The Role of History in Community Dynamics: A Modelling Approach

Abstract

Recent history plays an important role in the physiology, behavior, and ecology of individuals, and in the dynamics of populations and assemblages of species. In this paper, we examine the impact of history on the species composition of intertidal reef corals, by comparing simulation 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 long—term study spanning three decades at Heron Island, Great Barrier Reef. Rates of colonization (transitions from free space) varied 20—fold for different species groups, while mortality (transitions to free space) ranged fivefold, reflecting a wide range of life histories among the coral assemblage. Virtually all species groups could undergo reciprocal transitions (e.g., from A to B, and B to A) in a single time interval, indicating the lack of a single competitive dominant that was capable of outcompeting all or most other species. Transition probabilities changed markedly as a function of history. For most species groups, the probability of persisting (i.e., transitions from A to A) increased with time. Thus, a colony that had occupied space for some time 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 from mortality agents such as competition and predation. However, three species groups showed the opposite pattern. Algae, Pocilloporid corals, and fragile tabular Acropora 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 unoccupied for some time. These results falsify a major assumption of standard first—order models, i.e., that transition probabilities are constant, 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 composition in both transitory and climax (equilibrium) phases. Thus, while recent history was important in determining transition probabilities, it had little effect on community dynamics and structure in this system. This discrepancy is due to the rapid turnover of corals and algae on shallow reef crests, where only a small proportion of colonies survive long enough to display effects of history. All models agreed that 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.