Lake Erie Water Snake

Nerodia sipedon insularum

Description

The Lake Erie water snake, Nerodia sipedon insularum, and the northern water snake, N. s. sipedon, are separate subspecies. Northern water snakes are common and widely distributed in eastern North America, including the Ohio and Ontario mainland, whereas Lake Erie water snakes (N. s. insularum ) have declined and occur primarily on the offshore islands of western Lake Erie. Lake Erie water snakes have reduced or no color patterns, while northern water snakes have sharply defined band patterns. Lake Erie water snakes occur on rocky limestone and dolomite shorelines; northern water snakes use more heavily vegetated locations with soil, mud, or clay. Lake Erie water snakes also have a different diet, a larger adult body size, lower growth rates, and shorter tails compared to northern water snakes.

Lake Erie water snakes are uniformly gray or brown and have either no color pattern or have blotches or banding that are faded or reduced. Color pattern variations among Lake Erie water snakes are thought to result from the combined effects of both natural selection and gene flow. On the rocky shorelines of the western Lake Erie islands, water snakes with unbanded or reduced patterns appear to have a survival advantage compared to fully patterned water snakes. Female Lake Erie water snakes grow up to 3.5 ft (1 m) long and are larger than males. Newborn Lake Erie water snakes are the size of a pencil when born during late summer or early fall.

Lake Erie water snakes are preyed upon by gulls, herons, other birds, and other snakes.

Behavior

Lake Erie water snakes hunt for their prey of small fish in shallow water along shorelines. They will also scavenge dead fish. They mate in the springtime and give birth to "live" young in the late summer (i.e., they are ovoviviparous, meaning the female retains the eggs in her body, where they hatch). Lake Erie water snakes are not poisonous, but are irascible and will try to bite if captured.

Habitat

Lake Erie water snakes use habitat composed of shorelines that are rocky or contain limestone/ dolomite shelves and ledges for sunning and shelter. Shelter occurs in the form of loose rocks, piled rocks, or shelves and ledges with cracks, crevices, and nearby sparse shrubbery. Lake Erie water snakes are found less often on shorelines composed of small stones, gravel or sand. Certain types of riprap, armor stone, or docks made with rock cribs can serve as shelter for Lake Erie water snakes, provided adequate space exists in these structures that is above Lake Erie's water and ice levels.

Distribution

The current distribution of Lake Erie water snakes is small compared to their historic distribution. The historic range of the Lake Erie water snake (N. s. insularum ) included 22 or more offshore islands and rock outcrops of western Lake Erie, a portion of the Ontario mainland that includes Point Pelee, and shorelines of the Catawba/Marblehead Peninsula, Mouse Island, and Johnson Island in Ohio. Water snakes were found on Green Island in 1930 and early museum records initially confirmed water snakes on West Sister Island. Today, Lake Erie water snakes no longer occur on the Ontario mainland and four islands: West Sister Island, Green Island, Middle Sister Island, and North Harbour Island.

The Lake Erie water snake has declined in population abundance and in distribution. The current estimate for the U. S. population ranges from 1,530 to 2,030 adults and is restricted to only eight islands. Ninety-five percent of the Lake Erie water snake population is currently restricted to an area with a diameter of less than 25 mi (40 km) comprising 12 western Lake Erie offshore islands in the United States and Canada combined. The U. S. Lake Erie offshore islands and rock outcrops include, but are not limited to, the islands called Kelleys, South Bass, Middle Bass, North Bass, Sugar, Rattlesnake, Green, Gibraltar, Starve, Gull, Ballast, Lost Ballast, and West Sister. Canadian Lake Erie offshore islands and rock outcrops of Lake Erie include, but are not limited to, the islands called Pelee, Middle, East Sister, Middle Sister, North Harbour, Hen, Chick, Big Chicken, and Little Chicken.

The offshore islands are isolated from the Ohio and Ontario mainland by approximately 3-9 mi (4-14 km) of water. Although not a complete barrier, the distance from offshore islands to the mainland (and the near-shore islands) creates a natural barrier. This barrier maintains the integrity of the Lake Erie water snake gene pool by limiting interbreeding between offshore island Lake Erie water snakes and mainland and near-shore northern water snakes. Thus, species experts believe that the genetic pool on the western Lake Erie offshore islands is primarily Lake Erie water snake and the genetic pool on the mainlands and near-shore islands is predominately northern water snake (N. s. sipedon ).

