Calcium cycling and trophic relationships of arthropods in a ⁴⁵Ca-Tagged cornfield

Summary

The objectives of this study were to investigate (1) feeding relationships of arthropods on corn (Zea mays L.) and (2) cycling of calcium in a corn field.

In May, 1966, a 0.6 hectare field on the Savannah River Plant, Aiken, South Carolina, was planted with ⁴⁵Ca tagged corn seeds. Corn plants and weeds were periodically sampled at random. Arthropods from corn were sampled from randomly selected circular quadrats (0.25 m²) centered on corn hills, using a portable suction insect sampler. Ashed samples were counted on a Tracerlab thin window gas flow detector equipped with sample changer.

⁴⁵Ca per corn plant decreased four-fold from June 2 to July 6. Leaching and the loss of the older (and more radioactive) leaves during growth were probably the sources of the calcium loss. Presence of ⁴⁵Ca in the soil, and an increasing standing crop of ⁴⁵Ca in the weeds from June through August, and in the corn during July and August, indicate that isotope lost to the soil by leaching was taken up again by the vegetation in the field. Distribution of ⁴⁵Ca within the corn plants was uneven, being much greater in old than in young leaves, and in leaves than in stems.

Activity densities (amounts of isotope/gram of biomass) of the arthropods, because of the changing activity densities of the corn, could not by themselves show when they had attained equilibrium with the ⁴⁵Ca in the food plant. Therefore, the ratio of animal activity density/corn activity density was used in determining uptake curves, which showed the attainment of equilibrium when the ratio approached a constant value.

The ratio of ⁴⁵Ca per gram of insect/⁴⁵Ca per gram of corn may illustrate differences in feeding behavior in three ways: (1) in the time (the inflection point) at which the uptake curve reaches equilibrium, (2) the level at which the ratio reaches equilibrium (the mean activity density ratio), and (3) the variability between sampling dates in the ratio after it reaches equilibrium.

The date of inflection, though two weeks earlier in herbivores than in insect predators, occurred one month earlier in the pooled spiders, indicating that they became part of food chains beginning with corn much sooner than the other arthropods. Spiders, being predaceous, probably had fed on microarthropods not obtained in the samples.

Mean activity density ratios (weighted average of activity density ratios beginning with the inflection date) were less than one (0.2 to 0.5) in most groups of arthropods, probably because of feeding on material with a lower activity density than the entier corn plant, or because of smaller calcium concentrations in the arthropods than in the plants.

Variability of activity density ratios between sampling dates seemed correlated with the variability in activity densities of the food eaten. Thus the ratios for plant sap feeders were relatively constant, while the feeders on leaves (which had highly variable activity densities), had highly variable ratios.

An estimated 0.5 per cent of the ⁴⁵Ca in the field was cycled by the arthropods during the entire study.