White on White

An invertebrate inventory of White Sands National Monument.

New, pale species and subspecies of moths blend into the white grains of gypsum sand.

Eric H. Metzler

The largest gypsum dune field on Earth—covering 275 square miles—undulates across the Tularosa Basin in south central New Mexico. Approximately 40 percent of the field falls within the protected area of the White Sands National Monument in Otero County. The unusual feature started forming about 8,000 years ago, when water evaporated from the surface of large lakes near the southwestern boundary of the current dune field, and gypsum crystallized out of solution.
Over time, weathering degraded the crystals to sand-size particles, and winds, predominately from the southwest, blew the gypsum sand from the now-dry lake bed onto the dune field, which can rise as high as sixty feet. The blowing sand moves the dune crests from the southwest to the northeast as much as twenty-nine feet per year, covering and uncovering plants and soils as they move.

Plants respond to the harsh conditions of shifting pure gypsum soils in several ways. They add stem length rapidly to accommodate encroaching dunes; they send out rhizomes (lateral roots) so new shoots can sprout up sixty feet away from the original plant; and they further bolster their root systems to avoid being taken over by a passing dune. Many animals have adapted to life in the white dunes by evolving modified coloration. Such White Sands species and subspecies as the southwestern fence lizard (Sceloporus cowlesi) and the endemic bleached earless lizard (Holbrookia maculata ruthveni) are paler than closely related populations that live outside the gypsum dunes. Animals that are naturally white or pale in color elsewhere may reside at White Sands to take advantage of the pale substrate. A lycosid, or wolf, spider maintains production of its natural darker pigments, but secretes a waxy substance to appear white.

Before 2006, almost nothing was known about the invertebrate fauna in White Sands, or in Carlsbad Caverns National Park, another protected area of the northern Chihuahuan Desert about 190 miles from White Sands. As a result, the United States National Park Service invited me to conduct a ten-year study of the lepidoptera in the dunes of the Monument and within Carlsbad Caverns National Park. A primary purpose of the study was to survey the moths in various habitats within Carlsbad and White Sands, and to describe new species discovered during the study.

Since the study began, in January 2007, we have recorded more than 600 species of moths, including 26 species new to science. The first, Protogygia whitesandsensis, was discovered in February 2007. The most recent was found last year.

Black-light traps collect moths and other insects in the southeastern corner of White Sands National Monument.

Eric H. Metzler
The moths have been collected and recorded using black-light traps along a less-than-two-mile transect in the southeastern corner of the dunes of the Monument. The number of endemic species of moths at White Sands, when compared to all of North America, is the highest for a single location. Given the small study area and the early stages of the study, these numbers seem impressive.

About 50 percent of the new moth species are white or display washed-out colors. No quantifiable data exists to explain the high incidence of paleness, just a working hypothesis that it offers protection to those species that are more apt to be on the white sand during the day when they are not flying. Most moths fly at night when color doesn’t matter. Diet could also be a factor, since all moths start out as caterpillars, and 99 percent of caterpillars eat only plants. Thanks to the unique soil chemistry of gypsum compared to other nearby soils, the same species of plants inside and outside the dunes have different chemical signatures and different endophytes, or resident microbes.

Halfway through the study, I am continuing to detect, classify, and give names to new species. Descriptions of species from units of the National Park Service are important because names facilitate the cataloging and protecting of vulnerable species and the communication of information about their significance. The information gives the Park Service a baseline from which to measure the effects of changes in climate, air pollution, water tables, and a host of other variables. Figuring out why white moths are white, however, will have to be the task of an ongoing investigation. In the meantime, back to the field, where surely more new moth species await.

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