Camera trapping in the trees

Detecting elusive species

Wildlife researchers are expanding the use of camera traps—remote cameras triggered by animal movement—to study arboreal animals and their use of the forest canopy.

Camera traps provide photographic evidence of species that are rare, elusive, nocturnal or otherwise difficult to photograph in person. Since the 1990s, scientists have used camera traps to inventory vertebrate communities, confirm the presence of rare or cryptic species, determine a species’ range limits, examine mammal community structure, compare seasonal dynamics of animal presence, and estimate abundance of species with markings unique to individuals (e.g. tigers).

Camera trap use is relatively cost-effective, low-effort, and non-invasive, and traps collect data day and night on a wide range of species, including those that avoid people.

Some three quarters of tropical forest vertebrates and a large proportion of the mammals live in the forest canopy. Arboreal animals may be particularly vulnerable to forest loss and degradation, yet they are difficult to study and monitor. Vegetation can be dense, and seeing animals 20 to 40 meters above the ground is challenging at best.

Standard line transect surveys conducted on foot or in a vehicle can detect most primate species, which are mainly diurnal and often conspicuous, but rarely find other canopy species, especially those that are nocturnal or cryptic. Transect surveys are also labor intensive, requiring a trained team to cover many hundreds of kilometers to obtain sample sizes large enough to estimate population density.

Seeing the forest (animals) for the trees

Camera traps offer an alternative approach to surveying and studying canopy vertebrates. A few early studies, including a 2004 study of buff-headed capuchin monkeys and a 2007 investigation of kinkajou foraging, used remote cameras to study a single species, at relatively low strata in the canopy.

Scientists are now testing the utility of camera traps to inventory arboreal mammals, model species’ local distributions relative to hunting pressure, and assess animals’ use of canopy. They have also compared the relative cost and effort of camera traps, compared to ground-based inventories, to detect arboreal species’ presence. Mongabay-Wildtech reviewed several of these studies, all of which took place in Peru, and reports on some of the challenges and solutions to date in deploying cameras in the canopy.

1. Elevating camera technology to new heights

Dr. Tremaine Gregory of the Smithsonian Institution’s Conservation Biology Institute is one of the first to bring a set of remote cameras up into the tropical forest canopy.

In a 2014 publication, Gregory and colleagues assessed the usefulness of standard camera traps to document animal movements across 13 natural canopy bridges over a clearing for natural gas pipeline in Peru’s Lower Urubamba region.

“Initially,” explained Gregory, “we considered radio collaring members of primate groups in order to follow their movements…. we realized that monitoring the bridges themselves would give us a much more focused view of their use. Sitting under the bridges for 24 hours a day didn’t seem like a viable option, so we began to consider camera traps. Camera traps had never been used in the high canopy, [but] we were pleasantly surprised to see that they were highly effective in monitoring the bridges.”

The team placed a camera to monitor all the connecting points in each bridge, so some bridges had up to four cameras on them. They also placed camera traps on the ground to see if arboreal mammals crossed without using bridges and to compare crossing rates between the bridges and the ground to see which the arboreal mammals preferred.

With 25 cameras set up for more than 3,600 camera trap days during six months, the team logged over 215,000 images. Over 8,200 of these images were target vertebrate species. Most of the remaining 97% of photo events were false triggers caused by moving leaves, branches, or insects, far more than for corresponding ground-based camera traps, where surrounding vegetation had been cleared.

The researchers set the cameras up to take three rapid images of each passing animal, for a total of 1,522 “photo events” of a target animal passing in front of the cameras. In these events, the researchers recorded 47 vertebrate species—20 mammals, 23 birds and four reptiles—most of which differed from the species captured in similar camera traps placed on the ground below each canopy bridge.

The research team found that the rates at which animals passed in front of the arboreal cameras did not decrease over time, which suggests that cameras did not cause a negative response among the target species. If cameras are generally non-invasive (but see Schipper (2007)), they can be used to monitor a range of different species in a variety of environments at all times, especially where the presence of researchers affects animals’ use of locations or resources.

