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Try out PMC Labs and tell us what you think. Learn More. Supporting Information Table S1: Pride-specific information used to calculate season-habitat reference densities. Supporting Information Table S2: of follow hours in hours and cumulative distance moved in km used to estimate average speed of lion movement. The random encounter model REM is a novel method for estimating animal density from camera trap data without the need for individual recognition. It has never been used to estimate the density of large carnivore species, despite these being the focus of most camera trap studies worldwide.
In this context, we applied the REM to estimate the density of female lions Panthera leo from camera traps implemented in Serengeti National Park, Tanzania, comparing estimates to reference values derived from pride census data.
More specifically, we attempted to for bias resulting from non-random camera placement at lion resting sites under isolated trees by comparing estimates derived from night versus day photographs, between dry and wet seasons, and between habitats that differ in their amount of tree cover. Overall, we recorded and independent photographic events of female lions from 7, and 12, camera trap days carried out in the dry season of and the wet season ofrespectively.
Although all REM models considered over-estimated female lion density, models that considered only night-time events resulted in estimates that were much less biased relative to those based on all photographic events. We conclude that restricting REM estimation to periods and habitats in which animal movement is more likely to be random with respect to cameras can help reduce bias in estimates of density for female Serengeti lions.
We highlight that accurate REM estimates will nonetheless be dependent on reliable measures of average speed of animal movement and camera detection zone dimensions. A common aim of many camera trap surveys is estimating the density of a target species within an area of interest. To this end, recent spatially explicit capture-recapture SECR methods, which combine both the spatial and temporal information contained in photographs of recognizable individuals, have provided unbiased estimates of density for marked species, i.
Spatially explicit capture-recapture analysis is now supported by a substantial body of literature mainly focusing on spotted and striped felids Sollmann et al. In contrast, no well-established methods exist for estimating the density of unmarked species using camera traps Carbone et al.
Rowcliffe et al. The REM requires a species encounter rate sensu Carbone et al. A key assumption of the model is that cameras are placed randomly with respect to animal movement Rowcliffe et al. Studies using the REM have been implemented by deploying cameras in systematic or fully randomized arrays Rovero and MarshallManzo et al. To date, however, the REM has never been used to estimate the density of a large carnivore species Foster and Harmsen We applied the model to estimate the density of female lions Panthera leo from an extensive camera trap survey implemented in Serengeti National Park, Tanzania, comparing estimates to reference values derived from pride census data SchallerBygott et al.
As a threatened and unmarked large carnivore for which reliable estimates of abundance or density are difficult to obtain Ogutu et al. Indeed, given the decline of the species across Africa over the past decades Riggio et al. Importantly, the Serengeti camera trap survey was not deed with the REM in mind.
Despite the use of a gridded de, the low tree density encountered in grassland habitat resulted in cameras often being placed on isolated trees whose shade attracted lions during the day, thus representing a potential violation of the random placement assumption. We assessed the effect of this known source of bias by comparing the accuracy of REM estimates derived from night-only versus all photographs, between wet and dry seasons, and across habitat types that differed in their amount of tree cover.
During the night, during the wet season, and in more densely wooded habitat, lions are less likely to seek shade under trees, and we expected camera placements on trees to be closer to random with respect to lion movement. We anticipated that REM estimates derived from data filtered by these factors would show improved accuracy when compared to reference densities.
It is marked by a southeast-to-northwest gradient of rainfall and soil type Norton-Griffithswhich creates a transition from short-grass plains in the southeast hereafter, grassland to woodlands in the north Packer et al.
Lion density is largely limited by food availability in the dry season when prey biomass is at an annual low SchallerBertramPacker et al. Lions live in gregarious groups known as prides, which are composed of related females, their dependent offspring, and 1 or more males Scheel and PackerPacker et al. The latter form coalitions that can reside in, and distribute their time across, more than 1 pride SchallerBygott et al. In contrast, nom do not maintain a territory and move great distances through the ecosystem Schaller The population has been monitored continuously since Bygott et al. Since1 female member of each study pride was radio-collared, with all subsequent monitoring relying on a combination of radio telemetry and opportunistic sightings Mosser et al.
We consider 2 3-month blocks, 1 in the dry season of Aug—Oct and 1 in the wet season of Mar—Mayduring which and camera locations were defined as active, respectively Fig. This cell size aimed to ensure a minimum of 5 traps per pride home range. In the case of the REM, spatial autocorrelation between neighboring cameras is not considered a problem because the approach focuses on contact rates between cameras placed randomly with reference to animal movement. We used a handheld global positioning system GPS device to locate cell centroids and placed each camera trap on the closest tree within a 1-km radius of the corresponding point.
If no trees were located within that distance, we attached cameras to man-made poles 8. Camera settings were chosen as part of a large-scale, multi-species survey and were not specific to lions Swanson et al. During the dry season, camera traps were programmed to take a sequence of 3 pictures per trigger during the day and at night. During the wet season, cameras took only 1 picture per trigger at night. We stress that this difference is unlikely to have resulted in bias because female lions were observed in the first of 3 photographs in As a result, the of pictures per trigger is unlikely to influence lion detection probability.
In both seasons, there was an arbitrary 1-minute delay between consecutive triggers. The date and time of capture were automatically stamped onto each image. Although the camera model used an incandescent flash for night-time pictures, it is unlikely to have modified the behavior of lions, which in the Serengeti are habituated to humans and research equipment. We initially processed camera trap images to extract and quality control date and time metadata. We then imported the images into the Snapshot Serengeti see www.
