Cost-effective abundance estimation of rare animals: Testing performance of small-boat surveys for killer whales in British Columbia

Top predators are often rare, subject to anthropogenic mortality, and possess life-history traits that make them inherently vulnerable to extinction. IUCN criteria recognise populations as Critically Endangered when abundance is <250 mature individuals, but estimating abundance of rare species can be more challenging than for common ones. Cost-effective methods are needed to provide robust abundance estimates. In marine environments, small boats are more widely accessible than large ships for researchers conducting sightings surveys with limited funds, but studies are needed into efficacy of small-boat surveys. This study compares line transect and mark-recapture estimates from small-boat surveys in summer 2004 and 2005 for ‘northern resident’ killer whales in British Columbia to true population size, known from censuses conducted by Fisheries and Oceans Canada. The line transect estimate of 195 animals (95% CI 27-559) used model averaging to incorporate uncertainty in the detection function, while the mark-recapture estimate of 239 animals (CI 154-370) used a simple two-sample Chapman estimator. Both methods produced estimates close to the true population size, which numbered 219 animals in 2004 and 235 in 2006, but both suffered from the small sample sizes and violations of some model assumptions that will vex most pilot studies of rare species. Initial abundance estimates from relatively low-cost surveys can be thought of as hypotheses to be tested as new data are collected. For species of conservation concern, any cost-effective attempt to estimate absolute abundance will assist status assessments, as long as estimates are presented with appropriate caveats.     

A gamma-shaped detection function for line-transect surveys with mark-recapture and covariate data

We have developed a procedure for estimating animal population size from aerial survey data collected simultaneously by two observers on the same sighting platform. We used a line transect sample design where transects follow elevation contours in mountainous terrain. Because our 10 data sets from aerial line transect surveys, conducted over a terrestrial environment, consistently show unimodal detection shapes, we chose a gamma-shaped detection function that is unimodal and admits covariates. We fit models separately to data from each observer, and then used all of the data to estimate the probabilities at the apex of the detection curves. We used a Horvitz-Thompson estimator to estimate the population size. We illustrate our procedure on a recently collected brown bear data set.     

Comparing population growth rates using weighted bootstrapping: Guiding the conservation management of Petrogale xanthopus xanthopus (yellow-footed rock-wallaby)

In this study we compare estimates of the long-term mean stochastic population growth rates, E[r] for Petrogale xanthopus xanthopus (yellow-footed rock-wallaby), a threatened Australian species. In fluctuating environments such as semi-arid areas, herbivore populations respond directly to changes in the biomass of food resources. Biomass is generally correlated with rainfall, so it is often useful to model annual population growth rates of herbivores directly with rainfall. Models of this nature are referred to as numerical response models. The factors that are thought to threaten this species include competition from introduced herbivores and predation from foxes. Annual aerial survey data collected from 1997 to 2004 over approximately 600 km of transect line were analyzed in seven zones within South Australia. Using the Ivlev numerical response model, the annual population growth rates were found to correlate best with the rain that fell in the seven-month period immediately prior to the surveys. Not surprisingly, positive growth rates were found to be associated with higher rainfalls in this period, while negative growth rates were associated with lower rainfalls. We also used weighted bootstrapping to calculate confidence intervals around our estimates of long-term mean stochastic population growth rates, E[r]. The findings suggest that the estimates of E[r] are positive in areas where there is fox and herbivore management. However, we found no evidence that this species will decline in the absence of these treatments.     

长江特大洪水原因与防灾方略探讨

’９８长江大洪水波及到长江中下游的湘、鄂、赣、皖、苏５省,受灾面积１３３３万多ｋｍ２,洪水前后持续２个月。发生大洪水的原因,既有天气因素的影响,也有上游森林破坏招致水土流失的作用,还有人为围湖造田带来的负效应的影响,但根本原因是天气因素所致。中下游地区湖泊水面积减小,人为在河道上圈地、设障,是导致洪水水位抬高、险情增加的重要原因。长江综合治理的目标与重点是,上游修建除三峡以外的控制性的调蓄工程群,实行蓄水、减沙与森林生态工程相结合,中下游进行固堤、河道清障,保护现有湖泊的调蓄功能。     

Distribution, status, and conservation of radiated tortoises (Geochelone radiata) in Madagascar

