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A new study in Science confirms that the planet's sixth mass extinction is underway. As with past mass extinctions, numbers of large animals (elephants, rhinoceroses, polar bears, etc.) are falling at the fastest rate, but nearly all animal species are in decline, including most insects and other invertebrates.
The study's lead author, Stanford biology professor Rodolfo Dirzo, says, "Since 1500, more than 320 terrestrial vertebrates have become extinct. Populations of the remaining species show a 25 per cent average decline in abundance." He notes that when large animals are lost, areas can become overwhelmed with rodents, which carry high levels of pathogens and increase the risks of disease transmission.
Professor Vaclav Smil of the University of Manitoba has calculated that humans now make up one-third of the total weight of land mammals, with the other two-thirds mostly domesticated animals -- cows, in particular. In the year 2000, at an average of 50 kg/human, the world's 6 billion people weighed about 300 million tonnes, representing about 55 million tonnes of carbon. Dr. Smil estimates that "human biomass" now vastly exceeds the roughly 5 million tonnes of carbon in all wild terrestrial mammals (from mice to elephants), and that the latter declined two-fold during the past century.
Dr. Dirzo explains that "size and meat mass" of larger animals has made them attractive targets for humans. Acquiring food through hunting, fishing and farming (including livestock and crop production) is at the root of the current mass extinction. Animal populations are declining in the oceans as well as on land. Modern fishing technologies have depleted essentially all edible marine fish stocks. For sharks -- large fish at the top of the food chain -- we catch them, rip off their fins, and discard the rest.
The fossil record indicates that after the pressures that trigger a mass extinction subside, life's diversity slowly recovers. But this can take as much as 30 million years and can involve major shifts in dominant life forms, such as the replacement of dinosaurs by mammals some 66 million years ago.
Given that the current mass die-off is caused by humans, it is hard to predict when or how the trend can be reversed and recovery may start. It's in our hands.
Perhaps because we humans are so highly dependent on other species currently found on the planet, and disease risks are increasing, our numbers will also crash. This would alleviate the human-caused pressures contributing to the sixth mass extinction (habitat loss, over-harvesting, pollution, introduction of invasive species, and climate change), all of which are related to how we produce and consume food.
A more hopeful possibility is that we can directly address these pressures. Dr. Smil suggests various options, falling under the general headings of reducing consumption and using resources more efficiently. Some steps are surprisingly mundane, such as tackling obesity and our "high degree of carnivory," and reducing food waste, which he estimates at 40-45 per cent of total food supply. He also recommends conserving forests through reduced paper consumption and more efficient use of timber resources. He concludes optimistically that "improvements in quality of life for the world's still-growing population can be made without exerting a perilously large claim" on the biosphere.
A complementary approach is to find ways to make human-dominated ecosystems -- including urban areas as well as farmlands -- more hospitable to a wider range of species. Much like eating less to conserve wildlife habitat and lose weight, this can involve "win-win" scenarios. Small wildland patches in the city provide prime pollinator habitat while improving yields for urban gardeners. Native vegetation buffers along watercourses in agricultural areas improve water quality for downstream residents, as well as providing habitat for a variety of frogs, turtles, birds and insects.
Well, yes, we have to eat. But how food is produced and consumed really matters to the species with which we share this planet, and to our own future.
Ole Hendrickson is a forest ecologist and current president of the Ottawa River Institute, a non-profit charitable organization based in the Ottawa Valley.
Photo: Michael Pollak/flickr
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