Dipodomys Spectabilis Research Articles

Distribution and prevalence of Sin Nombre hantavirus in rodent species in eastern New Mexico.

Orthohantaviruses are diverse zoonotic RNA viruses. Small mammals, such as mice and rats are common chronic, asymptomatic hosts that transmit the virus through their feces and urine. In North America, hantavirus infection primarily causes hantavirus cardiopulmonary syndrome (HCPS), which has a mortality rate of nearly 36%. In the United States of America, New Mexico (NM) is leading the nation in the number of HCPS-reported cases (N = 129). However, no reported cases of HCPS have occurred within eastern NM. In this study, we assessed the prevalence of Sin Nombre virus (SNV) in rodent assemblages across eastern NM, using RT-qPCR. We screened for potential rodent hosts in the region, as well as identified areas that may pose significant infection risk to humans. We captured and collected blood and lung tissues from 738 rodents belonging to 23 species. 167 individuals from 16 different species were positive for SNV RNA by RT-qPCR, including 6 species unreported in the literature: Onychomys leucogaster (Northern grasshopper mouse), Dipodomys merriami (Merriam's kangaroo rat), Dipodomys ordii (Ord's kangaroo rat), Dipodomys spectabilis (Banner-tailed kangaroo rat), Perognathus flavus (Silky pocket mouse), and Chaetodipus hispidus (Hispid pocket mouse). The infection rates did not differ between sexes or rodent families (i.e., Cricetidae vs. Heteromyidae). Generalized linear model showed that disturbed habitat types positively influenced the prevalence of SNV at sites of survey. Overall, the results of this study indicate that many rodent species in east New Mexico have the potential to maintain SNV in the environment, but further research is needed to assess species specific infectivity mechanisms and potential risk to humans.

Remotely sensed environmental measurements detect decoupled processes driving population dynamics at contrasting scales.

The increasing availability of satellite imagery has supported a rapid expansion in forward-looking studies seeking to track and predict how climate change will influence wild population dynamics. However, these data can also be used in retrospect to provide additional context for historical data in the absence of contemporaneous environmental measurements. We used 167 Landsat-5 Thematic Mapper (TM) images spanning 13 years to identify environmental drivers of fitness and population size in a well-characterized population of banner-tailed kangaroo rats (Dipodomys spectabilis) in the southwestern United States. We found evidence of two decoupled processes that may be driving population dynamics in opposing directions over distinct time frames. Specifically, increasing mean surface temperature corresponded to increased individual fitness, where fitness is defined as the number of offspring produced by a single individual. This result contrasts with our findings for population size, where increasing surface temperature led to decreased numbers of active mounds. These relationships between surface temperature and (i) individual fitness and (ii) population size would not have been identified in the absence of remotely sensed data, indicating that such information can be used to test existing hypotheses and generate new ecological predictions regarding fitness at multiple spatial scales and degrees of sampling effort. To our knowledge, this study is the first to directly link remotely sensed environmental data to individual fitness in a nearly exhaustively sampled population, opening a new avenue for incorporating remote sensing data into eco-evolutionary studies.

High-Quality Reference Genome for an Arid-Adapted Mammal, the Banner-Tailed Kangaroo Rat (Dipodomys spectabilis)

Kangaroo rats in the genus Dipodomys are found in a variety of habitat types in western North America, including deserts, arid and semiarid grasslands, and scrublands. Many Dipodomys species are experiencing strong population declines due to increasing habitat fragmentation, with two species listed as federally endangered in the United States. The precarious state of many Dipodomys populations, including those occupying extreme environments, make species of this genus valuable subjects for studying the impacts of habitat degradation and fragmentation on population genomic patterns and for characterizing the genomic bases of adaptation to harsh conditions. To facilitate exploration of such questions, we assembled and annotated a reference genome for the banner-tailed kangaroo rat (Dipodomys spectabilis) using PacBio HiFi sequencing reads, providing a more contiguous genomic resource than two previously assembled Dipodomys genomes. Using the HiFi data for D. spectabilis and publicly available sequencing data for two other Dipodomys species (Dipodomys ordii and Dipodomys stephensi), we demonstrate the utility of this new assembly for studies of congeners by conducting inference of historic effective population sizes (Ne) and linking these patterns to the species’ current extinction risk statuses. The genome assembly presented here will serve as a valuable resource for population and conservation genomic studies of Dipodomys species, comparative genomic research within mammals and rodents, and investigations into genomic adaptation to extreme environments and changing landscapes.

