Nest mortality in a population of small mammals

Female white-footed micePeromyscus leucopus (Rafinesque, 1818) and their dependent offspring were monitored in nest boxes to determine the extent and causes of nest mortality. The mortality of dependent young was high (561 of 838; 66%) and variable among years. Most mortality involved the loss of entire litters (112 of 183 litters; 61%), with half of these losses attributed to the death of lactating females before the young were weaned (59 of 112 litters; 53%). Most mortality was from unknown causes, although infanticide, energetic constraints and predation were identified in a small number of cases. Predation is likely the major source of mortality in this population.     

Differential detectability of primates between primary and selectively logged habitats and implications for population surveys

Much information has been published concerning the accurary of various techniques commonly used for censusing primates. The sources of bias may vary according to the technique used. Accuracy may depend on population density as well as on differential visibility and hence detectability between areas. This paper provides evidence that primates may alter their behavior between areas of primary and selectively logged habitat, which in turn could affect census results. Adoption of cryptic behavior, changes in behavioral profile, and reduction in calling rates reduce the detectability of certain species. Increases in the frequency of subgrouping or in group dispersion may introduce a danger of overestimation of other species, unless differences in the size of foraging units are taken into account.     

小型哺乳动物的母体效应及其在种群调节中的作用

母体效应是指双亲的表型影响其后代表型的直接效应。它是子代对环境异质性的一种表型反应,亦是进化动力的一个重要来源,还可能与小型哺乳动物种群调节机制有关。以小型哺乳动物为例,介绍了母体效应的概念及其产生和发展过程,以及影响母体效应的营养和非营养因素,特别强调了光周期和激素的作用。在种群水平上,对度量母体效应的备选指标进行了评价,认为种群内个体的平均体重能较好地代表种群质量的高低;概述了衰老母体效应假说的主要内容及其在小型哺乳动物种群动态调节中的作用,即在种群数量的周期性波动过程中,母体质量的变化会影响后代的生殖和存活,甚至持续达2～3个世代,它与由种群年龄结构偏移所导致的衰老效应共同起作用,可使某些小型哺乳动物种群处于低数量期。本文还对母体效应的进化适应意义进行丁阐述。     

Correlates of rediscovery and the detectability of extinction in mammals

Extinction is difficult to detect, even in well-known taxa such as mammals. Species with long gaps in their sighting records, which might be considered possibly extinct, are often rediscovered. We used data on rediscovery rates of missing mammals to test whether extinction from different causes is equally detectable and to find which traits affect the probability of rediscovery. We find that species affected by habitat loss were much more likely to be misclassified as extinct or to remain missing than those affected by introduced predators and diseases, or overkill, unless they had very restricted distributions. We conclude that extinctions owing to habitat loss are most difficult to detect; hence, impacts of habitat loss on extinction have probably been overestimated, especially relative to introduced species. It is most likely that the highest rates of rediscovery will come from searching for species that have gone missing during the 20th century and have relatively large ranges threatened by habitat loss, rather than from additional effort focused on charismatic missing species.     

Factors affecting detectability of river otters during sign surveys

Sign surveys are commonly used to study and monitor wildlife species but may be flawed when surveys are conducted only once and cover short distances, which can lead to a lack of accountability for false absences. Multiple observers surveyed for river otter (Lontra canadensis) scat and tracks along stream and reservoir shorelines at 110 randomly selected sites in eastern Kansas from January to April 2008 and 2009 to determine if detection probability differed among substrates, sign types, observers, survey lengths, and near access points. We estimated detection probabilities (p) of river otters using occupancy models in Program PRESENCE. Mean detection probability for a 400-m survey was highest in mud substrates (p = 0.60) and lowest in snow (p = 0.18) and leaf litter substrates (p = 0.27). Scat had a higher detection probability (p = 0.53) than tracks (p = 0.18), and experienced observers had higher detection probabilities (p > 0.71) than novice observers (p < 0.55). Detection probabilities increased almost 3-fold as survey length increased from 200 m to 1,000 m, and otter sign was not concentrated near access points. After accounting for imperfect detection, our estimates of otter site occupancy based on a 400-m survey increased >3-fold, providing further evidence of the potential negative bias that can occur in estimates from sign surveys when imperfect detection is not addressed. Our study identifies areas for improvement in sign survey methodologies and results are applicable for sign surveys commonly used for many species across a range of habitats. © 2010 The Wildlife Society     

