Variation in banding pattern of Lake Erie water
snakes
(Nerodia, formerly Natrix)
`Lake Erie Water Snakes (Nerodia
[Natrix] sipedon) were the subject of a famous
study by Camin
& Ehrlich (1958) that compared frequencies of
cryptic phenotypes on slate islands of Lake Erie vs those on sandy
mainlands. Category A snakes are unbanded,
Category D are strongly banded, and
Categories B & C are intermediate.
Snakes on the mainlands of peninsular Ohio and Ontario are almost
entirely banded. Snakes on Pelee & Middle Islands,
which are furthest from the Ohio and Ontario mainlands, are
predominantly unbanded, but there is a large fraction of banded
snakes that have migrated from the mainland. Snakes on
Kelley's Island [lower right] and the Bass Island complex, both of
which are closer to the southern mainland, show higher proportions
of the banded types.
`Despite its fame, Nerodia
sipedon insularum had declined to an adult population of
fewer than 1,500 adults when it was listed in 1999 as Threatened
by the Environmental Protection Agency (EPA)
under the US Endangered Species Act. Factors included the
usual culprits, human predation and habitat loss &
degradation. Establishment of a 120 hectare protected habitat on
Pelee Island and 18 km of protected shoreline on the smaller
islands allowed the species to recover to an estimated 11,980
animals by 2009. In 2011, it became notable as one of only 23
species or subspecies to have recovered sufficiently to be "downlisted"
[moved to a lower threat category] by EPA. The Committee on
the Status of Endangered Wildlife in Canada (COSEWIC)
in 2015 listed Nerodia
sipedon as of Special Concern.
HOMEWORK: Counts of banding
types for mainland Ohio, Kelley's Island, and the Bass Island
complex can be estimated from the bar graphs. Use the G-test
on the Excel spreadsheet to test the three pairwise count
comparisons for significant departures from random
expectation. Does the small count from Kelley's Island influence
the power of the calculations?
