Small Sampling Units Research Articles

FACTORS INFLUENCING INVESTMENT IN COMMERCIAL REAL ESTATE IN JOS PLATEAU STATE

The investors in real estate are faced with challenges as often stated of knowing if the investment they make will generate returns. The paper aimed to analyse factors influencing investment in commercial real estate in Jos Plateau State, with a view to determining the level of returns of real estate investment. The study adopts survey method using quantitative approach and survey strategy was adopted and data were collected through questionnaire survey. The study adopts multistage sampling because the sampling technique involves taking samples in stages from smaller sampling units. The study revealed that factors that influence residential and commercial real estate were availability of social amenities, return and profit from real estate investment, availability and cost of acquiring land for investment and location advantage of the area. Urban planners and developers should prioritise the inclusion of social amenities in new developments. Collaborate with local communities to identify their specific needs and preferences. Government agencies should invest in infrastructure and programs that support the development and maintenance of social amenities. This includes funding for schools, parks, and public transportation. Real estate investors should consider social amenities as a primary factor when evaluating investment opportunities. Conduct thorough research on the availability and quality of amenities in target neighborhoods.

High-Resolution Transect Sampling and Multiple Scale Diversity Analyses for Evaluating Grassland Resilience to Climatic Extremes

Diversity responses to climatic factors in plant communities are well understood from experiments, but less known in natural conditions due to the rarity of appropriate long-term observational data. In this paper, we use long-term transect data sampled annually in three natural grasslands of different species pools, soils, landscape contexts and land use histories. Analyzing these specific belt transect data of contiguous small sampling units enabled us to explore scale dependence and spatial synchrony of diversity patterns within and among sites. The 14-year study period covered several droughts, including one extreme event between 2011 and 2012. We demonstrated that all natural grasslands responded to droughts by considerable fluctuations of diversity, but, overall, they remained stable. The plant functional group of annuals showed high resilience at all sites, while perennials were resistant to droughts. Our results were robust to changing spatial scales of observations, and we also demonstrated that within-site spatial synchrony could be used as a sensitive indicator of external climatic effects. We propose the broad application of high-resolution belt transects for powerful and adaptive vegetation monitoring in the future.

Soil physicochemical properties shape distinct nematode communities in serpentine ecosystems

Serpentine ecosystems have unique plant communities that have overcome the chemical and physical challenges presented by these soils. However, such a specific soil fauna community has not been identified. It is presumed that such communities are affected directly by the chemical and physical properties of serpentine soils. Here, we focused on nematodes, which comprise a ubiquitous and an abundant group in soil fauna, for comparing their community structure in serpentine and in non-serpentine soils based on morphological approaches. Root biomass, Mg, and Ni were higher in serpentine habitats than non-serpentine ones. In contrast, the soil water content, NO3− and Ca/Mg ratio were higher at non-serpentine habitats than serpentine ones. Among the nematode community indices, the channel index, which indicates the degree of fungal participation in the decomposition channels of soil food webs, was greater in serpentine (94.5 ± 4.8) than in non-serpentine (44.8 ± 3.2) habitats, whereas the enrichment index, which indicates the abundance and activity of primary detrital consumers, was greater in non-serpentine (56.2 ± 3.7) than in serpentine (40.2 ± 2.9) habitats. A non-metric multidimensional scaling scatterplot showed that nematode communities clustered into both serpentine and non-serpentine habitats by a permutational multivariate analysis of variance. Ca/Mg ratio accounted for the most variation in nematode communities: 90 % of the variation between the sites by envfit. Furthermore, relative frequencies of nine taxa (Filenchus, Malenchus, Mesodorylaimus, Monhystera, Paratylenchus, Plectus, Prodesmodora, Teratocephalus, and Tripyla) were greater in serpentine than in non-serpentine habitats, and those of five taxa (Alaimus, Bastiania, Cervidellus, Diptherophora, and Rhabditidae) were greater in non-serpentine than in serpentine ones by indicator species analysis. Although our study was a small sample size and unit, our findings suggest that the structure of nematode communities reflects the physicochemical properties of serpentine soils.

