Limited Membership in Pleistocene Reef Coral Assemblages from the Huon Peninsula , Papua New Guinea : Constancy During Global Change

-One of the most intriguing questions in community ecology remains unanswered: Are ecological communities open assemblages with each species reacting individually to environmental change, or are they integrated units consisting of multispecies assemblages acting in concert? I address this question for marine organisms by examining the taxonomic composition and diversity of Indo-Pacific reef coral communities that have undergone repeated global change between 125 and 30 Ka (thousand years before present). Investigation of community constancy through time relies on two critical questions: (1) Are there significant differences in taxonomic composition among communities from different times? and if not, (2) Are the observed patterns in temporal similarity significantly different from expected patterns resulting from a random sampling of the available within-habitat species pool? Constancy in taxonomic composition and species richness of Pleistocene reef coral assemblages is maintained through a 95-k.y. interval in the raised reef terraces of the Huon Peninsula, Papua New Guinea. Fossil reef coral assemblages show limited membership in species composition despite repeated exposure to marked fluctuations in sea level (up to 120 m) and sea-surface temperatures (up to 6'C). During the 95-k.y. interval, the reefs experienced nine cycles of perturbation and subsequent reassembly with similar species composition. Spatial differences in reef coral species composition were greater among the three study sites than among reefs of different ages. Thus local environmental parameters associated with riverine and terrestrial sources had a greater influence on reef coral composition than global climate and sea level changes. The ecological dynamics of reef communities from Papua New Guinea are in marked contrast to those of Quaternary terrestrial and level bottom marine communities which appear to show unlimited community membership on both larger and smaller time scales. Differences in community assembly among ecosystems mean either that coral reefs are fundamentally different or that different ecological patterns and processes are occurring at different temporal scales. John M . Pandolfi. Center for Tropical Paleoecology and Archaeology, Smithsonian Tropical Research Institute, Apartado 2072, Balboa, Republic of Panamli Mailing address: Center for Tropical Paleoecology and Archaeology, Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948 U.S.A. Accepted: October 30, 1995 Introduction a fraction of the forms that could theoretically do so actually form a community at any one A central debate in community ecology has time. The way in which species are assembled generated decades of research. Is species in ecological communities has enormous sigmembership in ecological communities unnificance not only for community ecology limited, with communities composed of (May 1973; Connell and Sousa 1983; Roughloosely associated taxa and individual species garden 1989; Drake 1990; Cornell and Lawton reacting individually to environmental changes 1992), but also for both theoretical and empir(Gleason 1926), or is membership limited with ical studies of extinction in modern and anstable communities composed of integrated cient ecosystems (Jablonski 1991; Ricklefs and assemblages of species (Elton 1933)? Gleason Schluter 1993). (1926) argued that ecological communities As with most questions in ecology, our unrepresent loosely associated species assemderstanding of the degree to which memberblies that result from two factors: fluctuating ship in ecological communities is limited may and fortuitous species immigrations, and life depend upon the spatial and temporal scale of in equally fluctuating and variable environstudy (Jackson 1991; May 1994). For example, ments. Conversely, Elton (1933) proposed that on ecological time scales (tens to hundreds of communities have limited membership-only years) both coral reef and forest communities O 1996 The Paleontological Society. All rights reserved. 0096-8373/96/2202-0003/$1.00 153 REEF CORAL COMMUNITY ASSEMBLY appear unpredictable, showing ephemeral species composition with rapid turnover of short-lived colonizing species (Connell 1978; Hubbell and Foster 1986). But if the ecological dynamics of coral reef and forest ecosystems vary at different spatial and temporal scales (Chesson and Huntly 1989; Jackson 1991; Levin 1992; May 1994), their communities might be variable and unpredictable on the h e scale ("Gleasonian"), and increasingly predictable on large scales ("Eltonian"). But the long-term or historical perspective needed to investigate large-scale patterns is often lacking in modern coral reef ecological studies, which attempt to describe, on the basis of only a few years or decades of data, coral community structure through time when many corals live up to 1000 years or more. In addition, many coral reef species are capable of long-distance dispersal and are represented by large metapopulations in fragmented oceanic provinces. Previous studies of coral reef community dynamics have concentrated on small areas, generally for short duration, and may be inappropriate for evaluating concepts of long-term community membership (Jackson 1991). Where preservational biases can be properly evaluated, the fossil record can provide the appropriate temporal and spatial scales necessary to study long-term community dynamics in coral reefs. The fossil record has been used to examine patterns of community assembly in a variety of other marine and terrestrial settings (Davis 1986; Graham and Grimm 1990; Miller 1990; Overpeck et al. 1991; Jackson 1992,1994; Valentine and Jablonski 1993; Buzas and Culver 1994; Davis et al. 1994; DiMichele 1994; Walter and Patterson 1994; Van Valkenburgh 1995). Coral reef ecosystems are composed of highly diverse marine assemblages, which display a large degree of biotic interaction and are generally regarded as classic competition/ interactive communities (reviewed in Lang and Chornesky 1990). The degree to which reef communities are predictable and stable in space and time is of prime importance in understanding the role of disturbance in their ecology (Karlson and Hurd 1993) and, ultimately, in guiding predictions on reef response to human perturbations (Green et al. 1987; Jackson 1992). Although considerable effort has been devoted to the topic of whether living coral reefs represent stable, equilibrium communities or communities in a non-equilibrium state (e.g., Connell 1978; Hughes and Jackson 1985; Hughes 1989; Done 1992), few data are available on the past long-term ecological dynamics of living coral reef assemblages (Jackson 1992). The purpose of this paper is to examine species assembly in reef coral communities subjected to nine intervals of sea-level and climate change during a 95-k.y. late Pleistocene interval. Examination of coral species composition of communities from nine different time periods and three different sites along a 35-km coastline is based on comparison of temporal versus spatial variability. Limited coral species membership in reef communities through time should be expressed by spatial variance in species composition that is similar to or greater than the temporal variance in species composition. In addition, any temporal similarities should be evaluated by comparing them to expected similarity resulting from a random sampling of the available within-habitat species pool. By examining the constancy of taxonomic composition and species richness of corals within reef environments from repeated Pleistocene reef sequences from Papua New Guinea, it is possible to investigate community membership in coral reef ecosystems over time scales greater than the generation times of their inhabitants.