Kenneth G. Johnson

Hopping Hotspots: Global Shifts in Marine Biodiversity

Hotspots of high species diversity are a prominent feature of modern global biodiversity patterns. Fossil and molecular evidence is starting to reveal the history of these hotspots. There have been at least three marine biodiversity hotspots during the past 50 million years. They have moved across almost half the globe, with their timing and locations coinciding with major tectonic events. The birth and death of successive hotspots highlights the link between environmental change and biodiversity patterns. The antiquity of the taxa in the modern Indo-Australian Archipelago hotspot emphasizes the role of pre-Pleistocene events in shaping modern diversity patterns.

Formation of the Isthmus of Panama

Independent evidence from rocks, fossils, and genes converge on a cohesive narrative of isthmus formation in the Pliocene. The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Environmental change preceded Caribbean extinction by 2 million years

Paleontologists typically treat major episodes of extinction as single and distinct events in which a major environmental perturbation results in a synchronous evolutionary response. Alternatively, the causes of biotic change may be multifaceted and extinction may lag behind the changes ultimately responsible because of nonlinear ecological dynamics. We examined these alternatives for the major episode of Caribbean extinction 2 million years ago (Ma). Isolation of the Caribbean from the Eastern Pacific by uplift of the Panamanian Isthmus was associated with synchronous changes in Caribbean near shore environments and community composition between 4.25 and 3.45 Ma. Seasonal fluctuations in Caribbean seawater temperature decreased 3-fold, carbonate deposition increased, and there was a striking, albeit patchy, shift in dominance of benthic ecosystems from heterotrophic mollusks to mixotrophic reef corals and calcareous algae. All of these changes correspond well with a simple model of decreased upwelling and collapse in planktonic productivity associated with the final stages of the closure of the isthmian barrier. However, extinction rates of mollusks and corals did not increase until 3–2 Ma and sharply peaked between 2 and 1 Ma, even though extinction overwhelmingly affected taxa commonly associated with high productivity. This time lag suggests that something other than environmental change per se was involved in extinction that does not occur as a single event. Understanding cause and effect will require more taxonomically refined analysis of the changing abundance and distribution patterns of different ecological guilds in the 2 million years leading up to the relatively sudden peak in extinction.

The ecology of extinction: molluscan feeding and faunal turnover in the Caribbean Neogene.

Molluscan faunal turnover in the Plio–Pleistocene of the tropical western Atlantic has been attributed to drops in temperature or primary productivity, but these competing hypotheses have not been assessed ecologically. To test these alternatives, we compiled data on changing molluscan life habits and trophic composition over 12 million years derived from 463 newly made collections from the southwestern Caribbean. Shelf ecosystems have altered markedly in trophic structure since the Late Pliocene. Predatory gastropods and suspension-feeding bivalves declined significantly in abundance, but not in diversity, and reef-dwellers became common. By contrast, all other ecological life habits remained remarkably stable. Food-web changes strongly support the hypothesis that declining regional nutrient supply had an increasing impact on regional macroecology, culminating in a faunal turnover.

Origination preceding extinction during late Cenozoic turnover of Caribbean reefs

Statistical analyses of occurrence data derived from new collections through scattered Caribbean sections indicate that increased speciation preceded a pulse of extinction during regional turnover of the Caribbean reef coral fauna in Plio-Pleistocene time. The data are based on samples that were newly collected and identified to species using standardized procedures. Age-dates were assigned using high-resolution chronostratigraphic methods. The results show that coral species with a wide range of ecological traits originated and were added to the species pool as much as 1- 2 million years before extinction peaked at the end of the turnover interval. Local assemblages consisted of a mix of extinct and living species, which varied in composition but not in richness. Extinction was selective and resulted in a faunal shift to the large, fast-growing species that dom- inate Caribbean reefs today. The unusual relationship between origination and extinction may have been caused by changes in oceanic circulation associated with emergence of the Central American Isthmus, followed by the onset of Northern Hemisphere glaciation. The pattern of origination pre- ceding extinction may have been responsible for the stability of reef ecosystems during the intense climatic fluctuations of the late Pleistocene, and for the composition and structure of modern Ca- ribbean reef ecosystems.

Extinction selectivity and ecology of Neogene Caribbean reef corals

-We analyze a new compilation of Neogene to Recent (22-0 Ma) Caribbean coral occurrences to determine how ecological and life history traits at the population level affect long-term evolutionary patterns. The compilation consists of occurrences of 175 species and 49 genera in one continuous (> 5 m.y.) sequence and 22 scattered sites across the Caribbean region. Previous study of evolutionary rates using these data has shown that both extinction and origination were accelerated between 4 and 1 Ma, resulting in large-scale faunal turnover. Categories for three morphological and two reproductive variables (colony size, colony shape, and corallite size; and sex, and mode of embryonic development; respectively) are assigned to each species in the compilation. Comparisons of the ecological variables with evolutionary rates using randomization procedures and modified analysis of variance show that only colony size was strongly related to rates of extinction and origination during either normal background times or times of accelerated extinction. Extinction rates were lower in species with large colonies, because species with small massive colonies tend to live in small, short-lived populations with highly fluctuating recruitment rates. During turnover, extinction rates increased disproportionately in species with small colonies. Origination rates are found to be less related to ecological variables, although species with small massive colonies originated at higher rates prior to turnover. Accelerated turnover may have therefore involved an increase in local population extinction rates that caused increased rates of both species extinction and origination across the entire fauna. Since extinction rates accelerated disproportionately with respect to colony size, the overall result was a relative increase in species with large colonies. After severe disturbance, one might expect that populations of species with large colonies and high rates of fragmentation would be more likely to escape extinction, because of larger population sizes, longer generation times, and more constant rates of population increase. The modern Caribbean reef-coral fauna is therefore structured by large, long-lived colonies that are robust to regional environmental change. Many of the very taxa that allowed reef communities to escape collapse in the past are declining today in response to anthropogenic disturbances, suggesting that Caribbean reef communities may be less resilient in the future in response to ongoing environmental perturbations. Kenneth G. Johnson.* Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom Ann F. Budd. Geology Department, The University of Iowa, Iowa City, Iowa 52242 U.S.A. Thomas A. Stemann. Geologisches Institut, Universitat Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland *Present address: Department of Geology and Applied Geology, University of Glasgow, Lilybank Gardens, Glasgow G12 8QQ, United Kingdom Accepted: June 2, 1994

