Changes in trophic abundance of soil arthropods along a grass-shrub-forestgradient

Ecological theory suggests that along productivity gradients, abundances of organisms within trophic levels will increase in a stepwise pattern from p roducers to consumers. To test this theory I investigated changes in abunda nce of soil arthropods at three trophic levels: microphytophages, represent ed by Collembola, predacious mites (Acari) that feed on Collembola, and thr ee groups of macroarthropods (spiders, ants, and centipedes) that were obse rved to feed on mites. Changes in abundance were monitored along a gradient in vegetation structure from grass to shrub to forest in the Canadian prai ries. I controlled for temporal variation in abundance among years and surv eys within a year. As predicted, (i) numbers of Collembola did not change w ith increases in productivity; (ii) mite numbers were greatest in the shrub lands; and (iii) numbers of macroarthropod predators increased from grassla nd to shrubland, and there was a nonsignificant increase in numbers of spid ers and centipedes in forest habitat. Contrary to predictions, macroarthrop od numbers were not significantly greater in forest habitat, and ant number s actually declined. Possible explanations for the lack of increase in macr oarthropod predator abundance in the forest habitat with the greatest produ ctivity include decreased ground-level humidity and greater abundance of ma croarthropod predators and parasites in forest environments.