[Show abstract][Hide abstract] ABSTRACT: Modest increases in global temperature have been implicated in causing population extirpations and range shifts in taxa inhabiting colder environs and in ectotherms whose thermoregulation is more closely tied to environmental conditions. Many arid-adapted endotherms already experience conditions at their physiological limits, so it is conceivable that they could be similarly affected by warming temperatures. We explored how climatic variables might influence the apparent survival of the banner-tailed kangaroo rat (Dipodomys spectabilis), a rodent endemic to the Chihuahuan Desert of North America and renowned for its behavioral and physiological adaptations to arid environments. Relative variable weight, strength of variable relationships, and other criteria indicated that summer, diurnal land surface temperature (SD_LST) was the primary environmental driver of apparent survival in these arid-adapted rodents. Higher temperatures had a negative effect on apparent survival, which ranged from 0.15 (SE = 0.04) for subadults to 0.50 (SE = 0.07) for adults. Elevated SD_LST may negatively influence survival through multiple pathways, including increased water loss and energy expenditure that could lead to chronic stress and/or hyperthermia that could cause direct mortality. Land surface temperatures are predicted to increase by as much 6.5°C by 2099, reducing apparent survival of adults to ~0.15 in some regions of the species' range, possibly causing a shift in their distribution. The relationship between SD_LST and survival suggests a mechanism whereby physiological tolerances are exceeded resulting in a reduction to individual fitness that may ultimately cause a shift in the species' range over time.
[Show abstract][Hide abstract] ABSTRACT: ABSTRACT Recently, a conservation strategy developed to restore populations of black-tailed prairie dog (Cynomys ludovicianus) suggested reintroducing animals into the Chihuahuan Desert grasslands of the southwestern United States. Rainfall in desert habitats is lower and more variable compared to rainfall near the center of the prairie dog's range. Additionally, peak rainfall comes months after prairie dogs reproduce in these desert systems. Thus, southwestern populations may be less prolific and fluctuate more than those found in northerly climes. Using mark-recapture and mark-resight techniques, we estimated reproduction and monthly survival from 577 individuals inhabiting 6 reintroduced colonies from 2003 to 2005 in the northern Chihuahuan Desert. During 2003 precipitation was 64% of the long-term average, whereas both 2004 and 2005 had near-average precipitation. Probability that a female became pregnant, number of juvenile prairie dogs emerging from maternity burrows, and date of emergence were all correlated to adult female body mass. Adult monthly survival decreased from >0.95 during spring to 0.70 in summer 2003, following a rapid loss in adult body mass that coincided with low precipitation. In 2003 monthly juvenile survival was near zero on 2 of the 3 largest colonies and growth rates of juveniles were half that of subsequent years. Estimated population size declined by 68% (range = 18–91%) from 2003 to 2004, and 5 of 6 populations declined an average of 75% from their original introduction size. Prairie dog populations in desert environs may have a high risk of extirpation caused by weather patterns indicative of desert climates. Our results are important for those managers involved in the conservation of prairie dogs and we suggest that regional differences should be carefully considered prior to any reintroduction effort.
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Banner-tailed kangaroo rats (Dipodomys spectabilis) have been used as models to explore competition, examine mating systems, and assess interactions among biotic and abiotic forces shaping population dynamics. However, forces shaping variation in individual survival, a key element for modeling population dynamics and assessing risk of population extinction, have received little attention. By unraveling the sources of variation in survival it is possible to gain a more thorough understanding of the evolution of life-history traits and the potential conservation needs of this keystone species. Our objective was to use population-level and environmental drivers to explain the variation in apparent annual survival among distinct populations of D. spectabilis. We collected mark-recapture data (2004-2007) from 11 populations in southern New Mexico and estimated annual survival using a model selection framework. A two-step approach was employed: 1) survival was modeled using categorical sources of variation and 2) a priori hypotheses divided categorical variation into specific population-level and environmental drivers. Models were ranked according to QAICc and estimates were drawn from multiple models to reduce the effect of model uncertainty. The importance of categorical sources of variation was ranked using the sum of QAICc weights (w).
