Colloquium The current biodiversity extinction event: Scenarios for mitigation and recovery Michael J. Novacek*† and Elsa E. Cleland‡ *American Museum of Natural History, New York, NY 10024; and ‡Department of Biological Sciences, Stanford University, Stanford, CA 94305 The current massive degradation of habitat and extinction of altered or encroached on by human activity (i.e., protecting species is taking place on a catastrophically short timescale, and ‘‘hotspots’’). A fourth recovery scenario involves enlightened their effects will fundamentally reset the future evolution of the human intervention beyond simple measures of wilderness pres- planet’s biota. The fossil record suggests that recovery of global ervation, a strategy that embraces ecosystem management and ecosystems has required millions or even tens of millions of years. mitigation of the current alteration of global biogeochemical Thus, intervention by humans, the very agents of the current cycles. Here, strong preference is expressed for the last of these environmental crisis, is required for any possibility of short-term options. Clearly, the future of evolution of the planet’s biota recovery or maintenance of the biota. Many current recovery depends significantly on what we do now to minimize loss of efforts have deficiencies, including insufficient information on the species, populations, and habitats. At the same time, there is diversity and distribution of species, ecological processes, and acute recognition of the challenges and potential shortcomings magnitude and interaction of threats to biodiversity (pollution, of many attempts at remediation and recovery. It is hoped that overharvesting, climate change, disruption of biogeochemical cy- this panel’s consideration of major threats, their interaction, and cles, introduced or invasive species, habitat loss and fragmentation the linkage between science and conservation in mitigating these through land use, disruption of community structure in habitats, threats suggest some feasible recovery scenarios at several and others). A much greater and more urgently applied investment different scales. to address these deficiencies is obviously warranted. Conservation and restoration in human-dominated ecosystems must strengthen Lessons from the Past: Recovery as a Long-Term Phenomenon connections between human activities, such as agricultural or It is clear that the fossil record powerfully indicates the reality harvesting practices, and relevant research generated in the bio- of extinction on many scales, the magnitude as well as selectivity logical, earth, and atmospheric sciences. Certain threats to biodi- of effects, and the pattern of recovery and survival (11, 12). To versity require intensive international cooperation and input from what extent then does the fossil record help us in forecasting both the scientific community to mitigate their harmful effects, includ- scenarios for extinction and recovery in the current crisis? ing climate change and alteration of global biogeochemical cycles. Consideration of this question moves us to acknowledge that In a world already transformed by human activity, the connection there are several aspects of these past events that diminish their between humans and the ecosystems they depend on must frame relevance to the current situation. any strategy for the recovery of the biota. First, ancient mass extinction events have been documented over comparatively long or imprecise timescales. The current here is consensus in the scientific community that the current crisis has been extended through historical times, a matter of Tmassive degradation of habitat and extinction of many of the centuries or a millennium, with a greatly accelerated impact that Earth’s biota is unprecedented and is taking place on a cata- began during the 20th century with the exponential increase of strophically short timescale. Based on extinction rates estimated world human populations. Thus, a period of only 75 to 100 years to be thousands of times the background rate, figures approach- may be most critical to the transformation of the present biota. ing 30% extermination of all species by the mid 21st century are Second, mass extinction events of the past are typified by not unrealistic (1–4), an event comparable to some of the global scale ecological transformation. By contrast, the current catastrophic mass extinction events of the past (5, 6). The current event is typified by a ‘‘patchy’’ pattern involving habitat frag- rate of rainforest destruction poses a profound threat to species mentation and loss, where impacts vary markedly for different diversity (7). Likewise, the degradation of the marine ecosystems habitats and different regions of the world (13). There is a large (8, 9) is directly evident through the denudation of species that body of evidence that suggests global climate changes and were once dominant and integral to such ecosystems. Indeed, alteration of global biogeochemical cycles may cause widespread this colloquium is framed by a view that if the current global transformations of ecosystems, but significant biodiversity loss extinction event is of the magnitude that seems to be well has not yet been linked to these impacts. indicated by the data at hand, then its effects will fundamentally Third, data on mass extinction events in the fossil record often reset the future evolution of the planet’s biota. fail to provide a clear connection between a primary cause and The devastating impact of the current biodiversity crisis moves effect (14–16). In contrast, the current biodiversity crisis has one us to consider the possibilities for the recovery of the biota. obvious biotic cause: ourselves. Moreover, the source of the Here, there are several options. First, a rebound could occur trauma also has the presumed capacity to mitigate its own from a natural reversal in trends. Such a pattern would, however, deleterious impact. Although the extinction of many species may require an unacceptably long timescale; recoveries from mass be an irreversible outcome of the current event, certain aspects extinction in the fossil record are measured in millions or tens of of human-caused global change are reversible. millions of years (10). Second, recovery could result from unacceptably Malthusian compensation—namely, marked re- duction in the world population of human consumers. Third, This paper was presented at the National Academy of Sciences colloquium, ‘‘The Future of some degree of recovery could result from a policy that protects Evolution,’’ held March 16–20, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA. key habitats even with minimal protection of ecosystems already †To whom reprint requests should be addressed. E-mail: [email protected]. 5466–5470 ͉ PNAS ͉ May 8, 2001 ͉ vol. 98 ͉ no. 10 www.pnas.org͞cgi͞doi͞10.1073͞pnas.091093698 Downloaded by guest on September 23, 2021 All of the above distinctions are pertinent to any scenario for One danger addressed by such efforts is the mismatch between recovery that might be extracted from fossil and geological the scale of the effect and the cause. The devastation of the coral evidence. Various reviews suggest that replenishment and di- reefs, sea grasses, and kelps in the Caribbean has been promoted versification of the biota following mass extinction events re- by the loss of benthic producers whose viable populations in turn quired a recovery phase of millions or tens of millions of years may have been greatly reduced by pollutants in runoff released (10, 12, 15). Surely such estimates based on fossil data indicate through human activity along the shoreline (8, 9). What may at the time lag that might be expected for a natural recovery of the first appear to be a complex crisis of subtle ecological dynamics biota following the current extinction event. Nonetheless, such could have a very direct and efficiently corrected cause—namely, lessons from the past do not effectively inform our scenarios for the introduction of the pollutants in the first place. One con- either current extinction or recovery given the emphatic role of structive effort here is the continual refinement of categories of humans in both processes. pollutants according to both the scale (global and local) and intensity (degree of toxicity, mutagenic impact, etc.) of the Near-Term Scenarios for Recovery: A Strategy effects. This often requires exacting experimental work, as in the Given the limited applicability of the record of past extinction identification of a link between polyvinyl chlorides (PVCs) in events for examining the current environmental crisis, it seems packaging and carcinogenic chemicals (21). Such toxin detective appropriate to turn to near-term recovery scenarios—namely, work must be applied to a much broader range of potential cases. scenarios that relate to human intervention just as they flow from human causation. Such a consideration involves at least Overharvesting. There is of course a clear and overlapping three steps. First, we must identify the threats to the biota and relationship between overharvesting and other threats to biodi- the entities most vulnerable to these threats. Second, we must versity, such as land use, but the matter deserves distinction here. consider the scientific principles or strategies that inform pre- Overharvesting impacts natural habitats with food sources that scriptions to alleviate the threats. Third, we must apply feasible are less dominated by agriculture or other human activities that recovery