Habitat Fragmentation
Eastern indigo snakes have large home ranges and move long distances (especially males); thus, habitat connectivity needs to be maintained. Urbanization creates habitat fragmentation by reducing habitat patch sizes. Primary and secondary roads (such as interstates and highways) are prominent features of urbanized areas and can contribute to isolation and fragmentation of eastern indigo snake populations because they often avoid these type of roads (Bauder et al. 2018, p.751), but they may eventually cross these road types in search of food and mates when habitat patch sizes decrease (Breininger et al. 2004, 2011, and 2012).
As urbanization of natural areas progresses, the size of fragmented habitat patches become smaller sustaining fewer snakes and creating islands of fragmented habitat with little or no connectivity within a landscape of unsuitable habitat. However, eastern indigo snakes will likely persist in localities where large, contiguous patches of natural habitat remain. It has been suggested that eastern indigo snake populations that occur on federal, state, or other privately managed conservation lands with multiple patches of at least 2,500 ac (1,000 ha) (i.e. multiple patches is >5,000 ac (>2,023 ha)) may increase long-term viability (Moler 1992). However, high edge-area habitat patches (e.g. edges created by roads or human-altered habitats) have greater extinction risk due to direct mortality (Breininger et al. 2004, 2011, and 2012, entire). A recent study suggested 2,500 ac is too small to support even a single pair of eastern indigo snakes and suggested about 12,000 – 22,000 ac (5,000 – 9,000 ha) of unfragmented habitat is needed to sustain eastern indigo populations in central Florida (Bauder 2018, p. 160). Sytsma et al. (2012, pp. 39–40) estimated a reserve size of 10,000 ac (4,047 ha) could support a small population of eastern indigo snakes. However, Hyslop et al. (2014, p.109) reported that the collective extent of eastern indigo snakes studied (n=31) near Fort Stewart in Southeast Georgia, where the snakes are believed to travel the farthest distance, was about 20,000 to 35,000 ac (8,000 to 14,000 ha). These patch sizes reported in the literature provide insight to the degree of habitat fragmentation suitable for eastern indigo snakes.
Habitat Destruction
Throughout the eastern indigo snake’s current range (i.e. Florida and Georgia), the increasing trend of urbanization and agricultural development continues to destroy and degrade habitat. Because of its relatively large home range and low degree of home range overlap (Hyslop et al. 2014, Bauder et al. 2016a), the eastern indigo snake is especially vulnerable to habitat loss (Lawler 1977, Moler 1985b, Breininger et al. 2004, 2011, and 2012; Hyslop et al. 2012, Bauder et al. 2018).
Habitat impacts due to urbanization are increasing across the species range, particularly in Florida. In 1977, Lawler reported that the loss of natural habitat in Florida was increasing and eastern indigo snake habitat was being lost at a rate of 5 percent per year. Zwick and Carr (2006, p.2) predicted that by 2060 nearly 3 million acres of natural habitat in Florida would be lost to urbanization. In a more recent study Carr and Zwick (2016) projected Florida’s population to grow from about 18.8 million to approximately 33.7 million by 2070. The projected population growth is not evenly distributed and may be accommodated by more compact pattern of development and increased protected lands (Carr and Zwick 2016). Generally, central Florida is projected to experience much greater growth and therefore have the greatest increase in developed lands while the Panhandle region is predicted to have the lowest rate of development with significant open space predicted to remain (Zwick and Carr 2006, Carr and Zwick 2016). Although eastern indigo snakes may occupy areas of low density residential housing in the southern portions of its range in Florida, this also represents a potential negative influence to the species since there is increased likelihood of snakes being killed by humans and domestic pets (Breininger et al. 2012, p. 364). The effects of habitat destruction on the eastern indigo snake are likely most substantial along the Florida coasts, in the Keys, and along the high ridges of central Florida, where human population growth is expected to continue to accelerate. In Southeast Georgia, urbanization also is increasing but not as rapidly compared to Florida. Georgia is mostly forested (>57% in 2012), followed by agricultural land (>18% in 2012) and developed land (>12% in 2012); however developed land continues to increase (USDA 2016, p.3).
Solar energy developments can destroy or degrade habitat for eastern indigo snakes. Solar developments on sand ridges are a factor that is increasing significantly in both Georgia and Florida in recent years (EIA 2018a, Florida Department of Environmental Protection (FDEP) 2016). In 2010 Florida produced approximately 80,000 Megawatts (MW) increasing to 870,000 MW in 2017. In Georgia, solar development has increased to almost 2.5 times more than Florida, having only produced only 3,000 MW in 2012 increasing to 2,137,000 MW in 2017. By the end of 2016, Georgia ranked 8th in the nation in solar (EIA 2018b). Some solar utility developers and companies recognize the potential impact this type of development may have on rare species and their habitat and have begun working with conservation organizations to reduce impacts via strategic siting assessments (NASA Develop 2018).
