Response of Pleistocene coral reefs to environmental change over long temporal scales

Response of Pleistocene Coral Reefs to Environmental Change Over Long Temporal Scales1

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 CO2 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 natural 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 habitat reduction, and caused disruption of coral metapopulation structure. Threshold effects predicted by metapopulation dynamics may also explain the apparent paradox 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

Environmental change has had a profound effect on the Earth's biota throughout the history of life. Some of these changes have occurred on the very grandest of scales, such as the coalescence and subsequent breaking up of huge super-continents (Erwin, 1992), or the decimation of the dinosaurs by extra-terrestrial impacts (Raup, 1991). Reefs are not immune to such processes, and the history of their diversity throughout the past 600 million years of the Phanerozoic has been enormously affected by environmental changes in ocean chemistry, sea level and climate (Copper, 1994; Fagerstrom, 1987; Kauffman and Fagerstrom, 1993). Indeed, climate change has been an integral part of the development of reefs, both coral and non-coral, for almost the entire Phanerozoic Era. The recent past history of living species resident on today's reefs has been affected by multiple cycles of global environmental change throughout the past 2 million years (Ma) of the Quaternary.

Here, I provide two examples from the Pleistocene fossil record of coral reefs that illustrate the way in which an understanding of the history of reefs might provide relevant information to the ecology of living reefs in the face of environmental change. I follow the distinctions made by Buddemeier and Smith (1999) and use "climate change" to denote variation in sea level, atmospheric COz concentration, inhabitable tropical benthic area, and possibly temperature associated with glacial cycles; "local environmental change" to denote variation at point sources in sediment load, turbidity, light etc., including anthropogenic influences such as pollution, nutrient content, overfishing etc. "Cumulative changes" are confined to local reefs, but have global importance because they occur at a number of discrete locations. I also use the terms "local" and "regional" when I discuss the importance of various ecological controls occurring in coral reef communities. Local controls include biotic interactions, niche diversification, and disturbance, whereas regional influences include species dispersal capabilities and colonization potential related to size of the species pool. Finally, in this paper I use the term local for distances 4,000 km.

In the first example, local environmental differences had a greater influence on Pleistocene Indo-Pacific reef coral communities from Papua New Guinea than environmental variations associated with different climate stages. Local environmental differences produced locally distinct coral communities, but these different communities consistently reoccurred throughout a 95 ka (thousand years) interval which included major changes in global climate and sea level. Moreover, the communities do not appear to be dispersal-limited (Pandolfi and Jackson, 1997), so local processes were important in their community dynamics. The importance of local controls on both local composition and regional diversity (as suggested in PNG), and regional controls on local composition and diversity (as suggested in both coral reefs and rain forestsHubbell, 1997 a, b; Cornell and Karlson, 1996; Karlson and Cornell, 1998, 1999; Caley and Schluter, 1997) means that both local and global environmental change can play important roles in altering coral species distribution patterns. Future study needs to be geared toward understanding how environmental changes on different scales will impact reef communities in various habitats.