Recent data also show declines in population density on three of the four U. S. islands most important to the water snake's long-term survival. When compared to the 1986 population estimate, the 1998 estimate indicates that the overall Lake Erie water snake population remains small. Small population size makes the Lake Erie water snake population vulnerable to extinction or extirpation.

Threats

Persecution by humans is the most significant and well documented factor in the decline of Lake Erie water snakes. During the 1800s, pigs were released on some islands to exterminate snakes. All snake species were eradicated from Rattlesnake Island by 1930, but a few water snakes recently moved to the island. The persecution of island water snakes was severe, and persecution by humans is still a serious problem on several islands. The effects of past and current persecution are evident today and are a threat to the continued existence of the water snake.

The current low population densities and insular distribution of Lake Erie water snakes make them vulnerable to extinction or extirpation from catastrophic events, demographic variation, negative genetic effects, and environmental stresses such as habitat destruction and extermination. Though populations naturally fluctuate, small populations are more likely to fluctuate below the minimum viable population threshold needed for long-term survival. Likewise, chance variation in age and sex ratios can cause death rates to exceed birth rates, causing a higher risk of extinction in small populations. Finally, decreasing genetic variability in small populations increases the vulnerability of a species to extinction due to inbreeding depression (decreased growth, survival, or productivity caused by inbreeding) and genetic drift (loss of genetic variability that takes place as a result of chance). A recent study of snakes in Sweden found that inbreeding depression in isolated populations resulted in smaller litter size, higher proportion of deformed and stillborn offspring, and lower degree of genetic heterozygosity, which in turn cause reduced fertility and survivorship. Thus, in small populations, environmental, demographic, and genetic changes can result in an accelerating slide toward extinction.

Although some water snakes were documented to contain or be adversely affected by certain pollutants, the role of pollution in the decline of Lake Erie water snakes is not clear. To date, comprehensive pollution toxicity studies have not been conducted. The impact of scientific collecting on the Lake Erie water snake population is also unknown. The number of museum collections and the numerous reports of collections within scientific literature suggest the Lake Erie water snake population can withstand some level of scientific collection.

Conservation and Recovery

Three preserves exist in Ontario, Canada, which are inhabited by Lake Erie water snakes and protected from habitat loss. On Pelee Island, Ontario, the Lake Erie water snake is protected by Provincial preserves at Fish Point and Lighthouse Point. The Essex Region Conservation Authority also set aside preserve land on Pelee Island which benefits water snakes and local plant species. East Sister Island is a Lake Erie water snake Provincial preserve, but the population of water snakes on the island is small and declining.

Certain types of artificial habitat (rip-rap, certain armor stone, rock piles, or docks made with rock-filled cribs) may provide shelter for Lake Erie water snakes. However, the extent to which such artificial refugia benefit Lake Erie water snakes is currently unknown. The conservation of Lake Erie water snakes can also be aided by incorporating rock-oriented designs into shoreline developments and associated erosion control structures. Such measures have already been adopted by one developer on Johnson Island. These structures, however, are unlikely to precipitate the expansion of the Lake Erie water snake (N. s. insularum ) population because of outside pressures such as habitat degradation, natural selection, and natural gene flow from the northern water snake.

Some possible recovery actions are as follows: 1) continuation of a public outreach program directed toward island residents and visitors; 2) habitat protection measures, as needed; 3) voluntary conservation agreements with landowners; 4) design and testing of artificial refugia; 5) increased law enforcement efforts; 6) voluntary land acquisition or conservation easements from willing sellers; 7) monitoring studies; 8) winter hibernation studies; 9) reintroduction of Lake Erie water snakes to appropriate locations; and 10) captive rearing.

A public outreach program has been active on the Lake Erie islands since 1994. The program encourages a "live and let live" attitude for snakes living among island residents and visitors. A poster contest, outdoor sign campaign, and personal contacts are helping island residents and visitors realize that Lake Erie water snakes are not poisonous and pose little threat to people.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1 Federal Drive
BHW Federal Building
Fort Snelling, Minnesota 55111
Telephone: (612) 713-5360
http://midwest.fws.gov/

Reynoldsburg Ecological Services Field Office
6950 Americana Parkway, Suite H
Reynoldsburg, Ohio 43068-4132
Telephone: (614) 469-6923
Fax: (614) 469-6919

Reference

U. S. Fish and Wildlife Service. 30 August 1999. "Threatened Status for Lake Erie Water Snakes (Nerodia sipedon insularum ) on the Offshore Islands of Western Lake Erie." Federal Register 64(67): 47126-47134.