Phylogeography of the ground squirrel subgenus Xerospermophilus and assembly of the Mojave Desert biota

Aim The Mohave ground squirrel (Xerospermophilus mohavensis) is one of a few endemic species of the Mojave Desert of south‐western North America. We describe phylogeographic patterns within this species and its sister taxon (Xerospermophilus tereticaudus) and test hypotheses concerning their biogeographical history using genetic signatures of stable versus expanding populations. We compare these patterns with those of other Mojave species to evaluate the role of vicariance in producing phylogeographic structure during the assembly of the Mojave Desert biota. Location The Mojave Desert and adjacent desert regions of south‐western North America. Methods Complete cytochrome b gene sequences of X. mohavensis (46 individuals representing 11 localities) and X. tereticaudus (38 individuals representing 14 localities) were analysed using Bayesian methods to infer phylogenetic relationships. Genetic signals of stable or expanding populations were examined based on the distribution of recent mutations and pairwise differences, as well as with a coalescent‐based approach. Results The two species are reciprocally monophyletic and may have diverged in response to the late Pliocene–early Pleistocene uplift of the Transverse Ranges and Mojave block. Little phylogeographic structure is evident within X. mohavensis, but there is a signature of northern expansion from a presumably full‐pluvial refugium in the Mojave River basin. Four geographic subgroups are evident within X. tereticaudus, and there is a signature of northern expansion from a presumably full‐pluvial refugium in the Sonoran coastal plains. Roughly congruent phylogeographic patterns are found within five arid‐adapted taxa, indicating a strong element of vicariance during the assembly of the generally transitional Mojave Desert biota. Main conclusions We present a preliminary model for the historical assembly of the Mojave Desert biota that indicates a strong vicariant element producing autochthonous lineages (including X. mohavensis) that diverged during the major geological and climatic events of the last 5 Myr. Phylogeographic partitioning within the Mojave Desert underscores the necessity of immediate conservation measures for this unique and fragile arid ecosystem that is locked between two large metropolitan population centres and is the target of continued adverse environmental impact.     

Effects of low water supplementation and nutrient addition on the aboveground biomass production of annual plants in a Chilean coastal desert site

Summary The effects of low water supplementation and nutrients on the aboveground biomass production of annual plants was tested in the field by mimicking small rainfall events of 5mm per month and by adding fertilizers to experimental quadrats. Field measurements were made during an extremely dry year, so the potential additional effects of rainfall probably had no important effect on plant responses. Biomass of non-native species was higher in irrigated than in non-irrigated quadrats. No significant responses to irrigation treatments were detected in native species. This lack of response may be due to higher thresholds of watering being required for either germination and/or growth. Because of the low water inputs, fertilizer additions did not promote any biomass response in either native or non-native species. Responses of non-native species to low and frequent pulses of water, which is characteristic of this arid system, may be important for the persistence of these species in this environment.This is a contribution of the Program of Arid zone Studies of Universidad de La Serena     

Seasonal diet by a generalist raptor: the case of the variable hawk (Geranoaetus polyosoma) at Atacama Desert,northern Chile

We determined the seasonal diet of the variable hawk, Geranoaetus polyosoma, in a hyperarid and threatened habitat in the Atacama Desert, northern Chile, by analysing the contents of the bird's pellets. We estimated the biomass and number of individuals of each prey species consumed. We compared our results with those of other studies on G. polyosoma in South America. In general, the diet was characterised by extremely low number of prey, low diversity of species consumed and high dietary breadth. In contrast with other biomes of Chile, in our study reptiles formed the dominant food item and accounted for the highest percentage of biomass consumed from autumn to spring, with rodents being a seasonally significant major food item. Arthropods were also consumed, but the biomass ingested was negligible. Statistically significant differences were noted across seasons in the prey items consumed, reflecting a flexible and opportunistic response to the scarcity of prey available. Our findings add to the data present on the basic natural history of G. polyosoma and can aid in its conservation in the Atacama Desert.     

