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Science
Volume 40 Issue 10 • July 8-14, 2010
now in our 40th season

Special on Nantucket


There are many things that are unique about Nantucket that make it both a destination and a home. If someone asks “what is special about Nantucket?” the answers might range from the number of wetlands, to its sheltered harbor, independent people, exceptional history, cobblestone streets, to the relative peace and quiet.

Many of the factors that make Nantucket special can be traced to its location.  The climate we enjoy and relatively pristine water and soil is directly a result of our distance from the mainland and from being surrounded by water.  The Gulf Stream’s warm waters reach close enough to provide warm breezes and strange creatures, while the surrounding water prevents Nantucket from becoming too hot when the mainland bakes each summer. Water is a transparent medium and land is opaque.

ater allows light to penetrate to depth, leaving the surface layers cooler than they would be if the surface was opaque. A cooler water surface results in cooler air temperatures above. When solar radiation strikes land, the energy is absorbed in a thin layer that heats relatively rapidly. Likewise, it readily gives up its heat to the atmosphere. This is something you do not have to tell someone in inland Massachusetts right now.

This is first of two articles discussing some of Nantucket unusual creatures, unique species and subspecies and environmental niches that are often a result of what normally occurs in island colonization due to island biogeography.  Island biogeography is the study of the distribution and dynamics of species in island environments.  Due to their isolation from more widespread continental species, islands are ideal places for unique species to evolve. Islands, however, are also places of concentrated extinction.  Of 724 known animal extinctions in the last 400 years, about half were of island species, and of the bird species that have become extinct in that period, at least 90 percent were island dwellers.

Island species are especially vulnerable to extinction because they have a small geographic range.  They are limited to the island or a particular part of the island, and they usually have low population numbers.  These factors make them more likely to become extinct as a result of natural factors such as disease, fire, and normal population fluctuations. If the population is small to begin with, a natural occurrence may occasionally kill enough individuals so there is no longer a viable population of that species.

This dynamic is exacerbated when introduced species such as humans, their domesticated animals, pests, and diseases arrive on the island. Native species that have evolved without contact with these new organisms are often unable to compete or defend themselves. Habitat destruction, direct hunting, competition for food, and other factors put intense pressure on island species. In the continental setting, a species may still have other undisturbed populations located in other areas, or the local population may be augmented by incoming individuals from other populations not experiencing the same pressures. In the island setting, there are no other populations to draw from, and the species may very well become extinct.

From R.P. Filson’s excellent online activity and primer, “Island Biogeography and Evolution: Solving a Phylogenetic Puzzle using Molecular Genetics” (www.ucmp.berkeley.edu/fosrec/Filson.html - an excellent curriculum piece for high school students) we learn there are some basic rules for island colonization of plants and animals.

  1. The closer the island to another land mass, the higher the probability of colonization.
  2. The older the island, the more likely it will be colonized.'
  3. The larger the island, the more species are likely to be established.
  4. Geographic isolation reduces gene flow between populations.
  5. Over time, colonial populations become genetically divergent from their parent population due to natural selection, mutation, and/or genetic drift.

These make sense:  it is easier to travel over here if given the time and the distance is short; more species can live peacefully on more land; over time we start looking like our parents; and if we become too isolated, we all start looking alike.  Okay, even at high school level, some of these terms are over my head. I’ll blame my ignorance on the fact that evolution was still a controversy 100 years ago when I was in high school, so let’s go over some quick definitions. First, what the heck is phylogenetics? It sounds vaguely Greek.  Phylogenetics is the study of evolutionary relatedness among various groups of organisms (for example, species or populations), which is discovered through molecular sequencing data and morphological data matrices (basically via DNA or how things look).  The term is of Greek origin from the terms “phyle/phylon,” meaning "tribe, race," and “genetikos,” meaning "relative to birth" from genesis "birth") which explains that Greek-like feeling. 

Gene flow is a type of migration and it occurs whenever there is any movement of genes from one population to another. Gene flow includes lots of different kinds of events, such as pollen being blown to a new destination or people moving to new cities or countries.

Let’s look at a couple of real world examples of island derived/evolved creatures.  Nantucket has very few mammals, but we do have several species that spend a great deal of their time living a fossorial or underground life, such as moles, voles, and shrews.  Voles are members of the order Muridae (rodents) which includes mice, rats, voles, and lemmings. The most common specimen found on Nantucket is the meadow vole (Microtus pennsylvanicus).  But, found only on Muskeget Island is Microtus breweri otherwise known as the beach vole.  M. breweri became isolated from its closest relative, the meadow vole, about 3,000 years ago during a glacial melt and rise in sea level.

