New published research: Indigo snake conservation units  

A large male Eastern Indigo Snake crossing a sand road in southern Georgia – Houston Chandler

A fundamental goal in wildlife management and conservation programs is to understand the current distribution of a species and the habitats that it relies on. This type of information is often summarized into some type of spatial conservation or management units — think shaded areas on a map that indicate something important about a species distribution. Spatial conservation units will have slightly different meanings depending on the goals of a project and methods used to draw them, but they are ultimately a tool that can be used by managers to help make decisions. For example, these types of spatial units have been employed to guide quotas for sustainable harvests, to identify areas where populations are most vulnerable to future threats, and to identify areas where translocations of rare species may be possible.

In partnership with the U.S. Fish and Wildlife Service, we began working on a range-wide conservation planning project for Eastern Indigo Snakes (Drymarchon couperi) all the way back in 2020. The goals of this multi-year project are to better understand the current status of indigo snake populations and provide tools that Fish and Wildlife Service and other stakeholders can use to make decisions about indigo snake management. The first product from this work was a range-wide habitat suitability model that was published at the beginning of the year (Chandler et al. 2022). This model simply predicts where suitable indigo snake habitat occurs across the species historic range. The next goal of this project was to use the results of this habitat model to draw conservation units for indigo snakes. These units were drawn to represent all areas where current data suggest that there was enough habitat to support at least one indigo snake home range as measured by previous research (Bauder et al. 2020). 

Conceptual diagram showing how areas of suitable habitat (green polygons) were connected if they were within an indigo snake’s movement potential (7.5 km) and if the resistance of the landscape was low, allowing a snake to move across the landscape. – Javan Bauder

To draw conservation units for indigo snakes, we used a landscape resistance approach to assess how habitat patches were connected. Basically, this approach used the habitat suitability model results to predict places where indigo snakes could more easily move across the landscape (i.e., the higher the habitat suitability value to lower the resistance to indigo snake movement). We also accounted for indigo snake’s ability to make seasonal movements such that suitable habitat patches would be connected only if they were within 7.5 km of one another (Hyslop et al. 2014). Finally, we made a handful of small adjustments to account for landscape features known to act as barriers to seasonal indigo snake movements (large roads and rivers). Summing up, our conservation units represented patches of indigo snake habitat that are connected by a landscape that indigo snakes could theoretically move through on an annual basis.

An example of the process used to draw conservation units. Landscape data were used to make a habitat model that was then turned into a resistance surface before finally being used to calculate a connectivity surface that represented patches of connected indigo snake habitat. – Javan Bauder

Using this approach, we identified 255 indigo snake conservation units across the species distribution. This included 53 units in Georgia, 29 in north Florida, 41 in the Panhandle, and 132 in Peninsular Florida. Conservations units ranged in size from approximately 200 hectares to over a million hectares, and the largest units contained upwards of 550,000 hectares of suitable indigo snake habitat. Furthermore, there were 47 conservation units that contained at least 5,000 hectares of suitable habitat (a size that has been suggested as needed to support a viable indigo snake population). The number of conservation units with contemporary (post-2000) indigo snake records was highest in Georgia (42%) and Peninsular Florida (47%) and lowest in North Florida (28%) and the Panhandle (2%). The low number in the Panhandle reflects the historic range contractions experienced by indigo snakes.

Final indigo snake conservation units (shaded polygons) and ‘population units’ created by Fish and Wildlife Service. – Houston Chandler

Overall, the size, the amount of suitable habitat, and the amount of land protected within conservation units was highly variable across the study area. One of the goals of the project was to compare habitat-based units (this study) with units that were delineated based solely on a distance surrounding indigo snakes records as part of the recent Species Status Assessment (USFWS 2019). We found that distance-based units often extended into areas where there was likely no suitable habitat for indigo snakes or stopped abruptly when habitat likely continued on in a linear fashion (i.e., along river corridors). These differences highlight the importance of accounting for landscape differences when designing spatial planning tools. There are many potential application for the indigo snake conservation units created in this study, including guiding future survey work, guiding land protection efforts, identifying populations that are the most vulnerable to a changing landscape, and using conservation units as a framework for modeling population viability (Bauder et al. 2022).

Literature Cited

Bauder, J.M., D.R. Breininger, M.R. Bolt, M.L. Legare, C.L. Jenkins, B.B. Rothermel, and K. McGarigal. 2020. Movement barriers, habitat heterogeneity or both? Testing hypothesized effects of landscape features on home range sizes in eastern indigo snakes. Journal of Zoology 311:204–221.

Bauder, J.M., H.C. Chandler, M.L. Elmore, and C.L. Jenkins. 2022. Incorporating habitat suitability, landscape distance, and resistant kernels to estimate conservation units for an imperiled terrestrial snake. Landscape Ecology.

Chandler, H.C., C.L. Jenkins, and J.M. Bauder. 2022. Accounting for geographic variation in species-habitat associations during habitat suitability modeling. Ecological Applications 2022:e2504.

Hyslop, N.L., J.M. Meyers, R.J. Cooper, and D.J. Stevenson. 2014. Effects of body size and sex of Drymarchon couperi (Eastern Indigo Snake) on habitat use, movements, and home range size in Georgia. Journal of Wildlife Management 78:101–111.

U. S. Fish and Wildlife Service. 2019. Species status assessment report for the eastern indigo snake (Drymarchon couperi). Version 1.1, July, 2019, Atlanta, Georgia, USA.

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