Response of Pleistocene Coral Reefs to Environmental Change Over Long Temporal Scales

SYNOPSIS. TWO studies from the Pleistocene coral reef fossil record demonstrate the sensitivity of reef communities to both local environmental parameters and habitat reduction. In the first study, Pleistocene reef coral assemblages from Papua New Guinea show pronounced constancy in taxonomic composition and species diversity between 125 and 30 ka (thousand years). Spatial differences in reef coral community composition during successive high stands of sea level were greater among sites of the same age than among reefs of different ages, even though global changes in sea level, atmospheric CO 2 concentration, tropical benthic habitat area, and temperature varied at each high sea level stand. Thus, local environmental variation associated with runoff from the land had greater influence on reef coral community composition than variation in global climate and sea level. Proportional sampling from a regional species pool does not explain the temporal persistence and local factors likely played a major role. Examination of coral reef response to global change should not only involve regional diversity patterns but also local ecological factors, and the interactive effects of local and global environmental change. In the second study, Pleistocene extinction of two widespread, strictly insular species of Caribbean reef corals, Pocillopora cf. palmata (Geister, 1975) and an organ-pipe growth form of the Montastraea "annularis" species complex, was nat- ural and did not involve gradual decrease in range and abundance, but was sudden (thousands of years) throughout the entire range. One explanation is that sea level drop at the Last Glacial Maximum (LGM—18 ka) resulted in a threshold of hab- itat reduction, and caused disruption of coral metapopulation structure. Threshold effects predicted by metapopulation dynamics may also explain the apparent par- adox of the large amount of degraded modern reef habitat without any known modern-day reef coral extinctions. The rapid extinction of widespread Pleistocene species emphasizes the vulnerability of reef corals in the face of present rapid environmental and climatic change. INTRODUCTION throughout the past 600 million years of the Environmental change has had a pro- Phanerozoic has been enormously affected found effect on the Earth's biota throughout by environmental changes in ocean chem- the history of life. Some of these changes lstr y' se a leve l and climate (Copper, 1994; have occurred on the very grandest of Fagerstrom, 1987; Kauffman and Fager-