The radiated tortoise, Geochelone radiata, one of Madagascar’s four endemic tortoises, occupies a narrow band of xeric spiny forest along the island’s southwest coast. Traditionally avoided by indigenous tribes, these tortoises are now routinely harvested for food. An accurate assessment of human exploitation remains problematic, however, hindered by limited, dated statistics available on tortoise populations. To update the radiated tortoise’s status and distribution, we established a series of line transects at seven localities across its range and implemented a mark-recapture study at one of these localities (Cap Sainte Marie). Tortoises currently range from south of Tulear to east of Cap Sainte Marie, at density estimates spanning 27-5744 tortoises/km². The mark-recapture estimate for Cap Sainte Marie (1905-2105 tortoises/km²) was substantially higher than its transect estimate (654 tortoises/km²) though comparable to actual tortoise captures (1438) there. Thus, our transect density values probably err as underestimates, and from these data, we calculate a conservative total population size of 12 million radiated tortoises. We also examined mitochondrial DNA sequences (ND4 gene) for two individuals/locality in a preliminary assessment of genetic variation across the species’ range. Only two ND4 haplotypes were recovered, the more common haplotype representing 13 of the 14 individuals. We offer several conservation recommendations in light of our survey results.     

Zigzag survey designs in line transect sampling

Zigzag survey lines are frequently used in shipboard and aerial line transect surveys of animal populations. Analysis proceeds assuming that coverage probability through the survey region is uniform. We show that the two types of zigzag design that are in wide use do not generally have this property, and explore the degree of bias in abundance estimates that can be anticipated. We construct a zigzag design for convex survey regions that has even coverage probability with respect to distance along a design axis. We also provide Horvitz-Thompson estimators that allow coverage probability to vary by location through the survey region.     

Likelihood-based inference for clustered line transect data

The uncertainty in estimation of spatial animal density from line transect surveys depends on the degree of spatial clustering in the animal population. To quantify the clustering we model line transect data as independent thinnings of spatial shot-noise Cox processes. Likelihood-based inference is implemented using Markov chain Monte Carlo (MCMC) methods to obtain efficient estimates of spatial clustering parameters. Uncertainty is addressed using parametric bootstrap or by consideration of posterior distributions in a Bayesian setting. Maximum likelihood estimation and Bayesian inference are compared in an example concerning minke whales in the northeast Atlantic.     

Estimating Distance Sampling Detection Functions When Distances Are Measured With Errors

Distance sampling methods assume that distances are known but in practice there are often errors in measuring them. These can have substantial impact on the bias and precision of distance sampling estimators. In this paper we develop methods that accommodate both systematic and stochastic measurement errors. We use the methods to estimate detection probability in two surveys with substantial measurement error. The first is a shipboard line transect survey in the North Sea in which information on measurement error comes from photographically measured distances to a subset of detections. The second is an aerial cue-counting survey off Iceland in which information on measurement error comes from pairs of independently estimated distances to a subset of detections. Different methods are required for measurement error estimation in the two cases. We investigate by simulation the properties of the new estimators and compare them to conventional estimators. They are found to perform better than conventional estimators in the presence of measurement error, more so in the case of cue-counting and point transect estimators than line transect estimators. An appendix on the asymptotic distributions of conditional and full likelihood estimators is available online.     

Rapid assessment of orangutan density

In order to design effective conservation measures for the orangutan, accurate information on their distribution and densities are needed. Nest counts along line transects allow for fairly accurate assessment of orangutan density. However, large sample sizes are needed to obtain accurate estimations of the decay rate of nests, preventing a rapid estimate. Moreover, due to the structure of tropical forests, nest numbers above and near the trail are underestimated. In this study, a rapid assessment method for the estimation of orangutan density is developed, allowing for the estimation of density with a single survey. Procedures are provided for estimating a correction factor which adjusts for underestimating nest numbers near the transect trail. Furthermore, we provide a shortcut to rapidly assess nest decay rate without the need for consecutive surveys. Finally, recommendations are made for determining orangutan density using two different techniques. These techniques incorporate correction factors and a shortcut to accurately assess orangutan density with only one or two visits to an area.     

Spatial models for line transect sampling

This article develops methods for fitting spatial models to line transect data. These allow animal density to be related to topographical, environmental, habitat, and other spatial variables, helping wildlife managers to identify the factors that affect abundance. They also enable estimation of abundance for any subarea of interest within the surveyed region, and potentially yield estimates of abundance from sightings surveys for which the survey design could not be randomized, such as surveys conducted from platforms of opportunity. The methods are illustrated through analyses of data from a shipboard sightings survey of minke whales in the Antarctic.     

