Dynamics of plant and arthropod diversity during old field succession

The successional dynamics of arthropod diversity in 18 abandoned agricultural fields (age 15-54 yr) at Cedar Creek, MN, USA were determined using sweep net sampling (44833 individuals of 618 species). Total arthropod species richness and equitability (J), but not abundance, increased significantly with field successional age. Herbivore and parasite species richness, but not detritivore and predator species richness, also increased significantly with field age. All of these arthropod variables were significantly positively correlated with plant species richness in the fields. When plant species richness was included as a covariate in regressions, there were no longer any significant effects of field age. These results supported the hypothesis that increases in arthropod diversity with field age are influenced by increases in plant diversity. The additional significant positive dependence of herbivore species richness on predator species richness suggests that predator-prey interactions may also influence the successional dynamics of arthropod diversity. Nine of the ten most common arthropod species decreased in abundance with field age, two of them significantly. The abundances of these two generalist forb-feeding species, Melanoplus femurrubrum (Orthoptera: Acrididae) and Scaphytopius acutus (Homoptera: Cicadellidae), each depended significantly on amount of forbs. The average body size of arthropod species (total and herbivores) decreased significantly with field age. An efficiency vs specialization hypothesis predicts such a decrease. Because plants in later secondary succession are generally less palatable, a diversity of smaller, potentially more specialized herbivores may have an advantage over larger and more efficient herbivores in later succession.