Heterothermy as a mechanism to offset energetic costs of environmental and homeostatic perturbations

Environmental and biotic pressures impose homeostatic costs on all organisms. The energetic costs of maintaining high body temperatures (Tb) render endotherms sensitive to pressures that increase foraging costs. In response, some mammals become more heterothermic to conserve energy. We measured Tb in banner-tailed kangaroo rats (Dipodomys spectabilis) to test and disentangle the effects of air temperature and moonlight (a proxy for predation risk) on thermoregulatory homeostasis. We further perturbed homeostasis in some animals with chronic corticosterone (CORT) via silastic implants. Heterothermy increased across summer, consistent with the predicted effect of lunar illumination (and predation), and in the direction opposite to the predicted effect of environmental temperatures. The effect of lunar illumination was also evident within nights as animals maintained low Tb when the moon was above the horizon. The pattern was accentuated in CORT-treated animals, suggesting they adopted an even further heightened risk-avoidance strategy that might impose reduced foraging and energy intake. Still, CORT-treatment did not affect body condition over the entire study, indicating kangaroo rats offset decreases in energy intake through energy savings associated with heterothermy. Environmental conditions receive the most attention in studies of thermoregulatory homeostasis, but we demonstrated here that biotic factors can be more important and should be considered in future studies.

Shrub encroachment creates a dynamic landscape of fear for desert lagomorphs via multiple pathways

AbstractShrub encroachment is transforming arid and semiarid grasslands worldwide. Such transitions should influence predator–prey interactions because vegetation cover often affects risk perception by prey and contributes to their landscape of fear. We examined how the landscape of fear of two desert lagomorphs (black‐tailed jackrabbit,Lepus californicus; desert cottontail,Sylvilagus audubonii) changes across grassland‐to‐shrubland gradients at Jornada Basin Long Term Ecological Research site in the Chihuahuan Desert of southern New Mexico. We test whether shrub encroachment shapes risk differently for these two lagomorphs because of differences in body size and predator escape tactics. We also examine whether an ecosystem engineer of grasslands (banner‐tailed kangaroo rat,Dipodomys spectabilis) mediates risk perception through the creation of escape refuge and whether trade‐offs exist between shrub encroachment and the local reduction of banner‐tailed kangaroo rats caused by shrub expansion. We measured perceived predation risk with flight initiation distances (FIDs) and then used structural equation modeling to tease apart the hypothesized direct and indirect pathways for how shrub encroachment could affect perceived risk. A total negative effect of shrub cover on FID was supported for jackrabbits and cottontails, suggesting both species perceive shrubbier habitat as safer. Increases in fine‐scale concealment also reduced risk for cottontails, but not jackrabbits, likely because cottontails rely on crypsis to avoid predator detection whereas jackrabbits rely on speed and agility to outrun predators. Perceived risk was reduced when individuals were near kangaroo rat mounds only for cottontails because the smaller species can use banner‐tailed kangaroo rat mounds as refuge. Shrub encroachment greatly reduced the availability of mounds. Thus, a trade‐off exists for cottontails in which shrub encroachment directly reduced perceived risk, but indirectly increased perceived risk through the local extirpation of an ecosystem engineer. Our work illustrates how the expansion of shrub encroachment can create a dynamic landscape of fear for populations of prey species involving direct and indirect pathways contingent on prey body size, escape tactics, and activities of an ecosystem engineer.

Rodents' responses to manipulated plant litter and seed densities: implications for restoration.

