The Effects of Slope Exposure on Arthropod Distribution Patterns

Abstract

Pitfall traps were used to collect over 39,000 specimens of 132 species and higher taxa of arthropods from eight different aspects (slope exposures) of a grassy bald in eastern Temqnessee-western North Carolina. Slopes were characterized as to available moisture and temperature; climatic descriptions were based on measurements of rainfall, temperature, prevailing winds, solar radiation and vegetation cover. Southern and western exposures were categorized as warm and dry; northern and eastern exposures were cool and wet. Of the 34 most abundant species of insects and arachnids, 32 were significantly (p < 0.05) more abundant on one or more of the eight aspects. The S-facing slope was the most preferred aspect for arthropod activity. The SE and NW slopes, both windswept, had least activity. INTRODUCTION Little detailed information has been published as to how slope exposure affects the distribution of arthropods. This stands in sharp contrast to the wealth of information available on the effects of slope exposure on plant population distributions. Ayyad and Dix (1964) cite 22 studies dealing with slope exposure and associated vegetation, and plant ecology texts invariably cover the vegetational zonation on different aspects of a mountain (Odum, 1959; Billings, 1970; Whittaker, 1970). While slope-exposure effects on arthropods have not gone completely unnoticed (Bristowe, 19,39 on spiders; Cole, 1940 on ants; Cannon, 1965 on spiders, Geiger, 1965 on grasshoppers, moths and beetles; Lowry, 1972 on desert arthropods), these studies, with the exception of Lowry (1972), are mainly descriptive. This article provides detailed information on slope exposure as it pertains to arthropod distribution. Arthropod distribution was determined with pitfall traps. Pitfall trapping is an appropriate means of measuring slope-exposure effects since it measures ground surface activity and approximates, in many instances, relative abundance (Duffey, 1962). Duffey showed excellent correlation between the activity of ground-dwelling spiders (measured by pitfall traps) and relative density (measured by Tullgren funnel extraction). Surface activity and relative population density are related to the number of ground-dwelling arthropods that cross the pitfall area (Mitchell, 1963), as reported by Aheam (1971). SITE DESCRIPTION The study area was located on the Tennessee-North Carolina boundary in Carter and Mitchell counties. It was situated geographically at 360 06' N latitude and 820 0V W longitude in the Unaka Mountains, a section of the Southern Appalachians. The study site was located at approximately 1750 m (altitude) on