Among the tiniest of snails…

Bitter Lake National Wildlife Refuge is home to 8 species that are listed as Federally Threatened or Endangered, and to several additional species listed as Threatened or Endangered by the State of New Mexico.  Among the Federally Endangered species is a tiny snail, so small it went unnoticed until 1987.  Pecos assiminea (Assiminea pecos) ranges in size from 1.4 to 2.2 mm.  It occurs near the wetted edges (mud-water interface) of certain sinkholes, springs, and spring-runs (also known as cienega) on the Refuge.  Pecos assiminea was listed as Endangered in 2005.  In addition to Bitter Lake NWR, the snail is found on The Nature Conservancy’s Diamond Y Springs in west Texas.

Sketch of Pecos assiminea, excerpted from Taylor, 1987 (Figure 2):


Due in large part to its tiny size, it is extremely difficult to survey.  Imagine trying to find a small brownish snail that is often less than 2 mm long in a shaded, uneven brown background; this difficulty is compounded because the habitat being searched is also full of small brown seeds, bits and pieces of small brown vegetation, and little mud stained salt crystals.  Search efforts, thus, have traditionally been hindered because they were extremely time-consuming and tedious.  Researchers have to lie on the ground with their face only a few inches above the ground, and several hours might be required to cover only a small area. Additionally, large-scale sampling efforts have been hindered by the lack of a feasible monitoring technique.

Searching for Pecos assiminea along Bitter Creek, Bitter Lake NWR, photo by Patricia Obryon
Searching for Pecos assiminea along Bitter Creek, Bitter Lake NWR, photo by Patricia Obryon.

In 2011, Bitter Lake National Wildlife Refuge decided to pursue a monitoring effort that would help it better understand the species distribution, habitat associations, and density on the refuge.  Field work for this project was largely carried out by Elizabeth (Beth) Roesler, who joined Bitter Lake NWR as an intern working for the Student Conservation Association.  Roesler’s field work involved systematically searching areas of the Refuge where Pecos assiminea is known and suspected to occur.  Roesler searched small quadrats of known size, and the quadrats were regularly spaced in searched habitats.  She also collected habitat information concurrent with each quadrat searched, such as vegetation type, soil texture/saturation, ground temperature, and water quality (from adjacent spring flows).  This information was collected in order to better understand habitat parameters associated with the species, and will eventually be incorporated into models that help predict where the species might occur.  However, even with a scientifically rigorous sampling design, positive finds of Pecos assiminea were few and searches still proved to be a tedious process.

In fall of 2012, Roesler incorporated a second technique into her sampling efforts.  This technique involved using known dimension wood tiles, and was suggested by Brian Lang, invertebrate biologist with New Mexico Department of Game and Fish.  Known dimension clay tiles are often used to sample aquatic snails, but Lang’s prior experience using clay tiles in attempts to sample Pecos assiminea had proved unsuccessful.  However, since Pecos assiminea are sometimes seen attached to small wood fragments, wood tiles seemed like they warranted a try.

Wood tile used to survey Pecos assiminea
Wood tile used to survey Pecos assiminea.

Initially, wood tiles were placed on the ground only along the edges of Sinkhole 31, which is believed to have the highest densities of Pecos assiminea on the Refuge.  Roesler’s use of wood tiles proved almost immediately successful.  Within a few weeks, Pecos assiminea began to attach to the wood tiles, and detection rates with tiles were much higher than detection rates from visually searching quadrats of known area.

As opportunity would have it…

In 2012, Bitter Lake NWR received an “Invasives with Volunteers” grant, which enabled them to initiate an ambitious phragmites (pronounced frag-mit-ies) plant removal program to restore a 1 mile spring run (approximately 7 acres of cienega habitat), known as Bitter Creek, to conditions that existed prior to phragmites invasion.  One of the goals of removing phragmites was to alter habitat in favor of Pecos assiminea and the 5 other federally listed species that occur there.  Pecos assiminea snails rarely occur in phragmites dominated habitats and declined along Bitter Creek after the spread of this invasive plant species.

Subsequent to receiving the “Invasives with Volunteers” grant, Texas Tech University received a Science Support Grant in partnership with U.S. Geological Survey and the Refuge.  The purpose of this grant was to monitor the response of Pecos assiminea to phragmites removal on Bitter Creek.  As Roesler was already experimenting with two monitoring techniques (visual searches of small quadrats and wood tiles) to monitor this species, and had already fully sampled Bitter Creek the prior year using quadrat searches, she was the best choice to make the leap from Student Conservation Association intern to graduate student.  In spring of 2013 she enrolled as a graduate student at Texas Tech University to work on the Science Support Grant funded project.  Two main goals of her graduate project are to monitor the effects of phragmites removal on Pecos assiminea, and to concurrently examine the efficacy of the two monitoring techniques mentioned above.  Unlike most graduate students in the field of conservation science, Roesler actually entered graduate school as one of the leading experts on her study species.  Thanks to her Student Conservation Association internship, perhaps only 1 or 2 people in the world have observed and sampled more Pecos assiminea than Roesler.

Beth Roesler, current graduate student at Texas Tech University surveying Pecos assiminea along Bitter Creek
Beth Roesler, current graduate student at Texas Tech University, surveying Pecos assiminea along Bitter Creek.  Photo by Patricia Obryon.

So what does Bitter Lake NWR gain from this project?  First, the Refuge aims to benefit an endangered species through its management actions/prescriptions, and the monitoring effort will help the Refuge learn how the species responds.  Secondly, the Refuge will get answers concerning the efficacy of two potentially suitable sampling techniques that can each be used to answer questions about Pecos assiminea distribution and density.  Hopefully one of the techniques can be used for “operational” monitoring for the species.  From a general conservation standpoint, the U.S. Fish and Wildlife Service will get estimates of Pecos assiminea density in multiple habitats, which will be among the first abundance estimates for this species!  Upon completion of the work, the Refuge will have enough information to make reasonable predictions about the response of Pecos assiminea to phragmites control if additional removal efforts are pursued, and, lastly, the Refuge should have enough information to start modeling likely occurrence of Pecos assiminea based on its habitat parameters.  This project is adaptive management at its best.

Citation:  Taylor, D.W.  1987.  Fresh-water mullusks from New Mexico and vicinity.  New Mexico Bureau of Mines and Mineral Resources Bulletin 116.