Annual survival was higher for females (Φ = 0.35 – 0.39 [SE = 0.04 – 0.06]) than for males (Φ = 0.28 – 0.31 [0.04 – 0.05]) and higher for adults (Φ = 0.35 – 0.37 [0.03 – 0.06]) than for subadults (Φ = 0.13 – 0.23 [0.06 – 0.09]). Average annual survival at each site increased through time, from a low of 0.17 (0.07) to a high of 0.52 (0.09). Recapture probability was high (0.92 [0.05]) and constant. Age (w = 0.91) had overwhelming support as an important source of variation. Further decomposition of age revealed important interactions with time (trend effect: w = 0.39) and space (w = 0.35). Differences in survival between the sexes may be related to mate seeking activities of males whereas the importance of age likely reflects subadult naïvet and dispersal, which are predicted to increase mortality. Temporal trend effect may have been related to the cessation of a 3-year drought (2001-2003) that was followed by 4 years of successively increasing rainfall (2004-2007). A bimodal distribution of precipitation drives primary production in the Chihuahuan Desert and topography yields heterogeneous patterns of vegetation; hypotheses based on these environmental drivers are currently being examined.
[Show abstract][Hide abstract] ABSTRACT: Large mammalian carnivores are ecologically important because relatively few individuals can cause strong predation-driven direct effects or fear-driven indirect effects that can ripple through communities and, ultimately, influence ecosystem structure and function. Most mammalian carnivores are not large, however, but are small to midsized species collectively termed “mesocarnivores.” Mesocarnivores are more numerous and more diverse than larger carnivores, and often reside in closer proximity to humans, yet we know little about how they influence communities and ecosystems. In this article we review the ecological role of the mesocarnivore and present examples where mesocarnivores drive community structure and function in roles similar to, or altogether different from, their larger brethren. Together, these examples substantiate the need for an assessment of the ecological role of mammalian carnivores beyond an examination of only the largest species. In particular, we emphasize the need to study the trophic penetrance of mesocarnivores and examine how ecological context modulates their functional role.
[Show abstract][Hide abstract] ABSTRACT: Biometricians have made great strides in the generation of reliable estimates of demographic rates and their uncertainties from imperfect field data, but these estimates are rarely used to produce detailed predictions of the dynamics or future viability of at-risk populations. Conversely, population viability analysis (PVA) modelers have increased the sophistication and complexity of their approaches, but most do not adequately address parameter and model uncertainties in viability assessments or include important ecological drivers. Merging the advances in these two fields could enable more defensible predictions of extinction risk and better evaluations of management options, but only if clear and interpretable PVA results can be distilled from these complex analyses and outputs. Here, we provide guidance on how to successfully conduct such a combined analysis, using the example of the endangered island fox (Urocyon littoralis), endemic to the Channel Islands of California, USA. This more rigorous demographic PVA was built by forming a close marriage between the statistical models used to estimate parameters from raw data and the details of the subsequent PVA simulation models. In particular, the use of mark-recapture analyses and other likelihood and information-theoretic methods allowed us to carefully incorporate parameter and model uncertainty, the effects of ecological drivers, density dependence, and other complexities into our PVA. Island fox populations show effects of density dependence, predation, and El Nino events, as well as substantial unexplained temporal variation in survival rates. Accounting not only for these sources of variability, but also for uncertainty in the models and parameters used to estimate their strengths, proved important in assessing fox viability with different starting population sizes and predation levels. While incorporating ecological drivers into PVA assessments can help to predict realistic dynamics, we also show that unexplained process variance has important effects even in our extremely well-studied system, and therefore must not be ignored in PVAs. Overall, the treatment of causal factors and uncertainties in parameter values and model structures need not result in unwieldy models or highly complex predictions, and we emphasize that future PVAs can and should include these effects when suitable data are available to support their analysis.
