Biodiversity surveys

Biodiversity measures are based on the ability to accurately count the number of species within a given area and usually some measure of their relative or absolute abundance. Population management of both common and rare species relies on accurate measures of population numbers or at least a way to measure population trends. Most mammal populations or communities are too complex for every individual to be counted, therefore a sample is often taken of the population and the number is estimated based on that sample. Unfortunately, obtaining these estimates is not an easy task. More than other vertebrate groups, mammals occupy a wide array of habitats and possess a broad range of body sizes. These factors make them difficult to survey as a group, and survey techniques have to be tailored for a specific species, or suite of species. Planning a biodiversity survey is a two-step process: the first step is to determine the level of information needed to meet objectives, and the second step is to tailor a survey to fit the attributes of the species.

Three levels of information that can be obtained from a survey include a species list, a relative index of abundance for each species, and an absolute density for each species. Generally, there is increasing cost and complexity as the level of information increases. For some mammal species, it is prohibitively expensive to estimate absolute density because of the habits of the mammal or the habitats it occupies. Solitary bats, which live in trees under strips of bark or in crevices, are a good example of a suite of species whose density estimate is logistically difficult to obtain. When planning a survey, the first consideration should be how necessary the increased information is to the management or research objectives. The initial survey of a park would not start with a density estimate of each mammal species, but rather a list of species found in the park. Often a mammal's relative density is adequate information to track changes in abundance within a park, and the saved money can be used for other conservation tasks such as patrolling. Within broad conservation plans for an area, mammal surveys should reflect a nested subset design. For example, following a complete species list for the area, some species from this list are monitored through an abundance index, and select animals from this group are targeted for detailed population and ecology studies that might include a density estimate.

If field technicians are working with a rare species or a harvested species, they might not be concerned with the higher levels of organization and start with a focal study on the target species. However, even under these circumstances, an index survey over a larger area may be more appropriate than a density estimate at one site. Project goals and financial logistics usually produce a compromise in how much information can be gathered. It is important the data collected are not stretched beyond their purpose, when compromises are made. Unfortunately, the scientific literature is full of indexes used to calculate densities and species lists used as indexes. Surveys are powerful tools in wildlife conservation and management, but when stretched beyond their ability they convey more confidence in the trends then the data warrant.

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