Road exposure and the detectability of birds in field surveys

Road ecology, the study of the impacts of roads and their traffic on wildlife, including birds, is a rapidly growing field, with research showing effects on local avian population densities up to several kilometres from a road. However, in most studies, the effects of roads on the detectability of birds by surveyors are not accounted for. This could be a significant source of error in estimates of the impacts of roads on birds and could also affect other studies of bird populations. Using road density, traffic volume and bird count data from across Great Britain, we assess the relationships between roads and detectability of a range of bird species. Of 51 species analysed, the detectability of 36 was significantly associated with road exposure, in most cases inversely. Across the range of road exposure recorded for each species, the mean positive change in detectability was 52% and the mean negative change was 36%, with the strongest negative associations found in smaller-bodied species and those for which aural cues are more important in detection. These associations between road exposure and detectability could be caused by a reduction in surveyors’ abilities to hear birds or by changes in birds’ behaviour, making them harder or easier to detect. We suggest that future studies of the impacts of roads on populations of birds or other taxa, and other studies using survey data from road-exposed areas, should account for the potential impacts of roads on detectability.     

Modeling biotic and abiotic influences on population size in small mammals

Most previous work in population ecology has modeled density-dependent effects in isolation. In this paper, we concurrently modeled the effect of density-dependent and density-independent factors on the rate of population change (R _t ) in Peromyscus leucopus (white-footed mouse), using a Ricker equation expanded to include weather and seasonality. From 1973 to 1996, we live-trapped P. leucopus monthly in a 2-ha Ohio woodlot. Population peaks (July to August) varied from 27 to 181 individuals, while troughs (December to March) varied from 4 to 46 individuals. We used time-delayed densities to act as surrogates for unobserved density-dependent factors, and principal components to represent 12 highly collinear weather variables. We identified time-delayed correlations by season between R _t and the independent variables (i.e., previous densities and weather principal components) using transfer function analysis. In summer, when P. leucopus densities were above the seasonal mean for the month, R _t was lower in the following 2 months; however, in winter, R _t was greater in the first but lower in the second succeeding month. R _t also correlated positively in autumn with contemporaneous precipitation, and was negatively correlated with `extreme' weather in summer with 2- and 3-month lags and in winter with a 3-month lag. We hypothesize that precipitation affected juveniles by influencing food resources and that `extreme' weather affected breeding. Our model explained 65% of the variability in R _t , and density-dependent and density-independent factors explained an equal percentage of that variability. This model created good forecasts of population density up to 12 months in the future. Received: 12 December 1996 / Accepted: 11 August 1997     

Patterns of diversification and genetic population structure of small mammals in Taiwan

Taiwan is a mountainous island off the coast of the Asian continent. The island is located on the continental shelf and rises to an elevation of nearly 4000 m. It became an island approximately 4 million years ago and has been connected to the continent more than once since its emergence. Therefore, the elevational zonation of two Taiwanese pairs of congeners in the rodent genera Apodemus and Niviventer can be explained by one of two competing hypotheses. One hypothesis assumes that speciation of the two congeners occurred in situ after an ancestral species migrated from the continent. In contrast, the second hypothesis argues the zonation resulted from separate incursion events during the connections. The phylogenetic analyses, which are based on electrophoretic allozyme data, reject the first of these hypotheses. Furthermore, genetic population structure and gene flow of three species of Taiwanese rodents (Apodemus semotus, Niviventer culturatus and Microtus kikuchii) are studied in relation to their patterns of elevational distribution. The genetic structure of A. semotus and N. culturatus, which have extensive elevational distribution, is rather homogeneous and no genie discontinuity or cline in allele frequency was detected. Gene flow among subpopulations for these two species is rather substantial; therefore, the potential isolating effect imposed by deep river valleys is minimal. Conversely, isolation of populations of M. kikuchii on different mountaintops is quite complete as mirrored by the high level of genie differentiation and low gene flow.     

Survival and detectability bias of avian fence collision surveys in sagebrush steppe

We used female ring-necked pheasant (Phasianus colchicus) carcasses as surrogates for greater sage-grouse (Centrocercus urophasianus) to study factors influencing survival and detection bias associated with avian fence collision surveys in southern Idaho, USA, during spring 2009. We randomly placed 50 pheasant carcasses on each of 2 study areas, estimated detection probability during fence-line surveys, and monitored survival and retention of carcasses and their associated sign over a 31-day period. Survival modeling suggested site and habitat features had little impact on carcass survival, and constant survival models were most supported by the data. Model averaged carcass daily survival probability was low on both study areas and ranged from 0.776 to 0.812. Survival of all carcass sign varied strongly by location, and the top sign survival model included a site effect parameter. Model averaged daily survival probability for collision sign on the 2 study sites ranged from 0.863 to 0.988 and varied between sites. Logistic regression modeling indicated detection probability of carcasses during fence-line surveys for avian collision victims was influenced by habitat type and microsite shrub height at the carcass location. Carcasses located in big sagebrush (Artemisia tridentata) habitats were detected at a lower rate (0.36) than carcasses in little (A. arbuscula) and black sagebrush (A. nova) habitats (0.71). Increasing shrub height at the carcass location from the little sagebrush mean of 16.5 cm to the big sagebrush mean of 36.0 cm reduced detection probability by approximately 30%. Avian fence collision surveys in sagebrush-steppe habitats should be conducted at ≤2-week sampling intervals to reduce the impact of survival bias on collision rate estimates. Two-week sampling intervals may be too long in areas with low carcass and sign survival, therefore survival rates should be estimated on all study areas to determine the appropriate sampling interval duration. Researchers should be aware of the effects of local vegetation on detection probabilities, and methods to correct detection probabilities based on collision site attributes should be applied to ensure more accurate collision rate estimates. Additionally, caution should be used when aggregating or comparing uncorrected collision data from areas with differing vegetation, as detection probabilities are likely different between sites. © 2011 The Wildlife Society.     

