Measurement of the degree of intransitivity in a community of sessile organisms

Abstract The word “stand-off” has been used with different meanings by different authors in studies of interspecific space competition by sessile organisms. Most of them have treated stand-off as an outcome of secondary importance to those of wins and losses in the interactions. Here, it is postulated that stand-off should be treated as an important competitive outcome equal in status to wins and losses. On this assumption, a new type of index system composed of three indices: win, loss, and stand-off is proposed to measure the degree of intransitivity among sessile organisms at a given locality. Intransitivity can be defined as the state in which all species have equal probability to be overgrown by others. Here, a community is divided into five according to the competitive ranking of the organisms, namely, network due to stand-off, neutral system due to equal win-loss and stand-off, hierarchically dominant, network due to win-loss dominant and a neutral system of the last two types. The method of its determination and its merits over the index proposed by earlier and are discussed with the help of field data collected on interspecific interactions among sessile organisms on PVC panels suspended in Tomioka Bay, Japan. Data presented by other investigators, which differed each other in the number of stand-offs, were also used to test the merits of our index system.