Prevalence of soil - borne nematode infections among residents living in urban/town areas of China in 2015

To understand the prevalence of soil-borne nematode infections among residents living in urban/town areas of China, so as to provide insights into the control and elimination of soil-borne nematodiasis. A total of 5 epidemic areas were classified in China according to the prevalence of human Clonorchis sinensis infections captured from the 2014-2015 national survey on major human parasitic diseases in China, and the total sample size was estimated according to the binomial distribution and Poisson's distribution. Then, the total sample size was allocated proportionally to each province (autonomous region, municipality) of China based on the percentage of residents living in urban and town areas, and the number of survey sites in each province (autonomous region, municipality) was proportionally assigned according to the percentages of residents living in urban and town areas. Then, stratified sampling was performed at county, township and community levels according to the number of sampling sites in each province (autonomous region, municipality), and the survey site (community) was defined as the smallest sampling unit. All permanent residents in the survey sites were selected as the study subjects, and their stool samples were collected for identification and counting of parasite egg using a Kato-Katz technique. The prevalence and intensity of each parasite species were calculated. From 2014 to 2015, among the 133 231 residents detected in 31 provinces (autonomous regions, municipalities) of China, the overall prevalence of soil-borne nematode infections was 1.23% (1 636/133 231), and the prevalence rates of hookworm, Ascaris lumbricoides and Trichuris trichiura infections were 0.77% (1 032/133 231), 0.32% (426/133 231) and 0.17% (224/133 231), respectively. The highest prevalence of soil-borne nematode infections was seen in Jiangxi (4.03%, 82/2 034) and Chongqing (4.03%, 524/13 012), followed by in Hainan (3.47%, 72/2 075). The prevalence of soilborne nematode infections was 1.07% (662/62 139) in men and 1.37% (974/71 092) in women, and the greatest prevalence was found in residents at ages of 65 to 70 years (2.56%, 219/8 569). With regard to occupations and education levels, herdsmen (2.47%, 2/81) and illiterate residents (3.33%, 226/6 795) were found to have the highest prevalence of soil-borne nematode infections, respectively. In addition, mild infections were predominantly identified in hookworm-, A. lumbricoides- and T. trichiura-infected individuals (all > 90%). The overall prevalence of soil-borne nematodiasis remains low in urban and town areas of China; however, human infections are widespread. According to the epidemiological features, health education combined with deworming are recommended to reduce the prevalence of soil-borne nematode infections among residents living in urban and town areas of China.

Accuracy Assessment of the FROM-GLC30 Land Cover Dataset Based on Watershed Sampling Units: A Continental-Scale Study

Land cover information plays an essential role in the study of global surface change. Multiple land cover datasets have been produced to meet various application needs. The FROM-GLC30 (Finer Resolution Observation and Monitoring of Global Land Cover) dataset is one of the latest land cover products with a resolution of 30 m, which is a relatively high resolution among global public datasets, and the accuracy of this dataset is of great concern in many related researches. The objective of this study was to calculate the accuracy of the FROM-GLC30 2017 dataset at the continental scale and to explore the spatial variation differences of each land type accuracy in different regions. In this study, the visual interpretation land cover results at 20,936 small watershed sampling units based on high-resolution remote sensing images were used as the reference data covering 65 countries in Asia, Europe, and Africa. The reference data were verified by field survey in typical watersheds. Based on that, the accuracy assessment of the FROM-GLC30 2017 dataset was carried out. The results showed (1) the area proportion of different land cover types in the FROM-GLC30 2017 dataset was generally consistent with that of the reference data. (2) The overall accuracy of the FROM-GLC30 2017 dataset was 72.78%, and was highest in West Asia–Northeast Africa, and lowest in South Asia. (3) Among all the seven land cover types, the accuracy of bareland and forest was relatively higher than that of others, and the accuracy of shrubland was the lowest. The accuracy for each land cover type differed among regions. The results of this work can provide useful information for land cover accuracy assessment researches at a large scale and promote the further practical applications of the open-source land cover datasets.