[Show abstract][Hide abstract] ABSTRACT: We characterized 15 polymorphic tetranucleotide microsatellite markers for the ringtail, Bassariscus astutus. We tested these loci in 21 individuals captured in Arizona and Texas and found six to 19 alleles per locus. Observed and expected heterozygosities ranged from 0.381 to 1.000 and from 0.381 to 0.941, respectively. All loci were in Hardy-Weinberg equilibrium, and none were in linkage disequilibrium. These markers may be used to investigate population genetics and mating patterns in this species.
[Show abstract][Hide abstract] ABSTRACT: Invasive species are recognized as a primary driver of native species endangerment and their removal is often a key component of a conservation strategy. Removing invasive species is not always a straightforward task, however, especially when they interact with other species in complex ways to negatively influence native species. Because unintended consequences may arise if all invasive species cannot be removed simultaneously, the order of their removal is of paramount importance to ecological restoration. In the mid-1990s, three subspecies of the island fox Urocyon littoralis were driven to near extinction on the northern California Channel Islands owing to heightened predation by golden eagles Aquila chrysaetos. Eagles were lured to the islands by an abundant supply of feral pigs Sus scrofa and through the process of apparent competition pigs indirectly facilitated the decline in foxes. As a consequence, both pigs and eagles had to be removed to recover the critically endangered fox. Complete removal of pigs was problematic: removing pigs first could force eagles to concentrate on the remaining foxes, increasing their probability of extinction. Removing eagles first was difficult: eagles are not easily captured and lethal removal was politically distasteful.
Using prey remains collected from eagle nests both before and after the eradication of pigs, we show that one pair of eagles that eluded capture did indeed focus more on foxes. These results support the premise that if the threat of eagle predation had not been mitigated prior to pig removal, fox extinction would have been a more likely outcome.
If complete eradication of all interacting invasive species is not possible, the order in which they are removed requires careful consideration. If overlooked, unexpected consequences may result that could impede restoration.
PLoS ONE 01/2009; 4(9):e7005. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Habitat loss, introduced disease, and government-sponsored eradication programs have caused population declines in all 5 species of prairie dogs. Black-tailed prairie dogs (Cynomys ludovicianus) currently occupy only about 2% of an extensive geographic range (160 million hectares) and were recently considered for listing under the United States Endangered Species Act. Accurate estimates of density for populations of prairie dogs would be valuable for estimating range-wide abundance and for determining threats to species persistence, yet estimates of density using robust approaches (e.g., complete enumeration or mark-recapture) are rarely undertaken. We introduce a novel approach to estimating density of prairie dogs using mark-resight methods. Using mark-resight, mark-recapture, and 3 other indices, we estimated the abundance of prairie dogs on 3 reintroduced colonies over a 3-year period (2003-2005). We show that mark-resight is a superior approach to estimating abundance of prairie dogs, that average density estimates from the southern extremity of the species' range are considerably lower (11.3 prairie dogs/ha) than estimates from more northerly climes (X = 18.3-90.3 prairie dogs/ha), and that population densities can fluctuate widely in accordance with local environmental conditions. We propose that resource agencies use mark–resight methods to obtain density estimates of prairie dog populations within diverse ecoregions, and couple these estimates with an assessment of the area occupied by prairie dog colonies to determine range-wide abundance.
[Show abstract][Hide abstract] ABSTRACT: Factors governing the rate and direction of prairie dog (Cynomys spp.) colony expansion remain poorly understood. However, increased knowledge and ability to control these factors may lead to more effective reintroductions of prairie dogs and restoration of grassland habitats. We present density and directional analyses of the establishment of new burrows on three reintroduced colonies of Black-tailed prairie dog (Cynomys ludovicianus) in southern New Mexico; the study colonies had been subjected to mow and burn treatments in the second year of the study. Our hypotheses were that prairie dogs will preferentially dig new burrows in the treatment plots versus control plots and that the colonies will expand in the direction of the treatment plots. The results support these hypotheses; analysis of burrow counts by site and treatment shows that prairie dogs preferentially colonized both mow and burn treatments compared to untreated areas at the periphery of the colonies. Directional analysis showed a significant posttreatment orientation of new burrows toward the treatment plots for all colonies. Our results show that the direction of expansion of prairie dog colonies can be manipulated. Effective control of the expansion of prairie dog colonies may lead to more successful reintroductions.
