Estimating lizard population density: an empirical comparison between line-transect and capture–recapture methods

2013 ◽  
Vol 40 (7) ◽  
pp. 552 ◽  
Author(s):  
J. Ruiz de Infante Anton ◽  
A. Rotger ◽  
J. M. Igual ◽  
G. Tavecchia
Keyword(s):  
Population Density ◽  
Spatial Data ◽  
Natural Populations ◽  
Line Transect ◽  
Sampling Errors ◽  
Density Estimates ◽  
Mark Recapture ◽  
The Real ◽  
Podarcis Lilfordi ◽  
Capture Recapture

Context In most natural populations, exhaustive counts are not possible and estimates need to be derived from partial sampling by using analytical methods that account for biological processes, sampling errors and detection probability. The methods available have contrasting pitfalls and payoffs in relation to the assumptions made but are seldom contrasted on the same population. Aims We compared density estimates derived by different sampling methods. Despite the real density being unknown, the ‘soft’ validation of density estimates might help to better understand the possible pitfalls and payoffs of each method. This was done in three closed populations and with three different habitat typologies to disentangle the effects of different capture-detection processes to those introduced by the method itself. Methods We considered the problem of estimating population density of the endemic Balearic lizard, Podarcis lilfordi, in three island populations. We compared estimates derived by distance sampling (LT) in three types of habitat with those calculated from a simultaneous 3-day capture–mark–recapture study. Capture histories of marked individuals were used to estimate density using spatially explicit capture–recapture models (SECR) and a capture–mark–recapture model without spatial data (CMR). Moreover, we empirically assessed the influence of survey duration by extending the survey in the largest island to five occasions. The real population density was unknown and absolute accuracy of each method cannot be assessed; nevertheless, relative estimates might be informative. Key results LT estimates had the greatest coefficient of variation in vegetated habitats, corresponding to possible departures from model assumptions. SECR estimates differed among islands and were from 12% to 37% lower than those derived by LT but only in the largest islands with high and dense vegetation. CMR estimates depended on the number of occasions whereas SECR did not and showed lower variance. LT and SECR estimates showed differences across islets. Conclusions Line-transect and capture–recapture methods gave comparable results but the interaction between recapture processes and habitat types should be considered when inferring density to the whole area. We found density estimates between 1500 and 2500 individuals ha–1, being a higher value than those found for lizards in continental regions. Implications Pitfalls and payoffs of each method are discussed to optimise experimental design in estimating population density.

The influence of various factors on some methods of estimating fibre and follicle population density in the skin of Merino sheep. I. Methods of delineating area of natural skin

1958 ◽  
Vol 9 (2) ◽  
pp. 237 ◽  
Author(s):  
HB Carter ◽  
HN Turner ◽  
MH Hardy
Keyword(s):  
Population Density ◽  
Body Region ◽  
Skin Area ◽  
Sampling Errors ◽  
Density Estimates ◽  
Merino Sheep ◽  
Coefficients Of Variation ◽  
Biopsy Punch ◽  
The Mean ◽  
Influence Measurement

Many factors may influence measurement of skin area for estimation of fibre or follicle population density. This paper analyses the influence of method of delineation (hairpin calliper, Hardy clipper, or Carter biopsy punch), type of sheep (wrinkled or plain), body region, and sheep individuality on the mean density estimated and on the error of estimation. With either a biopsy punch or a 1 in.2 hairpin calliper in the midside region, sampling errors of the order of 8–11 per cent. were found for density estimates in a series of field observations on Merino and Corriedale ewes, with between-sheep coefficients of variation ranging from 13 to 18 per cent.

scholarly journals Assumptions about fence permeability influence density estimates for brown hyaenas across South Africa

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Kathryn S. Williams ◽  
Samual T. Williams ◽  
Rebecca J. Welch ◽  
Courtney J. Marneweck ◽  
Gareth K. H. Mann ◽  
...  
Keyword(s):  
South Africa ◽  
Population Density ◽  
Wildlife Management ◽  
Management Decisions ◽  
Density Estimates ◽  
Future Studies ◽  
Dominant Feature ◽  
Capture Recapture ◽  
Number Of Individuals ◽  
Density Results

AbstractWildlife population density estimates provide information on the number of individuals in an area and influence conservation management decisions. Thus, accuracy is vital. A dominant feature in many landscapes globally is fencing, yet the implications of fence permeability on density estimation using spatial capture-recapture modelling are seldom considered. We used camera trap data from 15 fenced reserves across South Africa to examine the density of brown hyaenas (Parahyaena brunnea). We estimated density and modelled its relationship with a suite of covariates when fenced reserve boundaries were assumed to be permeable or impermeable to hyaena movements. The best performing models were those that included only the influence of study site on both hyaena density and detection probability, regardless of assumptions of fence permeability. When fences were considered impermeable, densities ranged from 2.55 to 15.06 animals per 100 km2, but when fences were considered permeable, density estimates were on average 9.52 times lower (from 0.17 to 1.59 animals per 100 km2). Fence permeability should therefore be an essential consideration when estimating density, especially since density results can considerably influence wildlife management decisions. In the absence of strong evidence to the contrary, future studies in fenced areas should assume some degree of permeability in order to avoid overestimating population density.