Spatial pattern of intraspecific trait variability inSphagnum fuscum

Recent studies have shown that intraspecific variability is a mechanism by which species respond to environmental heterogeneity, and that intraspecific variation can have large implications for ecological processes. Here, we studied whether there is meaningful intraspecific variation in the ecohydrological traits, biomass allocation, and decomposability in Sphagnum moss, and if so, to explore the spatial pattern of variability. We implemented a hierarchical design in which we quantified traits of S. fuscum at three spatial scales: (i) between individuals within 8 cm2patches; (ii) between replicate patches located within a single hummock or hollow location; and (iii) between hummocks. Although we focused on S. fuscum, we also compared the variability in some morphological features of S. fuscum and S. magellanicum. If growth is affected by density, we expected variability to be lowest at the patch level. Contrary to our expectation, most of the variability in both species occurred within-patch, which is our smallest sampling unit. Variability was generally higher in the traits for S. magellanicum compared with the variability in the traits for S. fuscum, which was generally negligible. Also, the pattern of variability observed for some of the traits such as the capitulum mass suggests that the mechanisms controlling different traits may be operating at different spatial scales.

Assessing Post-Harvest Regeneration in Northern Hardwood and Mixedwood Stands: Evolution of Species Composition and Dominance within 15-Year-Old Group Selection and Patch Cutting

Multi-cohort forest management in northern hardwood stands may well be the best way to successfully regenerate tree species of intermediate shade tolerance, such as yellow birch (Betula alleghaniensis Britt.). The creation of large enough gaps in the canopy favors increased light availability within the opening, while soil scarification provides suitable germination seedbeds. Evidence of these methods’ success nonetheless remains mostly the purview of experimental studies rather than operational tests. In Quebec, Canada, the multi-cohort methods promoted include group selection cutting and patch cutting. The present study tested their implementation at an operational scale and over a large territory in both hardwood-dominated and mixedwood stands. We assessed their efficacy in promoting natural regeneration of commercial hardwood trees, notably yellow birch and sugar maple (Acer saccharum Marsh.). We conducted regeneration surveys at 2, 5, 10, and 15 years after harvest. Overall, group selection and patch cuttings were successful in regenerating the target species. Yellow birch, for instance, showed a mean stocking around 60% and a mean sapling density around 3400 stems ha−1 after 15 years. We compared several variables for measuring regeneration in early years, and found that the relative abundance, the stocking based on one stem per sampling unit, and the mean maximum height were good predictors of the relative presence of yellow birch and sugar maple in 15-year-old canopy openings. Using smaller sampling units (6.25 m2 rather than 25 m2) and waiting until year 5 may be more useful for making such predictions. In addition, there was an important turnover in vertical dominance in these openings. Non-commercial woody competitors were frequently dominant in early years but were often replaced by commercial hardwoods, notably yellow birch. We propose certain thresholds for assessing the success of post-harvest regeneration and for evaluating the need for a cleaning treatment.

Influence of light availability and soil productivity on insect herbivory on bilberry (Vaccinium myrtillusL.) leaves following mammalian herbivory.

Vegetative parts of bilberry (Vaccinium myrtillus) are important forage for many boreal forest mammal, bird and insect species. Plant palatability to insects is affected by concentration of nutrients and defense compounds in plants. We expected that palatability of bilberry leaves to insect herbivores is influenced by light availability and soil productivity (both affecting nitrogen concentration and constitutive carbon-based defense compound concentration) and herbivory by mammals (affecting nitrogen concentration and induced carbon-based defense compound concentration). We studied bilberry leaf herbivory under different light availability, soil productivity and mammalian herbivory pressure in small sampling units (1m x 1m) in boreal forest in Norway. We used generalized linear mixed models and generalized additive mixed models to model insect herbivory on bilberry leaves as a function of shade, soil productivity and mammalian herbivory. Observed insect herbivory on bilberry leaves increased with increasing shade levels. Predicted insect herbivory increased with increasing previous mammalian herbivory at high shade levels and this response was magnified at higher soil productivity levels. At low to intermediate shade levels, this response was only present under high soil productivity levels. Our results indicate that light availability is more important for variation in bilberry leaf palatability than soil nutrient conditions.

