The cod is not in immediate danger of extinction, yet populations of cod in the Atlantic have been so badly depleted that fishing communities in North America and Europe have lost a traditional source of livelihood for generations to come.
The blue spruce is not likely to be wiped off the face of the earth soon, yet every time a large forest is clear-cut, the loss affects not only the animals and other species that lived there, but communities damaged by downstream erosion and landslides, and the global balance of greenhouse gases that the trees would have helped to absorb.
That is why the loss of populations of plants, animals and other species may be as, or more, significant than the extinction of an entire species, three Stanford scientists say in an article in the Oct. 24 issue of the journal Science. While species are being lost globally with alarming speed - the highest rate since the mass extinction that included the dinosaurs - they calculate in their study that separate populations that make up various species are going extinct at a rate three to eight times faster.
"You could destroy all of a species' populations but one, and the species still exists," said Jennifer Hughes, lead author of the study. "However, you would have lost the benefits supplied by those populations. This is a tremendously important dimension of biodiversity which is often ignored."
Hughes is an ecologist at Stanford's Center for Conservation Biology. She conducted the study with Gretchen Daily, Bing Interdisciplinary Research Scientist, and Paul R. Ehrlich, the Bing Professor of Population Studies, at Stanford.
Hughes and Daily and will speak on Oct. 28 and 29 in Washington, D.C., at the National Academy of Sciences' second forum on biodiversity, "Nature and Human Society: The Quest for a Sustainable World."
Their paper in Science is linked to an article on species extinction by Sean Nee and Robert May of Oxford University, and a "Perspectives" essay by British ecologist Norman Myers. "[A] mass extinction is now overtaking the world's biodiversity," Myers writes. In their study, Hughes, Daily and Ehrlich looked at the loss of biodiversity on a different scale - that of localized populations that make up a species - from the scale that scientists and policy-makers usually have considered.
Biodiversity is the variety of life found at all levels of biological organization, ranging from individuals and populations to species, communities and ecosystems. "Although much of the current public and scientific concern over the extinction of biodiversity emphasizes the loss of species, species are only one aspect of biodiversity," Hughes said.
"While species are important, many of the benefits that biodiversity confers upon humanity are delivered through populations. This means that species extinction rates do not accurately represent the loss of the benefits of biodiversity," she said.
In the study, the Stanford scientists estimated the current rate of population extinction. A population is a group of individuals in a given location of the same species - a population that is genetically different from other such groups. Species are made up of one or more populations.
The scientists calculated that there were one billion to six billion populations on Earth. They then estimated that, by a conservative calculation based on known rates of habitat loss, populations are going extinct at a rate of 0.8 per cent per year, or 1,800 populations per hour, in tropical forests alone.
In contrast, species loss in tropical forests has been predicted in other studies to occur at a rate between 0.1 and 0.3 per cent each year, or 2 to 5 species per hour. The loss of populations therefore is occurring 3 to 8 times faster than species loss.
The loss of populations is significant because most of the benefits provided by individual species, or species working together in an ecosystem, are local and regional. For example, each population of a seafood or timber species that survives is available as a stock to be harvested. Each population of a particular species of plant has a slightly different genetic makeup - genetic material that may make a difference in the development of pharmaceuticals or the improvement of agricultural crops. "About half of the annual increase in crop production comes from the incorporation of new genes from populations of wild relatives that confer enhanced resistance to pests, disease, soil salinity and so on," Hughes said.
Perhaps the most important benefits that populations provide are in the form of ecosystem services, the authors write. "Natural ecosystems supply a wide array of services to society whose full value is enormous, but often ignored," Daily said. These include purification of air and water, stabilization of climate, detoxification of waste, generation and maintenance of soil fertility and pollination of crops.
Daily's presentation at the biodiversity forum will discuss society's dependence on natural ecosystems, and how to factor that dependence into decision-making. One method for doing that, she said, is to factor into planning programs the cost to society when ecosystem services are lost.
Local populations also deliver global ecosystem services, the Stanford scientists said. The impact of the loss of populations from an area will not always be restricted to the immediate area, but will often affect a wider region, and at times, the entire globe.
"Although populations operate at a local scale, they are responsible for producing services that are far-reaching," Hughes said. The blue spruce is an example: "The large-scale destruction of tree populations from a Canadian coniferous forest would influence the global balance of greenhouse gases in the atmosphere even if no species were exterminated."
The authors conclude that current conservation strategies, which focus on individual species, will not be sufficient to protect the benefits that humanity derives from biodiversity. "It is critical to go beyond saving certain species. Habitats must be conserved for the preservation of biodiversity and the life support systems that maintain human civilization," Hughes said.
In his "Perspectives" essay, Myers states that Hughes' findings about population loss raise critical questions for the foreseeable future. "If we lose, say, half of all species plus 90 percent of the populations of surviving species, which will be more detrimental for the biggest [ecosystem] service of all, environmental maintenance of the biosphere?"
Materials provided by Stanford University. Note: Content may be edited for style and length.
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