Use of pitfall traps
Pitfall trapping is a sampling technique which is widely used in studies of seasonal occurrence, to examine spatial distribution patterns, to compare relative abundance in different micro-habitats, to study daily activity rhythms, and in community surveys.
The use of pitfall traps was considered by the Animal Research Review Panel's Wildlife Advisory Group following concerns raised to the Panel 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. There is no rapid loss of consciousness before drowning and preserving.
The Wildlife Advisory Group identified two issues:-
- the impact of pitfall traps on non-target vertebrates.
- whether wet traps are acceptable, including those set to trap invertebrates.
1.1 Dry Pitfall Traps
The pitfall trap is an adaptation by the ecologist of a common hunting technique: the use of a pit in the ground into which an animal falls and cannot escape. The ecologist's pitfall trap consists basically of a glass, plastic or metal container, sunk into the soil so that the mouth is level with the soil surface. Many ground dwelling animals fall into the trap and are unable to escape.
Dry pitfall traps used to collect reptiles or frogs are described by Harold Cogger as generally consisting "of jars, tins or drums which are buried in the ground with their lips flush with the ground's surface. The openings are covered by a slightly raised lid or stone, or other object to keep out predators and prevent trapped animals from being overheated (during the day) or drowned (when it rains)
To be effective they should be placed along known 'runs', where they are most likely to be encountered by the animals to be trapped." (Cogger, H. "The Reptiles and Amphibians of Australia" 1986 page 24). In addition to being positioned along known ‘runs’, traps are often used in conjunction with drift fences for enhanced effectiveness.
1.2 Wet Pitfall Trap
A wet pitfall trap is defined as a dry pitfall trap containing a solution designed to trap, kill and preserve an animal or animals. Aqueous solutions used in these traps include; formalin (10% formaldehyde), alcohol, methylated spirits, trisodium phosphate and picric acid.
Pitfall traps are used for sampling animal populations by:
- capturing species which are difficult to obtain by other methods;
- estimating relative abundances and species richness or for catching particular types of animals;
- determining movement patterns of individual animals.
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 areas can be compared. It is a "passive" form of sampling which relies on the animal rather than the observer making the action that leads to capture and enumeration.
There are three basic approaches to using pitfall traps
- For survey work, traps that catch the animal randomly - animals foraging on the ground 'accidentally' fall into the trap.
- Traps that are used in conjunction with barriers - a 'drift fence' barrier can be used to direct foraging animals towards the trap; traps set up on known runs, to collect specific animals; or adaptations such as a lid or cover that encourages behavioural responses in certain animals to take refuge and therefore fall in.
- Baits used to attract certain species or animal groups.
2.0 Justification for Use
2.1 Dry pitfall traps
2.1.1 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.
2.1.2 The advantages of using these traps include the following:
- they are simple, cheap and cost effective;
- have no moving parts;
- do not kill the animals (except inadvertently);
- collect large numbers of animals;
- are safe for the operator;
- are often the only practical alternative.
2.1.3 The disadvantages include the following, that:
- they require deactivating;
- they are fairly non-selective;
- they do not prevent trapped animals from killing each other;
- catch size is influenced by population sizes, activity levels, weather, size and nature of trap.
2.2 Wet Pitfall Traps
Wet pitfall traps are routinely used to trap invertebrates, and are acceptable for this purpose. They are currently unacceptable for vertebrates, however, as the preservative solutions used do not kill humanely. Furthermore, traps used for invertebrates can pose a significant risk to small non-target vertebrates, such as lizards, frogs and even small mammals.
3.1 Management of Dry Pitfall traps
Dry pitfall traps must be managed to minimise the impact on trapped animals by taking into account issues such as:
- time animals will spend in the trap
- the possibility of trapping animals which may prey upon or parasitise other trapped animals
- environmental effects such as dehydration and hyperthermia in hot weather, hypothermia or drowning
- deprivation of food and water
- deactivation of traps when no longer required
- appropriate size of trap - diameter, depth
construction of trap - conformation of the walls, lids, covers or grids
- possible non-target species - bearing in mind that small vertebrates may in fact be smaller than large invertebrates
- traps should not be set in areas where there is a possibility of them filling with water such as low lying areas or wetlands
3.2 Modifications to enhance the operation of traps:
- pitfall traps may be fitted with rain guards to prevent flooding and polystyrene "floats"
- shade covers 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 added to the trap from the site provides shelter and moisture which prolongs survival of trapped animals. A saturated sponge provides high moisture levels for trapped amphibians
- PVC tubing can be used to provide shelter inside the trap
- insecticides may be used where ants are prevalent and cause a problem by attacking trapped animals (eg 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
3.3 Wet pitfall traps
The Wildlife Advisory Group considers that the designs 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.
4.0 General References
General consideration of trapping techniques
Southwood, T.R.E. (1978) Ecological methods. Chapman and Hall. London. 524pp.
Papers examining particular aspects
Gist, C.S. and Crossley, D.A. (1973) A method for quantifying pitfall trapping.
Environmental Entomology 2, 951-2.
Greenslade, P. And Greenslade, P.J.M. (1971) The use of baits and preservatives in pitfall traps. Journal of the Australian Entomological Society. 10, 253-60.
Hobbs, T.J. (in prep) Influence of shade covers for pitfall traps on reptile and small mammal capture success in Arid Australia. Wildlife Research.
Jansen, M.J.W. and 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.
Upton, M.S. (1991) Methods for collecting, preserving, and studying insects and allied forms. The Aust Entomological Soc. Miscellaneous Publication No. 3, 4th edition.
Members of the Wildlife Advisory Group to be thanked who participated in the preparation of this document are; Mr Dan Lunney, Mr Walter Boles, Dr Burt Sheridan, Dr Jack Giles, Dr George Russ, Dr Andrew Braid, Dr Grant Singleton, Dr Leslie Reddacliff, Associate Professor Margaret Rose and particularly, Dr Alan York.
Animal Research Review Panel Guideline 6