Although to a lesser extent than urbanization, conversion of suitable, natural eastern indigo snake habitat to agricultural land uses (including crop, pasture and timber land) also contributes to habitat destruction and degradation throughout much of Georgia and Florida (Enge et al.2013, USDA 2016 entire, Carr and Zwick 2016). These anthropogenic land uses have variable influences on eastern indigo snakes, but may provide important habitat for eastern indigo snakes (e.g. Ceilley et al. 2014, GDNR 2017). However, these land uses are subject to relatively frequent alteration (e.g. herbicides, plowing) and heavy equipment as a result of various production needs (harvesting, planting, ditching, etc.) that may negatively influence eastern indigo snakes (e.g. Godley and Moler 2013, p. 363, Enge et al. 2013). Nevertheless, eastern indigo snakes are known to inhabit extensive canal systems in central and south Florida. Efforts to restore natural wetlands at these agricultural sites may adversely impact eastern indigo snakes (Ceilley et al. 2014). Agricultural land use practices (e.g. heavy herbicide use, bedding, planting dense stands of Pinus sp.) can reduce herbaceous groundcover and negatively influence gopher tortoise populations (CCA 2012, Enge et al. 2013) and the availability of gopher tortoise burrows as shelter sites for eastern indigo snakes. Loss of thermally stable, below-ground shelter sites can negativity influence eastern indigo snakes, especially in the northern portion of its range (Enge et al. 2013). While agricultural lands present some risk to eastern indigo snake populations, negative impacts may be offset by conservation of agricultural lands. For example, conserved agricultural land (e.g. conservation easements, Sustainable Forestry Initiative) may reduce impacts from urbanization, improve wildlife habitat, and maintain connectivity among eastern indigo snake populations.
Mining for resources such as sand, limestone, phosphate and heavy metals continues to increase in Georgia and Florida (GEPD 2017, FDEP 2018) and adversely impact eastern indigo snake habitat. In Georgia, multiple sand and heavy metal mines within the range of the eastern indigo snake have been permitted since 2008 (GEPD 2017). In Florida, mining is widespread across eastern indigo snake habitats; for example, phosphate mines disturb between 3,000 and 6,000 acres (1,200- 2,400 ha) annually in Florida (FDEP 2003). Generally, resource mining causes intensive land disturbance over relatively large areas over time. In an effort to reduce overall environmental impacts from mining, mitigation and reclamation of mined lands are often implemented. Land protection (mitigation) in strategic areas may help offset impacts to habitat loss; however, effectiveness of reclaiming retired mines and restoring habitat suitability for eastern indigo snakes is not known.
Habitat modification from any of the above activities can lead to direct mortality from impacts due to equipment and/or hazardous materials. Heavy equipment can kill or injure snakes. Construction debris can also cause harm to individuals. For example, snakes are particularly vulnerable to entanglement in plastic netting that is often used in matting for erosion control on construction projects (Stuart and Watson 2001, pp. 162-164) and eastern indigo snake entanglement has been documented (Enge et al. in press).
Habitat Degradation (inadequate fire management)
Eastern indigo snakes use a variety of habitats, and patterns of habitat use may shift seasonally. However throughout its range, eastern indigo snakes show a strong affinity for upland habitat types, especially longleaf pine habitats. Most of these upland habitat types depend on reoccurring periodic fire to maintain good quality. Natural fires are now often suppressed, and many habitats are degraded from inadequate fire management (Wear and Greis 2002), however the number of states offering education and training to certify prescribed fire managers has increased over time increasing the capacity for prescribed fire (Melvin 2015, p. 1). The inability to meet prescribed fire goals is likely to be influenced with expanding urbanization and climate change. Changes in climate are predicted to increase wildfire risk and limit the number of suitable burn days due to warming temperatures and regional drying via evapotranspiration regardless of changes in precipitation (Ingram et al. 2013, p. 166). In addition, to reduce “non-essential” carbon emissions, the possibility of additional air quality restrictions (PM 2.5) further limit prescribed fire. In 2014, state forestry agencies in the southeast United States ranked air quality/smoke management and wildland urban interface/population growth higher than the national percentage as impediments limiting prescribed fire (Melvin 2015, p. 17).