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There are many, many methods used for sampling wildlife populations. The method chosen will depend on the information the wildlife manager wishes to collect. They may wish to conduct counts of populations within a particular region, record the presence of species within a study area or gather population information on a specific species among others. Below are some of the most common methods used.
Aerial surveys are usually undertaken by wildlife managers to assist with improving management techniques of wildlife populations, to gather large amounts of data on different aspects such as habitat, wildlife and ecosystems. It is the most popular survey technique for estimating the size of populations of larger animals. Think of trying to conduct a population survey of deer over a huge area of countryside. Flying over the top of the animals to count them is an effective survey method in this instance and also allows managers to gather other population information such as density, age and sex ratios.
When doing aerial survey work, managers need to first set out the boundaries of the survey area. The plane can then be flown up and down the survey area, and these runs become the transects of the survey.
It is a difficult task to estimate herd numbers, or even to spot some species from the air and takes keen observational skills and a lot of practice to become an expert aerial surveyor.
There are various types of traps used by wildlife managers and researchers. These will vary depending on the species you are trapping, the information you wish to collect and the habitat you are working in. Some different types of traps include:
- Elliot style folding traps – these are commonly used for capturing small terrestrial mammals such as rats, mice,
- Mist net traps – these are generally used for capturing bats and birds. There are various designs ranging in size of gauges of mesh and type of material. The choice of mist net will depend on the target species.
- Cage traps – there are a wide range of cage traps used for various species from small mammals, water birds, rabbits, foxes and larger animals such as feral pigs.
- Pit fall traps – these are usually used in conjunction with drift nets to direct small terrestrial animals such as reptiles into pits in the ground.
- Two-stage roost trap – first compartment captures bird which will then walk up to the second compartment to house the birds for up to 2 days. This style of trap is used to capture pest bird species.
There are also traps which kill the captured species. Obviously, live trapping is preferable with desirable populations. When trapping problem species, killing during trapping is an option. However, the actual removal of these individuals from the population does effect the survey data. Ideally, where possible, pest species should be trapped live and removed for disposal.
Transects are used in both plant and animal surveys. They vary, but as a general rule, transects are straight lines that an observer moves along, or along which traps are placed. The positioning of a transect can effect the data collected. It is best to survey across the grain of a country than along it, such as surveying across a river or over a slope. This will provide more information on the variation in habitats and species within a study area.
When undertaking transect surveys, tally counters move along the transect recording the animals seen or heard. This type of counting requires skilled observers, as animals maybe difficult to see, identify (think of some bird species for example) or easily frightened.
Indirect methods are those methods used to calculate the size of a population when the animal itself isn’t actually counted or is counted but does not depend on accurate animal counts. It may be based on observing their activity such as burrowings or excreted faeces.
Another example of a method in where animals are not counted is where some small mammals can be counted by putting out PVC pipes that have a sticky inside. The fur of the mammals gets caught and stuck to the sticky lining, but the animal moves on. Managers can identify the animals by fur analysis and thus get an indirect picture of the population. The presence of animals can also be noted by observing burrows, nests, footprints, and scats or using call-back recordings to survey birds. The use of call-back recordings is especially useful for locating birds of prey such as owls that are difficult to locate visually.
Examples of methods where animals are counted but accuracy is not needed include the change of ratio method, the mark-recapture method and incomplete counts.
Mark recapture methods are just as the name suggest – a sample of the animal population are caught, tagged and released. There are a number of different ways of carrying out mark recapture surveys. These include the Peterson model, frequency of capture models, estimation of density, many counts and two counts.
Data collected from mark-recapture surveys can be used to estimate the size of a population and identify trends in population growth or decline.
Roadside or Call Counts
This type of survey is often used with birds. Usually, volunteers are asked to fill out forms detailing sitings or calls of a particular species in a particular area. Information is then collated and analysed. Researchers can use Call Counts to estimate population sizes, identify bird species within a specific study area or use over time to observe the population trends of a particular species.