Precision Of Population Estimates Research Articles

Mark‐Recapture Distance Sampling for Aerial Surveys of Ungulates on Rangelands

ABSTRACTAerial surveys are an efficient technique for counting animals over large geographic areas such as rangelands. In southwestern rangelands, aerial surveys are routinely conducted for ungulates, with the implicit understanding that abundance estimates represent an undercount. Distance sampling can correct for visibility bias, but assumes perfect detection on the survey line, a condition often violated in aerial surveys of ungulates. The incorporation of mark‐resight methods into the distance‐sampling framework, termed mark‐recapture distance sampling (MRDS), corrects for both visibility bias and imperfect detection on the survey line. However, the use of MRDS introduces logistical and technical constraints that may not be practical for aerial surveys. We conducted aerial surveys of ungulates on rangelands in south Texas, USA, to evaluate the feasibility and performance of MRDS relative to conventional distance sampling (CDS) and multiple covariate distance sampling (MCDS). We conducted surveys prior to and after leaf‐fall in 2013–2015 to test the hypothesis that distance sampling corrected for changes in visibility bias. We surveyed white‐tailed deer (Odocoileus virginianus), nilgai (Boselaphus tragocamelus), collared peccary (Pecari tajacu), and feral swine (Sus scrofa) on 4 sites. Each site was surveyed seasonally for 2 years; twice both prior to (Nov) and after leaf‐fall (Feb). Probability of detection on survey lines for each species was high (range = 0.82–0.97) and the average for each species was similar between seasons (0.89 and 0.92 during pre and post leaf‐fall, respectively). The MRDS density estimates often were only ~10% greater than CDS and MCDS estimates; all population estimates had overlapping 95% confidence intervals. Further, CDS and MCDS estimates were similar indicating that measured covariates (seat position, vegetation type, and cluster size) contributed little towards detection probabilities. Despite similar average probability of detection for all species before and after leaf‐fall (0.47 and 0.51, respectively), deer and nilgai population estimates were 22–59% lower during fall surveys than winter surveys. The discrepancy between consistent probability of detections with different population estimates suggests that availability bias, which cannot be addressed with distance sampling, was an issue. Overall, the MRDS technique addressed imperfect detection on the survey line and generated probabilities of detection in the survey area consistent with previous studies done in Texas. However, the extra costs (~US$13,000) and logistical hurdles to preserve observer independence for a small increase in precision of population estimates may not be justifiable. © 2020 The Wildlife Society.

The Shetland Bird Survey: population trends for widespread breeding birds 2002–2019

ABSTRACT Capsule Population trends are estimated for the first time for 12 species based on Shetland Breeding Bird Survey data collected between 2002 and 2019. Aim To provide population trends for a selection of commoner breeding waders and passerines on the Shetland Isles, the most northerly region of the UK, which has historically been under-represented in national bird monitoring schemes. Methods One hundred and forty-five self-selected 1-km2 squares were surveyed between 2002 and 2019 using similar methods to the BTO/JNCC/RSPB Breeding Bird Survey (BBS), the main breeding bird monitoring scheme in the UK. Results Over 17 years, Eurasian Oystercatcher Haematopus ostralegus, Northern Lapwing Vanellus vanellus, and Common Redshank Tringa totanus populations declined whilst Eurasian Curlew Numenius arquata and Common Snipe Gallinago gallinago showed no significant changes in their population trends. For passerines, Common Skylark Alauda arvensis and Northern Wheatear Oenanthe oenanthe populations declined, whilst Common Starling Sturnus vulgaris, Common Blackbird Turdus merula, and House Sparrow Passer domesticus increased. Winter Wren Troglodytes troglodytes and Meadow Pipit Anthus pratensis showed no significant population changes. Comparisons with BBS trends for the UK and for Scotland are provided. For Eurasian Curlew, Common Starling, and House Sparrow, trends were more favourable in Shetland than elsewhere in UK. Conclusion The Shetland Breeding Bird Survey provides useful trends for several bird species, which include important breeding numbers of Winter Wren and Common Starling (both endemic subspecies), as well as nationally important wader populations. We recommend increasing the number of survey squares sampled each year, which would increase the precision of population estimates. The inclusion of a random or stratified element to square selection would help to ensure that these trends are representative of the wider region.

