Use of pitfall traps
Animal Research Review Panel Guideline 6
Revised: January 2020
Pitfall trapping is a sampling technique that is widely used to examine species occurrence during surveys, spatial distribution patterns, compare relative abundance in different micro-habitats, study daily activity rhythms, and study seasonal occurrence. Pitfall traps tend to be used more in low rainfall, arid habitats than in mesic environments.
The use of pitfall traps was considered by the Animal Research Review Panel following concerns it had regarding the use of formalin as a killing agent in wet pitfall traps. Solutions such as formalin are used in wet pitfall traps because they preserve the specimen, not because they are humane because there is no rapid loss of consciousness before drowning and preserving. The Panel has made the following considerations in relation to the use of dry and wet pitfall traps.
Drift fence – A length of fence, often made with aluminium or nylon fly-wire mesh, which runs across the centre of a pitfall trap. This helps to funnel animals into the trap in order to increase the numbers of animals captured.
Dry pitfall trap - A hole in the ground into which a bucket or pipe is placed into the soil so that the mouth is level with the soil surface. These traps are often used to capture small mammals, reptiles and frogs that fall into the pit and cannot escape due to the smooth walls. The openings of the traps are often covered by a slightly raised lid, stone or other object to keep out predators and prevent trapped animals from being overheated (during the day) or drowned (when it rains).
Wet pitfall trap – A pitfall trap containing a solution designed to trap, kill and preserve an animal or animals. Aqueous solutions used in these traps may include: formalin (10% formaldehyde), alcohol, methylated spirits, trisodium phosphate or picric acid. Wet pitfall traps cannot be used for vertebrates so the main issue is to ensure that vertebrates are not caught as by-catch in wet traps set for invertebrates.
2. General Principles
Pitfall traps are used for several reasons including:
1) capturing species that are difficult to obtain by other methods;
2) estimating relative abundances and species richness or for catching particular species of animals;
3) estimating daily activity of different animals;
4) determining movement patterns of individual animals;
5) determining microhabitat use.
The pitfall trap is a relative method of estimating animal numbers and species, thus it cannot be used to estimate absolute population sizes or overall species richness of an area. It produces an "index" by which several geographic locations can be compared.
There are several basic approaches to using pitfall traps:
1) For survey work, traps that catch the animal randomly - animals foraging on the ground 'accidentally' fall into the trap.
2) Traps that are used in conjunction with a 'drift fence' barrier can be used to direct animals towards the trap. Traps can be set up on known runs, to collect specific species.
3) Traps can be set up with a lid or cover that encourages behavioural responses in certain animals to take refuge and therefore fall into the trap.
4) Baits can be used to attract certain species or animal groups.
3.1 Dry pitfall traps
In certain isolated locations, dry pitfall traps are the only practical method of catching small, ground dwelling vertebrates and invertebrates. An example of this would be trapping for ground dwelling spiders in a remote part of a forest.
The advantages of using dry pitfall traps include the following:
- Simple, cheap and cost effective.
- Typically do not kill the animals (except inadvertently).
- Each trap can collect multiple animals.
- Safe for the operator.
- Often the only practical alternative.
The disadvantages of using dry pitfall traps include the following:
- Labour intensive to install.
- Fairly non-selective in the species caught.
- Do not prevent trapped animals from injuring each other (e.g. predator & prey).
- Require deactivation at the end of the trapping period.
3.2 Wet Pitfall Traps
Wet pitfall traps are routinely used to trap invertebrates, and are acceptable for this purpose. They are currently unacceptable for vertebrates, as the preservative solutions used do not kill humanely.
Wet pitfall traps used for the capture of invertebrates, however, can pose a significant by-catch risk to small non-target vertebrates such as lizards, frogs and small mammals.
4. Management of dry pitfall traps
4.1. Types of impacts
Potential animal welfare impacts of pitfall trapping include:
- Distress caused by confinement, discomfort, social isolation, separation from mother, or exposure to predation by other animals captured.
- Predation of captured animals from animals entering pit including ants (that are not captured).
- Flooding of the traps.
- Trauma associated with capture or handing.
4.2 Mitigation of potential animal welfare impacts
Dry pitfall traps must be managed to minimise the impact on trapped animals by taking into account the following:
- Pitfall trapping is undertaken by individuals with recognised competency in the construction of traps and the handling the animals caught.
- Minimise the time animals spend in the trap by inspecting traps at least twice per day (typically dawn and afternoon) and more often in extreme weather conditions.
- Closing the traps during extreme temperatures.
- Provide moisture in the trap with the use of a wet cloth or small lid of water under leaves or bark.
- Minimising the handling time before release. Ideally within five minutes.
- Providing shelter on the bottom of the pitfall trap in the form of soil, leaf litter, large leaves or bark to reduce the potential risk of exposure due to hypothermia, hyperthermia and dehydration.
- Not placing traps in the vicinity of ant nests if possible.
- Not placing traps in areas of likely water runoff so they do not fill with water if it rains.
- Deactivation of traps after each trapping session by remove everything inside the trap to ensure all animals have been removed and properly closing the lid.
Modifications to reduce the animal welfare impacts of pitfall traps may include:
- Traps being fitted with rain guards to prevent flooding and include polystyrene "floats" that act as rafts in the event of water filling the traps. This is a better option in loamy or clay soils.
