Invertebrate Research

Are Earthworms Nantucketers or Transplants?

Mustard extraction of earthworms

Mustard water irritates earthworms underground and they crawl to the surface where researchers can count and identify them.

In 2008 and 2009, MMA scientists used a process called mustard extraction (see picture) to survey the Islands for earthworms. They found six non-native species and no native species. Why all non native species? Nantucket was formed by a glacier 20,000 years ago and the crushing ice and nonexistent topsoil at that time probably wiped out any native earthworm species. Earthworms only move into new territory at a rate of approximately 8 meters (26 feet) per year. Earthworms from the south probably did not have enough time to migrate to Nantucket before the ocean surrounded it. You can download the 2008 report that we submitted to the Nantucket Biodiversity Initiative.

 

Rollie Pollies Everywhere!

Counting isopods is just one aspect of Nantucket ResearchMMA scientists counted over 17,000 isopods (rollie-pollies and pill bugs) from special traps set in a sandplain grassland habitat. They found that the more grass covering the ground, the fewer the number of isopods.

Some traps had over 900 isopods, others had zero. Interestingly the counts of isopods follow Bendford’s Law to some degree. This law states that in any group of naturally collected numbers (e.g. lengths of rivers), 30% of the numbers will start with 1, 18% will start with 2, 12% start with 3, and so on. Only 4.6% should start with 9. These percentages come from logarithmic formulas. On a hunch we ran the numbers on the isopod data and it came out very close!

First Digit Benford’s Law Isopod Count
1 30.1% 25.1%
2 17.1 19.2
3 12.5 10.8
4 9.7 9.6
5 7.9 8.4
6 6.7 6.6
7 5.8 8.4
8 5.1 4.2
9 4.6 7.8

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