Rodent populations in arid grasslands do not always track seed production, possibly due to high levels of plant litter. When natural disturbances are suppressed, litter accumulates becoming physically complex, causing rodents to harvest fewer seeds per equivalent time foraging. It also alters security from predation. Restoring natural disturbances may be an important element in conserving rodent communities. The aim of this study was to assess the influence of two levels of plant litter cover and seed densities on nocturnal rodent population characteristics in a semiarid grassland. Specifically, I hypothesized that kangaroo rats, pocket mice, grasshopper mice, and total rodents would be higher in the sparse plant litter treatment than dense litter, whereas deer mice would be lower in sparse plots. I further hypothesized that kangaroo rats and deer mice would be higher in the seed augmented treatment compared to the unseeded treatment. A prescribed fire removed litter in four of eight plots prior to sowing native seeds 1 year postfire into two burned and two unburned plots. Rodents were live-trapped during spring and fall 1 year. Sparse litter treatment had higher total rodent abundance, biomass, and frequency of offspring compared to dense plots indicating use of stored seeds. Banner-tailed kangaroo rats had higher abundance, implying reduced predation risk. Pocket mice body mass was greater in dense plots. After winter, seeded plots had higher kangaroo rat body mass and grasshopper mice abundance than unseeded, reflecting the use of stored seeds. These short term results demonstrate litter’s physical complexity may be equivalent to seed pulses on the responses of nocturnal rodents. Managers might positively influence grassland rodents by providing a mosaic of varying levels of plant litter.

Ecological engineers’ nests benefit plant recovery following fire in a semiarid grassland, New Mexico, USA

AbstractQuestionsAnts and rodents are recognized as ecosystem engineers worldwide creating soil‐modified nests and adjacent edges with superior resources. The influence of Dipodomys spectabilis (rat) and Pogonomyrmex rugosus (ant) on nests and edges would be similar in burned and unburned habitat on functional groups’ cover, density, number of inflorescences and species number. We hypothesize that annual and perennial dicots would recover faster on rat patches than on burned grassland, dicots and grasses would recover faster on ant patches than on burned grassland, and annual dicots would recover faster on rat than on ant patches, whereas grasses would recover faster on ant than on rat patches.LocationSemiarid grassland, Sevilleta National Wildlife Refuge, New Mexico, USA.MethodsWe analyzed annual and perennial dicots, grasses and subshrubs in burned and unburned plots at 80 nests during cool‐ and warm‐seasons two years post‐fire. Data were collected at nest top (mound), edge, and 20 m (control) from the edge.ResultsCover, density and species richness of different functional groups were low on animal mounds in unburned and burned plots. Ant edges had greater perennial dicot cover, grass cover and richness while rat edges had lower grass density and richness than control sites at unburned and burned plots. Perennial dicot density and richness recovered faster at rat mounds and edges than control. Ant edges yielded faster recovery of grass inflorescences than control. Dicots recovered faster on rat than ant mounds whereas grasses grew faster on ant edges.ConclusionsRats create nests enhancing density of recovering dicots and ant edges facilitate reproduction of recovering grasses. Equal regeneration between animal nests and grassland occurred commonly when fire reduced grass competition. Dysochory, granivory and fire intensity may influence recovery. Nests may be sources of reestablishment and seed dispersal. Recovery of grassland may be amplified as plants infill from engineered nests.

Inbreeding load and inbreeding depression estimated from lifetime reproductive success in a small, dispersal-limited population.

The fitness consequences of inbreeding and the individual behaviors that prevent its detrimental effects can be challenging to document in wild populations. Here, we use field and molecular data from a 17-year study of banner-tailed kangaroo rats (Dipodomys spectabilis) to quantify the relationship between inbreeding, mate kinship, and lifetime reproductive success. Using a pedigree that was reconstructed using genetic and field data within a Bayesian framework (median probability of parental assignment = 0.92, mean pedigree depth = 6 generations), we estimated both inbreeding coefficients and kinship between individuals that produced offspring (mean inbreeding coefficient = 0.07, mean mate kinship = 0.08). We also used the pedigree, in combination with census data, to generate a series of fitness estimates, ranging from survival to reproductive maturity to lifetime reproductive success. We found that the population's inbreeding load was low to moderate (0.98-4.66 haploid lethal equivalents) and increased with the time frame over which fitness was estimated (lowest for survival to maturity, highest for adult-to-adult reproductive success). Fitness decreased with increasing inbreeding coefficients. For example, lifetime reproductive success was reduced by 24% for individuals with inbreeding coefficients greater than twice the population mean. Within full sibling pairs, the sibling with less-related mates produced an average of 30% more offspring over its lifetime. These data further illustrate that inbreeding can have a negative effect on lifetime reproductive success.

