Allometric Equations for Estimation of Ash-Free Dry Mass from Length Measurements for Selected European Earthworm Species (Lumbricidae) in the Western Great Lakes Region

Article excerpt

ABSTRACT.-In the last decade the invasion of European earthworms into previously wormfree glaciated areas of North America has stimulated research into their impacts on native forest ecosystems in the region. As a first approximation, the impacts of invading earthworms are related to their biomass. However, direct measurements of biomass can be significantly affected by the moisture conditions under which the earthworms are collected and their relative gut contents. Ash-free dry mass is the best standardized measure of earthworm biomass, but requires the destruction of specimens. This paper presents five allometric equations that allow for estimation of ash-free dry biomass based on length (mm) measurements for European earthworm species (Lumbricidae) commonly seen in the United States.

INTRODUCTION

In the last decade the invasion of exotic earthworms into previously worm-free glaciated areas of North America has stimulated research into their impacts on native ecosystems in the region (Hendrix and Bohlen, 2002). European earthworms have been invading the North American continent since European settlement (Gates, 1982). In the previously worm-free glaciated regions of North America their appearance in native forest ecosystems is often associated with a cascade of changes in ecosystem function and structure (Hendrix, 1995; Burtelow et al., 1998; Gundale, 2002; Bohlen et al., in press). Alban and Berry (1994) documented rapid changes in soil morphology, carbon and nitrogen in response to earthworm invasion of a birch-aspen forest. Scheu and Parkinson (1994) demonstrated how earthworm invasion can affect plant growth in an aspen forest. In lodgepole pine forests, McLean and Parkinson (1997) documented decreases in organic matter content, total nitrogen, carbon, basal respiration and metabolic quotient (qCO^sub 2^) with increasing earthworm biomass. To a first approximation, these changes are proportional to earthworm biomass. Research investigating the effects of the invasion by European earthworms on a wide variety of ecological parameters and ecosystems necessitates an efficient determination of earthworm biomass. Preservation of collected specimens for future reference is often desirable. Therefore, an alternative to direct measurement is needed which does not result in the destruction of the specimens. Individual earthworms within a species vary in size and body proportions, making the total number of individuals a poor measure of total biomass of a species (McLean and Parkinson, 1997).

Earthworm fresh mass can vary greatly depending on the moisture status of the environment in which they are found (Lee, 1985). Earthworms preserved in formalin commonly lose a significant proportion of their fresh body mass (Lee, 1985). Variability in gut content can account for up to 20% of both fresh and dry mass measures of biomass (Lee, 1985; Edwards and Bohlen, 1996). For these reasons, ash-free dry mass measures of biomass which remove gut contents from the dry mass measurement are the most accurate and provide data that is comparable between different sites and under different conditions (Edwards, 1998). Although models relating length to biomass for some invertebrate taxa are available in the literature (Rogers et al., 1977; Schoener, 1980), including a few equations relating length to either fresh or dry mass for selected genera or species of terrestrial Oligochaeta (Lumbricidae) (Lee, 1985; Collins, 1991), equations relating length to ash-free dry mass measures of biomass do not exist. This paper presents five allometric equations that allow for estimation of ash-free dry biomass based on length (mm) measurements for European earthworm species (Lumbricidae) commonly seen in the United States.

Earthworm assemblages in a given site can range widely in species composition. Different earthworms within a species vary in both size and proportion. Therefore, specific equations relating length to ashfree dry mass are needed for each species or group of closely related species (Bohlen, pers. …