Lake Erie Watersnake Biology and Conservation
Modified from: King, R. B., A. Queral-Regil, and K. M. Stanford. 2006. Population size and recovery criteria of the threatened Lake Erie watersnake: Integrating multiple methods of population estimation. Herpetological Monographs 20:83-104.
(article.pdf, fig2.pdf, appendices.pdf)
The Lake Erie watersnake is a non-venomous live-bearing medium-sized colubrid snake that occurs only in the island region of western Lake Erie, straddling the U.S. – Canada international boundary. The 18 islands in this region consist of dolomite and limestone bedrock that resisted glaciation. The islands range in size from 1 to more than 4000 ha and span an area less than 40 km in diameter, giving the Lake Erie watersnake one of the most restricted geographic distributions of any North American vertebrate taxon.
Island watersnakes differ in color pattern from watersnakes on the nearby mainland. Whereas mainland watersnakes are regularly banded along their entire length, island watersnakes have reduced and highly variable dorsal banding. This difference caught the attention of Roger Conant (then curator of herpetology at the Toledo Zoo) and William Clay (then a PhD student at the University of Michigan) and led them in 1937 to designate island watersnakes as a subspecies, Nerodia sipedon insularum, distinct from the mainland form, Nerodia sipedon sipedon. Studies since have demonstrated that color pattern variation has a genetic basis and that this difference in color pattern is maintained by a dynamic balance between natural selection favoring uniformly gray unbanded snakes in island populations and gene flow from mainland populations where banded snakes are favored. Visual predators overlook snakes with reduced patterns along exposed rocky island shorelines. In more heavily vegetated mainland habitats, it is the regularly patterned snakes that are more difficult for predators to see. Thus, color pattern variation in N. s. insularum serves as a textbook example of the combined effects of multiple microevolutionary processes (e.g., Evolutionary Analysis by Freeman and Herron, Genetics of Populations by Hedrick, Biological Science by Freeman).
Courtship and mating occur in spring and early summer and offspring are born in late summer and fall. Sexual maturity is reached in 2-3 years and longevity may exceed 10 years. Female watersnakes exceed males in body size. Summer activity is largely restricted to island shorelines but hibernation sites sometimes occur well inland. Historically, the diet of Lake Erie watersnakes consisted of native fishes and amphibians (mudpuppies) captured from the nearshore waters of Lake Erie. A dramatic change in diet composition took place starting in the mid-1990’s and Lake Erie watersnakes now feed almost exclusively on round gobies, an invasive fish now abundant in the island region.
Historic Population Trends
Early European visitors were impressed with the abundance of snakes in the region, dubbing the islands the “Les Isles aux Serpentes” and reporting “myriads of watersnakes basking in the sun. Although pigs released by European settlers may have reduced snake populations, scientific investigators in the early- to mid-1900s reported high watersnake densities; three workers caught 234 in just four hours in 1935 and seven workers caught about 400 in five hours in 1949. Published reports raising concerns that watersnake populations were declining first appeared in the 1960s with statements that “[a] campaign of extermination is being waged against the snakes” and that “[t]he Lake Erie Water Snakes appear to be much less abundant now around the islands than they were in 1936.” In 1982, it was noted “population densities of insular Nerodia have been drastically reduced over the past 40 years” and that “building and clearing activities along the shoreline and the increased influx of tourists . . . are contributing to the degradation of the shoreline habitat that Nerodia inhabits.” Such observations were echoed in comments by biologists teaching at the Ohio State University F. T. Stone Laboratory on South Bass Island in the early 1980s. The first quantitative estimates of Lake Erie watersnake population size were based on field work at 12 study sites on 7 islands from 1980-1984 provided further evidence of population declines: (1) Capture rates during 1980-1984 were far lower than those reported by earlier workers. (2) The adult population on Middle Island, a small island sampled in its entirety, numbered ca. 35 adults in the early 1980s whereas about 400 watersnakes were captured there in 1949. (3) No watersnakes were found on four small islands (Green, West Sister, North Harbour, Middle Sister) during repeated visits between 1980 and 1997 despite their presence in the 1930s and 1940s, suggesting that they had been extirpated from these islands.
Legal protection of the Lake Erie watersnake was extended to Canadian populations in 1977 and the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) listed it as endangered in 1991. In the U.S., the USFWS designated the Lake Erie watersnake as a category 2 candidate species in 1985, a category 1 species in 1991, and proposed it for listing as threatened in 1993. Listing was delayed when, in 1995, Congress enacted a moratorium on all final listing actions under the U.S. Endangered Species Act. In 1999, a USFWS final rule designated the Lake Erie watersnake as threatened. The Ohio Department of Natural Resources Division of Wildlife listed the Lake Erie watersnake as state endangered in 2000. Restricted geographic distribution and declining population size, attributed to human persecution and habitat loss, were identified as primary factors in extending legal protection to the Lake Erie watersnake.
