Importance of timing of panel exposure on the competitive outcome and succession of sessile organisms

Abstract

Competitive interaction among sessile organisms was studied by suspending PVC (polyvinylchloride) panels in 4 different seasons at Tomioka Bay, south Japan. Panels were suspended in such a way that only 1 side received direct sunlight. Panels were monitored by intermittent photography for a maximum period of 14 mo. On the sunlit side of the panels, 35 species belonging to 8 taxonomic groups were encountered. A total of 7980 interspecific interactions among these organisms were monitored and the competitive relationships among them were ascertained. Competitive relationships on panels suspended in November 1991 (autumn) and August 1992 (summer) and monitored until December 1992 resulted in a simple hierarchy. On the other hand, panels suspended in February (winter) and April 1992 (spring) and monitored until December 1992 did not result in a hierarchy. Observation of panels after 4 mo of initiation in all 4 seasons showed different levels of competitive dominance for the same set of species. At the end of the study, the colonial ascidians Diplosoma mitsukurii and Didemnum moseleyi dominated on the panels initiated in autumn and summer, whilst no such hierarchy could be observed on the panels initiated in winter and spring. However, at the end of 4 mo of initiation, hierarchy could be detected in all 4 seasons. The branching bryozoan Zoohotryon pellucidum occupied the topmost position in the hierarchical order in panels initiated in winter and spring when observed after 4 mo of initiation. Reversal of organisms' positions in the hierarchical order with the change in panel initiation season could be observed on the panels at the end of study. The results showed the competitive interactions among these organisms, the time of panel initiation and/or the initial colonizers of the panel surfaces which significantly affected the subsequent species composition and the succession of species on these panels.