The Biodiversity Crisis
Scientists, conservationists, and hunters have been concerned about species extinction, primarily vertebrate species, at the local and regional scales for decades. Toward the end of the nineteenth-century in North America and Europe, bird plumage on hats was a dominant middle class fashion statement. Exotic and colorful bird feathers were in great demand. As a consequence, many bird species were hunted to near extinction. By the turn of the century, public outrage at the scale of the slaughter prompted local and national governments to crack down on the trade and led to the passage of regulatory laws and to the formation of conservationist organizations, notably the Audubon Society. At the height of European colonialism in the early twentieth century, hunter naturalists based in Europe feared that certain game animals were being pushed to the brink of extinction in Africa and South Asia. Hunters and conservation activists pressurized colonial governments to pass restrictive hunting laws and establish protected areas, such as game reserves and national parks. By the second half of the century, scientists increasingly conceptualized the problem of species extinction as a global one. This sentiment crystallized at the National Forum on Biodiversity, where extinction was framed as a global biodiversity crisis. In framing extinction as a worldwide crisis, conservation biologists constructed biodiversity as a global commons problem.
Trends in Global Biodiversity
The basic unit of analysis for assessing biodiversity trends at any geographic scale is the species. Thus, the number of species measured per unit area is taken as a representative measure of biodiversity. Conversely, species extinction is taken as a representative measure of biodiversity loss. Two important sources for assessing biodiversity at the global scale are the Secretariat of the CBD and the International Union for the Conservation of Nature and Natural Resources (IUCN; World Conservation Union). The Secretariat, among other duties, collects and disseminates information, notably through a series of reports titled, 'Global Biodiversity Outlook', first published in 2001. In publishing these reports, the Secretariat does not generate data from its own research, but rather serves as an information clearinghouse. Data come from individual scientists, governments, and major international nongovernmental organizations (NGOs). The IUCN, in the 1960s, began evaluating and cataloging species that have become extinct since AD 1500 or that were currently at risk of global extinction. It publicizes its findings through its trademarked Red List of Threatened Species, the most recent edition (2004). Similar to the CBD Secretariat, the IUCN derives its assessments of biodiversity loss from data collected by others (including partner organizations and members of the Red List Consortium) much of which is previously published in monographs, edited volumes, and web based databases.
Based on the IUCN and the Secretariat's most recent published assessments and their supporting sources – which, it must be stressed, are continually reevaluated as new data are obtained – it is widely accepted that the global rate of species extinction has increased. The increase is largely due to human activities. Of the estimated total of 14 000 000 species of all living organisms on Earth, approximately 1 900 000 have been described within scientific (Linnaean) taxonomies. Of the described species, 20% of mammals are threatened with extinction, 12% of birds, and 32% of amphibians. The Red List contains 784 documented extinctions since AD 1500 and, based on known extinctions of birds, mammals, and amphibians in the past 100 years, estimates that current extinction rates are 50–500 times higher than extinction rates in the fossil record. Since it is difficult to prove that a species extinction has occurred, scientists created a category of 'possibly extinct' species. If possibly extinct species are included, current extinction rates are 100–1000 times above average or 'background' extinction rates. Background extinction rates are determined by tracing rates of speciation and extinction of life forms in the fossil record. The evolutionary trend has been toward higher levels of biodiversity (i.e., speciation rates higher than extinction rates), and it is estimated that current global biodiversity is near an all time high in planetary history. The fossil record also shows, however, that global biodiversity has fluctuated greatly over time, punctuated by periods of relatively sudden (1–10 million years) mass extinctions. Scientists have identified five mass extinction events where a high proportion of species across a wide range of taxa disappeared from the fossil record, the first occurring 440 million years ago and the most recent, 65 million years ago. Some biologists suggest that the planet may be experiencing a sixth episode of mass extinction, largely induced by human activities.
Though many scientists are convinced that the world is facing a biodiversity crisis, there is a great deal of uncertainty involved in quantifying trends. First, there is a paucity of baseline data, beginning with the absence of a comprehensive species inventory. For example, it is estimated that only 13.6% of all existing species have been described and cataloged in scientific taxonomies, although the actual number of existing species is unknown. For some taxa, the difference between what is known and what (probably) exists is even more extreme. For example, only 0.4% of the estimated total bacteria species have been classified. The lack of basic data means that biodiversity assessments vary wildly by two or three orders of magnitude between what is known and what is estimated as possible or likely. Second, the methodologies used for assessment can vary, so that results are not comparable or only partially compatible. For example, the Secretariat's 'Global Biodiversity Outlook' considers bacteria species and cultivars in its assessment, while the IUCN's Red list does not. More troublingly, there are large epistemic differences between science based inventories and locally based bodies of knowledge that can make them incompatible for the purpose of biodiversity assessment. Third, there is little time series data that would allow scientists to quantify trends in biodiversity over a significant geographic area. Most of the data on species have been collected on an ad hoc basis in static, one time inventories. Finally, it is difficult to know the degree to which our assessment of biodiversity reflects actual trends in the Earth's biosphere or is an artifact of improved monitoring.
Proximate and Ultimate Causes of Biodiversity Loss
Beyond a general agreement that increased biodiversity loss is associated with human activities – ranging from the human colonization of the Americas and Australia 15 000–25 000 years ago to the industrial revolution – there is a great deal of debate about precise, causal mechanisms. A typical list of causes includes disparities in wealth and poverty, insecure property rights, overpopulation, habitat loss, overconsumption, and, increasingly, climate change. Differences in assessing causation are driven partly by whether one is considering proximate or ultimate causes. For example, a common proximate causal explanation of the decline in global biodiversity is habitat loss driven by population pressure, especially in the Global South. An ultimate causal explanation might point to government land tenure and taxation policies that promote the concentration of land ownership and the expansion of large scale capitalist agriculture at the expense of poor family farmers who are forced to clear new land that is agriculturally marginal, but biodiverse. Thus proximate explanations, driven by a neo Malthusian ideology, often single out demographic forces, while ultimate explanations might suggest complex political economic processes through which class interests are pursued. Sometimes, the latter are referred to as root causes – processes that drive biodiversity loss, but which are spatially and temporally distant from where the loss occurs. An analysis that seeks to look beyond proximate causes, therefore, must examine cultural, social, and political economic processes operating at multiple temporal and geographic scales.