powering our way to extinction

Most references to biodiversity are in relation to how we are reducing plant diversity by taking over habitats either by expanding the built environment or by extending agricultural areas. What is rarely considered is the role of humans as animals (technically 'megafauna') within the ecosystem. An article published in the August 12 edition of PNAS presents a fascinating argument about the risks of population collapse (ours and other megafauna) in the context of energy and climate change.

The creation of biomass, whether as plants or animals, depends on energy flow through ecosystems, and the source of that energy is the sun. Until the industrial revolution, humans relied on solar energy directly, just as other species do - then we extended our ability to grow as a population by drawing from stored energy sources: wood, coal, oil and natural gas. So our current population growth trajectory depends on finite energy reserves. People have written before about the limits to growth - notably the Club of Rome - and their estimates of the limits of earth's carrying capacity have been shown to be inaccurate. But this new analysis approaches the issue from a different angle: the trade-off of biomass among megafauna.

Researcher Anthony D. Barnosky has looked at the total megafauna biomass through different periods of Earth's history, and found that the growth in human biomass (literally, the total mass of humans on Earth) has been accompanied by a reduction in the biomass of other species of megafauna.

There has been one major extinction event while humans have occupied the planet. The Quaternary Megafauna Extinction (QME) killed more than 178 species of the world's largest mammals, those weighing at least 44 kg. It took place at a time when the human population was rapidly expanding during a global warming episode, between 50,000 and 7,000 years before now.

After the crash, the global ecosystem gradually recovered, but with a lower overall diversity and a greater concentration of biomass around one species, humans, instead of being distributed across many species. Precrash biomass levels were finally reached just before the Industrial Revolution began, then skyrocketed above the precrash baseline as humans augmented the energy available to the global ecosystem by mining fossil fuels.

Humans co-opted the energy previously shared among other megafauna, and when that source became insufficient we tapped new sources. This means that, apart from the broad ecological niche we are occupying in terms of physical space, there is no energy available for the recovery of other megafauna. They are gone for as long as we remain abundant.

This puts into perspective our efforts at protecting the odd bullfrog here and red data species there. I am not against protecting vulnerable species. The point is that we talk about protecting biodiversity as if preserving a few wetlands would make a difference.

We didn't cause the previous mass extinction, but when it happened we spotted an opportunity and filled the gap. The crash wiped out literally half of all megafauna species, and virtually all the increase in biomass since then has been in human expansion. And despite turning to stored energy to fuel that growth, we are still increasing our portion of the energy budget available to other megafauna, with the result that extinctions continue.

Barnosky says that, if we don't make extraordinary efforts to conserve remaining biodiversity, we may be heading for another crash - and we are not immune to the devastating results that would ensue.

I used to think that it didn't matter how much energy we use, as long as we develop alternatives to nonrenewable and carbon-intense sources. At the very least, we will have to develop those alternatives in order to ensure our own survival in the long term, but this research suggests that there is a greater challenge at stake, since preserving our own current population levels (apart from future growth) means that biodiversity will be kept low. We are at risk of experiencing a repeat of the previous mass extinction:

Today, we stand at a similar crossroads, because growth of human biomass in the past few decades has moved us to the point where we are beginning to co-opt resources from, further displace, and cause extinctions of species with whom we have been coexisting for 10,000 years. At the same time, Earth's climate is warming even faster than the rates of climate change that characterised the QME.