Title: Coral population
thresholds and vital rates
Description:
Coral-population dynamics are influenced by a suite of processes that act on individual colonies at all life-history stages, from differential reproduction, recruitment and survival, through to large-scale processes involving herbivory and predation, which are all influenced by regional oceanography. Yet, we know little about vital-population rates and how they vary spatially, seasonally, and under different environmental circumstances. Our research sought to understand the dynamics of coral populations by examining a long-term photograph record from the reefs of southern Japan.
Specifically, our objectives were to: (1) quantify the dynamics of coral growth, partial mortality and whole-coral colony mortality and (2) ascertain whether universal functions and probability distributions exist for the three parameters. Vital population processes are in clear need of qualification. We also need to discern whether there are universal-rate models for coral growth, partial mortality and total coral colony mortality. Corymbose Acropora colonies were tracked within 2 m by 2 m at three depths (i.e., 1 m, 3-4 m, and 6-7 m), at four stations (two were leeward and two were windward) on Akajima Island, southern Japan (26.2oN, 127.3oE). In the lab corymbose Acropora colonies were measured to the nearest centimeter and tracked through time. The data from each depth were pooled across stations, after comparing size-frequency distributions with Kolmogorov-Smirnov tests. Maximum diameter was compared using initial and subsequent measurements (i.e., ‘96-‘97, ‘97-‘98). Increases in maximum diameter were categorized as growth, whereas reductions were categorized as colonies with inflicted partial mortality. Further distinctions were made to filter colonies undergoing fragmentation. If a colony underwent splitting, or fragmentation, the largest fragment retained the initial colony identification, whereas the newly formed fragment was given a new identification number. The initial colony size and relative growth rates were compared with a well known suite of functions using curve fitting and error analysis (in Mathematica®). Size-frequency distributions were allocated to a set bin density, which were analyzed for concordance with known frequency distributions.
Coral colony growth rates followed the function, f(x)=m/x, where m is the mean relative growth and x is the initial size of a colony, and a chi-squared probability distribution with three degrees of freedom. Partial coral colony mortality rates followed a quadratic function whose vertex is at (m*, rmax ) where m*is the mean of the initial sizes of the colonies that experienced partial mortality and rmax is the maximum partial mortality rate, and a chi-square probability distribution with one degree of freedom. Total mortality followed a similar chi-squared probability distribution with one degree of freedom, giving the probability of a colony with a given initial size being dead the following year. In conclusion, (1) the smaller the coral colony, independent of coral species, the higher the relative growth rate and the higher the chance of total mortality, and (2) the probability of partial coral mortality increased with coral-colony size.