Estimation of detection probability in aerial surveys of antarctic pack-ice seals

We use line transect detection functions together with generalized linear and additive models to estimate detection probability when detection on the line (“g(0)”) may not be certain. The methods provide a flexible way of modeling detection probability for independent observer surveys, and for investigating the effects of explanatory variables. Analysis of data from an aerial survey of pack-ice seals produced g(0) estimates substantially below 1 for some observers (it varied from 0.80 to 0.98), demonstrated a fairly complex dependence of detection probability on covariates, and showed negative correlation between observers’ search width and their g(0). In addition to illustrating the utility of generalized additive models for capturing the effect of covariates on detection probability, the analysis suggests that detection functions may be sufficiently variable that use of g(0) correction factors obtained from other surveys would be inadvisable. We recommend that estimation of g(0) be considered for all aerial surveys; if g(0) is found to be very close to 1, estimation from subsequent surveys under the assumption that it is 1 may be reasonable, but without any estimation of g(0), the assumption that it is 1 is a matter of faith.     

Distribution, status, and conservation needs of Asian elephants (Elephas maximus) in Lampung Province, Sumatra, Indonesia

In the mid 1980s, Asian elephant (Elephas maximus) populations were believed to persist in 44 populations on the Indonesian island of Sumatra. Twelve of these populations occurred in Lampung Province, but our surveys revealed that only three were extant in 2002. Causal factors underlying this decline include human population growth, changes in land use, and human-elephant conflict. Nevertheless, our surveys in the Province’s two national parks, Bukit Barisan Selatan and Way Kambas, produced population estimates of 498 (95% CI = [373, 666]) and 180 (95% CI = [144, 225]) elephants, respectively. The estimate for Bukit Barisan Selatan is much larger than previous estimates; the estimate for Way Kambas falls between previous estimates. The third population was much smaller and may not be viable. These are the first estimates for Southeast Asian elephant populations based on rigorous sampling-based methods that satisfied the assumptions of the models used, and they suggest that elephant numbers in these parks are of international importance. While our results suggest that Sumatra’s remaining elephant populations may be larger than expected, they also suggest that the future for these animals is bleak. Human-elephant conflict was reported around all three areas in Lampung and their elephant populations are currently threatened by habitat loss and poaching. Local solutions are possible, but will require much greater commitment by all stakeholders.     

湖北长江特大洪水防御对策研究

通过总结湖北省1998年长江防洪基本特点,分析了湖北长江防洪存在的主要问题和症结,一是防洪标准低,二是江湖关系恶化,三是堤防自身存在薄弱环节,四是分蓄洪区运用难。从而有针对性地提出了加高加固堤防、扩大河道行洪能力、建设蓄滞洪区安全设施、主要支流上兴建控制性水库、实施平垸行洪、退田还湖以及提高非工程措施防洪水平等防御特大洪水的综合性对策,并对三峡工程建成后的长江防洪作了展望。     

关于长江中上游水土流失基本问题探讨

在全国土壤抗侵蚀特征原位测试的基础上,结合长江中上游地区水土流失的基本问题诸如水土流失的严重性、坡地土壤侵蚀的基本过程、水土流失与江河泥沙的关系等进行了初步论证,指出长江中上游地区水土流失是严重的,但与江河泥沙的关系并不密切。长江中上游地区水土流失有其本身的特殊性,水土保持亦应提倡“排水保土”,而不是“水土保持”的观点。     

Distribution, abundance and conservation status of dugongs and dolphins in the southern and western Arabian Gulf

The marine mammals of Saudi Arabia, Bahrain, Qatar and the United Arab Emirates (UAE) were censussed by three strip-transect aerial surveys. The Arabian Gulf supports a population of ca. 5800 dugongs (Dugong dugon), which is the largest known outside Australia. The most important habitats occur (1) around Murawah Island (UAE), (2) between Qatar and Bahrain and (3) between Qatar and the UAE. Surveys of the UAE were repeated 13 years apart. The two estimates of the dugong population were not significantly different, suggesting a stable population of ca. 3000 between 1986 and 1999. In the region between Kuwait and Oman, the Indian Ocean bottlenose dolphin (Tursiops aduncus) is the most common cetacean (71% of groups and individuals), followed by the Indo-Pacific humpback dolphin (Sousa chinensis; 27%) and finless porpoise (Neophocaena phocaenoides; 2%). The estimates of cetacean abundance in the UAE differed significantly between 1986 and 1999 and indicate a population decline of 71%. At least two die-offs of marine mammals occurred between these surveys. The countries of the southern Gulf are developed and affluent and are well positioned to take a lead in marine conservation in the region. A coordinated series of protected areas could greatly enhance the long-term prospects for marine mammals and other components of biodiversity in the region.     

