SPECIES COEXISTENCE, KEYSTONE SPECIES, AND SUCCESSION - A SENSITIVITYANALYSIS

One of the major questions in ecology is, what controls the structure of communities? We used projection matrix models to examine community dynamics and patterns of succession. The inputs of the model are trans ition probabilities of species replacements that were measured repeate dly during a long-term (1962-1989) study of diverse coral assemblages on Heron Island, Great Barrier Reef. Transitions varied strikingly amo ng species and sites, reflecting differences in recruitment, growth, l ongevity (persistence), and the rate of replacement of one species by another. Species that had a poor ability to persist (e.g., algae and P ocilloporid corals) were generally good colonists. The observed number of transitions expressed as a proportion of the maximum number possib le provides an index of the complexity of interactions in an assemblag e, analogous to the concept of connectance in food-web analysis. Trans itions occurred to and from nearly every species group, indicating tha t there was no competitive dominant in this system. We use the models in simulations to track transitory changes in species abundance and co mmunity composition following a major disturbance (e.g., due to a cycl one or outbreak of crown-of-thorns starfish). Some species showed a ra pid initial increase followed by a decline to lower equilibrium levels , while others increased smoothly to a generally higher equilibrial ab undance. The length of time required to reach a climax assemblage usin g the same matrix recurrently (approximate to 20 yr) is far greater th an the observed interval between major disturbances, supporting nonequ ilibrium theories of coral reef communities. Climax assemblages were h ighly diverse and varied in composition from site to site. The ''inter mediate disturbance hypothesis'' does not fully predict successional c hanges in these shallow-water coral assemblages since diversity remain ed very high at equilibrium (i.e., long after a major disturbance), Co mpetitively inferior species were not eliminated because routine morta lity ensured that some space always remained available for colonizatio n. We also present a novel method for quantifying the relative importa nce of each species interaction to community composition and the rate of succession, based on a sensitivity analysis of the transition matri x. The analysis shows that the importance of a species to the dynamics of a community may be unrelated to its abundance at equilibrium, with some rare species groups having a greater impact than more common one s. Sensitivity analysis of this type will provide a powerful means of identifying ''keystone'' species in complex assemblages where experime ntal manipulation of each species is impossible.