Pj. Edmunds, Patterns in the distribution of juvenile corals and coral reef community structure in St. John, US Virgin Islands, MAR ECOL-PR, 202, 2000, pp. 113-124
There is increasing awareness that coral reefs exhibit a high degree of spa
tio-temporal variability, yet the processes that create these patterns are
poorly known. In this study, benthic community structure and scleractinian
recruitment on shallow reefs (5 m depth) were quantified at 18 sites along
10 km of the coast of St. John, US Virgin Islands. The goal was to test 2 m
echanisms that could create spatio-temporal variation-coral recruitment and
early life-history events-by addressing 3 questions: (1) Is the percent co
ver of scleractinians correlated with the density of juvenile corals? (2) H
ow is the density of juvenile corals affected by coral reef community struc
ture? (3) What are the rates of mortality and growth of juvenile corals and
how do they map onto the patterns of variation in juvenile density? Commun
ity structure was assessed as the percentage cover of the 4 major substratu
m components, and coral recruitment was estimated from the abundance of juv
enile scleractinians. Temporal variation in the population of juvenile cora
ls was examined over 4 yr (1994 to 1997) at 5 sites. The same 5 sites were
used to assess the growth and survivorship of juvenile corals over 1 yr. Ov
erall, the results demonstrate that there is a high degree of spatio-tempor
al variability in the shallow reefs of St. John. The community structure va
ried significantly among sites, and the density and taxonomic composition o
f juvenile corals varied significantly among sites and years. However, the
density of juvenile corals was not correlated with the percentage cover of
scleractinians, and the overall community structure did not explain a signi
ficant portion of the variation in the density of juvenile corals. Similar
results were obtained when the juvenile corals were separated by reproducti
ve mode (brooders vs spawners). The variation in density of juvenile corals
was unlikely to have resulted from differential growth, as growth rates di
d not vary among sites. Additionally, mortality was not correlated with den
sity of juvenile corals, although it did vary among sites. Together, these
results suggest that the community structure of coral reefs is related only
loosely to the contemporaneous distribution of juvenile corals and early l
ife-history events (i.e., survivorship and growth of juvenile corals). Stud
ies on larger spatial (>10 km) and temporal (>4 yr) scales probably are nec
essary to quantify the relationships between coral recruitment and coral co
mmunity structure.