According to the succinctly named article “Microtus breweri” by Robert Tamarin and Thomas H Kunz in the June 28, 1974 publication of Mammalian Species (no 45. pages 1-3 + figures), the species was first written about by Baird in 1857 and substantiated by Miller in 1896 as a distinct species with a variant pelage (fur) and skull size.  The name is a commemorative name to honor Dr. T. M Brewer, its discoverer. It is the only mammal endemic to Massachusetts, the result of genetic isolation from the mainland meadow vole.  It differs from the M. pennsylvanicus in having a lower rate of reproduction, smaller litter size, greater body weight, and longer life span. It differs from almost all other voles by maintaining relatively constant population densities; voles are well known for fluctuations in density.

Beach grass provides the major food source and habitat for this rodent, which is also known as the beach mouse or the beach meadow mouse (sounds like a good band name). Subspecies and species offshoots occur when there is genetic isolation which is a natural result of mammals living on islands separated by water.  The beach vole is larger, more grizzled (sort of Clint Eastwood-ish), and pale brown when compared to its darker, slightly smaller cousin. Perhaps due to less predators, the Muskeget beach vole is a better parent and less aggressive than Microtus pennsylvanicus. This species has been separated out and lumped together with its cousins many times. According to Don E. Wilson’s 2003 book, “The Smithsonian Book of North American Mammals,”  the beach vole is indeed a separate species as established by a statistical analysis of skull and tooth characteristics and that is good enough for me.

Now it is time to bust a commonly held myth that I always believed, that Nantucket was the furthest northern extent for the prickly pear cactus. This most common species on Coatue, Opuntia humifusa [(Raf.) Raf.] also known as the Indian fig and eastern prickly pear, has been observed further north on the mainland on the Outer Cape all the way to Montana and similar species are found in parts of  Canada. Prickly Pear cactus grows in sprawling clumps 2 to 3 feet (0.6 -0.9 m) across and generally less than a foot high. It has flat, fleshy green pads covered with clusters of short reddish-brown barbed bristles. Flowers are bright yellow, 2 to 3 inches wide (5-8 cm), and bloom from the tops of the pads, opening only for several days in July. The large red fruits are pear-shaped and 1 to 2 inches (3-5 cm) long, juicy; sweet, and edible. In Mexico the fruits are called “tuna.”  According to Massachusetts Natural Heritage program, the range extends from Massachusetts to Minnesota and west to Alabama and Oklahoma. It is the only cactus which is widespread in the East. Prickly pears are also known as "nopal" or nopales, from the Nahuatl word nōpalli for the pads, or nostle, from the Nahuatl word nōchtli for the fruit; or paddle cactus (from the resemblance to the ball-and-paddle toy). Opuntia are the most cold-tolerant of the lowland cacti, extending into western and southern Canada; one subspecies, Opuntia fragilis var. fragilis, has been found growing along the Beatton River in central British Columbia, southwest of Cecil Lake at 56° 17’ N latitude and 120° 39’ W longitude. Member of the Opuntia cactus family are some of the most useful plants known to man, serving as food and water sources, as medicine, to make alcohol, as a hangover remedy, as a psychoactive compound and as a source of dye for the Aztecs and Mayans. The cacti host a scale insect called Cochineal (Dactylopius coccus) which is a crazy looking parasite that produces carminic acid which is used to make the red dye, carmine.

Opuntia is given the distinction of being the first formally described botanical species in Eugene Bicknell’s 1914 paper “the ferns and flowering plants of Nantucket – XII” which is found in the Bulletin of the Torrey Botanical Club (Volume 41, no 2- Google Books online). You may recall Bicknell’s florid prose from previous editions of this column and I groaned and then laughed when this reference came up on a Google search.  Bicknell, in his typical manner, declares that “we do not know if it was ever seen on Nantucket by civilized man prior to its discovery there by Mr.  Thomas A Green of New Bedford on whose authority it was announced as a Nantucket plant as long ago as 1833 by Professor Edward Hitchcock in his “Report on the Geology Mineralogy Botany and Zoology of Massachusetts.”  Apparently no other “civilized man” had set foot on Nantucket or seen this plant.

Bicknell’s goes on to give this cactus its primary due by stating that although “other plants of Nantucket find record in the same work but the prickly pear under the name Cactus Opuntia L has priority of place thus eighty years ago marking the starting point of exact Nantucket botany.”  Huzzah.  Bicknell also writes that “On Nantucket this cactus is at the extreme northeastern limit of its range and is native only on that long arm of sand known as Coatue which reaches along the western side of the harbor protecting it from Nantucket Sound.  It abounds there in sandy openings among the low red cedars taking so strong a growth as to form close assurgent clusters sometimes three to five feet across.” So, we can blame him for the misinformation and give our northern mainland cousins their prickly due.

Believe it or not, we have only spied the tip of the evolutionary and island-specific iceberg. Next time we’ll look at some of the phenotypical variations in spiders, plants, and snakes (spreading the phobia factor around equally).

 

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