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Spruce Fir Moss Spider
Microhexura montivaga

FAMILY: Dipluridae

STATUS:Endangered, Federal Register, February 6, 1995

DESCRIPTION: The spruce-fir moss spider was originally describedby Crosby and Bishop (1925) based on collections made from a mountain peak in western North Carolina in 1923 (Coyle 1981). It is one of the smallest members of the primitive suborder of spiders that are often popularly referred to as "tarantulas" (Harp 1991, 1992). Adults of this species measure only 0.10 to 0.15 inch (about the size of a BB) (Coyle 1981). Coloration of the spruce-fir moss spider ranges from light brown to yellow-brown to a darker reddish brown, and there are no markings on its abdomen (Harp 1991, 1992). The most reliable field identification characteristics for the spruce-fir moss spider are chelicerae that project forward well beyond the anterior edge of the carapace, a pair of very long posterior spinnerets, and the presence of a second pair of book lungs, which appear as light patches posterior to the genital furrow (Harp 1992).

RANGE AND POPULATION LEVEL: The spruce-fir moss spider, Microhexura montivaga, is known fromonly the highest mountain peaks (at and above 5,400 feet in elevation) in the Southern Appalachian Mountains of North Carolina and Tennessee. It has been recorded from Mount Mitchell, Yancey County, North Carolina; Grandfather Mountain, Watauga, Avery, and Caldwell Counties, North Carolina; Mount Collins, Swain County, North Carolina; Clingmans Dome, Swain County, North Carolina; Roan Mountain, Avery and Mitchell Counties, North Carolina, and Carter County, Tennessee; Mount Buckley, Sevier County, Tennessee; and Mount LeConte, Sevier County, Tennessee.

Recent and ongoing surveys indicate that reproducing populations of the spruce-fir moss spider still survive on Grandfather Mountain in North Carolina (Harp 1992; personal observation, 1995; Jane Thompson, The Nature Conservancy, pers. comm., 1997); Mount LeConte in Tennessee (Coyle 1997); and Mount Buckley (Coyle, pers. comm., 2000) and Roan Mountain in North Carolina and Tennessee (Coyle 1999). The Mount Mitchell population is believed to be extirpated (Harp 1992), and both the Mount Collins and Clingmans Dome populations, if still present, are extremely small, with only one spruce-fir moss spider having been found at each of these two sites in recent years (Harp 1991, 1992). The occurrences of the species on Mount LeConte, Mount Collins, Clingmans Dome, and Mount Buckley are all within the boundaries of the GSMNP, administered by the NPS; the sites supporting the species on Roan Mountain are within the boundaries of the Pisgah National Forest and are managed by the USFS; and the area on Grandfather Mountain that still supports the spruce-fir moss spider is privately owned and is managed by The Nature Conservancy through an agreement with the landowner.

Recent work by Coyle (1997) indicates that Mount LeConte currently supports the healthiest of the surviving populations of the spruce-fir moss spider. In his study of the species on Mount LeConte, Coyle (1997) recorded the species from four small, separate areas of rock outcrop. The Grandfather Mountain population is restricted to small patches of suitable microhabitat occurring on a single rock outcrop and a nearby boulder (Harp 1992; personal observation, 1995). The Mount Buckley population is restricted to scattered patches of suitable microhabitat on separate rock outcrop sites within an area roughly 0.20 hectare (0.5 acre) in size. On Roan Mountain, Coyle (1999) recorded scattered occurrences of the spruce-fir moss spider at 12 small, separate rock outcrop sites but found more than two spiders living in the same discrete patch of moss/liverwort on only three occasions. As stated above, individual spruce-fir moss spiders have been observed in recent years, one on Mount Collins and one on Clingmans Dome, indicating extremely low population levels. Coyle (in litt., 1991) reported that the spruce-fir moss spider wascommon at a site on Clingmans Dome as late as 1983 but was extremely rare by 1988, which he suspected was largely due to deterioration of the forest canopy at the site.

HABITAT:The spruce-fir moss spider is known only from Fraser firand red spruce forest communities of the highest elevations of the southern Appalachian Mountains in western North Carolina and eastern Tennessee (Coyle 1981, 1997, 1999; Harp 1991, 1992). The typical habitat of this spider is found in damp, but well drained, moss mats growing on rock outcrops and boulders in well shaded situations within these forests (Coyle 1981, 1997, 1999; Harp 1992). The moss mats cannot be too dry (the species is very sensitive to desiccation) or too wet (large drops of water can also pose a threat to the spider). The spider constructs tube-shaped webs in the interface between the moss mat and rock surface. There is no record of prey having been found in the webs of the spruce-fir moss spider, nor has the species been observed taking prey in the wild, but the abundant springtails (collembolans) in the moss mats provide the most likely source of food for the spider (Coyle 1981, Harp 1992).

REASONS FOR CURRENT STATUS:The primary factor determining therarity of the spruce-fir moss spider is the current rarity, and continuing decline in the quality, of the species habitat. As stated above, this spider is known only from high-elevation spruce-fir forest communities of the southern Appalachian Mountains in western North Carolina and eastern Tennessee. Most of these forests have suffered extensive changes and declines in size and/or vigor during the past century as the result of a number factors including past logging and burning practices and storm damage (Peart et al. 1992); and possibly atmospheric pollution (Bruck 1988; Johnson et al. 1992) and other factors not yet fully understood. The portion of the spruce-fir forests where the spruce-fir moss spider has been found are dominated by Fraser fir. In recent years, Fraser Fir throughout the southern Appalachian Mountains have suffered extensive mortality due to infestation bthe balsam wooly adelgid (Adelges piceae), a non-native insect pestbelieved to have been introduced into the United States from Europe (Kotinsky 1916; Eagar 1984). Most mature Fraser fir are easily killed by the adelgid (Amman and Speers 1965) with death occurring within two to seven years of the initial infestation (Eagar 1984). The remaining trees become more susceptible to storm and other damage. The death and thinning of the forest canopy results in locally drastic changes in microclimate including increased temperatures and decreased moisture leading to desiccation of the moss mats on which the spider depends for its survival. The spruce-fir moss spider is very sensitive to desiccation and requires situations of high and constant humidity (Coyle 1981; Harp 1991, 1992) and as the mats dry out, so does the spider.

All existing data (Coyle 1981, 1997, 1999; Harp 1991, 1992,U.S. Fish and Wildlife Service 1995, 1998) indicate that suitable habitat for the spruce-fir moss spider is extremely limited and restricted to small areas of rock outcrops occurring in forest stands dominated by fir trees, providing the shelter and organic substrata required by the spider. This restricted range of each of the surviving populations of the spruce-fir moss spider also makes it extremely vulnerable to extirpation from a single event or activity, such as a severe storm, wildfire, land-clearing or timber operation, pesticide/herbicide application, etc. In addition, the spider and the moss mats it inhabits are very fragile and easily destroyed by human trampling or other disturbance. Many of the high-elevation areas where the spider occurs are frequented by tens of thousands of visitors each year. Because of their small size, disturbance of the moss mats or damage to the surrounding vegetation shading the mats could result in the extirpation of entire spruce-fir moss spider populations and/or population fragments.

MANAGEMENT AND PROTECTION:Recovery actions for the spruce-firmoss spider will include efforts to protect and manage remaining populations in the wild. However, because of the difficulties in controlling the adelgid and reversing the decline of the species' habitat, efforts are being made to establish a captive breeding program for the spider in order to secure the species for potential reintroduction into historic sites, if it becomes feasible to do so.

Studies on the spruce-fir moss spider's life history, habitat requirements, and threats to its habitat will continue and may provide further insight into recovery actions.


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