Within-tree distribution of Ricania shantungensis (Hemiptera: Ricaniidae) eggs on chestnuts

Abstract Within-tree distribution of an insect pest is useful to determine sample unit, sampling location, and required sample number for this pest within a tree. This study was conducted to characterize the within-tree distribution pattern and determine an efficient sample unit for eggs of Ricania shantungensis (Hemiptera: Ricaniidae) on a chestnut tree. A total of 60 chestnut trees were randomly selected from chestnut fields in Gongju-Si and Buyeo-Gun, Korea in 2019. For each tree, 12 branches were cut at three vertical heights (low, middle, and high) in four cardinal directions (east, west, south, and north). The cut branches were observed for counting the number of egg masses per 10 cm from the branch tip. Coefficient of variation (CV) and coefficient of determination (r2) were calculated and used to determine an optimal sample unit for eggs of R. shantungensis on chestnut trees. Numbers of R. shantungensis egg masses were not significantly different (P > 0.05) vertically and horizontally within a tree. Based on CV and r2 values, the 50 cm branch tip would be the optimal sample unit for R. shantungensis eggs. Using this small sample unit for sampling of R. shantungensis eggs could be economical and efficient.

Detecção automática de árvores em unidades amostrais a partir de varredura simples com laser terrestre

O objetivo deste estudo foi a identificação automática de árvores em unidades amostrais circulares de 200, 300 e 400 m² em povoamentos de Eucalyptus spp. a partir de dados de TLS (Terrestrial Laser Scanning) no estado do Mato Grosso do Sul. Foram testadas três idades de plantio: árvores com 2 anos, sendo consideradas com desrama natural (E. urophylla) e sem desrama natural (E. grandis); árvores com 4,5 e com 5,5 anos, sem desrama (hibrido E. urophylla x E. grandis). A varredura laser em campo foi realizada com o equipamento Trimble TX5. Para a identificação automática das árvores, foi realizado um recorte na nuvem de pontos a 1,30 m do solo (DAP) e desenvolvido o algoritmo denominado fatia 2D. Foi possível a detecção de 98,3, 98 e 93,9% das árvores para as unidades amostrais de 200, 300 e 400 m², respectivamente. A distribuição espacial das árvores no plantio influenciou a identificação, ocasionando sombreamento das árvores mais distantes do equipamento laser. A adoção de varreduras simples deve ser aplicada somente em unidades amostrais pequenas, pois unidades amostrais acima de 200 m² acarretam maiores erros de detecção de árvores, o que pode gerar informações equivocadas nas estimativas dendrométricas da floresta.

Plants on small islands revisited: the effects of spatial scale and habitat quality on the species–area relationship

Understanding how species diversity is related to sampling area and spatial scale is central to ecology and biogeography. Small islands and small sampling units support fewer species than larger ones. However, the factors influencing species richness may not be consistent across scales. Richness at local scales is primarily affected by small‐scale environmental factors, stochasticity and the richness at the island scale. Richness at whole‐island scale, however, is usually strongly related to island area, isolation and habitat diversity. Despite these contrasting drivers at local and island scales, island species–area relationships (SARs) are often constructed based on richness sampled at the local scale. Whether local scale samples adequately predict richness at the island scale and how local scale samples influence the island SAR remains poorly understood. We investigated the effects of different sampling scales on the SAR of trees on 60 small islands in the Raja Ampat archipelago (Indonesia) using standardised transects and a hierarchically nested sampling design. We compared species richness at different grain sizes ranging from single (sub)transects to whole islands and tested whether the shape of the SAR changed with sampling scale. We then determined the importance of island area, isolation, shape and habitat quality at each scale on species richness. We found strong support for scale dependency of the SAR. The SAR changed from exponential shape at local sampling scales to sigmoidal shape at the island scale indicating variation of species richness independent of area for small islands and hence the presence of a small‐island effect. Island area was the most important variable explaining species richness at all scales, but habitat quality was also important at local scales. We conclude that the SAR and drivers of species richness are influenced by sampling scale, and that the sampling design for assessing the island SARs therefore requires careful consideration.

