Observation-guide
Wildlife surveys: why a count is not the same as abundance
An animal record depends on how and when the search is conducted. Detections help establish whether a species is present, but failing to detect it during a survey does not, by itself, prove that it does not live there.
Detecting is not counting the whole population
A survey records what its methods manage to detect—not necessarily every animal that is present. If a species does not appear in a survey, it may have gone unnoticed: this is known as imperfect detection. For that reason, a count of observations should not automatically be confused with the actual abundance of wildlife. Information gathered through field surveys helps support the conservation and management of national parks. (National Park Service)
More searches increase confidence, but do not guarantee certainty
The probability of detecting a species in a single survey depends on whether it is present and is found. Repeating searches increases the cumulative probability of detecting it; even so, there may remain a small possibility that it is present but was not recorded. The benefit of adding surveys diminishes as that cumulative probability approaches 100%. Therefore, scientists can rarely claim that a species is truly absent, although sufficient effort can give them considerable confidence in that conclusion.
The method and conditions matter
Surveys may be visual or acoustic, use cameras or drones, or look for environmental DNA in water or soil. Each method records what it can detect: for example, a camera set up for animals the size of a rodent or larger may miss insects and other tiny species. Species may also be missed if searches take place at a time of day or year when they are not active. Interpreting a count therefore requires keeping in mind which method was used, when, and with what effort; on their own, these data do not provide a universal measure of abundance.
A non-detection can be a signal, not proof
The absence of a species that was expected to be found can warn of an environmental problem, such as degraded habitat or a new stressor that makes survival more difficult. But a non-detection can also be due to the animals not being found during the surveys. Responsible interpretation keeps both possibilities open and avoids turning a missing record into a definite claim of absence or decline.
Sources
- Who lives here? · National Park Service