration and denitrification in the Mojave Desert: the role of harvester ant nests and vegetation

Seed harvesting ants ( Pogonomyrmex rugosus ) concentrate organic matter and nutrients near their nests and create biogeochemical hotspots in desert soil. We examined factors regulating denitrification and soil respiration in a Mojave Desert ecosystem to determine the role harvester ant colonies play in nitrogen loss and carbon mineralization. Organic matter and nutrient storage were significantly greater in colonies than under the dominant vegetation (i.e., Pleuraphis rigida , a bunch grass) and in bare soil, with standing stocks of inorganic nitrogen in colonies nearly 4-fold greater than in the other microhabitats. Soil respiration, measured with laboratory incubations, was below detection limits under ambient soil conditions. Respiration rate in soil from bare patches and under grass was limited by water and labile organic carbon, with secondary nitrate limitation evident only once carbon limitation was alleviated. Soil respiration in ant nest soil was limited by water and labile organic carbon only. Denitrification, measured by the acetylene block technique, was elevated in bare soil and under grass with the addition of nitrate. Ant nest soil also responded to a nitrate addition; however, denitrification rate was greatest with addition of glucose. Ordinarily desert soil has low rates of respiration and denitrification due to dry ambient conditions. However, ant colonies likely function as important sites for nitrogen loss and carbon mineralization following rain storms, especially if storms coincide with seed set and the temporally pulsed input of organic matter into colonies.     

Influence of soil frost on infiltration of shrub coppice dune and dune interspace soils in southeastern Nevada

The influence of soil frost on the infiltration rate of shrub coppice dune and dune interspace soils was evaluated near Crystal Springs, Nevada, using simulated rainfall. The infiltration rate of the coppice dune soil was greater than the dune interspace soil under frozen or unfrozen conditions. Because of different vegetation cover and surface soil characteristics, coppice dune and dune interspace soils responded differently to freezing, thus imposing a spatial and temporal response to infiltration rate. Infiltration rate of soils with porous concrete frost increased as the soils thawed during simulated rainfall, but soils with nonporous concrete frost allowed very little infiltration to occur. Both coppice dune and dune interspace soils that were classified in January as having granular frost had a higher infiltration rate than the same unfrozen soils in March.     

Productivity, food habits, and associated variables of Barn Owls utilizing nest boxes in north central Utah

Productivity and food habits of the Barn Owl ( Tyto alba ) utilizing nest boxes in Juab, Utah, and Salt Lake counties, Utah, During 1979-1984 were examined. Average clutch size was 5.8 eggs for the 6-yr period; mean number fledged was 3.9 young per successful nest. While severe weather during the 1981-82 winter did not result in significant decrease in productivity during the 1982 breeding season, it may have resulted in a significant overproduction of female young. Barn Owls in north central Utah fed almost exclusively on mammalian species, particularly Microtus spp. Difference in clutch size between areas and years may be a response to availability as well as abundance of prey.     

Effects of rotational grazing on rodents and raptors in a coastal grassland

We conducted a 4-year experiment to assess the impacts of rotational cattle grazing on rodents and raptors in a mesic coastal grassland in northwestern California. Live-trapping indicated that rodent abundance declined by 69% on the grazed area and increased by 14% on the ungrazed area. Raptor use of the grazed area declined by 15% and increased on the ungrazed area by 63%. Measures of giving-up density indicated that rodents perceived a 25% higher predation risk on grazed area than on ungrazed area, but raptor hunting surveys indicated that risk of depredation from raptors was 2.5 times lower in the grazed habitat, suggesting that rodents use indirect vegetative cues to assess risk.     

Competition Between Adult and Seedling Shrubs of Ambrosia dumosa in the Mojave Desert, Nevada

Seeds of the perennial shrub Ambrosia dumosa germinated in abundance following 11 days of rain during August 1983 at a study site in the northern Mojave Desert. Seedling establishment, growth, and reproduction were observed in natural vegetation and in an area that had been previously cleared of vegetation. For 5,527 A. dumosa seedlings, percent survival in April 1986 averaged 3% in the undisturbed vegetation and 58% in the denuded area. Seedlings occupying the cleared area had grown to sizes up to 0. 1 m 3 m by October 1984; some produced flowers and fruit in the spring of 1985. Surviving seedlings in the undisturbed vegetation were all smaller than 0.001 m 3 and did not reproduce. These pronounced differences in growth, survival, and reproduction associated with the presence or absence of adult shrubs demonstrated an intense competition that is incompatible with indications of mild competition from nearest-neighbor analyses. I therefore hypothesize that competition for water occurred, not by competition for water in two dimensions but by rapid use of a common resource, as if several people were drinking with straws from a common cup. This temporal mechanism would strongly favor adults over seedlings.     