Bayesian degradation assessment of CNC machine tools considering unit non-homogeneity

Field reliability assessment and prediction is critical for the estimation, operation and health management of CNC machine tools. The classical methods for field reliability of CNC Machine Tools assessment and prediction are challenged with the issues of expensive reliability tests, small sample size and unit non-homogeneity. In order to solve these problems, this paper introduces a degradation analysis based reliability assessment method for CNC machine tools under performance testing. Since the degradation is an independent increment process, the gamma process is employed to characterize the degradation process of CNC machine tools. The random effects are introduced to accommodate performance degradation model with unit non-homogeneity. The parameters of model are updated by Bayesian estimation approach. As a case study, the CNC Machine Tools is studied to illustrate the approach. And the proposed method is demonstrated precise for practical use.

Two-Level Hybrid Sampled Simulation of Multithreaded Applications

Sampled microarchitectural simulation of single-threaded applications is mature technology for over a decade now. Sampling multithreaded applications, on the other hand, is much more complicated. Not until very recently have researchers proposed solutions for sampled simulation of multithreaded applications. Time-Based Sampling (TBS) samples multithreaded application execution based on time—not instructions as is typically done for single-threaded applications—yielding estimates for a multithreaded application’s execution time. In this article, we revisit and analyze previously proposed TBS approaches (periodic and cantor fractal based sampling), and we obtain a number of novel and surprising insights, such as (i) accurately estimating fast-forwarding IPC , that is, performance in-between sampling units, is more important than accurately estimating sample IPC , that is, performance within the sampling units; (ii) fast-forwarding IPC estimation accuracy is determined by both the sampling unit distribution and how to use the sampling units to predict fast-forwarding IPC; and (iii) cantor sampling is more accurate at small sampling unit sizes, whereas periodic is more accurate at large sampling unit sizes. These insights lead to the development of Two-level Hybrid Sampling (THS) , a novel sampling methodology for multithreaded applications that combines periodic sampling’s accuracy at large time scales (i.e., uniformly selecting coarse-grain sampling units across the entire program execution) with cantor sampling’s accuracy at small time scales (i.e., the ability to accurately predict fast-forwarding IPC in-between small sampling units). The clustered occurrence of small sampling units under cantor sampling also enables shortened warmup and thus enhanced simulation speed. Overall, THS achieves an average absolute execution time prediction error of 4% while yielding an average simulation speedup of 40 × compared to detailed simulation, which is both more accurate and faster than the current state-of-the-art. Case studies illustrate THS’ ability to accurately predict relative performance differences across the design space.

Accuracy or precision: Implications of sample design and methodology on abundance estimation

Sampling by spatially replicated counts (point-count) is an increasingly popular method of estimating population size of organisms. Challenges exist when sampling by point-count method, and it is often impractical to sample entire area of interest and impossible to detect every individual present. Ecologists encounter logistical limitations that force them to sample either few large-sample units or many small sample-units, introducing biases to sample counts. We generated a computer environment and simulated sampling scenarios to test the role of number of samples, sample unit area, number of organisms, and distribution of organisms in the estimation of population sizes using N-mixture models. Many sample units of small area provided estimates that were consistently closer to true abundance than sample scenarios with few sample units of large area. However, sample scenarios with few sample units of large area provided more precise abundance estimates than abundance estimates derived from sample scenarios with many sample units of small area. It is important to consider accuracy and precision of abundance estimates during the sample design process with study goals and objectives fully recognized, although and with consequence, consideration of accuracy and precision of abundance estimates is often an afterthought that occurs during the data analysis process.

