THE ROLE OF HISTORY IN COMMUNITY DYNAMICS - A MODELING APPROACH

Recent history plays an important role in the physiology, behavior, an d ecology of individuals, and in the dynamics of populations and assem blages of species. In this paper, we examine the impact of history on the species composition of intertidal reef corals, by comparing simula tion models that incorporate four different levels of knowledge about the recent past (over a time scale of 1-27 yr). The models are Markov or semi-Markov transition probability matrix models, based on rates of colonization, persistence, and species replacement measured from a lo ng-term study spanning three decades at Heron Island, Great Barrier Re ef. Rates of colonization (transitions from free space) varied 20-fold for different species groups, while mortality (transitions to free sp ace) ranged fivefold, reflecting a wide range of life histories among the coral assemblage. Virtually all species groups could undergo recip rocal transitions (e.g., from A to B, and B to A) in a single time int erval, indicating the lack of a single competitive dominant that was c apable of outcompeting all or most other species. Transition probabili ties changed markedly as a function of history. For most species group s, the probability of persisting (i.e., ''transitions'' from A to A) i ncreased with time. Thus, a colony that had occupied space for some ti me was generally more likely to continue to do so than a new arrival. This result is consistent with an escape in size for older colonies fr om mortality agents such as competition and predation. However, three species groups showed the opposite pattern. Algae, Pocilloporid corals , and fragile tabular Acroporn showed marked increases in transitions to free space after 3-5 yr, reflecting a more ephemeral suite of life history traits. Similarly, free space that had recently been generated had a higher rate of colonization than substratum that had been unocc upied for some time. These results falsify a major assumption of stand ard first-order models, i.e., that transition probabilities are consta nt, and that history is irrelevant. Although the changes in transition probabilities as a function of history were often striking, the four different models we employed show only minor variation in community co mposition in both transitory and climax (equilibrium) phases. Thus, wh ile recent history was important in determining transition probabiliti es, it had little effect on community dynamics and structure in this s ystem. This discrepancy is due to the rapid turnover of corals and alg ae on shallow reef crests, where only a small proportion of colonies s urvive long enough to display effects of history. All models agreed th at the length of time required for this system to reach an equilibrium community structure is far longer than the observed interval between recurrent disturbances from tropical cyclones.