Density, biomass, and diversity of grasshoppers (Orthoptera: Acrididae) in a California native grassland

A native California perennial grassland was sampled for grasshopper populations. The grassland is managed for the preservation of the native perennial bunchgrass, Nassella pulchra Hitch. Grasshopper density, biomass, diversity, and richness were measured from July 1993 to October 1994. Average density of all grasshoppers was 2.30 hoppers/m 2 (0.66 s ) for 1994 (June through August). Overall forage consumed for 1994 was 140 kg/ha, suggesting that grasshopper populations exist at economically damaging levels. Grasshoppers do not appear in the grasslands until late spring, after annual grasses have set seed. Biomass of grasshoppers peaks in July when adults are predominant. Both grasshopper density and biomass were higher in 1993 than in 1994, and a total of 5 species were found throughout the study. Melanoplus sanguinipes Fabricus dominated the acridid communities and accounted for more than 95% of the individuals.     

Experimental manipulations of fertile islands and nurse plant effects in the Mojave Desert, USA

In a mixed desert shrub community we removed and added shrub canopies to examine above- and belowground influences of 3 species of shrubs on islands of soil fertility and the survival of transplanted Ambrosia dumosa seedlings. Soils sampled under shrubs in the wet season had higher pH, water content, organic matter, and both total and mineralizable nitrogen than soils in adjacent open areas, confirming a widely established pattern in arid lands. However, we also found species differences in soil parameters. Soils under Coleogyne ramosissima had highest pH, soils under A. dumosa had highest water content and nitrogen mineralization rates, and soils under Larrea tridentata had lowest water content. Soils sampled under shrubs in the dry season, 7 months after experimental shrub removal, maintained higher organic matter and total and mineralizable nitrogen content than adjacent open soils, but pH and water were altered by shrub manipulations. Species differences persisted only in soil water levels ( A. dumosa soils were driest). Over a 1-year period, transplanted A. dumosa seedlings had highest survivorship in shrub removal and open treatments and died most rapidly under control shrubs of all 3 species, suggesting that shrubs had a strong negative effect on seedling survival, even in the presence of higher organic matter, nutrients, and (initially) higher water content of fertile islands. Our results suggest that nurse plants and islands of soil fertility have the potential to facilitate growth of other species by nutrient additions, but that the net effect of nurse plants can be negative due to shading and/or root competition.     

Effects of arthropods on root:shoot ratio and biomass production in undisturbed and modified mountain shrub habitats

In 1987 the effects of arthropods on root:shoot ratios and biomass production were examined in southeastern Wyoming mountain shrub habitats that were undisturbed or had undergone conventional shrub management by mowing, burning, or herbicide application during the previous year. In habitats with and without shrub canopy, treatment with insecticides generally increased foliage biomass and consistently decreased root biomass, regardless of the shrub management practice. In open areas the effects of insecticide treatments on the root and shoot biomass were relatively greater in undisturbed plots than in any of the managed plots. With only one exception, the root:shoot ratio increased following application of insecticide.     

Brood parasitism and nest survival of Brown-headed Cowbird hosts at high-elevation riparian sites in the eastern Sierra Nevada, California

Deciduous riparian ecosystems in the western United States provide habitat for a higher density of breeding birds than reported for any other avian habitat type and provide habitat for more breeding bird species than adjacent uplands. On the east slope of the Sierra Nevada, riparian ecosystems make up Molothrus ater )—an obligate brood parasite that forages on bare ground and feedlots but typically commutes to distinct shrubland or woodland habitats for breeding. We examined nest survival, brood parasitism, breeding phenology, and causes of nest failure for birds at North Lake and Rock Creek: 2 high-elevation (>2500 m) riparian breeding habitats adjacent to recreational development and within cowbird commuting distance to additional potential foraging sites. Nest survival tended to be higher for host species at Rock Creek than for those at North Lake, but parasitism rates were not significantly different between plots. Of 21 open-cup nesting species, 12 were parasitized. We found the highest rate of parasitism (92%) for Warbling Vireos ( Vireo gilvus ) at North Lake, and parasitism contributed to lower total nest survival there (14%). For nearly all species, parasitized nests were less successful and produced fewer young than nonparasitized nests. However, predation was the leading cause of complete nest failure across all species and contributed to the lowest total nest survival estimates for Western Wood-Pewees ( Contopus sordidulus, 11%) and Dusky Flycatchers ( Empidonax oberholseri, 15%) at North Lake and for Dark-eyed Juncos ( Junco hyemalis, 15%) at Rock Creek. Nest survival was relatively high for Western Wood-Pewees (41%) at Rock Creek and for Yellow Warblers ( Dendroica petechia, 47%) at North Lake. We noted whether the arrival of pack animals at pack-station corrals contributed to variation in cowbird numbers at corrals or in parasitism rates at the 2 sites. Cowbirds occupied corrals before and after pack-stock arrival, and most host clutches were completed prior to pack-stock arrival at nearby corrals, suggesting that the presence of pack animals did not directly affect cowbird host species.     

