Decreased diversity caused by differential inhibition among artificial phytoplankton communities in an undisturbed environment

Abstract

Cultures of freshwater algae were assembled into artificial communities consisting of 2, 3, 4 and 8 species. The same species were also grown simultaneously in unialgal cultures in separate containers. Different combinations of algae in the same-sized artificial communities were grown three times for 17 weeks. Data obtained from the algae grown separately were treated in the same manner as those from algae grown in the artificial communities. The algae grown as unicultures in separate containers were found to grow in a descending order of abundance: a hierarchical order. When they grew as part of a community, several things happened: (1) all species were inhibited in growth; (2) the inhibition among the species was not uniform; (3) the species often exchanged numerical position in the hierarchy; and (4) the hierarchical order always increased, the degree of hierarchy being determined from the formula -2:.Pi Inpi’ The greater hierarchy in the communities means there was less evenness of numerical distribution and, therefore, less diversity. This study corroborated Connell’s intermediate disturbance hypothesis from the aspect of the increase in hierarchy. Normally, this hypothesis has referred to the reduction of the number of species in an undisturbed community. © 1995 The British Phycological Society.