Spatial-typological structure and organization of the population of small mammals in the Prealtai Plain

Basing on materials collected in 1963, 1998, and 2009–2016, we describe subzonal changes in the population of small mammals in the Prealtai Plain, provide their classification, investigate the spatial–typological structure, and estimate the dependence of heterogeneity of communities on environmental factors. We compare the heterogeneity rate of the populations of West Siberian and Prealtai plains and mountain provinces of Altai.     

小型哺乳动物种群周期性波动的外因调节假说

对小型哺乳动物种群数量周期性波动的外因调节假说进行介绍 ,概述了食物假说、捕食假说和复合因子假说的主要内容和研究进展。在少数生存环境严酷的小型哺乳动物种群中 ,食物假说能解释它们的周期性数量波动现象 ,可能作为调节因子起作用 ,但难以说明低数量期的确切机制 ,对于大多数小型哺乳动物而言 ,它更可能作为限制因子。捕食假说解释了北欧芬诺斯坎底亚地区某些种群的周期性波动 ,尤其是捕食的间接效应已引起许多学者的关注 ,但也有不支持该假说的证据。对于复合因子假说 ,近年颇受学者重视 ,其中验证食物和捕食交互及累加作用的实验证据较多 ,有的研究还包括气候、种间竞争、空间或社会行为等因素。有关复合因子的实验研究 ,尽管工作是困难和艰巨的 ,花费也是巨大的 ,但所得结果却极有价值 ,为深入理解种群动态调节理论提供了一个合理而有效的手段     

New regression formula to estimate the whole population for recapture-addicted small mammals

We have devised a census formula of curvilinear regression suitable for capture-recapture data of recapture-addicted populations of the Japanese field vole (Microtus montebelli) obtained under a grid-plan with single-catch traps in order to estimate the whole population. The equation is founded on the assumption that the trappable population on the initial day is increased in way of an exponential curve until it reaches to the whole during one trapping period. The effect of trap-preoccupation by marked and multiple collisions is considered in the formula. As a result of its application to field data of the vole, it has turned out that the equation is required for the data gained under the trapping plan with trap spacing 10m, but not for those under the plan with spacing 5m, to estimate the whole. A convenient method of analysis of the formula is offered here, but we have been yet unable to introduce assymptotic variance of estimates.     

Measurement errors in line transect surveys where detectability varies with distance and size

When using bivariate line transect methods to estimate the biomass density of a tightly clustered biological population, it is generally assumed that both the perpendicular distance from the trackline to the cluster and the cluster size, or biomass, are measured without error. This is unlikely to be the case in practice. In this article, assuming additive mean zero errors in distance and multiplicative errors in size, we develop an estimator of density that corrects for these errors. We use the method of moments for the case of gamma cluster size, randomly placed transect lines, and the generalized exponential detection function. We derive results that show that it may not be necessary to correct for errors in distance or size when the distance and size estimates are not biased. When the size estimates are biased, the biomass density estimate has approximately the same bias as the size estimates. The work is illustrated in the context of annual aerial surveys for juvenile southern bluefin tuna in the Great Australian Bight.     

Trypanosomes of small mammals in Iran

Trypanosomes in the subgenera Herpetosoma, Schizotrypanom and Megatrypanum were found in 31 small mammals representing nine species of the orders Rodentia, Insectivora and Chiroptera in Iran.     

The population dynamics of small mammals living in set-aside and surrounding semi-natural and crop land

Small mammals were found on set-aside and surrounding farmland. Wood mice showed a different pattern of demography at two sites, with higher densities, greater survival, heavier weights and longer breeding seasons at Aldroughty than at Newburgh. These differences were thought to be due to a difference in habitat productivity at the two sites. Field voles showed less of a difference in population dynamics between the two sites, and both species of vole maintained populations at low densities.