The effects of water deprivation and salt load on water conservation efficiency in two Indian desert gerbils

Summary The effects of water deprivation and intraperitoneal salt loading on urine volume and on various urinary constituents have been examined in two gerbil species of the Rajasthan desert, the Indian desert gerbil (Meriones hurrianae Jerdon) and the Indian gerbil (Tatera indica indica Hardwicke). During summer, hydratedT. indica excreted 0.782 ml urine·100 g^-1·d^-1 which was about 60.5% higher than the volume of urine excreted by hydratedM. hurrianae (0.487 ml·100 g^-1·d^-1). During winter, both species excreted around 1.5 ml urine ·100 g^-1·d^-1. The experimental treatments caused reductions in urine volume inM. hurrianae from 40 to 76% during summer and from 35 to 71% in winter. Similar treatments inT. indica caused reductions in urine volume of 50–82% in summer and 5–60% in winter. The mean increase in urine osmolarity following various salt loading treatments inT. indica ranged from 3800 to 5761 mosmol·l^-1 and from 4034 to 6255 mosmol·l^-1 during summer and winter, respectively. The mean values of urine osmolarity for hydratedT. indica were 2831 and 3189 mosmol·l^-1 during summer and winter, respectively. InM. hurrianae salt loading treatments caused increases of urine osmolarity between 3381 and 5646 mosmol·l^-1 and between 4032 and 5434 mosmol· l^-1, during summer and winter, respectively, over the values recorded for hydrated animals (summer=3292; winter=3294 mosmol·l^-1). A maximum urine osmolarity of around 7000 mosmol·l^-1 was found in both species when subjected to 2% salt-loading treatment. The treatments used in this study increased urinary urea level in bothT. indica (3039–4056 mM) and inM. hurrianae (1900–2180 mM) compared to the level in their respective hydrated controls (T. indica=1628 mM;M. hurrianae=1372 mM). The results indicate thatT. indica may be better adapted to produce more concentrated urine thanM. hurrianae.     

The effect of grazing on granivory patterns in the temperate Monte Desert, Argentina

Structural variations result in a marked heterogeneity of processes in arid environments. Disturbances have noticeable impacts on ecosystems, and a solid way of assessing their effect is to analyze how ecological processes operate. In deserts, granivory has been considered as a keystone process. The objective of our study was to determine the effect of domestic grazing on seed removal rates by birds, ants and rodents as an indirect measure of the process of granivory. This study was carried out in grazed and ungrazed habitats in the central Monte desert ecoregion in Argentina, during February and July 2002. We analyzed three major habitats: mesquite forest, creosotebush shrubland, and sand dunes under two different treatments (grazed, ungrazed). We found significant effects of grazing and non-grazing treatments on total seed removal rates, among granivorous taxa, as well as interactions with season and habitats. Ants and birds were the taxa that differed most under grazing pressure whereas seed removal by rodents showed no significant responses towards grazing. In conclusion, grazing affects plant structure and the assemblage of granivorous taxa, which in turn translates to the granivory process, adding another source of variation to the seed removal patterns reported for the temperate Monte desert.     

Environmentally cued parturition in a desert rattlesnake,Crotalus atrox

Embryonic development in animals is dynamically regulated by physiological, behavioural, and environmental factors (temperature, precipitation, humidity), which in turn influence the timing of birth or hatching. In the present study, we provide evidence that parturition in a large‐bodied North American pitviper, the western diamond‐backed rattlesnake (Crotalus atrox), is environmentally cued. Specifically, we tested the hypothesis of births coinciding with rainfall events during the second‐half of the monsoon season (late July to mid September) using randomization modelling. Twenty‐one adult females surgically implanted with radio‐transmitters were tracked for extended periods from 2001–2010. From 2003 to 2007, the 21 females gave birth to 38 litters, generating sufficient data to test our hypothesis. In all years, births were restricted to a 4‐week period from 5 August to 7 September, which spanned between 6 and 19 days (mean ± SD, 15 ± 5.2 days). Most births (92.1%) occurred in August. Births were significantly associated with rainfall events in 2007, although births in 2003 and 2005 occurred closer to rain events than randomly generated births for respective years. However, when birth events across all 5 years were pooled, the model indicated a significance difference in mean rain‐days versus random rain‐days. Hence, births occurred more closely to rain events than random days. Other variables associated with monsoon events (increases in cloud cover and humidity; changes in barometric pressure) were not measured but constitute potential cues. The present research is the first long‐term, individual‐based radio‐telemetric study of a snake species to investigate environmental cues related to parturition using procedures of randomization modelling. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 866–877.     

