Coyote diets in the Chihuahuan Desert, more evidence for optimal foraging

With 7 years data on coyote prey use and availability, we tested three predictions based on optimal foraging: (1) the proportion eaten of the most profitable item is constant regardless of availability, (2) under a constant density of higher ranked prey, there is no relationship between the proportion eaten of low profitability items and their density, (3) the ratio of number consumed to availability for the highest ranking prey item should be negatively related to availability. We tested these predictions with regression analyses. We found no significant linear relationship between lagomorph availability and use. There was no significant linear relationship between percent frequency of occurrence of rodents nor arthropods and their respective availabilities for two levels of availability of lagomorphs. We found a significant negative relationship between number consumed/number available to availability for lagomorphs but not for rodents nor arthropods. The results supported the three predictions and we concluded that coyotes are foraging optimally.     

The summer diet of the Little Owl (Athene noctua) in the desert of north-west China

The summer diet of Little Owls (Athene noctua) in Minqin Desert Experimental Research Station of northwestern China was analysed. A total of 293 individuals representing seven species of rodents, one bird, three reptiles, two families of invertebrates and unidentified invertebrates were identified in 113 pellets and 29 prey remains collected during June and July of 2005. By frequency, invertebrates constituted 77.5% of the diet, mammals 17.8%, reptiles 3.8% and birds 1.0%. In term of biomass consumption, mammals comprised 93.7% of the diet, invertebrates 3.9%, birds 1.9% and reptiles 1.1%. The results suggested that the Little Owls were active at day and night and fed mainly on rodents and beetles related to the desert habitats.     

Do predator and prey foraging activity patterns match? A study of coyotes (Canis latrans), and lagomorphs (Lepus californicus and Sylvilagus audobonii)

Many aspects of an animal’s ecology are associated with activity patterns. One important and controversial one is in the predator/prey relationship. We examined temporal patterns of coyotes (Canis latrans) and lagomorphs (Lepus californicus, Sylvilagus audobonii), their main prey in the Chihuahuan Desert. We test the hypothesis that a predator and prey will have non-random distribution of activity relative to each other; this non-random activity could be a substantial overlap or the lack of synchrony depending on the competing theories. We used GPS collars to track coyotes; we identified areas of activity and determined daily patterns. We installed lagomorph feeding stations and game cameras in coyote activity areas to assess lagomorph patterns. All three species were mainly crepuscular. Black-tail jackrabbits and Audubon’s cottontail rabbits had different hourly patterns of activity (P < 0.001). Activity peaks for jackrabbits occurred between 0400-0700 h, and 1800-2000 h. Cottontail rabbit activity peaks occurred between 0500-0700 h, and 1800-2000 h. The coyote evening activity peak was synchronous with lagomorph activity, but the morning peak occurred from 0700 to 1000. We found partial lack of synchrony in the activity patterns of the predator and its prey.     

Isotopic composition of carbon and oxygen in desert fauna: investigations into the effects of diet, physiology, and seasonality

Carbon and oxygen isotope ratios of bone apatite were measured in 14 endothermic and ectothermic vertebrates native to the Chihuahuan Desert and collected in June and July of 1999 and 2000. The δ⁸O values of most reptiles were very high, up to 44‰ (standard mean ocean water (SMOW), some of the highestδ¹⁸ O values ever measured for an animal. The δ¹⁸O values of rodents and birds were lower (32±5‰ vSMOW), and the earless lizard Holbrookia maculata were the lowest of all species analysed (25‰). Omnivorous grasshopper mice (Onychomys torridus) had lower δ¹⁸O values than granivorous rodents. Results from oxygen analysis likely reflect variation in diet and body water flux differences between endotherms and ectotherms. Carbon isotope analysis revealed a dramatic shift in diet from C₃ plants in 1999 to C₄ plants in 2000 in most rodents and birds. Kangaroo rats and reptiles did not change, having a constantδ¹³ C value indicative of a C₃-based diet in both years. This suggests reliance on winter annual plant seed caches for kangaroo rats, but not other rodents. The carbon isotope data can be explained in terms of seasonal differences within and between years in the timing and intensity of the seasonal rainfall events, and the productivity of summer and winter annual plants. This study illustrates that stable isotope analysis is a powerful method for tracking dietary change and feeding behavior in desert vertebrates.     

