INDIRECT METHODS
Explanation:
To estimate population density there are two possible methods: Direct methods and indirect methods.
Abundance indexes are the parameters related to density, and they reflect changes that might happen in the target population density. They are commonly used to compare population sizes, for instance, the abundance of an insect species during the winter and the summer. These indexes are easy to get, so it is possible to make many estimations and make statistical analyses to detect differences.
There are two types of abundance indexes:
The ones based on direct observation and/or capture → DIRECT METHODThe ones based on traces and frequencies → INDIRECT METHOD
INDIRECT METHODS:
When talking about traces, we are referring to any mark or evidence left by animals, such as footprints, holes, feces, sediments, hair, marks, pathways, nests, shelters, dead bodies, food leftovers, songs, noises, among many others. Traces are directly related to population abundance.
The advantages over the indexes that use capture and observation are that traces are not influenced by detectability, they are much easier to use, cheaper, and they turn to be significant with less visible species.
The paths are useful, but they last for a long time in the habitat, so they do not entirely reflect the recent presence of the animal in a specific moment. However, on the paths, there can be other traces such as footprints, hair, and others that are easier to identify and which reflect a recent pass. These can be used to determine abundance.
In shy species, the alternative to evaluate abundance is to constate their presence in a sampling unit, this is, their frequency. Frequency can be used as an abundance index and, under certain conditions, might be statistically tested to evaluate abundance differences.