Reproductive ecology and inbreeding depression in Opuntia rastrera (Cactaceae) in the Chihuahuan Desert: Why are sexually derived recruitments so rare?

We evaluated the influence of the reproductive ecology on low recruitment of sexually derived progeny observed in Opuntia rastrera Weber in the Southern Chihuahuan Desert, in two vegetation types. The flowers are diurnal, remaining open 9–10 hr. Pollen is released in the morning and at the same time the stigmas become receptive. Nectar is produced all day, but the production rate is higher at noon. The flowers are visited by insects, mainly solitary bees (Diadasia sp. and Lithurge sp.). Floral characteristics and the pollen/ovule ratio suggest that Opuntia rastrera is a facultative xenogamous species. Controlled pollinations indicate that it is not apomictic and pollinators are required to set fruit; it is also self-compatible, but there is strong inbreeding depression for fruit set. However, we were unable to demonstrate differences in reproductive characters between the populations in both vegetation types. The average density of adults per hectare was one order of magnitude higher in the nopalera (Opuntia-dominated scrublands) than in the grassland. However, the average density of plants that originated from seeds was one order of magnitude lower in the nopalera. Opuntia rastrera produces abundant flowers, fruits, and seeds in both vegetation types in natural conditions. The low success in the recruitment of new genets cannot be ascribed to the reproductive ecology.     

Vagabond males and sedentary females: spatial ecology and mating system of the speckled rattlesnake (Crotalus mitchellii)

We used radiotelemetric data and behavioural observations to characterize seasonal (mating versus post‐mating seasons) and sexual variation in movement patterns, as well as to examine some of the ecological factors contributing to the evolution of the mating system in a venomous predator from the Mojave Desert of North America, the speckled rattlesnake, Crotalus mitchellii. Mating occurs in spring from late April to early June, shortly after emergence from hibernation, when snakes are predictably aggregated around the dens. Males and females travelled further per unit time in the mating season compared to the post‐mating season. Males also travelled longer distances per unit time than females in the mating and post‐mating seasons, and males with larger home ranges during the mating season had more potential mating partners. The results obtained suggest that males actively locate females during the mating season, and that the drastic increase in distance travelled by males during the mating season may be caused by strong male–male competition for access to females, probably because of the limited availability of sexually receptive females. Furthermore, males fight for access to females, and males of larger size are more likely to acquire females. Therefore, sexual selection apparently acts on two different male phenotypic traits: investment in mate‐searching activities and male body size. The present study demonstrates that combining quantitative spatial analyses and behavioural observations in an explicit temporal context can significantly advance our understanding of the ecology and evolution of organismal mating systems. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 681–695.     

NOTES ON THE AVAILABILITY AND CHEMICAL COMPOSITION OF WATER FROM THE GRAVEL PLAINS OF THE NAMIB-NAUKLUFT PARK

SUMMARY

Data are presented on the occurrence of a surprising number of permanent water sources on the gravel plains of the Namib-Naukluft Park, South West Africa. As a consequence, no game animal is further than ca 25 km away from the nearest watering-place. Within a broad range of ionic contents, the surface waters of the desert display a marked similarity in their chemical composition, being mainly dominated by Na⁺>Ca⁺⁺>Mg⁺⁺: Cl^?>SO₄ ^?>HCO₃ ^??CO_3- ^?. Evaporation, precipitation and rock dominance seem to be the main controlling mechanisms involved in determining the chemical composition of the waters.