Experimental effects of density and food on growth and mortality of the southern Appalachian land gastropod, Mesodon normalis (Pilsbry).

INTRODUCTION

Most models of population regulation in land snails (Williamson et al., 1976, 1977; Oosterhoff, 1977) were developed around relatively high density European species and incorporate strong interrelationships among adult density, adult body (shell) size, juvenile growth rate, and reproduction. Negative correlations between density and adult shell diameter have been reported for Cepaea nemoralis (Williamson et al., 1976), Arianta arbustorum (Baur, 1988) and Helicella itala (Tattersfield, 1981). Numerous authors have also shown that snails living at higher densities have lower growth rates and are smaller as adults than animals growing in lower density or lower food per capita conditions (e.g., Wolda, 1970; Dan and Bailey, 1982; Lucarz and Gomot, 1985). Animals at low density may begin laying eggs later in life, lay fewer eggs (Carter and Ashdown, 1984; Baur and Baur, 1992) and may have higher rates of mortality (Dan and Bailey, 1982) than those living in lower density conditions. The actual mechanisms of density limitation are far from clear (Perry and Arthur, 1991), but hypotheses range from intraspecific exploitative competition for food (Peake, 1978; Baur and Baur, 1990), chemical interference by mucus (Oosterhoff, 1977; Dan and Bailey, 1982; Baur and Baur, 1990) and inter- and intraspecific aggression (Rollo and Wellington, 1979). Separating the effects of interference and exploitative competition has proved difficult (Baur and Baur, 1990; Perry and Arthur, 1991).

Mesodon normalis (Pilsbry) is a large land snail of the forests of the southern Appalachian Mountains of the U.S.A (Pilsbry, 1940; Hubricht, 1985; Emberton, 1988). It is active at dawn and dusk (Asami, 1993; Emberton, 1994). Monoecious adults mate during the spring and early summer, then lay clutches of 15 to 100 eggs in the leaf litter. Young snails emerge throughout the summer. Juvenile snails require approximately 2 yr to reach adult size which can range 27 to 36 mm diam, and they often live 3 or more yr as adults (pers. observ.). In contrast to many European helicids such as Cepaea nemoralis where densities range up to 10 [m.sup.-2] (Williamson et al., 1977; Perry and Arthur, 1991), adult Mesodon population densities are quite low ([less than]0.2 [m.sup.-2]) (Foster and Stiven, 1994; Stiven and Foster, in press). In addition, we have found a significant negative association between body size and field densities in populations of Mesodon normalis across eight different sites (Stiven and Foster, in press). We have also shown that reproductive output (total eggs produced and hatching success) is independent of age and body size (Foster and Stiven, 1994). This contrasts with many European land snails (e.g., Baur and Baur, 1992). Finally, no significant correlation of body size with abiotic environmental factors was found (Stiven and Foster, in press) even though they have been reported for other species of land snails (review by Goodfriend, 1986).

In this experimental laboratory investigation of population processes in the land snail Mesodon normalis, we first ask whether concurrent increases in density and decreases in per capita food negatively affect shell diameter, growth rate, survival and size at adulthood. These are traits predicted by the density-dependent mechanisms (Osterhoff, 1977; Williamson et al., 1977) to be important to the population biology of snails, including reproductive output (Baur and Baur, 1990). Second, body size hierarchies, which have been used in both plant (Solbrig, 1981; Ellison and Rabinowitz, 1989) and animal population research (Wall and Begon, 1987), may be caused by interference from neighbors (Weiner, 1985; Weiner and Thomas, 1986; Huston and DeAngelis, 1987). We explore variation in the differential responses of individuals in the experimental treatments in an attempt to infer the nature of possible competitive interactions.

METHODS

Laboratory techniques. - Stock adult Mesodon normalis were collected in the Nantahala National Forest from two sites off N.C. 106 highway approximately 7 and 8 km SW of Highlands, N.C., Macon County (lat and long of 35.01.41N, 83.16.072W and 36.02.23N 83.14.34W with elevation at 1128 and 1164 m, respectively), and from two sites in the U.S. Forest Service Coweeta Hydrologic Laboratory (U.S. For. Serv., 1972) ca. 26 km W of Highlands, N.C. (lat and long of 35.01.57N 83.27.02W and 35.02.25N 83.27.06W and elevation at 1000 and 1167 m, respectively). These snails were placed in 15 cm diam chambers containing approximately 3 cm of soil and 2 to 3 layers of leaves and covered with insect mesh fiberglass screening. When nine large clutches of [greater than]50 eggs were laid within 3 days of each other by nine different adults, these eggs and the resulting young snails were treated as follows. Each clutch was removed from the soil and placed in 9-cm petri dishes lined with moist paper toweling. Eggs hatched in 15 to 21 days, and all eggs of a clutch hatched within 24 h of each other. Seven days later, snails from each clutch were individually marked (numbered with india ink, covered with cyanoarylic glue), then randomly allocated to a replicate of a food-density treatment.

The growth chambers were plastic chambers (8 x 8 x 15 cm) with open fiberglass mesh tops. The snails were given daily rations of lettuce dusted with calcium carbonate following Cowie and Cain (1983), and the chambers brushed clean with hot water and the towel linings replaced weekly. There was no indication that paper towel linings were eaten, and mite infestations were not observed in the culture chambers (cf., Emberton, 1994). Temperatures in the laboratory at the Highlands Biological Station were about ambient during the spring, summer and autumn (mean monthly temperatures at Highlands from May through October are 15.1, 18.3, 20.0, 19.5, 16.9 and 11.4 C, respectively, Southeastern Regional Climate Center), but during the winter months the experimental temperature was ca. 20 C. The light-dark regime paralleled natural conditions. At the start of the experiment in September, shell diameters were measured with a calibrated reticule and dissecting microscope. All measurements after …