Effects of fire and environmental variables on plant structure and composition in grazed salt desert shrublands of the Great Basin (USA)

Fire in shrub-dominated portions of the Great Basin, largely fueled by non-native annuals such as Bromus tectorum, has become an important structuring force altering vegetation composition and soil characteristics. The extent to which fire affects native species in drier portions of the Great Basin, termed salt desert, is poorly documented. We conducted a survey of grazed salt desert habitat in northwestern Nevada 5 years after wildfires burned 650,000 ha, with the goal of investigating community response to fire and factors correlating with post-fire recovery. We found that recruitment of a dominant shrub, Artemisia spinescens, is severely restricted following fire: it occurred in only 2 of the 24 burned sites. The co-dominant shrub, Atriplex confertifolia, occurred in most burned sites although on average its percent cover was one-third lower than adjacent unburned sites. Biotic soil crust cover was four times lower, and non-native species cover 5 times higher, in burned sites compared to unburned. Ordination analyses confirmed differences among plant communities in burned versus unburned sites, with environmental variables soil conductivity, plant litter, soil potassium (K⁺) and pH explaining 38% of the variance in community composition. However, we found no environmental predictors of recovery for native species in burned sites. Future recruitment is likely to be further limited, as fire frequency in the salt desert is expected to increase with invasion by non-native annual grasses and with global climate change.     

Effects of body mass and temperature on standard metabolic rate of the herbivorous desert lizardUromastyx philbyi

The standard metabolic rate ofUromastyx philbyiwas measured at 20–40 °C, using constant pressure manometric respirometers. Standard metabolic rate was mass-dependent. Values for the mass exponent ‘b’ ranged from 0·75 at 20 °C to 0·82 at 40 °C. Standard metabolic rate increased as temperature increased with high Q₁₀values at low temperatures (20–25 °C). Low standard metabolic rates in this species are probably an adaptation to herbivory in hot desert environments.     

Ant communities and livestock grazing in the Great Basin, U.S.A.

The objectives of this study were to determine if metrics for ant species assemblages can be used as indicators of rangeland condition, and to determine the influence of vegetation and ground cover variables, factors often influenced by livestock grazing, on ant communities. The study was conducted in two areas in the Great Basin: a sagebrush-steppe in southeastern Idaho (n=30 sites), and a salt-desert shrub in western Utah (n=27 sites). Sites were selected based on known rangeland condition (i.e. good, fair, poor) associated with livestock grazing. Ant communities differed considerably between the two study areas. Collectively, more ant species occurred at the Idaho sites (30) than at the Utah sites (21), relatively few species (eight) occurred in both areas, species richness was significantly greater at the Idaho sites (mean=12·0 species) than the Utah sites (mean=6·9 species), and Formica spp. were diverse (total of 15 species) at the Idaho sites but rare (one species) at the Utah sites. In Idaho, all species collectively, generalists, and Formica spp. were significantly less abundant on sites in poor condition than that on sites in good or fair condition, whereas in Utah, seed harvesters and Pheidole spp. were significantly more abundant on sites in poor condition than that on sites in good or fair condition. In Idaho, species richness was significantly lower on sites in poor condition. In Idaho, species richness and relative abundances of several ant groups were significantly related to bare patch size and parameters for cover or species richness of several vegetation groups. In contrast to the comparisons involving sites in poor condition, no differences in ant communities in either Idaho or Utah were evident between sites in good and fair condition. Thus, the ant communities responded only to large changes in rangeland condition and to large differences in climatic/edaphic conditions between the two areas. Hence, ant community metrics appear to have limited utility as indicators of rangeland condition in the Great Basin.     

Evolutionary convergence of the highly adapted desert rodent Tympanoctomys barrerae (Octodontidae)

Octodontid rodents have a long evolutionary history in arid landscapes of South America. The red vizcacha rat, Tympanoctomys barrerae , is a monotypic, micro-endemic species that inhabits salt pan-sand dune habitats in west-central Argentina. Its natural history is almost unknown. We present an analysis of the ecology, morphology, behavior, and physiology of the red vizcacha rat and assess the overall degree of convergence of this species with rodents that inhabit similar habitats in different deserts. Our results show that Tympanoctomys barrerae is highly adapted to salt flat basin habitats. Its diet of halophytic vegetation with a high salt content, and physiological and anatomical traits related to salt consumption, are similar to those found in some members of the families Heteromyidae (Dipodomys microps) of North America, and Muridae (Psammomys obesus and Rhombomys opimus) of Africa and Asia. Similarities include feeding behavior, diet composition, kidney morphology, and urine concentration, among other traits. Tympanoctomys barrerae is more similar to these desert rodents than it is to confamilials that do not feed on halophytes.     