[Show abstract][Hide abstract] ABSTRACT: An Allee effect (AE) occurs in populations when individuals suffer a decrease in fitness at low densities. If a fitness component is reduced (component AE), per capita population growth rates may decline as a consequence (demographic AE) and extinction risk is increased. The island fox (Urocyon littoralis) is endemic to six of the eight California Channel Islands. Population crashes have coincided with an increase in predation by Golden Eagles (Aquila chrysaetos). We propose that AEs could render fox populations more sensitive and may be a likely explanation for their sharp decline. We analyzed demographic data collected between 1988 and 2000 to test whether fox density (1) influences survival and reproductive rates; (2) interacts with eagle presence and affects fox fitness parameters; and (3) influences per capita fox population trends. A double component AE simultaneously influenced survival (of adults and pups) and proportion of breeding adult females. The adult survival AE was driven by predation by eagles. These component AEs led to a demographic AE. Multiple-component AEs, a predation-driven AE, and the simultaneous occurrence of both component and demographic AEs in a mammal are all previously unreported processes. Populations below 7 foxes/km(2) could have suboptimal population growth rates due to the demographic AE, and AEs may have contributed to the dramatic declines in three fox populations. Because fox densities in critically endangered populations are well below this level, removing Golden Eagles appears necessary to prevent a predation-driven AE. Conservationists should also be aware of AEs when planning the release of captive foxes. More generally, our findings highlight the danger of overlooking AEs in the conservation of populations of rare or threatened species.
[Show abstract][Hide abstract] ABSTRACT: Las California Channel Islands, USA, y la islas del Pacífico de Baja California (en adelante islas de California) son conocidas por sus altos niveles de biodiversidad y mundialmente importantes colonias de aves marinas. Documentamos la historia, impactos y manejo de los mamíferos introducidos y reportamos el estado actual de los mamíferos nativos no voladores en las islas de California. De las 26 especies de mamíferos nativos de las islas de California, incluyendo 6 especies y 41 subespecies endémicas, ≥10 poblaciones han sufrido de extinción local o global. Todas las extirpaciones y extinciones recientes resultaron directamente de depredadores mamíferos introducidos o indirectamente por la modificación de hábitat causada por herbívoros introducidos. En vista de los efectos devastadores que los mamíferos introducidos han tenido en las biotas nativas insulares de las islas de California, una variedad de organizaciones ha colaborado y ha erradicado 44 poblaciones de los mamíferos no nativos de 19 islas de California. La documentación de estos efectos y la implementación oportuna de los esfuerzos de erradicación son esenciales para la conservación de estos ecosistemas y los de otras islas.
The Southwestern Naturalist 01/2007; · 0.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Large vertebrates are strong interactors in food webs, yet they were lost from most ecosystems after the dispersal of modern humans from Africa and Eurasia. We call for restoration of missing ecological functions and evolutionary potential of lost North American megafauna using extant conspecifics and related taxa. We refer to this restoration as Pleistocene rewilding; it is conceived as carefully managed ecosystem manipulations whereby costs and benefits are objectively addressed on a case-by-case and locality-by-locality basis. Pleistocene rewilding would deliberately promote large, long-lived species over pest and weed assemblages, facilitate the persistence and ecological effectiveness of megafauna on a global scale, and broaden the underlying premise of conservation from managing extinction to encompass restoring ecological and evolutionary processes. Pleistocene rewilding can begin immediately with species such as Bolson tortoises and feral horses and continue through the coming decades with elephants and Holarctic lions. Our exemplar taxa would contribute biological, economic, and cultural benefits to North America. Owners of large tracts of private land in the central and western United States could be the first to implement this restoration. Risks of Pleistocene rewilding include the possibility of altered disease ecology and associated human health implications, as well as unexpected ecological and sociopolitical consequences of reintroductions. Establishment of programs to monitor suites of species interactions and their consequences for biodiversity and ecosystem health will be a significant challenge. Secure fencing would be a major economic cost, and social challenges will include acceptance of predation as an overriding natural process and the incorporation of pre-Columbian ecological frameworks into conservation strategies.
