Discus macclintocki

Discus macclintocki

The Iowa Pleistocene Snail (Discus macclintocki), also known as the Pleistocene disk, is a tiny air-breathing land snail, a terrestrial pulmonate gastropod mollusk.

This snail lives in a few specialized locations in Iowa and Illinois.

The Iowa Pleistocene Snail is an endangered species, in danger of becoming extinct.

A relict from the ice age, this tiny snail makes its home on cool rocky slopes, near the coldwater streams, cliffs, valleys, and sinkholes that make up the Driftless Area National Wildlife Refuge in Iowa.

Known from fossil records to have existed 400,000 years ago, it is one of many glacial relict species that found refuge in the region of northeast Iowa, northwest Illinois, southeast Minnesota, and southwest Wisconsin called the driftless area. The rugged seemingly driftless area was so called because of early geologist’s inability to find evidence of glacial drift. Much of the area was indeed covered by glaciers about 500,000 years ago.

The Iowa Pleistocene Snail found its current home with desirable temperature, moisture, and food resources about 10,000 years ago as ice age conditions moderated. Certain slopes, usually north facing, have loose rock that allows ice-cooled air to exit from underground cracks and fissures. Upland sinkholes contribute to the air flow regime and are an important component of a unique system called an algific talus slope, meaning cold producing rocky slope. Even when outside air temperature is 90 degrees Fahrenheit (30 °C), ground temperatures on these slopes can be close to freezing. This air flow provides a climate similar to what was prevalent in glacial eras. Freezing winter temperatures are moderated on the slopes giving a year round range of about -10 to 10 °C (14 to 50 °F).

The Iowa Pleistocene snail now occurs nowhere else in the world but 37 algific talus slopes in Iowa and Illinois. The Iowa Pleistocene snail was thought to be extinct, until it was discovered in 1955 by a scientist working in northeast Iowa. It was listed as endangered in 1977.

It is no bigger than a shirt button, with adults ranging in size from 5 to 7 millimeters in diameter. They live in the leaf litter preferring a diet of birch and maple leaves. The snail shares its habitat with a host of rare and disjunct plants and animals associated with cool habitats. Balsam fir (Abies balsamea), Canada yew (Taxus canadensis) and yellow birch (Betula alleghaniensis) are common to algific talus slopes. The threatened Northern monkshood plant (Aconitum noveboracense) also grows on these sites.

The 775 acre (3.14 km²) Driftless Area National Wildlife Refuge was established in 1989 to permanently protect populations of the Iowa Pleistocene Snail and threatened Northern monkshood. These species’ habitat cannot be restored once lost and the primary objective of their respective recovery plans is providing protection for remaining colonies. The invasion of garlic mustard (Alliaria petiolata) onto algific talus slopes has emerged as a threat in recent years.

The Refuge consists of scattered tracts of land in northeast Iowa ranging from 6 to 208 acres (24,000 to 842,000 m²). Algific talus slopes range in size from a few square meters to perhaps ½ mile (800 m) in length. More than just the algific talus slope is targeted for acquisition. Sinkholes that feed the system can occur some distance away from the slope and need protection to ensure long term integrity of the site. Buffer areas around the slope are included when possible. The Refuge contributes to lands already protected by The Nature Conservancy, the Iowa Department of Natural Resources, County Conservation Boards in Iowa, and publicly owned sites in Ohio and New York.

At least eight other snail species, considered glacial relicts, are also protected on these sites. Some of these species like the midwest pleistocene vertigo (Vertigo hubricti hubricti), are even smaller and perhaps more rare than the Iowa Pleistocene Snail. Protection of algific talus slopes may help prevent the need for threatened or endangered status for these snails and plants like the golden saxifrage (Chrysosplenium iowense).

  • Appearance - These small land snails are only about 1/4 inch (6 mm) in diameter. Their shells are brown or greenish white.
  • Habitat - The snails live in the leaf litter of special cool and moist hillsides called algific talus slopes. Cool air and water, from underground ice, flow out of cracks in the slopes and keep the ground temperatures below 50 °F (10 °C) in summer and above 14 °F (−10 °C) in winter.
  • Reproduction - Iowa Pleistocene snails breed from late March to August. Two to six eggs are laid among the leaf litter and hatch in about 28 days. The snail's life span is about five to seven years.
  • Feeding Habits - The snails eat the fallen leaves of birch and maple trees and dogwood shrubs.
  • Range - These snails have only been found at about 30 sites in Iowa and Illinois. Fossilized shells indicate they were once much more widespread during cooler glacial periods.

Causes of endangered status

  • Habitat Loss or Degradation - The major long-term cause of snail population decline is climate change. The most immediate habitat threats are from logging, quarrying, road building, sinkhole filling and contamination, human foot traffic, livestock grazing and trampling, and misapplication of pesticides.

Protection

  • The Iowa Pleistocene snail was listed as an endangered species in 1978.
  • The Driftless Area National Wildlife Refuge was created to protect this snail and a plant.
  • Habitat Protection - The U.S. Fish and Wildlife Service, state, county, and private conservation agencies are all working to preserve the snail and its habitat. Some private landowners have entered into voluntary protection agreements.

References

  1. Database entry includes justification for why this species is of least concern
  2. Iowa Pleistocene Snail fact sheet. .
  3. Species Profile for Iowa Pleistocene snail. .
  4. Iowa Pleistocene snail, Links, Endangered Species: U.S. Fish & Wildlife Service, Links, Endangered Species: U.S. Fish & Wildlife Service. .
  5. Driftless Area National Wildlife Refuge: U.S. Fish and Wildlife Service. .

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