Kidney structure and allometry of Argentine desert rodents

Two large groups of rodents inhabit South America: the hystricognaths and the murids. It has been postulated that while the former show a high degree of specialization to desert habitats, the murids are not well adapted to xeric conditions. We studied the renal structure and function of selected desert-dwelling murid and octodontid (hystricognath) rodents from Argentina to evaluate levels of adaptation to aridity. Our results show that the murids Salinomys, Andalgalomys, Calomys and Eligmodontia have the highest renal indices and urine concentration among Argentine desert rodents. The octodontid Tympanoctomys barrerae shows higher renal indices and urine osmolarity than those of its close relativesOctomys mimax and Octodontomys gliroides . We compare the renal traits of the Argentine desert murids with those of other world-desert rodents such as North American heteromyid rodents. The results show that these Argentine murid rodents posses renal adaptations to conditions which are as impressive as those of both octodontids and the classic desert-adapted heteromyids.     

Seed bank dynamics in a Great Basin salt playa

Salt playa are common in the Great Basin area with halophytes as the dominant flora. The relationship between the seed bank flora and existing vegetation was studied. The seed bank flora showed a high degree of similarity with that of perennial and annual halophytic vegetation. Both above and below ground components of the community are dominated by halophyte species like Allenrolfea occidentalis, Holosteum umbellatum, Salicornia rubra and Salicornia utahensis. Seasonal variation in the seed bank was also studied for the 1995–1996 and 1996–1997 growing seasons. The seeds of A. occidentalis persist in the soil of the upper zone in large densities. Salicornia utahensis was much higher in the upper zone throughout the study period. Most species demonstrated the persistent nature of the seed banks with their numbers increasing after seed dispersal and decreasing in subsequent months. The variation illustrates the transient nature of the seed bank. We concluded that A. occidentalis is more dependent on a large seed bank for maintaining its population than the two annual halophytes. The significance of the seed bank and their variation due to environmental conditions for perennial and annual species of the Great Basin salt playa community is discussed.     

Food habits of the Barn Owl (Tyto alba) in a steppe area of Tunisia

The diet of the Barn Owl (Tyto alba) is well known in different habitats but a lack of data persists for arid and subarid environments. We provide preliminary information on food selection of barn owls in Tunisia by the analysis of the composition of pellets. The diet consists largely of rodents and the three-toed Jerboa (Jaculus jaculus) that accounted for more than 50% of total prey biomass. Results suggest that barn owls show a nonrandom tendency toward rodent species especially small and young individuals.     

Feeding ecology of the Andean hog-nosed skunk (Conepatus chinga) in areas under different land use in north-western Patagonia

We studied the feeding ecology of the Andean hog-nosed skunk (Conepatus chinga) in three areas with different land use in Patagonia. Two areas were heavily grazed by sheep or horses and wild exotic herbivores and the third area was lightly grazed by cattle. In each area, we assessed skunk diet, prey abundance and carrion availability. Skunks were generalist feeders, with invertebrates composing the bulk of their diet, but also consumed small vertebrates and carrion. Differences in diet composition were found among areas, mainly because skunks consumed more beetle larvae in the sheep area and more cricetines and spiders in the exotic-herbivore area. All prey but rodents were significantly more abundant in spring and summer than in fall and winter, but skunk diets were similar among seasons. In the sheep area, where the soil was most disturbed, we found reduced abundances of invertebrate prey and rodents. High consumption of beetles during autumn and winter when these were least abundant and in the sheep area indicates that skunks had a preference for beetles and their larvae. We conclude that Andean hog-nosed skunks in Patagonia might be negatively affected by reductions of their main prey due to soil and plant alterations by livestock.     

Timing of reproduction of a cold desert viperid snake from North America,the Great Basin rattlesnake (Crotalus lutosus)

In this study, we examined the reproductive tracts of preserved specimens of the Great Basin rattlesnake (Crotalus lutosus) to investigate the reproductive strategies used by the only species of rattlesnakes widely occurring in a cold desert. The vitellogenic cycle of female rattlesnakes spanned over two consecutive years: females started vitellogenesis in late spring/early summer, and ovulated in the summer of the next year. This contrasts with the female reproductive cycle of other rattlesnake species from warmer regions of North America, which typically initiate vitellogenesis in summer/fall, and ovulate in late spring of the next year. On the other hand, the reproductive cycle of male C. lutosus conformed well to those described for other rattlesnakes; that is, testes were active (i.e., sperm formation) in summer. The sexual segments of the kidneys of males were hypertrophied in summer, suggesting an unimodal summer mating season. Our findings suggests that the relatively short active season (i.e., 4–5 months) experienced by these snakes in the Great Basin of North America has implications for the timing of female reproductive events, such as an early onset of vitellogenesis, a late time of ovulation, and possibly a restricted period for giving birth.     

