It is now widely accepted that many species are not perfectly detectable during an ecological survey. This means that, sometimes, a species that is present at a site will not be detected by an observer (or observers) during a survey of that site. The probability that the species will be detected if it is present (its ‘detectability’) is influenced by many factors. One of the most important factors is the level of effort put into the survey. In general, the more effort that is expended, the higher the chance of detecting the species.
Why do we care about this? Well, there are many reasons.
Imperfect detectability affects our ability to determine a range of important ecological metrics, such as the size of a population and the spatial extent or distribution of a species. It also makes it difficult to detect changes in these metrics, which is particularly important when we invest valuable funds in programs designed to address things such as declining population size and shrinking ranges.
But the implications of imperfect detectability can be particularly severe when we are considering the potential impacts of development on a threatened species. Falsely assuming the species is absent may mean that decisions about the future use of the site will cause unknown impacts on the species. In many cases, the species will be lost from the site and, at worst, the chance of the species going extinct will be increased.
Despite this, most environmental impact assessment regulations do not specify requirements for survey effort to ensure that the probability of detecting threatened species is high. CEED researcher Georgia Garrard has undertaken a range of studies on various aspects of detectability. In this most recent study she led an analysis that sought to demonstrate how to use detectability estimates to set minimum survey effort requirements for environmental impact assessments.
She considered two methods for determining minimum survey effort requirements for threatened species during environmental impact assessments. One method allows the regulator to specify the survey effort required to ensure that the species will be detected (with some probability) if it is present. This method uses a simple relationship between detectability and survey effort to estimate the probability of detection given the species is present.
The second method allows the regulator to place the burden on the developer to demonstrate that the species is absent from the site. This requires information on detectability and survey effort, as well as information on the belief – prior to conducting surveys – that the species is present. The basic premise of this method is that, when surveying a site that you have good reason to believe is occupied by the species (for example, because the species has previously been recorded there, or because the habitat is thought to be suitable), a greater investment in survey effort is required to convince you (or anyone else!) that the species is truly absent if it is not detected than if you were surveying a site that was deemed unlikely to contain the species. She applied each of these methods to estimate survey effort requirements for Pimelea spinescens, a critically endangered native grassland plant species.