Spatial Clustering of Aedes aegypti (Diptera: Culicidae) and Its Impact on Entomological Surveillance Indicators.

Spatial clustering of Aedes aegypti (L.) was assessed in 0.25-km2 sampling plots located in three different neighborhoods in the city of Rio de Janeiro, Brazil Monitoring was carried out with two oviposition traps (collecting either. eggs or adult mosquitoes) over a consecutive 80-wk period. Taylor power laws (TPL) were used to assess clustering in both adult and egg collections. The clustering level observed in both TPL models was sufficient to increase by two to three times the sampling effort required to achieve a desired precision of population estimates of egg and adults collections, respectively. The relationship between positivity and density indices was also affected by this clustering level, and virtually no changes in positivity data were observed for variations of adult densities over two mosquitoes per trap. General TPL model parameterizations for dealing with clustering of egg and adult populations of Ae. aegypti are proposed. Accounting for clustering-related issues is necessary for an appropriate design and interpretation of trap-based entomological surveillance data.

Detection and stratification approaches for aerial surveys of deer in prairie–parklands

Context For management decisions that require accurate and precise estimates of large mammal population numbers, aerial surveys are considered reliable despite their cost. However, aerial surveys may still suffer from underestimation because of undetected animals and low precision as a result of inefficient sampling designs. Aims We assess detection of two species of deer in prairie-parkland communities of western Canada and evaluate a suite of survey design features for improving the accuracy and precision of population estimates from aerial surveys. Methods Modelling detection of deer was based on 100 sightability trials involving 54 radio-collared white-tailed and 46 mule deer. We used empirical survey data to simulate surveys under three alternative stratification approaches, schemes for grouping strata, and allocations of survey effort and compared the precision and accuracy of the resulting population estimates. Key results We observed deer in 83 of the 100 trials, with detection decreasing with small group size, reduced deer activity, low snow cover, high forest cover and observer fatigue. Survey precision and accuracy were highest when stratification was based on natural breaks, calculated via Jenks optimisation, in the values of resource-selection function (RSF), although improvement was less pronounced for estimates of mule deer abundance. Optimal or equal allocation of sampling effort among strata outperformed proportional allocation of sampling effort. Use of RSF for stratification reduced the coefficient of variation (CV) in estimates of deer numbers from 38% to 23% for white-tailed deer and from 33% to 27% for mule deer compared with past approaches. Conclusions Stratification based on RSF values improved precision of deer surveys the most; however, using even simple measures related to habitat selection can improve population estimates. Where deer are highly aggregated, reliably recording all variables needed to implement sightability models can prove problematic; however, survey detection adjustments are nevertheless important to account for the relatively small, but still significant, proportion of missed animals in open prairie–parkland environments. Implications Field experiments to assess aerial survey design are impractical because of cost. We illustrate how simulated surveys using empirical data can be useful to evaluate alternative survey designs to improve population estimates in a region when high accuracy or precision are needed to address management questions or to calibrate more cost-effective approaches.

Sampling variability of ichthyoplankton surveys—Exploring the roles of scale and resolution on uncertainty

We examine the accuracy of predictions from a conceptual model dealing with survey design in a variable environment using data collected from a number ichthyoplankton collections carried out off Newfoundland, Canada. We test predictions concerning the effects of spatial scale, survey resolution, as well as the impacts which variations in environmental forcing and survey duration have on the precision of population estimates. Although the size of the survey area does not appear to influence the precision of population estimates, the distance between stations, the variability in wind forcing of currents, and the time taken to cover the area of interest all have notable effects on precision. The significance which each of these variables may have on the ability to detect changes in population abundance is likely to vary depending on the underlying circulation that characterizes different ecosystems. Our findings, however, point to the potential benefits that may be derived from evaluating the possible influence of such variables on the precision of population estimates based on long standing monitoring programs. This may serve to explain why it has been so difficult to detect substantial changes in vital rates obtained in process-oriented research.