Quantifying the effect of sampling plot size on the estimation of structural indicators in old-growth forest stands

There is increasing awareness that structure-based indicators should be considered for assessing the biological value of late successional forests. In order to increase the unique habitat features critical for old-growth associated species, it is important to identify and rank candidate potential forest sites on the basis of their distinctive structural features. Data on living and deadwood components for the identification of old-growth condition are usually acquired in the considered forest stands by two sampling survey: (i) census performed in relatively large monitoring sites; (ii) network of small sampling units, on which inventory practices are usually based. Several authors argued that choosing between these survey strategies might have substantial effects on the values of common indicators of old-growth condition. Our study aims at (i) assessing the total estimate differences among old-growth structural indicators measured in field plots with different sizes, and (ii) defining the optimal sample size for the reliable assessment of such indicators. The study was carried out in six beech dominated forest stands on the Apennines range in Italy. In each stand, living and deadwood components were surveyed and geocoded in 1-ha square areas. Based on these dataset, circular plots with radii ranging from 4m up to 20m were then considered in order to quantify the effect of sampling plot size on the estimation of four structural indicators: (1) number of living trees; (2) number of large trees (dbh⩾50cm); (3) total deadwood volume; (4) number of deadwood elements (snags, dead standing trees; lying dead trees, lying deadwood) with dbh (or average diameter for lying deadwood) ⩾ 30cm. We found that the size of the sampling plots should be at least 500 m2 in order to establish a database for the assessment of the investigated indicators. The census approach should be preferred to the sampling plot approach for old-growth forest stands smaller than 3–5ha. The achieved results contribute to define assessment protocols for characterizing and ranking the degree to which forest stands approximate old-growth condition based on standardized indicators.

Canopy height model characteristics derived from airbone laser scanning and its effectiveness in discriminating various tropical moist forest types

Mapping tropical forests to a sufficient level of spatial resolution and structural detail is a prerequisite for their rational management, which however remains a largely unmet challenge. We explore the degree to which a forest canopy height model (CHM) derived from airborne laser scanning (ALS) can discriminate between five forest types of similar height but varying structure or composition. We systematically compare various textural features (Haralick, Fourier transform-based, and wavelet-based features) and various classification procedures (linear discriminant analysis (LDA), random forest(RF), and support vector machine (SVM)) applied to two sizes of sampling units (64 m × 64 m and 32 m × 32 m). Simple height distribution statistics achieve at best 70% classification accuracy in our sample set comprising 120 sampling units of 64 m × 64 m. Using w avelet-based features, this accuracy increases to 79% but drops by 10% with smaller sampling units (32 m × 32 m). Classifier performance depends on the texture feature set used, but SVM and RF tend to perform better than LDA. High discrimination rates between forests types of similar height indicate that the ALS-derived CHM provides information suitable for mapping of tropical forest types. Wavelet-based texture features coupled with a SVM classifier was found to be the most promising combination of methods. Ancillary data derived from laser scans and notably topography could be used jointly for an improved segmentation scheme.

Geostatistical analysis of adult Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) in wheat stored at constant temperatures

Insect monitoring and sampling programmes are used in the stored grains industry for the detection and estimation of insect pests. At the low pest densities dictated by economic and commercial requirements, the accuracy of both detection and abundance estimates can be influenced by variations in the spatial structure of pest populations over short distances. Geostatistical analysis of Rhyzopertha dominica populations in 2-dimensions showed that, in both the horizontal and vertical directions and at all temperatures examined, insect numbers were positively correlated over short (0–5 cm) distances, and negatively correlated over longer (≥10 cm) distances. Analysis in 3 dimensions showed a similar pattern, with positive correlations over short distances and negative correlations at longer distances. At 35 °C, insects were located significantly further from the grain surface than at 25 and 30 °C. Dispersion metrics showed statistically significant aggregation in all cases. This is the first research using small sample units, high sampling intensities, and a range of temperatures, to show spatial structuring of R. dominica populations over short distances. This research will have significant implications for sampling in the stored grains industry.