Vertical leaping mechanics of the Lesser Egyptian Jerboa reveal specialization for maneuverability rather than elastic energy storage

BackgroundNumerous historical descriptions of the Lesser Egyptian jerboa, Jaculus jaculus, a small bipedal mammal with elongate hindlimbs, make special note of their extraordinary leaping ability. We observed jerboa locomotion in a laboratory setting and performed inverse dynamics analysis to understand how this small rodent generates such impressive leaps. We combined kinematic data from video, kinetic data from a force platform, and morphometric data from dissections to calculate the relative contributions of each hindlimb muscle and tendon to the total movement.ResultsJerboas leapt in excess of 10 times their hip height. At the maximum recorded leap height (not the maximum observed leap height), peak moments for metatarso-phalangeal, ankle, knee, and hip joints were 13.1, 58.4, 65.1, and 66.9 Nmm, respectively. Muscles acting at the ankle joint contributed the most work (mean 231.6 mJ / kg Body Mass) to produce the energy of vertical leaping, while muscles acting at the metatarso-phalangeal joint produced the most stress (peak 317.1 kPa). The plantaris, digital flexors, and gastrocnemius tendons encountered peak stresses of 25.6, 19.1, and 6.0 MPa, respectively, transmitting the forces of their corresponding muscles (peak force 3.3, 2.0, and 3.8 N, respectively). Notably, we found that the mean elastic energy recovered in the primary tendons of both hindlimbs comprised on average only 4.4% of the energy of the associated leap.ConclusionsThe limited use of tendon elastic energy storage in the jerboa parallels the morphologically similar heteromyid kangaroo rat, Dipodomys spectabilis. When compared to larger saltatory kangaroos and wallabies that sustain hopping over longer periods of time, these small saltatory rodents store and recover less elastic strain energy in their tendons. The large contribution of muscle work, rather than elastic strain energy, to the vertical leap suggests that the fitness benefit of rapid acceleration for predator avoidance dominated over the need to enhance locomotor economy in the evolutionary history of jerboas.

Rapid genetic restoration of a keystone species exhibiting delayed demographic response.

Genetic founder effects are often expected when animals colonize restored habitat in fragmented landscapes, but empirical data on genetic responses to restoration are limited. We examined the genetic response of banner-tailed kangaroo rats (Dipodomys spectabilis) to landscape-scale grassland restoration in the Chihuahuan Desert of New Mexico, USA. Dipodomys spectabilis is a grassland specialist and keystone species. At sites treated with herbicide to remove shrubs, colonization by D. spectabilis is slow and populations persist at low density for ≥10 years (≥6 generations). Persistence at low density and low gene flow may cause strong founder effects. We compared genetic structure of D. spectabilis populations between treated sites and remnant grasslands, and we examined how the genetic response to restoration depended on treatment age, area, and connectivity to source populations. Allelic richness and heterozygosity were similar between treated sites and remnant grasslands. Allelic richness at treated sites was greatest early in the restoration trajectory, and genetic divergence did not differ between recently colonized and established populations. These results indicated that founder effects during colonization of treated sites were weak or absent. Moreover, our results suggested founder effects were not mitigated by treatment area or connectivity. Dispersal is negatively density-dependent in D. spectabilis, and we hypothesize that high gene flow may occur early in the restoration trajectory when density is low. Our study shows genetic diversity can be recovered more rapidly than demographic components of populations after habitat restoration and that founder effects are not inevitable for animals colonizing restored habitat in fragmented landscapes.

Comparative analysis of active retrotransposons in the transcriptomes of three species of heteromyid rodents

Virtually every eukaryotic genome is replete with transposable elements (TEs). TE activity, or lack thereof, is of considerable evolutionary interest as TE insertions actively drive genome evolution by altering gene expression and/or function through different mechanisms. Herein, we take a comparative approach to better understand the variation in TE transcriptional activity. Our goals were to identify transcriptionally active TEs and to evaluate their relative expression levels in an effort to identify key determinants of TE activity. We do so in three related non-model rodent species (Dipodomys spectabilis, Chaetodipus baileyi, and Heteromys desmarestianus). We used these species (a) because rodents have long been productive models for the study of TEs; (b) because the known and dated relationships among these species permit strong phylogenetic inference (e.g., ancestral character states); and (c) because we have previously characterized key genes that underlie evolutionary adaptations in these species (e.g., osmoregulation). We used RNA-seq to characterize the transcriptomes of two different tissues – kidney and spleen – in each of these three species. Our data revealed a diversity of retrotransposons that were actively transcribed in these rodents (including LINEs, retroviruses/ERVs, and SINEs). We also identified tissue-specific differences in retrotransposon-activity in about half of the retrotransposons detected. We interpret these differences in TE activity in light of overall gene expression, with the transcribed SINEs enriched in differentially expressed sequences in all three species. These transcribed TEs may simply represent transpositionally active TEs. Alternatively, they may be TEs that have been co-opted to represent functionally important components of the host transcriptome (i.e., exaptations). Overall, our results contribute to a growing appreciation for the evolutionary interplay between TEs and their hosts.

