Sunday, December 4, 2011

Indigo Snake Survival Demographics

The Indigo Snake, Drymarchon couperi JCM

The Eastern Indigo snake, Drymarchon couperi has been federally listed as threatened since 1978. It is one of the largest North American colubrids (if not the largest) reaching 2.63 m and has been the focus of intensive research and conservation efforts throughout its range in the southeastern United States. Indigo snakes frequently encounter humans because they  actively forage for food during the day. They prey on a wide range of vertebrates and have been reported to act as scavengers.  Breeding occurs from October to March when the snakes are using sandhill habitats. Gestation lasts 100–150 days and a single clutch of 4–12 relatively large eggs are normally laid in May or June, and they hatch about 90 days later. Drymarchon couperi exhibits male-biased sexual size and require 3 to 4 years of growth before they reach sexual maturity. Indigos often the use xeric sand ridge communities (or sandhills), especially during the fall-winter breeding season and frequently inhabit burrows of the gopher tortoise, Gopherus polyphemus for protection from environmental extremes, fires, and predators.

Hyslop et al. (2011) examined indigo snake demographic data collected over 11 years of capture-mark-recapture (CMR)  studies as well as  2.5 years of radiotelemetry data from snakes in southeastern Georgia, to estimate apparent survival, capture and transition probabilities, and evaluate the factors that influencing these parameters. They estimated the annual survival probability is 0.700 (±0.030 SE), a number  comparable to a knonw fate analysis (radiotelemetry) done at the same site. Body size positively influenced survival, regardless of sex, with larger snakes having a greater chance of surviving a year. Their model averaged estimate of annual adult survival was 0.738 ± 0.030 and 0.515 ± 0.189 for subadults. Population growth rate ranged from 0.96 to 1.03 depending on the value of the probability of transitioning from subadult to adult stage. Their results suggest that protecting adult snakes and their habitats will result in the highest likelihood of long-term population stability and growth. This is excellent news for a species that has been greatly reduced in numbers over the last 50 years. Now, if we can just keep its habitat intact.

Hyslop N, Stevenson D, Macey J, Carlile L, Jenkins C, et al. 2011. Survival and population growth of a long-lived threatened snake species, Drymarchon couperi  (Eastern Indigo Snake). Population Ecology: 1-12.

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