Limited effects of dominant ants on assemblage species richness in three Amazon forests

1. Ants are highly interactive organisms and dominant species are considered to be able to control the species richness of other ants via competitive exclusion. However, depending on the scale studied, inter-specific competition may or may not structure biological assemblages. To date, ant dominance–richness relationships have only been studied in small sample units, where a few dominant colonies could plausibly control most of the sample unit. 2. We conducted a comprehensive survey of terrestrial ant assemblages using bait, pitfall, and litter-sorting methods in three sites in Brazilian Amazonia. Using a spatially structured rarefaction approach, based on sampling units with linear dimensions ranging from 25 to 250 m, the mesoscale patterns of ant dominance–richness relationships (sampling units covering hundreds of meters separated by kilometers) were investigated. 3. Interference–competition models (parabolic or negative linear relationships between species richness and the abundance of dominant ants) tended to be more frequent in smaller sample units or in assemblages sampled with interactive methods, such as baits. Using more inclusive sampling methods, the relationship was generally asymptotic rather than parabolic, with no reduction in species diversity because of the presence of dominants. Random co-occurrence patterns of species within sites support the interpretation of a limited role for present-day competition in structuring these assemblages. 4. Competition from dominant species may reduce species richness in small areas, especially when artificial baits are used, but appears to be less important than environmental constraints in determining ant species richness across scales of hectares and greater in these Amazon forests.

Using multilevel spatial models to understand salamander site occupancy patterns after wildfire

Studies of the distribution of elusive forest wildlife have suffered from the confounding of true presence with the uncertainty of detection. Occupancy modeling, which incorporates probabilities of species detection conditional on presence, is an emerging approach for reducing observation bias. However, the current likelihood modeling framework is restrictive for handling unexplained sources of variation in the response that may occur when there are dependence structures such as smaller sampling units that are nested within larger sampling units. We used multilevel Bayesian occupancy modeling to handle dependence structures and to partition sources of variation in occupancy of sites by terrestrial salamanders (family Plethodontidae) within and surrounding an earlier wildfire in western Oregon, USA. Comparison of model fit favored a spatial N-mixture model that accounted for variation in salamander abundance over models that were based on binary detection/non-detection data. Though catch per unit effort was higher in burned areas than unburned, there was strong support that this pattern was due to a higher probability of capture for individuals in burned plots. Within the burn, the odds of capturing an individual given it was present were 2.06 times the odds outside the burn, reflecting reduced complexity of ground cover in the burn. Ther was weak support that true occupancy was lower within the burned area. While the odds of occupancy in the burn were 0.49 times the odds outside the burn among the five species, the magnitude of variation attributed to the burn was small in comparison to variation attributed to other landscape variables and to unexplained, spatially autocorrelated random variation. While ordinary occupancy models may separate the biological pattern of interest from variation in detection probability when all sources of variation are known, the addition of random effects structures for unexplained sources of variation in occupancy and detection probability may often more appropriately represent levels of uncertainty.

Nested sampling: an improved visual-census technique for studying reef fish assemblages

Our aim was to devise visual sampling protocols to estimate densities in a morphologi- cally and behaviorally diverse assemblage of reef fishes. The focus was on a design that is cost- effective and produces relatively accurate and precise density estimates for the largest number of species. We evaluated the 2 most widely used shapes of sampling units (strip transects and stationary cylinders) of several dimensions on a fringing reef at Fernando de Noronha Archipelago, tropical West Atlantic. Small sampling units produced the best density estimates for small fishes, while large ones produced the best estimates for large fishes, regardless of sampling unit shape. The sampling unit with the lowest cost was the stationary cylinder. We further compared accuracy and precision of density and richness estimates from several combinations of stationary cylinders and fish size. A simple improvement in stationary censuses, i.e. a sampling unit composed of 2 nested cylinders (2 m and 4 m radii, respectively), enhanced the accuracy and precision of estimates. Counts of small fish (≤10 cm) were made in the smaller cylinder and counts of larger fish (>10 cm) were made in the larger. Such nested cylinders reduce the bias towards large and conspicuous species and allow for separate density estimates for adults and juveniles of large species.