Effects of grassland restoration efforts on mound-building ants in the Chihuahuan Desert

Shrub encroachment is a serious problem in arid environments worldwide because of potential reductions in ecosystem services and negative effects on biodiversity. In southwestern USA, Chihuahuan Desert grasslands have experienced long-term encroachment by shrubs including creosotebush (Larrea tridentata). Land managers have attempted an ambitious intervention to control shrubs by spraying herbicides over extensive areas to provide grassland habitat for wildlife species of conservation concern. To provide a broader assessment of how these restoration practices affect biodiversity, we evaluated responses by four mound-building ant species (Pogonomyrmex rugosus, Aphaenogaster cockerelli, Myrmecocystus depilis, and Myrmecocystus mexicanus). We compared colony densities between 14 pairs of treated areas (herbicide applied 10–30 years before sampling) and untreated areas (spatially matched and dominated by creosotebush). P. rugosus and A. cockerelli responded positively to restoration treatments likely due to an increased abundance of seeds associated with increased grass cover. Variation in P. rugosus densities among different-aged treatments suggests a substantial time lag in response that could reflect temporal changes in habitat quality or facilitation by a keystone rodent, Dipodomys spectabilis. Colony densities of the scavenging ant M. mexicanus were reduced on treated areas, and M. depilis exhibited a similar trend, likely reflecting a reduction of liquid food resources associated with shrubs. Our results demonstrate that ongoing efforts to restore Chihuahuan Desert grasslands are having both positive and negative effects on non-target taxa such as ants and support the need for a landscape mosaic approach to restoration.

Multiple socioecological factors influence timing of natal dispersal in kangaroo rats

Natal dispersal is an important event in the life history of many species. Timing of natal dispersal can influence survivorship and subsequent reproductive success. A variety of individual proximal factors determine if and when offspring disperse from the natal territory by influencing the costs of dispersing and the benefits of delaying dispersal. I examined the influence of multiple factors on dispersal age in the banner-tailed kangaroo rat (Dipodomys spectabilis), a solitary species lacking extreme sex-biased dispersal. I used an information theoretic approach to compare Cox proportional hazards regression models of dispersal age for 121 offspring over a 3-year period consisting of low and high population densities. The top-ranked models indicated that dispersal age was influenced by a combination of socioecological factors related to resource competition, environmental conditions, kin competition, and a lesser extent sex. Circumstances that likely reduced the probability of successful dispersal such as intense resource competition at high population density and being born earlier in the breeding season when environmental conditions were poor lead to longer delays in natal dispersal. Offspring in larger litters also dispersed earlier possibly to avoid competition with kin. Sex was weakly supported in top models but may only influence dispersal age at high population densities. These results suggest that the proximal factors that trigger dispersal are influenced by a combination of multiple effects related to the costs of dispersing and the benefits of remaining at home, even in species that do not form long-term social groups or have extreme sex-biased dispersal.