scholarly journals Estimating the density of small population of leopard Panthera pardus using multi-session photographic□sampling data

2020 ◽  
Author(s):  
Mohammad S. Farhadinia ◽  
Pouyan Behnoud ◽  
Kaveh Hobeali ◽  
Seyed Jalal Mousavi ◽  
Fatemeh Hosseini-Zavarei ◽  
...  
Keyword(s):  
Population Density ◽  
Arid Regions ◽  
Animal Movement ◽  
Small Population ◽  
Conservation Priorities ◽  
Large Carnivores ◽  
Panthera Pardus ◽  
Demographic Stochasticity ◽  
Density Estimates ◽  
Capture Recapture

AbstractWest Asian drylands host a number of threatened large carnivores, including the leopard (Panthera pardus) which is limited to spatially scattered landscapes with generally low primary productivity. While conservation efforts have focused on these areas for several decades, reliable population density estimates are missing. Spatially-explicit capture-recapture (SECR) methodology, incorporating animal movement in density estimates, is widely used to monitor populations of large carnivores. We employed multi-session SECR modeling to estimate the density of a small population of leopard (Panthera pardus) in a mountainous stretch surrounded by deserts in central Iran. During 6724 camera trap nights, we detected eight and five independent leopards in 2012 and 2016 sessions, respectively. The top performing model demonstrated density estimates of 1.6 (95% CI = 0.9-2.9) and 1.0 (95% CI = 0.6-1.6) independent leopards/100 km2 in 2012 and 2016, respectively. Both sex and season had substantial effects on spatial scale (σ), with larger movements for males and during winter. Currently available estimates in arid regions represent some of the lowest densities across the leopard global range. These small populations are vulnerable to demographic stochasticity. Monitoring temporal changes in population density and composition can inform conservation priorities.

scholarly journals Spatial Mark-Resight for Categorically Marked Populations with an Application to Genetic Capture-Recapture

2018 ◽  
Author(s):  
Ben C. Augustine ◽  
Frances E. C. Stewart ◽  
J. Andrew Royle ◽  
Jason T. Fisher ◽  
Marcella J. Kelly
Keyword(s):  
Individual Heterogeneity ◽  
Positive Bias ◽  
Telemetry Data ◽  
Detection Function ◽  
Noninvasive Methods ◽  
Data Set ◽  
Density Estimates ◽  
Mark Recapture ◽  
Capture Recapture ◽  
Scr Model

AbstractThe estimation of animal population density is a fundamental goal in wildlife ecology and management, commonly met using mark recapture or spatial mark recapture (SCR) study designs and statistical methods. Mark-recapture methods require the identification of individuals; however, for many species and sampling methods, particularly noninvasive methods, no individuals or only a subset of individuals are individually identifiable. The unmarked SCR model, theoretically, can estimate the density of unmarked populations; however, it produces biased and imprecise density estimates in many sampling scenarios typically encountered. Spatial mark-resight (SMR) models extend the unmarked SCR model in three ways: 1) by introducing a subset of individuals that are marked and individually identifiable, 2) introducing the possibility of individual-linked telemetry data, and 3) introducing the possibility that the capture-recapture data from the survey used to deploy the marks can be used in a joint model, all improving the reliability of density estimates. The categorical spatial partial identity model (SPIM) improves the reliability of density estimates over unmarked SCR along another dimension, by adding categorical identity covariates that improve the probabilistic association of the latent identity samples. Here, we combine these two models into a “categorical SMR” model to exploit the benefits of both models simultaneously. We demonstrate using simulations that SMR alone can produce biased and imprecise density estimates with sparse data and/or when few individuals are marked. Then, using a fisher (Pekania pennanti) genetic capture-recapture data set, we show how categorical identity covariates, marked individuals, telemetry data, and jointly modeling the capture survey used to deploy marks with the resighting survey all combine to improve inference over the unmarked SCR model. As previously seen in an application of the categorical SPIM to a real-world data set, the fisher data set demonstrates that individual heterogeneity in detection function parameters, especially the spatial scale parameter σ, introduces positive bias into latent identity SCR models (e.g., unmarked SCR, SMR), but the categorical SMR model provides more tools to reduce this positive bias than SMR or the categorical SPIM alone. We introduce the possibility of detection functions that vary by identity category level, which will remove individual heterogeneity in detection function parameters than is explained by categorical covariates, such as individual sex. Finally, we provide efficient SMR algorithms that accommodate all SMR sample types, interspersed marking and sighting periods, and any number of identity covariates using the 2-dimensional individual by trap data in conjunction with precomputed constraint matrices, rather than the 3-dimensional individual by trap by occasion data used in SMR algorithms to date.