Measurement error and survey design in sightability model development

ABSTRACT Measurement error of explanatory variables used in sightability models can result in biased population estimates and associated measures of precision. We developed a Monte Carlo simulation procedure that can be implemented within the sightability model framework when measurement error is present. Additionally, we developed simulation and sample survey methods, for determining the optimal allocation of survey effort to maximize precision of population estimates for a fixed survey cost, when a complete survey of a study area is not feasible. We used data from aerial surveys of elk during 2004–2006 in Michigan to demonstrate the application of these techniques. By accounting for measurement error and applying appropriate survey design practices, managers employing sightability models may be able to generate more accurate and cost-effective population estimates and accompanying measures of precision than is possible if these techniques are ignored.

Aerial Survey Intensity as a Determinant of Estimates of African Elephant Population Sizes and Trends

Population estimates based on strip transect aerial surveys of large herbivores can aid management decisions if estimates are accurate and precise. Because sampling intensities may influence precision, we simulated survey intensities to determine which could yield accurate and precise population estimates and detect population changes for several African elephant (Loxodonta africana) populations. Simulated surveys of hypothetical elephant populations had to cover 5-20% of a study area to yield accurate estimates, but this depended on how density and distribution varied. Precise estimates, however, needed survey intensities greater than 50%. In addition, the detection of typical rates of population change needed greater sampling intensities than those currently used for most elephant surveys. Survey intensity plays an important role in estimating the accuracy and precision of population estimates and the detection of population trends. Population managers should consider existing information on population density, distribution, and survey intensity to design aerial surveys that will yield both accurate and precise estimates.

SPATIO-TEMPORAL AND DEMOGRAPHIC VARIATION IN THE HARVEST OF BLACK BEARS: IMPLICATIONS FOR POPULATION ESTIMATION

Biologists have monitored black bear (Ursus americanus) populations using annual Lincoln-Petersen (L-P) estimates of population size derived from the fraction of marked bears recovered in the harvest. Although spatial, temporal, and demographic factors have been linked to variation in harvest rates of black bears, the effect of this heterogeneity on mark–recapture population estimates has not been evaluated. Failure to incorporate heterogeneity in harvest rates can result in biased population estimates and poor precision, which may lead to inappropriate management decisions. We used records of 6,982 bears captured during 1983–2001 in Pennsylvania, USA, to estimate the probability of harvest related to spatiotemporal, environmental, and demographic characteristics associated with individual bears. Harvest rates varied according to sex, age class, hunter density, and snow cover. In addition, harvest rates varied temporally (by year and month of capture) and spatially across Pennsylvania. Model selection based on Akaike's Information Criterion (AIC) supported a more complex harvest model based on a Horvitz-Thompson (H-T) estimator than the simpler model implicitly assumed by a series of annual L-P estimates. The H-T estimates of population size, which incorporated heterogeneity in harvest rates, were consistently lower than the annual L-P estimates because the H-T estimates accounted for mortality that occurred prior to the hunting season in addition to other sources of harvest heterogeneity. However, significant heterogeneity among breeding-age females could not be incorporated because we did not know the reproductive status (pregnant or with cubs) of each tagged and harvested female bear. Additional predictive variables of harvest rates of breeding-age females could further improve our model; however, an extension of our model that incorporates data from tagged cohorts may be an alternative means to improve the accuracy and precision of population estimates.