The food web of parasitoid wasps and their non-phytophagous fly hosts in birds' nests (Hymenoptera: Chalcidoidea,and Diptera: Cyclorrhapha)

We present a food-web study of the parasitoid wasps and their fly hosts found in the nests of cavity-nesting songbirds. This birds' nest parasitoid web is different from all parasitoid webs studied comprising exclusively non-phytophagous host species. A total of 490 nests were examined for host occurrence, abundance and parasitism rates. Thirty-two species of potential hosts and 10 species of parasitoid wasps were recorded in nests from 25 sites in northern Germany and from two additional sites in south-western Germany. Most of these species were rare and unspecialized. The only very common parasitoid was Nasonia vitripennis (Chalcidoidea: Pteromalidae). The records of 14 regular host species from northern Germany, which are connected to all 10 parasitoid species, were used for further calculation of food-web statistics. The parasitoid communities in birds' nests were compared with other known parasitoid communities. The birds' nests exhibited a lower species richness compared with communities of phytophagous hosts, but showed similarities with communities of necrophagous or coprophagous hosts, or of parasitoid hosts. The parasitism rate was comparable in all host–parasitoid-systems, supporting the idea that the parasitism rate is independent of species richness.     

Specialization in the choice and use of spider silk in the nest of the chaffinch (Fringilla coelebs) (Aves,Fringillidae)

The chaffinch, Fringilla coelebs, is one of a large number of birds from many families that use silk in the construction of their nests. Thirty-eight chaffinch nests collected from around the UK were examined to determine the nature and role of silk in nest construction. A regular survey of web, retreat and cocoon silk availability was made at a study site close to Glasgow, Scotland, over a 12 month period. The only spider web silk found in the nests was of the type produced by cribellate spiders. The majority of silk in nests, however, was spider cocoons, but there was no correlation between the amount of cocoon and web silk used. Nests with more lichen decoration contained more silk, and cocoon silk was particularly associated with the attachment of lichen. Nest construction at the study site took place from late April to mid-May. When nest building began, the availability of suitable web silk had doubled from its winter (lowest) level; however, its abundance continued to rise sharply until the end of May. The possible influence of silk availability on the timing of chaffinch nesting is discussed.     

Influence of substrate size, Cladophora , and caddisfly pupal cases on colonization of macroinvertebrates in Sagehen Creek, California

Colonization of stream macroinvertebrates onto artificial substrates was examined in Sagehen Creek, California, during 1993. Substrate size and presence or absence of pupal cases of the caddisfly Anagapetus (Glossosomatidae) were examined as possible influences on macroinvertebrate colonization. Macroinvertebrates were collected from substrates after 60 d of incubation and identified to the lowest feasible taxonomic unit. Dry weight of the filamentous green alga Cladophora also was determined for each substrate. A general linear model indicated a significant positive relationship between both substrate size and presence of pupal cases on richness ( P Anagapetus cases on Micrasema bactro , Epeorus , and Rhyacophila sp. 1, and of substrate size on Hydropsyche occidentalis and Epeorus . Neither cases nor substrate size had a significant effect on Cladophora density ( P > 0.05). There were significant negative correlations between Cladophora density and both Shannon H′ and J′ indices. There were significant positive relationships between Cladophora density with Baetis , Simulium , and Chironomidae density. Combined main effects of substrate size, Anagapetus pupal cases, and Cladophora have dynamic influences on this macroinvertebrate fauna. This study demonstrates the complex interplay of biotic and abiotic factors that determine macroinvertebrate distribution and abundance in this system.     

A new species of Allorhogas (Hymenoptera: Braconidae: Doryctinae) inducing ovule galls on Miconia chamissois Naudin,a potentially invasive shrub in the Brazilian cerrado

ABSTRACT

Allorhogas Gahan (Hymenoptera: Braconidae) is a mainly Neotropical doryctine wasp genus whose species have been associated with 11 vascular plant families. All species of Allorhogas whose feeding habits are known are confirmed or presumed to be phytophagous, either by being gall inducers on seeds, seeds predators or by inquilinism of other gall former taxa. Here we describe a new species of Allorhogas from Brazil and assess its phylogenetic affinities using one mitochondrial (COI) and one nuclear (28S) marker. Different from other described species of Allorhogas that induce galls on fruits/seeds of Melastomataceae, the new species attacks floral bud ovules of Miconia chamissois Naudin (Melastomataceae), forming ‘fruit-like’ galls. We recovered a clade exclusively composed of species of Allorhogas that attack Melastomataceae species. The new species could potentially be used as a biological control of M. chamissois populations to prevent the imbalance of the plant diversity of cerrado, which is one of the most endangered biomes in Brazil.

http://www.zoobnk.org/urn:lsid:zoobank.org:act:27ADE2CD-6C55-4712-92EC-B0F209FEFD10