2. Assessing efficiency and utility of cameras vs line transect surveys

A more recent (2016) study led by Dr. Andrew Whitworth compared the cost, effort, and species of larger mammals detected by arboreal camera traps with those associated with standard ground transects. This research team also assessed the ability of arboreal camera traps to record hard-to-find species in lower and upper canopy strata.

The researchers set up 30 cameras at two hunted sites in southeastern Peru over three months to inventory arboreal mammals. They positioned cameras at two heights in the canopy, for a total of over 2,900 camera trap days, setting the cameras to take both a photo and a video when triggered.

Although nine cameras failed due to depleted batteries, filled-up memory cards from false triggers, or batteries that dislodged during setup, the camera trap setup detected 18 arboreal mammal species in 339 records, while line transects (diurnal and nocturnal) detected 13 species in 862 records.

In this study, neither the camera trap inventories nor the transect inventories captured all expected mammal species, indicating that greater survey effort (more camera days or more transect walks) would be needed to detect all species.

The study’s cost-effort analysis suggested that the upfront equipment costs, training in climbing and camera use, and installation time needed for arboreal camera trapping was balanced by relatively little effort during data collection. Overall, costs were similar to those of line transect surveys, which require less training and equipment but much greater time and effort to cut the transects and walk them repeatedly.

3. Modeling species occupancy related to hunting pressure

A research team led by Dr. Mark Bowler also compared the use of camera traps to line transect surveys in inventorying medium-sized and large arboreal mammals in a hunted area along Peru’s Rio Napo. They further examined whether they could use camera trapping methods to reliably model the animals’ occupancy based on distances from human hunting access.

They placed 42 cameras at intersections of a large grid, in trees that were safe to climb and had branches touching those of at least one adjacent tree. They set the cameras to record 10-second infrared video clips, instead of still photos, to capture the characteristic movements of otherwise similar-looking nocturnal species.

Analyzing a survey effort of nearly 3,150 camera trap days, the team logged over 700 events (videos) of 18 medium-sized and large mammals, as well as a number of small mammals (e.g. rodents and opossums), birds, and reptiles. They set their cameras to record a photo at noon each day so they would know the dates of any camera failures.

The camera traps and the 2,014 kilometers of line transect surveys each detected all but two of the expected medium-sized and large diurnal mammal species. The cameras also detected eight nocturnal mammals; detecting these species during transect surveys would require an additional series of nocturnal survey walks. Models for most species showed a low rate of occupancy that tended to increase (not significantly so) with distance to the nearby village.

Challenges

Deploying camera traps in the forest comes with a set of specific challenges, some specific to setting up camera traps high up in the forest canopy, including the following:

CAMERA TRAPPING CHALLENGES	POTENTIAL SOLUTIONS
Setting up the cameras requires finding trees that can be climbed safely, with branches that connect with those of other trees, and camera positions and angles that allow capture of whole animals.	Place more than one camera in each location, such as at different angles or heights in the canopy, can improve likelihood of successfully detecting species with distinct movement patterns.
Teams need climbing gear, climbing expertise, and time to install, maintain, and take down cameras.	No way around this, though clearing paths to camera sites ahead time might shorten this time.  Gregory and colleague Farah Carrasco-Rueda learned to climb the trees and checked the cameras every couple of months.
Finding appropriate camera locations that remain stable and capture movement of various species in a 3-dimensional habitat makes positioning these cameras even more tricky. Unlike setting up a camera on the ground, Gregory said, monitoring the canopy requires the cameras to be angled in multiple directions—not just directly ahead.	Stability: place camera as close to trunk as possible to minimize movement of cameras and possible damage due to wind. Flexibility: Gregory et al. (2014) created custom mounts with two ball joints that enhanced versatility in positioning the camera. “We designed a mount with PVC tubing, nylon webbing, and a couple of universal joints that allowed us to aim the cameras in any direction,” she described and added that more companies may be designing mounts with these capabilities.
Arboreal camera traps incur false alarm triggers from the movement of leaves and branches. These images filled up 11 of the 4GB memory cards. This problem affects all camera traps, but the wind and branches in trees can trigger the camera and deplete battery and space on memory cards.	Gregory’s team reduced false triggering by 80% by moving leaves away from in front of the cameras. They also switched to 16 GB memory cards. These 16 GB and the 32 GB memory cards used by Bowler et al. (2016), did not reach capacity before being checked months after deployment.
Up to 30% of the arboreal cameras malfunctioned, due to the light sensor or other internal workings or to movement and damage by water or wildlife. Gregory’s cameras “were invaded multiple times by tiny ants and termites that entered through a small pressure-release valve on the casing of the camera.”	Simple tricks—such as surveying during dry season, putting silica gel inside cameras, or putting steel wool or petroleum jelly around the camera to keep out insects—can help keep cameras running correctly for longer periods.  Gregory added that Reconyx, the maker of their cameras, “has modified their camera design in order to prevent invasions of canopy insects” by eliminating the pressure-release valve, making the cameras more resistant both to insects and to humidity.
White flash can cause animals to avoid a camera and may impair the vision of nocturnal mammals, which have sensitive eyes for moving in darkness.	Two of the studies used an infrared flash, which produced images that were less clear but did not disturb the target wildlife.