Water requirements for germination and early seedling establishment in four African savanna woody plant species

The aim of this study was to determine the water requirements for germination and early seedling establishment of four African savanna tree species, namelyAcacia karroo, A. nilotica, A. tortilisandMundulea sericea. The acacias are characteristic of nutrient-rich, andM. sericeaof nutrient-poor savannas. Imbibition times of scarified seeds ofA. karroo(4 h),M. sericea(6 h) andA. tortilis(8 h) were rapid relative toA. nilotica(28 h) and were inversely correlated with seed size. Imbibed water is lost in about 2 h after drying at 25°C for all species exceptA. nilotica(only 70% moisture loss). Seeds ofA. karrooandM. sericeakept at 40°C dried to below their original moisture contents within 2 h. Seeds ofA. niloticaandA. tortilisdried at 40°C lost viability relative to (undried) control seeds, while there was no significant loss of viability forA. karrooandM. sericea. For fully imbibed seeds to germinate in sandy savanna soils, all four species required at least the equivalent of 3 mm rainfall every 2 days under mild greenhouse conditions. However, most ungerminated seeds were still viable despite drying-out from a fully imbibed state. Maintenance of soil at 50% field capacity (FC), or watering to FC every 9th day is the maintenance requirements for 2-week-old seedlings ofA. niloticaandM. sericeato continue growth for a further 5 weeks under greenhouse conditions, althoughA. niloticamostly survived 25% FC with one seedling surviving 12·5% FC. Root penetration was rapid in sandy soils, withA. niloticaandM. sericeaattaining a depth of 40 cm within 15 days. Frequent, but not necessarily high, rainfall appears to be essential for germination and seedling survival over the first 7 weeks.     

Responses of photosynthesis and water relations to rainfall in the desert shrub creosote bush (Larrea tridentata) as influenced by municipal biosolids

Responses of photosynthesis (P_n), stomatal conductance (g_s), pre-dawn leaf water potential (Ψ_lp) and leaf water content (ω_l) of creosote bush to 10 rainfall events in the Chihuahuan Desert were investigated. Infiltration of rainwater was manipulated by applying municipal biosolids. The responses of P_nand water relation parameters to rainfall (>10 mm) were mainly dependent upon drought severity: (1) following a moderate drought, P_n, g_s, Ψ_lpand ω_lrecovered to corresponding values of irrigated plants within 2 days after a 23-mm rainfall; (2) Ψ_lpand g_sresponded to a 15-mm rainfall within 2 days, following a 25-day drought, whereas responses of P_nand ω_lwere delayed for several days; (3) responses of P_n, g_s, Ψ_lpand ω_lto a 14·7-mm rainfall were all delayed for several weeks following a 110-day drought, but the delay was longer in P_n, g_sand ω_lthan in Ψ_lp. Creosote bush responded to small rainfall events (approximately 6 to 8 mm) with an increase in Ψ_lp, but without noticeable changes in g_sand P_n, suggesting a strong stomatal control of water loss even though xylem embolism was reduced. Biosolids applied at high rates (3·4 and 9 kgm⁻²) decreased the soil water by 2 to 4 mm following rainfall events, and this in turn delayed and decreased the responses of P_nand water relation parameters to rainfall.P_nand g_swere linearly related to ω_land exponentially related to Ψ_lp. With the generally coincidental responses of P_nor g_sand ω_lto rainfall, we concluded that the responses of P_nand g_sto rainfall were dependent on leaf rehydration which resulted from restored hydraulic conductance following drought.     

Effects of fire, grazing, and the presence of shrubs on Chihuahuan desert grasslands

Responses of herbaceous and suffrutescent species to fire, grazing, and presence of Prosopis glandulosa were examined in a Chihuahuan desert grassland in south-central New Mexico. Treatments were assigned randomly to eight 12×8 m plots within each of two blocks. Following fires in June 1995, unfenced plots were exposed to livestock grazing over 4 years. Plots were established that either included or excluded P. glandulosa. Perennial grass cover, primarilyBouteloua eriopoda , decreased by 13% in burned plots but increased 5% in unburned areas. Conversely, perennial forb cover was 4% greater after fire. Perennial grass frequency decreased 30% more and perennial forb frequency increased 10% more following burning. Further, increases in evenness after fire resulted in a 225% increase in species diversity. Grazing also resulted in a decrease in perennial grass cover while frequency decreased 22% more in grazed than ungrazed plots. Only frequency and not cover of perennial forbs and annual grasses increased more following grazing. Presence of P. glandulosa had no differential effect on responses of non-shrub species. Fires were conducted during near drought conditions while grazing occurred during years of precipitation equivalent to the long-term average. Precipitation immediately following fire may be critical for recovery of B. eriopoda -dominated desert grasslands; relationships between fire and post-fire precipitation patterns require future investigation.     