Comparison of survey estimates of the finite population variance

The Environmental Monitoring and Assessment Program (EMAP) of the U.S. Environmental Protection Agency has conducted several probability surveys of aquatic resources. Such surveys usually have unequal probability of including population elements in the sample. The Northeast lakes survey, which motivated this study of variance estimation, was such a survey. We examine ten estimators for the finite population variance using a Monte Carlo factorial experiment that considers three population characteristics. The results show that the correlation between the inclusion probabilities and the response is the most important factor that differentiates the estimators. Under conditions of low correlation (approximately <0.4), a common feature in environmental surveys, the sample variance is best, elsewhere, two ratio estimators, one based on consistency and the Horvitz-Thompson Theorem (HT) and the other based on the Yates-Grundy form, behave similarly and best.     

Increasing magnetic susceptibility of the suspended particles in Yangtze River and possible contribution of fly ash

The magnetic properties of soil have been increasingly applied as a rapid and economic way to monitor environment pollution. Sediments from a growing islet in the lower reach of Yangtze River as well as the suspended particles in the surrounding river water were used to identify anthropogenic influence on the magnetic susceptibility (MS) of the Yangtze River sediment. Results show that newly deposited sediments in 2004 have significantly higher MS (~ 150 × 10⁻⁸ m³ kg⁻¹) on average than that of the ancient deposit (~ 50 × 10⁻⁸ m³ kg^− 1). Scanning electron microscope (SEM) of the extracted magnetic particles from newly deposited sediments and fly ash samples indicates large contribution of fly ash for the samples with elevated MS. Dependence of MS on grain size is evident, which enable calculation of the MS of suspended particles from river sediment. A value of 32 × 10⁻⁸ m³ kg^− 1is inferred for the suspended particles in ancient Yangtze River. The records from 2004 to 2010 indicate progressive increase in the MS of the suspended particles in Yangtze River from 67 to 96 × 10⁻⁸ m³ kg^− 1, which is much higher than that of the ancient. Mass balance calculation based on the increasing MS suggests that at least 7% of the fly ash produced within the catchment of Yangtze River was released into the environment.     

长江上游流域径流变化

分析长江上游流域5个主要控制站：金沙江屏山、岷江高场、嘉陵江北碚、长江上游干流寸滩及乌江武隆的月流量变化。结果表明,高场和北碚站的径流显著减少,寸滩站径流则稍微减少。从年内分配上看,9-11月份减少较多。寸滩站径流减少主要受岷江和嘉陵江流域降水量减少和人类活动的影响。     

GIS支持下的长江上游降雨侵蚀力时空分布特征分析

降雨侵蚀力是土壤侵蚀评估模型中的一个基本因子,利用长江上游361个测站1961-2004年日雨量资料估算降雨侵蚀力R值,利用GIS空间分析功能,获得长江上游降雨侵蚀力分布图、降雨侵蚀力年际变化趋势图、各区域R值平均年内分配曲线,在此基础上分析长江上游降雨侵蚀力时空分布特征。研究表明长江上游降雨侵蚀力的地域差异十分显著,与降雨量空间分布近似,由东向西减少,且降雨侵蚀力大的区域与多雨中心和暴雨中心分布基本一致。降雨侵蚀力年际变化存在明显的空间差异性,在一些地区年降雨侵蚀力的变化与年降雨量的变化趋势不一致。各区域降雨侵蚀力年内分配曲线为尖峰状分布,降雨侵蚀力十分集中。     

三峡调蓄条件下长江河口地区滨海滨江土壤盐渍化状况研究

为了研究三峡调蓄过渡期长江河口地区秋季滨海滨江土壤的盐渍化状况,进行土样采集和盐分组成测定,利用统计分析软件对区域土壤盐渍化类型和特点进行主成分分析,对区域土壤盐分剖面类型进行聚类分析。分析结果表明:三峡调蓄过渡期河口地区秋季土壤以盐化潮土为主,阳离子组成Na+为主,阴离子组成SO42-型略多于Cl-型,土壤剖面盐分以上升积聚为主,盐分积聚既呈现短期盐峰又有长期积累现象,存在盐渍化趋势,并主要受干旱枯水因素影响。三峡调蓄的影响程度有待进一步研究。提出应有针对性地采取合理灌排措施和Na质盐碱土改良措施。