The American Naturalist 12/2006; 168(5):660-81. · 4.55 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Interspecific interactions are often difficult to elucidate, particularly with large vertebrates at large spatial scales. Here, we describe a methodology for estimating interspecific interactions by combining stable isotopes with bioenergetics. We illustrate this approach by modeling the population dynamics and species interactions of a suite of vertebrates on Santa Cruz Island, California, USA: two endemic carnivores (the island fox and island spotted skunk), an exotic herbivore (the feral pig), and their shared predator, the Golden Eagle. Sensitivity analyses suggest that our parameter estimates are robust, and natural history observations suggest that our overall approach captures the species interactions in this vertebrate community. Nonetheless, several factors provide challenges to using isotopes to infer species interactions. Knowledge regarding species-specific isotopic fractionation and diet breadth is often lacking, necessitating detailed laboratory studies and natural history information. However, when coupled with other approaches, including bioenergetics, mechanistic models, and natural history, stable isotopes can be powerful tools in illuminating interspecific interactions and community dynamics.
[Show abstract][Hide abstract] ABSTRACT: Introduced species can compete with, prey upon or transmit disease to native forms, resulting in devastation of indigenous communities. A more subtle but equally severe effect of exotic species is as a supplemental food source for predators that allows them to increase in abundance and then overexploit native prey species. Here we show that the introduction of feral pigs (Sus scrofa) to the California Channel Islands has sustained an unnaturally large breeding population of golden eagles (Aquila chrysaetos), a native predator. The resulting increase in predation on the island fox (Urocyon littoralis) has caused the near extirpation of three subspecies of this endemic carnivore. Foxes evolved on the islands over the past 20,000 years, pigs were introduced in the 1850s and golden eagles, historically, were only transient visitors. Although these three species have been sympatric for the past 150 years, this predator-prey interaction is a recent phenomenon, occurring within the last decade. We hypothesize that this interaction ultimately stems from human-induced perturbations to the island, mainland and surrounding marine environments.
[Show abstract][Hide abstract] ABSTRACT: Insular populations typically occur at higher densities, have higher survivorship, reduced fecundity, decreased dispersal, and reduced aggression compared to their mainland counterparts. Insularity may also affect mating system and genetic population structure. However, these factors have not been examined simultaneously in any island vertebrate. Here we report on the ecological, behavioural and genetic characteristics of a small carnivore, the island fox Urocyon littoralis, from Fraser Point, Santa Cruz Island, California. Dispersal distances in island foxes are very low (mean 1.39 km, sd 1.26, range 0.16–3.58 km, n=8). Home-range size is one of the smallest (mean annual home range=0.55 km2, sd 0.2, n= 14) and density is nearly the highest recorded for any canid species (2.4–15.9 foxes/km2). Similar to other fox species, island foxes are distributed as mated pairs that maintain discrete territories. Overlap among mated pairs was always high (mean 0.85, sd 0.05), while overlap among neighbours (mean 0.11, sd 0.13), regardless of sex, was low. Despite this high degree of territoriality, island foxes are not strictly monogamous. Four of 16 offspring whose parents were identified by paternity analysis were a result of extra-pair fertilizations. Mated pairs were unrelated, however, suggesting inbreeding avoidance. Substantial population differentiation was found between the Fraser Point subpopulation and one only 13 km away (Fst= 0.11). We suggest that the primary effect of finite island area is to limit dispersal, which then influences the demography, behaviour and genetic structure of island fox populations.