Accuracy and precision of population estimates of Verticillium dahliae on growth media in quantitative soil assays

Verticillium dahliae Kleb. is a serious pathogen of many plant species. Growth media used to measure population density of V. dahliae in soil were evaluated for high recovery of the pathogen, as well as accuracy and precision of population estimates from naturally and artificially infested sandy loam soil using the soil dilution method. Recovery of V. dahliae from naturally infested field soil was highest on soil pectate Tergitol agar (SPT), soil extract agar + sodium polypectate (SEAP), modified pectate agar (MPA), potato dextrose agar + streptomycin sulphate (PDAS), and Talboys' prune lactose agar (TPA); however, PDAS and TPA were overgrown with contaminating fungi, making enumeration difficult. Use of SPT medium resulted in the most precise population estimate with a standard error (SE) of 12.6% of the mean followed by use of pectate agar (PA) (SE = 14.8%) and SEAP (SE = 19.1%). Ethanol, biotin, and Dox salts enhanced recovery of V. dahliae from naturally infested soil, but combining Dox salts with ethanol and biotin significantly reduced population density. Soil extract had no significant effects on population density. Accuracy of V. dahliae population estimates from sterile artificially inoculated soil was highest on modified soil extract agar (MSEA) (64%) followed by SPT (58%). However, accuracy of V. dahliae population estimates from nonsterile artificially inoculated soil was highest on SPT (36%). Soil extract is not an essential ingredient and biotin may increase recovery of V. dahliae from soil.Key words: Verticillium dahliae, verticillium wilt, population density, recovery, accuracy, precision.

Sample Number and Colonization Patterns of Benthic Macroinvertebrates and Organic Material on Artificial and Natural Substrata

ABSTRACT We determined differences in colonization by benthic macroinvertebrates among two types of artificial substratum (single rocks and bricks) and natural substratum after it was physically disturbed. We also determined the number of samples required for the same level of precision using the artificial substrata and direct sampling of the natural substratum. The study was conducted in a slow moving riffle-run in the Battle River in Alberta, Canada. All measures of the macroinvertebrate fauna (densities of each taxon, total number of organisms, and total number of taxa) and quantities of organic material were greater on the natural substratum than on artificial substrata Physical disturbance of the natural substratum substantially reduced the macroinvertebrate community and quantity of organic material, and changed the composition and size of the natural substratum. But this disturbance to the substratum did not affect the colonization patterns of the macroinvertebrate fauna and organic material on the substratum types. Artificial substrata, similar those used in our study, are unlikely to give representative samples of macroinvertebrate fauna, especially for densities of zoobenthos taxa, or quantities of organic material on the natural substratum. The artificial substrata, in contrast to the natural substratum, required a larger number of samples for the same level of precision. Using the artificial substrata instead of direct sampling of the natural substratum therefore, does not appear to increase the precision of population estimates of benthic macroinvertebrates or quantities of organic material. Our results suggest that benthic macroinvertebrate communities and organic material in lotic systems might be sampled more efficiently using direct sampling of the natural substratum than the use of artificial substrata.

Sampling Shrimp Populations in Hatcheries

Abstract.– Shrimp populations were estimated with volumetric sampling techniques during different phases of hatchery operations. Factors considered which might effect precision of estimates included stage age, density and temperature. Estimates of nauplii were obtained with coefficient of variation values smaller than 3% when nauplii were concentrated in 20‐L buckets and smaller than 8% when released in 13,000 L of water in larval rearing tanks. In both cases estimates were of sufficient precision to make management decisions for routine hatchery work. A technique was developed for estimating shrimp populations in larval rearing tanks with coefficient of variation values smaller than 15%. Volumetric sampling of postlarvae older than PL4, in larval rearing tanks was highly variable due to the tendency of shrimp to congregate and attach to tank walls and bottoms. A direct relationship was observed between postlarval density and the precision of population estimates in harvest buckets. Water temperature and postlarval age did not affect precision at the densities and ages studied. These findings have important implications for commercial transactions. Industry standards utilizing coefficient of variation are recommended for larvae and postlarvae population estimates.