The latter combined multiple classifications of each image to yield high accuracy determinations of species see Swanson et al. We calculated reference densities for female lions in grassland and woodland habitats for both the dry season of and the wet season of At the time of study, 23 prides were known to use the study area and were being intensively monitored by the SLP.
Each pride is generally located using radio telemetry and observed directly at least once every 2 weeks. Unlike camera trap images, direct observation of lions allows for individual recognition from natural facial markings Packer and Puseyand thus enables near-perfect knowledge of pride size and composition. In particular, the size of the female component of each study pride excluding cubs is known with a very high level of confidence. Our study does not consider a small of transient nomadic females, which are known to remain in the study area for very short periods of time only Packer et al.
Failure to for this is likely to lead to bias in habitat-specific female densities.
Thus, for each pride, we obtained seasonal values of pwhich we assumed reflected seasonal changes in the contribution to female lion abundance in woodland and grassland habitats. We estimated pride UDs using reference bandwidths Silverman from functions implemented in the R package adehabitatHR Calenge For both grassland and woodland habitats, we calculated seasonal reference density of female lions D ref as:.
The latter was estimated by adding a buffer w equal to half the mean home range diameter of prides within the corresponding habitat to the camera trap grid hull i. We acknowledge that choice of buffer width for each habitat is the main source of uncertainty in our estimation of reference densities.
We used the following REM equation to obtain density estimates from camera trap encounter rates Rowcliffe et al. We considered only camera trap photographs taken in the dry season of Aug—Oct and the wet season of Mar—May. We did not include images of male lions in our analyses owing to our reduced ability to accurately estimate reference male densities. We defined an independent contact with a camera as a female lion entering and exiting the field of view.
Therefore, we considered consecutive photographic events of an individual lion remaining stationary in front of a camera as the same event. We calculated survey Speed dating lion compass as the total of camera hours, and obtained encounter rates by dividing the total of independent photographic events of female lions by total survey effort.
We defined night-time photographic events as those occurring between and We carried out ex-situ field trials to determine the dimensions of the camera detection zone. To estimate camera radius rwe approached a test camera directly from the front and on all fours 10 times, and measured the distance from the camera to the location at first trigger for each approach.
For each resulting location, we took a bearing using a compass placed on a flat surface directly below the camera.
We recorded detection angle as the angle formed by the mean compass bearings taken on each side. We estimated average distance moved per hour by female lions hereafter, speed from 4-day continuous follows of individual Serengeti prides carried out between September and December see Packer et al. We derived distance moved from car odometer readings. For each season, we averaged hourly movement rate across prides of the same habitat to obtain average speed of movement during the hour period V all and at night V n.
Although lions are considered a social species and are often encountered as part of a pride, we chose not to define individual events as group contact events whereby REM density is multiplied by average group size Rowcliffe et al. Our view is analogous to that put forward in the context of distance sampling of clustered animals.
Thomas et al. In this case, variance surrounding the REM estimates will be inflated, but estimates remain unbiased Thomas et al. We defined 4 season-habitat Speed dating lion compass for which to estimate female lion densities: dry season-grassland D-Gdry season-woodland D-Wwet season-grassland W-Gand wet season-woodland W-W.
We extracted habitat-specific camera points from corresponding habitat polygons Fig. We computed overall variance of REM density estimates using the delta method Seber The latter incorporated variance associated with the encounter rate estimated by re-sampling camera locations with replacement 10, times, as per Rowcliffe et al. We used percentage differences from reference densities to assess bias in REM estimates.
We carried out all analyses in R version 3. Prides with home ranges straddling both grassland and woodland habitat ed for Estimated reference female lion abundance was Estimated survey area for grassland habitat was For woodland habitat, estimated surveyed area was Mean home range area was greatest in grassland habitat during the wet season Table 1.
We obtained independent events of female lions from 7, camera trap days in the dry season, including 55 taken at night see Table 2 for a summary of events recorded per season-habitat combination. For the wet season, independent events were recorded over 12, camera trap days, including 73 at night.
We used follow data from 3 woodland and 4 grassland prides to estimate habitat-specific average speed of lion movement in the dry and wet season, respectively Table 2 ; Table S2. Average speed during both the hour period and at night was highest in grassland habitat during the wet and dry seasons Table 2.
Estimates for camera detection radius and angle were Camera effort in days and of independent photographic events of female Serengeti lions recorded in total and at night only for the different habitats considered during the dry season of and the wet season of Models that considered only night-time events resulted in estimates that were much less biased relative to those based on all photographic events. Estimates from all events were also ificantly different from reference density values for all season-habitat combinations except in woodland habitat during the wet season.
In contrast, confidence intervals associated with night-time REM estimates and reference densities overlapped for all season-habitat combinations. Although restricting data to night-time records had the strongest effect on accuracy, the effect of season was also notable, with wet season estimates based on all events being less biased than dry season estimates Fig.
When considering all events, estimates from woodland habitat were closer to reference values in both seasons. Absolute percentage errors associated with random encounter model REM estimates of female lion density in the Serengeti Lion Project study area during the dry season of and the wet season of Bars represent errors in the estimates derived from all light gray or night-time only dark gray photographic events. Season-habitat combinations are defined as follows: dry season-grassland D-Gdry season-woodland D-Wwet season-grassland W-Gand wet season-woodland W-W.
Estimates of animal density form the basis of many monitoring programs and often determine allocation of conservation efforts Jones et al. Although camera traps offer a cost-effective way of gathering information on multiple Speed dating lion compass, methods for estimating density remain largely focused on marked species.
In this context, the REM offers a simple framework that is potentially suited to a wider range of species.Speed dating lion compass
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