Caribbean Reef Development Was Independent of Coral Diversity over 28 Million Years

The relationship between natural variations in coral species diversity, reef development, and ecosystem function on coral reefs is poorly understood. Recent coral diversity varies 10-fold among geographic regions, but rates of reef growth are broadly similar, suggesting that diversity is unimportant for reef development. Differences in diversity may reflect regional differences in long-term biotic history in addition to environmental conditions. Using a combination of new and published fossil and stratigraphic data, we compared changes in coral diversity and reef development within the tropical western Atlantic over the past 28 million years. Reef development was unrelated to coral diversity, and the largest reef tracts formed after extinction had reduced diversity by 50%. High diversity is thus not essential for the growth and persistence of coral reefs.

Life in the last few million years

Abstract The excellent fossil record of the past few million years, combined with the overwhelming similarity of the biota to extant species, provides an outstanding opportunity for understanding paleoecological and macroevolutionary patterns and processes within a rigorous biological framework. Unfortunately, this potential has not been fully exploited because of lack of well-sampled time series and adequate statistical analysis. Nevertheless, four basic patterns appear to be of general significance. First, a major pulse of extinction occurred 1–2 m.y. ago in many ocean basins, more or less coincident with the intensification of glaciation in the Northern Hemisphere. Rates of origination also increased greatly but were more variable in magnitude and timing. The fine-scale correlation of these evolutionary events with changes in climate is poorly understood. Similar events probably occurred on land but have not been tested adequately. Second, rates of origination and extinction in the oceans waned after the pulse of extinction, especially during the past 1 m.y. Thus, most marine species originated long before the Pleistocene under very different environmental circumstances, suggesting that they are “exapted” rather than adapted to their present ecological circumstances. The same may be true for many terrestrial groups, but not for the mammals or fresh-water fishes that have continued to undergo speciation throughout the Pleistocene. Third, community membership of late Pleistocene coral reef communities was more stable than expected by chance. These are the only paleoecological data adequate to test hypotheses of community stability, so that we do not know whether community structure involving other taxa or environments typically reflects more than the collective behavior of individual species distributions. Regardless, the strong evidence for nearly universal exaptation of ecological characteristics argues strongly against ideas of coevolution of species in communities. Finally, ecological communities were profoundly altered by human activities long before modern ecological studies began. Holocene paleontological, archeological, and historical data constitute the only ecological baseline for “pristine” ecological communities before significant human disturbance. Holocene records should be much more extensively used as a baseline for Recent ecological studies and for conservation and management.

Climate Change and Biosphere Response: Unlocking the Collections Vault

Natural history collections (NHCs) are an important source of the long-term data needed to understand how biota respond to ongoing anthropogenic climate change. These include taxon occurrence data for ecological modeling, as well as information that can be used to reconstruct mechanisms through which biota respond to changing climates. The full potential of NHCs for climate change research cannot be fully realized until high-quality data sets are conveniently accessible for research, but this requires that higher priority be placed on digitizing the holdings most useful for climate change research (e.g., whole-biota studies, time series, records of intensively sampled common taxa). Natural history collections must not neglect the proliferation of new information from efforts to understand how present-day ecosystems are responding to environmental change. These new directions require a strategic realignment for many NHC holders to complement their existing focus on taxonomy and systematics. To set these new priorities, we need strong partnerships between NHC holders and global change biologists.

1200 year paleoecological record of coral community development from the terrigenous inner shelf of the Great Barrier Reef

Increased terrestrial sediment and nutrient yields are regarded as significant threats to coral reef health. Within the central Great Barrier Reef lagoon, where water quality has reportedly declined since European settlement (since ca. A.D. 1850), inner-shelf reef conditions have purportedly deteriorated. However, the link between reef decline and water-quality change remains controversial, primarily because of a lack of pre-European period ecological baseline data against which to assess contemporary ecological states. Here we present a high-resolution record of reef accretion and coral community composition from a turbid-zone, nearshore reef on the inner shelf of the Great Barrier Reef; the record is based on six radiocarbon date–constrained cores, and extends back to ca. 1200 calibrated yr B.P. Results demonstrate not only the potential for coral communities to initiate and persist in settings dominated by fine-grained terrigenous sediment accumulation, but also that a temporally persistent (but low diversity) suite of corals has dominated the reef-building community at this site for at least the past millennium. Furthermore, the coral assemblages exhibit no evidence of community shifts attributable to post-European water-quality changes. While extrapolation of these findings to other turbid-zone reefs must remain tentative, the study raises important questions about the resilience of inner-shelf reefs that are under terrestrial sediment influence and subject to elevated turbidity conditions, and demonstrates the potential to develop detailed, millennial time scale, coral community records from Holocene reef systems.