 

A role for camera traps in studying arboreal wildlife

Arboreal camera trapping offers the potential to better understand the occupancy dynamics, behaviors, and effects of forest disturbance on a suite of even secretive species.

“Camera traps were originally designed for use on the ground, but our study, and others, have demonstrated that they are also very useful in the canopy,” said Gregory. “In the canopy, the conditions seem to be harsher, and we experienced higher camera malfunction rates and ended up with many thousands more photos [triggered] by leaves and insects. However, they were very effective in capturing crossing events, and their nearly non-invasive data collection capabilities made them ideal for these circumstances.”

Nevertheless, she continued, “unlike on the ground, where cameras are sampling a plane, cameras in the canopy are only capturing events at specific branches, so the sampling is somewhat more biased.”

Other studies suggest that diurnal line transects may be more cost-effective than cameras for detecting most large diurnal species, such as primates, in non-hunted areas. Camera traps have lower cost per detection for hunted (wary) species, as well as nocturnal and cryptic species. Conducting both types of surveys will more effectively assess the whole mammal community.

While camera trapping can feasibly determine the presence of multiple arboreal species relative to environmental factors, its use cannot yet help estimate population abundance or density.

Some helpful references

Bowler, M. T., Tobler, M. W., Endress, B. A., Gilmore, M. P., & Anderson, M. J. (2016). Estimating mammalian species richness and occupancy in tropical forest canopies with arboreal camera traps. Remote Sensing in Ecology and Conservation.

Gregory, T., Carrasco Rueda, F., Deichmann, J., Kolowski, J., & Alonso, A. (2014). Arboreal camera trapping: taking a proven method to new heights. Methods in Ecology and Evolution, 5(5), 443-451.

Gregory, T., Carrasco-Rueda, F., Alonso, A., Kolowski, J., & Deichmann, J. L. (2017). Natural canopy bridges effectively mitigate tropical forest fragmentation for arboreal mammals. Scientific reports, 7(1), 3892.

Kays, R., & Allison, A. (2001). Arboreal tropical forest vertebrates: current knowledge and research trends. In Tropical Forest Canopies: Ecology and Management (pp. 109-120). Springer Netherlands.

Peres, C. A. (1999). General guidelines for standardizing line transect surveys of tropical forest primates. Neotropical Primates 7:11-16.

Rovero, F., Zimmermann, F., Berzi, D., & Meek, P. (2013). ” Which camera trap type and how many do I need?” A review of camera features and study designs for a range of wildlife research applications. Hystrix, the Italian Journal of Mammalogy, 24(2), 148-156.

Schipper, J. (2007). Camera-trap avoidance by Kinkajous Potos flavus: rethinking the “non-invasive” paradigm. Small Carnivore Conservation, 36, 38-41.

Whitworth, A., Braunholtz, L. D., Huarcaya, R. P., MacLeod, R., & Beirne, C. (2016). Out on a limb: arboreal camera traps as an emerging methodology for inventorying elusive rainforest mammals. Tropical Conservation Science, 9(2), 675-698.  (a 2015 mongabay.com post shows some of this study’s photos)