The Lake Erie Watersnake Recovery Plan
A recovery plan with the goal of removing the Lake Erie watersnake from the Federal list of Endangered and Threatened Wildlife was approved by the USFWS in 2003. Three criteria for delisting were identified. The first, Population Persistence, sets overall and island-specific population size requirements for the U.S. islands. The second, Habitat Protection and Management, sets overall and island-specific habitat protection requirements. The third, Reduction of Human-induced Mortality, seeks to reduce intentional and accidental human-induced mortality to the point where such mortality no longer represents a significant threat. Recovery tasks designed to achieve these criteria include population monitoring, land acquisition, habitat management, and public outreach.
The Population Persistence Criterion
The population persistence criterion specifies that “[e]stimated population size reaches or exceeds 5,555 adult Lake Erie watersnakes on the U.S. islands combined (Kelleys, South Bass, Middle Bass, North Bass, Rattlesnake, West Sister, Green, Ballast, and Gibraltar) for a period of six or more years.” The rationale for this criterion comes from the International Union for Conservation of Nature and Natural Resources (IUCN) Red List Categories and Criteria: Version 3.1. The population persistence criterion further specifies that “[s]ubpopulations on each of the 5 small islands capable of supporting Lake Erie watersnakes year-round (Rattlesnake, Sugar, Green, Ballast, and Gibraltar) persist during the same six or more year period as Criterion 1a, and the estimated population size reaches or exceeds the population size stated below for the four largest islands simultaneously during the same six or more year period as Criterion 1a.” Island-specific population criteria are: 1. Kelleys Island – 900 adults, 2. South Bass Island – 850 adults, 3. Middle Bass Island – 620 adults, and 4. North Bass Island – 410 adults. These criteria are intended to ensure that multiple Lake Erie watersnake subpopulations persist as a hedge against population decline or extinction due to catastrophic or stochastic events. The requirement that total and island-specific population criteria are met for six or more years is intended to ensure that there is sufficient time for recruitment of new adults within the recovery period.
Current Lake Erie Watersnake Population Size and the Population Persistence Criterion for Recovery
Estimates based on field work from 2000-2004 indicate that watersnake population size exceeds 6500 adults, more than meeting the overall population size criterion of 5555 adults specified in the Lake Erie Watersnake Recovery Plan. Estimates also exceed island-specific criteria on three of four islands, totaling more that 2100 adults on Kelleys Island, 1000 adults on South Bass Island, and 1900 adults on Middle Bass Island. The estimated population on North Bass Island, 385 adults, falls short of the criterion population size for that island. However, a portion of that island remains uncensused. Assuming population sizes remain stable or increase and other recovery criteria (Habitat Protection and Management, Reduction of Human-induced Mortality) are met, delisting of the Lake Erie watersnake might commence in the near future. Although caution is warranted lest delisting take place prior to true population recovery, provisions of the U.S. Endangered Species Act require population monitoring for a minimum of five additional years and mandate relisting should populations fall below criterion levels during that period.The delisting process itself also includes multiple safeguards, including a public comment period and review at regional and national levels. In addition, protections at the state level may remain should federal protection end. Although delisting of recovered species under the U.S. Endangered Species Act has not been common, some notable examples exist, including the American alligator, peregrine falcon, and brown pelican.
Other Recovery Criteria
Considerable progress has been made in achieving the Habitat Protection and Management criterion specified in the Lake Erie Watersnake Recovery Plan. Public lands managed by the ODNR are found on the four larger U.S. islands, including significant recent acquisitions on Middle Bass and North Bass Island (totaling ca. 87% of the latter island). In addition, Green Island is managed by the ODNR and West Sister Island is part of the Ottawa National Wildlife Refuge. Public lands on the Canadian Islands include the entirety of Middle Island and East Sister Island and multiple preserves on Pelee Island. Management plans are being developed for public lands on the U.S. islands by both the ODNR DOW. Further progress in habitat management and protection is being achieved through federally sanctioned Habitat Conservation Plans and state sanctioned Conservation Easements involving private landowners.
Although listed as threatened, local populations of Lake Erie watersnakes are sometimes remarkably dense, creating a dichotomy of perceptions regarding protection. Some island residents and visitors, aware of its local abundance, view the Lake Erie watersnake as interfering with recreational activities and its protection as imposing limits on local land use practices. In contrast, other residents, visitors, and wildlife managers recognize the Lake Erie watersnake as a unique component of the local biota that, despite its local abundance, is rare on a global scale. To these people, Lake Erie watersnake protection is seen to benefit other species and ecological communities and promote awareness of local environmental resources. Resolving these contrasting views is no small task and can probably never be fully achieved. However, through outreach efforts and land use recommendations, management agencies are seeking to foster greater appreciation of the Lake Erie watersnake and provide guidelines minimizing watersnake-human conflicts. Lake Erie watersnakes seem tolerant of many human activities. Medium to high density populations occur in close proximity to summer homes and high traffic areas (e.g., adjacent to public boat launches. In addition, Lake Erie watersnakes appear capable of rapid population growth. Many females produce large numbers of offspring annually and sexual maturity is attained in 2-3 years. These characteristics, combined with management activities outlined in the Lake Erie Watersnake Recovery Plan, should promote population persistence of this Lake Erie endemic.