Imperfect Detection: An important aspect of Eastern Indigo Snake Surveys



The 2017–2018 Eastern Indigo Snake (Drymarchon couperi) survey season is winding down. The majority of surveys will be finished by mid-February, and the snakes will slowly start moving away from sandhills as the weather warms up in late winter and early spring. This annual movement off of sandhills makes a species that is already difficult to find nearly impossible to encounter reliably until next winter. Survey sites always seem more familiar at the end of a season than they did at the beginning because the last visit marks the third time that we have been to that site in a three-month period. Some Indigo Snakes are recaptured at the exact same Gopher Tortoise (Gopherus polyphemus) burrow on multiple surveys, while others are observed just once. But, why do we survey each site three times in a single survey season?

Indigo Snake processed at Indigo Days. Photo: Michael Starkey

It all has to do with the Indigo Snake’s detection probability. As anyone who has spent time searching for reptiles and amphibians knows, it is often difficult to locate a species, even when they are definitely present at a location. Thus, species almost always have a detectability that is less than 100% because it is possible to search for them without finding them. Over the past 10–20 years estimating a species’ detection probability (the probability of finding it if it is present) has become increasingly recognized as an important aspect of wildlife research. Not accounting for detection probability while surveying can skew results by making animals seem rarer or less abundant than they actually are, which is especially problematic for species of conservation concern. For example, in terrestrial salamanders accounting for a species’ detectability significantly increased density estimates because these salamander species are commonly underground and not detectable using standard survey techniques (O’Donnell and Semlitsch 2015). Similarly, Indigo Snakes are often underground in tortoise burrows or hiding in other retreats, making it possible to miss them even on surveys with good conditions. Therefore, we need to estimate the detection probability for Indigo Snakes on a typical survey site.

The most common method used to estimate detection probability is to visit a survey site multiple times in a given survey season (typically a short window of time when animals are available for capture and are not moving between sites). During each survey the species of interest is either identified as ‘present’ or ‘absent’ (it is essentially impossible to identify a true absence for most herpetofauna in natural environments). The combination of detections and non-detections can then be used to estimate the probability of detecting that species if it is present (MacKenzie 2005). The estimated detection probability is then used to inform abundance, density, or estimates of other population parameters that are generally the values of most interest in conservation research.

Indigo Snake processed at Indigo Days. Photo: Michael Starkey

Accounting for imperfect detection also fits well into a broader approach to surveying known as occupancy monitoring. Many wildlife surveys are now designed around an occupancy framework (perhaps most notably the North American Amphibian Monitoring Program) where sites that meet specific qualifications are visited in an effort to determine whether or not they are occupied by a species or group of species. Occupancy surveys are useful because they allow researchers to monitor species using just presence/absence data, which are easier to collect than other types of population data (e.g., abundance data). This can often allow researchers to maximize the number of survey sites and reduce the effort required for each individual survey. However, surveying with this design is best used to monitor species at a broad scale and identify changes in species occupancy. Other survey techniques are needed to investigate other aspects of a species’ ecology.

The Orianne Society’s Indigo Snake surveys are a combination of occupancy and mark-recapture surveys. We survey sites for occupancy across southern Georgia and also mark all encountered snakes with a PIT tag. Survey sites are visited three times in a season and then are rotated, allowing us to maximize the number of suitable sites that we can visit in a time-frame that is still relevant to Indigo Snake populations. Occupancy surveys provide a snapshot of Indigo Snake occupancy across the state, while accounting for imperfect detection (approximately 40% in southern Georgia). Marking snakes allows us to follow individual snakes through their life and to estimate other important aspects of population biology (e.g., population size and growth rate). Continuing these surveys over long time periods is one key component of working towards insuring stable Indigo Snake populations in southern Georgia.

Header Photo of Eastern Indigo Snake: Matt Moore

Literature Cited

MacKenzie, D. I. 2005. Was it there? Dealing with imperfect detection for species presence/absence data. Australian and New Zealand Journal of Statistics 47:65–74.

O’Donnell, K. M., and R. D. Semlitsch. 2015. Advancing terrestrial salamander population ecology: The central role of imperfect detection. Journal of Herpetology 49:533–540.