Seed dispersal, habitat selection and movement patterns in the Amazonian tortoise, Geochelone denticulata

2008 ◽  
Vol 29 (4) ◽  
pp. 463-472 ◽  
Author(s):  
Pablo Stevenson ◽  
Adriana Guzmán
Keyword(s):  
Population Density ◽  
Seed Dispersal ◽  
Habitat Use ◽  
Plant Species ◽  
Forest Type ◽  
Movement Patterns ◽  
Line Transect ◽  
Mark Recapture ◽  
High Solar Radiation ◽  
Dispersal Quality

AbstractThe Amazonian tortoise Geochelone denticulata may play an important role in forest dynamics due to its highly frugivorous diet, ability to disperse viable seeds, and predilection for resting in forest gaps for thermoregulation. The purpose of our study was to evaluate the species' effectiveness as a seed disperser. We measured dispersal quantity (abundance of seeds in feces, frequency of droppings, and population density of the disperser) and dispersal quality (movement patterns, habitat use, germination rates of dispersed seeds, and recruitment probabilities of seedlings) in a SW Amazonian forest, in Peru. Population density was calculated by mark-recapture and line-transect methods. Eight individuals were radio-tracked to monitor habitat use. Diet was described from fecal samples, which were washed to count seeds and for germination experiments. Seedling survival in different environmental conditions was monitored for three plant species. Population densities with mark-recapture estimates (0.15-0.31 individuals/ha) were much higher than with line transects estimates (0.0025 individuals/ha). Diet included fruit of 55 different plant species. Dispersed seeds had high germination rates (average 76%). In spite of their low activity, we documented long seed dispersal distances (average 89.6 m). Tortoises showed a marked preference for the open-canopy swampy forest, where long term recruitment was not favorable for seedlings of the species examined. However, the high solar radiation in this forest type promoted survival of pioneer seedlings in the short term. In conclusion, while G. denticulata did not perform a very efficient role in terms of the quantity of seed dispersal, the species can be considered efficient in many aspects of dispersal quality.

scholarly journals Estimating the density of a small population of leopards (Panthera pardus) in central Iran using multi-session photographic‐sampling data

2021 ◽  
Author(s):  
Mohammad S. Farhadinia ◽  
Pouyan Behnoud ◽  
Kaveh Hobeali ◽  
Seyed Jalal Mousavi ◽  
Fatemeh Hosseini-Zavarei ◽  
...  
Keyword(s):  
Population Density ◽  
Density Estimation ◽  
Animal Movement ◽  
Small Population ◽  
Central Iran ◽  
Large Carnivores ◽  
Panthera Pardus ◽  
Demographic Stochasticity ◽  
Density Estimates ◽  
Capture Recapture

AbstractWest Asian drylands host a number of threatened large carnivores, including the leopard (Panthera pardus) which is limited generally to areas with low primary productivity. While conservation efforts have focused on these areas for several decades, reliable population density estimates are missing for many of them. Spatially explicit capture–recapture (SECR) methodology is a widely accepted population density estimation tool to monitor populations of large carnivores and it incorporates animal movement in the statistical estimation process. We employed multi-session maximum-likelihood SECR modeling to estimate the density of a small population of leopard in a mountainous environment surrounded by deserts in central Iran. During 6724 camera trap nights, we detected 8 and 5 independent leopards in 2012 and 2016 sessions, respectively. The top-performing model produced density estimates of 1.6 (95% CI = 0.9–2.9) and 1.0 (95% CI = 0.6–1.6) independent leopards/100 km2 in 2012 and 2016, respectively. Both sex and season had substantial effects on spatial scale (σ), with larger movements recorded for males, and during winter. The estimates from our density estimation exercise represent some of the lowest densities across the leopard global range and strengthen the notion that arid habitats support low densities of the species. These small populations are vulnerable to demographic stochasticity, and monitoring temporal changes in their population density and composition is a critical tool in assisting conservation managers to better understand their population performance.

scholarly journals Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii

2021 ◽  
Author(s):  
Konrad Lipkowski ◽  
Sophie Steigerwald ◽  
Lisa M Schulte ◽  
Carolin Sommer-Trembo ◽  
Jonas Jourdan
Keyword(s):  
Population Density ◽  
Sex Ratio ◽  
Natural Variation ◽  
Natural Populations ◽  
Social Background ◽  
Social Conditions ◽  
Background Information ◽  
Male Mating ◽  
Natural Environments ◽  
Male Mate

Abstract The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.

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