- Traps fitted with a flywire mesh base, which prevents animals from digging out and also allows free drainage of water in sandy soils. This is not a good option in loamy or clay soils.
- Where possible design the depth of the trap, cover and drift fence to target the focal species. E.g. some non-target rodents can be overly selected by the use of deep traps and drift nets compared with non-target species.
- Shade covers may be fitted to reduce midday pit temperatures (but may reduce trap success).
- Traps may have "exclusion barriers" such as a selective grid or "roof" to exclude unwanted fauna (predators, non-target species).
- Leaf litter may be added to the trap from the site to provide shelter and moisture which prolongs survival of trapped animals. If this is used the traps must be checked very carefully as small vertebrates such as lizards of the genera Lerista, Menetia and Morethia, which can readily hide under the leaf litter.
- A saturated sponge may be added to provide high moisture levels for trapped amphibians. These work best in warm, summer conditions when the capture of frogs is unlikely. During winter, a wet sponge is more likely to wet the fur of a small mammal and cause hypothermia.
- PVC tubing can be used to provide shelter inside the trap.
- Food and water may also be offered, however baits with honey will attract ants. Peanut butter and raisins may be better.
- Insecticides may be used where ants are prevalent and cause a problem by attacking trapped animals (e.g. in drier areas), for example Rid Roll On around the rim of the trap (however, as the effects of insecticides on most reptiles and amphibians are not known, insecticides should be used with caution). Coopex Insecticidal Powder dusted very lightly around trap entrances helps to stop ant attacks unless the trap has been positioned near an ant nest (when the trap should not be used). Coopex breaks down quickly, and does not appear to have any effects on frogs, reptiles or mammals.
5. Management of wet pitfall traps
ARRP considers that the design of wet pitfall traps and the solutions in current use are unacceptable for vertebrates because they cause an inhumane death. When used for the capture of invertebrates these traps must be managed so as to minimise the inadvertent capture of vertebrates. This may include the placement of mesh barriers around the pitfall traps to exclude vertebrates.
6. Possible alternatives to pitfall trapping
Though pitfall trapping is probably the most appropriate technique for the study of small animals. A potential alternative in some cases may be the placement of camera traps along drift fence lines as a sampling method.
7. Related Documents
Department of Parks and Wildlife, Species and Communities Branch (2013) Standards Operating Procedure. Dry Pitfall Trapping for Vertebrates and Invertebrates. Department of Parks and Wildlife Animal Ethics Committee, Perth. SOP No: 9.3.
Ellis, M. (2013) Impacts of pit size, drift fence material and fence configuration on capture rates of small reptiles and mammals in the New South Wales rangelands. Australian Zoologist 36, 404-412.
Ellis, M.V. & Bedward, M. (2014) A simulation study to quantify drift fence configuration and spacing effects when sampling mobile animals. Ecosphere 5(5)55, 1-20.
Gist, C.S. & Crossley, D.A. (1973) A method for quantifying pitfall trapping. Environmental Entomology 2, 951-952.
Greenslade, P. & Greenslade, P.J.M. (1971) The use of baits and preservatives in pitfall traps. Journal of the Australian Entomological Society 10, 253-260.
Hobbs, T.J. & James, C.D. (1999) Influence of shade covers on pitfall trap temperatures and capture success of reptiles and small mammals in Arid Australia. Wildlife Research 26, 314-349.
Hohnen, R., Murphy, B.P., Gates, J.A., Legge, S., Dickman, C.R. & Woinarski, J.C.Z. (2018) Detecting and protecting the threatened Kangaroo Island dunnart (Sminthopsis fuliginosus aitkeni). Conservation Science and Practice 2019, 1:e4.
Jansen, M.J.W. & Metz, J.A.J. (1979) How many victims will the pitfall make? Acta Biotheoretica 28, 98-122.
Luff, M.L. (1973) Some features influencing the efficiency of pitfall traps. Oecologia 19, 345-57.
Milton, D.A. (1980) A comparison of three techniques used in a reptile survey of the Conondale Ranges. Victorian Naturalist 97, 26-31.
New, T.R. (1999) By-catch, ethics, and pitfall traps. Journal of Insect Conservation 3, 1-3.
Pestell, A.J.L. & Petit, S. (2007) Methods and ethical considerations of pitfall trapping for the western pygmy possum (Cercartetus concinnus Gould) (Marsupialia : Burramyidae), with observations on capture patterns and nest sites. Wildlife Research 34, 296-305.
Read, J.L. (2016) Too hot to handle? Balancing increased trapability with capture mortality in hot weather pitfall trapping. Austral Ecology 41, 918-926.
Southwood, T.R.E. (1978) Ecological Methods. Chapman and Hall. London. 524pp.
Upton, M.S. (1991) Methods for collecting, preserving, and studying insects and allied forms. The Australian Entomological Society Miscellaneous Publication No. 3, 4th edition.
Stephen Jackson (NSW DPI) for the development of the current edition and Chris Dickman (University of Sydney) and Mike Fleming (DPIE) for proving comments on the draft document. The previous edition of these guidelines was developed with the assistance of Dan Lunney, Walter Boles, Burt Sheridan, Jack Giles, George Russ, Andrew Braid, Grant Singleton, Leslie Reddacliff, Margaret Rose and Alan York.