Constraints and time lags for recovery of a keystone species (Dipodomys spectabilis) after landscape restoration

Habitat restoration is typically focused on reestablishing suitable conditions at a local scale, but landscape constraints may be important for keystone species with limited dispersal. We tested for time lags and examined the relative importance of local and landscape constraints on the response of the banner-tailed kangaroo rat (Dipodomys spectabilis) to restoration of Chihuahuan Desert grasslands in New Mexico, USA. Dipodomys spectabilis is a keystone species that creates habitat heterogeneity and modifies the structure of plant and animal communities. We selected 21 sites and compared density of D. spectabilis between areas treated with herbicide to control shrubs (treated areas) and paired untreated areas. We evaluated whether density of D. spectabilis depended on treatment age, local habitat quality (vegetation and soil structure), and landscape factors (treatment area and spatial connectivity). Density was greater at treated areas than at untreated areas due to a direct effect of reduced shrub cover. However, the response of D. spectabilis to restoration was lagged by a decade or more. Structural equation modeling indicated the time lag reflected a dispersal constraint as opposed to a temporal change in habitat quality. This inference was corroborated by a positive relationship between density at treated areas and connectivity to source populations. Our results indicate that density of D. spectabilis depended strongly on the spatial configuration of treated areas, which supports a landscape mosaic approach to restoration. If keystone species commonly exhibit limited dispersal ability, landscape constraints may be broadly important for shaping ecosystem structure and function after habitat restoration.

Female mobility and the mating system of the banner-tailed kangaroo rat (Dipodomys spectabilis)

Like some species of pika (Ochotona) and tree squirrel (Tamiasciurus), the banner-tailed kangaroo rat (Dipodomys spectabilis) is a solitary, philopatric larder-hoarder that practices unisexual territoriality. Seeking to understand how this unusual combination of traits might influence the mating system, we used systematic trapping at mounds (burrows) and spool-and-line tracking to examine male and female visits to each other. Where genetic data exist, unisexual larder-hoarders such as D. spectabilis have been found to be polygynandrous. We asked whether the fact that females sometimes mate with more than 1 male is a passive side effect of imperfect mate defense by males, or whether females actively engage in behaviors that encourage multiple mating. During the nonbreeding season, adults of both sexes were rarely captured at mounds other than their own. In contrast, during the breeding season females as well as males were significantly more likely to be captured away from their home mounds, most often at mounds belonging to adults of the opposite sex. Spool-and-line tracking during the breeding season confirmed these results. Consistent with the expectation that they engage in competitive mate searching, adult males visited adult females' mounds significantly more than expected by chance and without regard to age, size, or relatedness. More surprisingly, adult females exhibited similar behavior, expanding their home ranges and disproportionately visiting the mounds of large adult males during the nights preceding estrus. Increased (and apparently directed) female mobility during the days preceding estrus may serve as a means of shaping the pool of competing mates and is worthy of more attention in this and other, ecologically similar species.

Microsatellite analyses across three diverse vertebrate transcriptomes (Acipenser fulvescens,Ambystoma tigrinum, andDipodomys spectabilis)

Historically, many population genetics studies have utilized microsatellite markers sampled at random from the genome and presumed to be selectively neutral. Recent studies, however, have shown that microsatellites can occur in transcribed regions, where they are more likely to be under selection. In this study, we mined microsatellites from transcriptomes generated by 454-pyrosequencing for three vertebrate species: lake sturgeon (Acipenser fulvescens), tiger salamander (Ambystoma tigrinum), and kangaroo rat (Dipodomys spectabilis). We evaluated (i) the occurrence of microsatellites across species; (ii) whether particular gene ontology terms were over-represented in genes that contained microsatellites; (iii) whether repeat motifs were located in untranslated regions or coding sequences of genes; and (iv) in silico polymorphism. Microsatellites were less common in tiger salamanders than in either lake sturgeon or kangaroo rats. Across libraries, trinucleotides were found more frequently than any other motif type, presumably because they do not cause frameshift mutations. By evaluating variation across reads assembled to a given contig, we were able to identify repeat motifs likely to be polymorphic. Our study represents one of the first comparative data sets on the distribution of vertebrate microsatellites within expressed genes. Our results reinforce the idea that microsatellites do not always occur in noncoding DNA, but commonly occur in expressed genes.

Fitness consequences of dispersal: Is leaving home the best of a bad lot?