Effect of day length during plant development and caryopsis maturation on flowering and germination, in addition to temperature during dry storage and light during wetting, ofSchismus arabicus(Poaceae) in the Negev Desert, Israel

Schismus arabicusis a common annual with an ‘escape’ strategy of caryopsis dispersal and ‘opportunistic’ strategy of germination. It inhabits the Negev and Saharo-Arabian and Irano-Turanian phytogeographical regions. It displays a quantitative long-day response for flowering and a day length response in caryopses germinability. Germination in the dark was much higher than in light. The longer the period of light, from 0·5 h to continuous light, the lower the percentage of germination. The low percentage of germination, which is also regulated by temperature during dry storage, is important for the survival ofS. arabicusunder the unpredictable distribution and small amounts of rain in its extreme desert habitats.     

Plant life-form and germination in a Mexican inter-tropical desert: effects of soil water potential and temperature

We investigated the effects of soil water potential (SWP) and temperature on seed germination of six coexisting species of an inter-tropical desert. These species include three life-forms; the shrubs Cercidium praecox andProsopis laevigata ; the columnar succulents Neobuxbaumia tetetzo and Pachycereus hollianus; and the arborescent semi-succulents Beaucarnea gracilis and Yucca periculosa. In the six species germinability increased and germination time and speed of germination (t⁵⁰) decreased as SWP decreased. The SWP treatments were 0 MPa, −0·12 MPa, −0·2 MPa, −0·41 MPa and −0·66 MPa. There was, however, a SWP threshold below which germination time and t⁵⁰increased and germinability decreased. The shrubs had the highest germinability whereas the columnar succulents had the lowest. The shrubs also had shorter germination time andt⁵⁰ than arborescent semi-succulents whereas seeds of the columnar succulents were indeterminate. In all species except P. laevigata germinability increased and the germination time and t⁵⁰decreased as temperature increased. The temperature treatments were 12°C, 20°C and 26°C. The shrubs had the shortest t⁵⁰and germination time and the highest germinability at all temperatures. Arborescent semi-succulents had the lowest germinability and longest germination time and t⁵⁰at the three temperatures treatments. Our results support the hypothesis that in desert environments different plant life-forms utilise different germination strategies to persist.     

LATE QUATERNARY CHANGES IN PINYON AND JUNIPER DISTRIBUTIONS IN THE WHITE MOUNTAIN REGION OF CALIFORNIA AND NEVADA

Data from packrat middens are used to reconstruct the migration of single-leaf pinyon (Pinus monophylla) and Utah juniper (Juniperus osteosperma) in the White Mountain region of California and Nevada for the last 20,000 years. Today this region is characterized by arid conditions with dry summers. Pinyon woodlands, which are favored by dry summer conditions, are dominant at middle elevations of the White Mountains. The midden record indicates that during the late Pleistocene and early Holocene, juniper was dominant at elevations now occupied by desert shrubs. A late Pleistocene-early Holocene record of desert shrubs was found only in the double rainshadow of the Sierra Nevada and the White Mountains. Pinyon entered the area during the Holocene (ca. 9000 yr B.P.) and subsequently became more important than juniper. During the late Holocene, desert shrubs became established at the lower elevation sites that they now occupy. The late Pleistocene-early Holocene record indicates that there may have been a more zonal upper-level wind flow and associated penetration of Pacific moisture, as indicated by the occurrence of desert shrubs downwind of the north-south-trending mountain ranges. Zonal flow probably became less important during the early Holocene based on the increasing amount of pinyon. The increase in pinyon also may indicate a change from an even seasonal distribution of precipitation to the dry summer conditions presently found in the region. [Key words: biogeography, packrat middens, Great Basin paleoenvironments, Pinus monophylla, Juniperus osteosperma, White Mountains.]     

Dispersal and germination ofProsopis flexuosa(Fabaceae) seeds by desert mammals in Argentina

The ‘algarrobo’Prosopis flexuosais an important food resource in the Monte Desert of Argentina. Native, domestic, and exotic mammals consume the fruit of this legume and disperse the seed through faeces. In the present study we analyse the effect that different dispersal agents (cattle, horse, European wild boar, rodents, gray fox) have on seed damage, viability, and germination. Cattle increase germination at the expense of reduced viability, whereas horses maintain viability but do not contribute to a prompt germination response. Among native mammals, the gray fox maintains seed viability without increasing germination, whereas rodents affect seed viability but enhance germination rates. The European wild boar, however, damages all of the seeds it consumes.