Using 20 years of demographic and genetic data from four populations of banner-tailed kangaroo rats (Dipodomys spectabilis), we asked whether dispersing individuals gain benefits during adulthood that might compensate for the substantial survival costs they experience as juveniles. Compared to philopatric animals, within- and between-population dispersers gained no measureable advantages in adult survival, fecundity, or probability of recruiting offspring to adulthood. Restricting analyses to members of two central populations living more than 15 times the median dispersal distance from the study site edge, and using peripheral populations only to detect dispersal or offspring recruitment "offsite," did not change this result. Population density during year of birth had small negative effects on adult survival and fecundity, but there were no interactions with dispersal phenotype. We found no evidence that dispersers gained access to superior habitat or that their offspring suffered less inbreeding depression. Our results are consistent with fitness advantages being indirect; by leaving, dispersers release their kin from competition. Our results are also consistent with the possibility that dispersal is the "best of a bad lot." If dispersal is a conditional strategy, then the benefits may be obscured in observational data that compare dispersing individuals to philopatric individuals rather than to individuals whose dispersal phenotype is experimentally manipulated.

IS LEAVING HOME THE BEST OF A BAD LOT?

Photographs by Peter Waser A tagged adult male banner-tailed kangaroo rat (Dipodomys spectabilis) is at the entrance to his mound during the summer monsoon season, 2007, a time of unusually profuse grass growth. The banner-tailed kangaroo rat (Dipodomys spectabilis) builds conspicuous earth mounds, in which they live and larder-hoard, and which they often pass from one generation to the next. Their absolute dependence on these mounds makes it possible to tag most juveniles on or near their mother's mound and to track their residency patterns throughout their lives. In this way, the effects of dispersal on fitness, population dynamics, and genetic population structure can be studied. The mound and part of the surrounding runway system belonging to an individual banner-tailed kangaroo rat in the study population, as well as the grassland habitat preferred by this species. The view is looking south toward Mexico from the Rucker study site on the Krentz Ranch in southeast Arizona. These photographs illustrate the article “Fitness consequences of dispersal: Is leaving home the best of a bad lot?” by P. M. Waser, K. M. Nichols, and J. D. Hadfield, tentatively scheduled to appear in Ecology 94(6), June 2013.

A robust design capture-recapture model with multiple age classes augmented with population assignment data

The relative contribution of in situ reproduction versus immigration to the recruitment process is important to ecologists. Here we consider a robust design superpopulation capture-recapture model for a population with two age classes augmented with population assignment data. We first use age information to estimate the entry probabilities of new animals originating via in situ reproduction and immigration separately for all except the first period. Then we combine age and population assignment information with the capture-recapture model, which enables us to estimate the entry probability of in situ births and the entry probability of immigrants separately for all sampling periods. Further, this augmentation of age specific capture-recapture data with population assignment data greatly improves the estimators’ precision. We apply our new model to a capture-recapture data set with genetic information for banner-tailed kangaroo rats in Southern Arizona. We find that many more individuals are born in situ than are immigrants for all time periods. Young animals have lower survival probabilities than adults born in situ. Adult animals born in situ have higher survival probabilities than adults that were immigrants.

Response of lizard community structure to desert grassland restoration mediated by a keystone rodent

Many grasslands in the Chihuahuan Desert have transformed to shrublands dominated by creosotebush (Larrea tridentata). Grassland restoration efforts have been directed at controlling creosotebush by applying herbicide over large spatial scales. However, we have a limited understanding of how landscape-scale restoration affects biodiversity. We examined whether restoration treatments in southern New Mexico, USA have influenced the community structure of lizards, which are sensitive to shrub encroachment. We compared lizard community structure on 21 areas treated with herbicide from 7 to 29 years ago with paired untreated areas that were dominated by shrubs and matched by geomorphology, soils, and elevation. To examine mechanisms underlying responses to restoration, we tested whether the abundance of a grassland specialist, Aspidoscelis uniparens, depended on time since treatment, treatment area and isolation, and local habitat quality. Because lizards use rodent burrows as habitat, we tested whether community structure and A. uniparens abundance depended on the abundance of the keystone rodent, Dipodomys spectabilis. Treated areas had reduced shrub cover and increased grass cover compared to untreated areas. Lizard community composition differed strongly between areas, with four species responding to treatments. Divergence in community composition between treated–untreated pairs was greatest for old treatments (≥22 years), and community composition was influenced by D. spectabilis. In particular, the abundance of A. uniparens was greatest on old treatments with a high density of D. spectabilis. Overall, our results demonstrate lizard community structure responds to grassland restoration efforts, and keystone species can shape restoration outcomes. Reestablishment of keystone species may be a critical constraint on the recovery of animal biodiversity after habitat restoration.