How much biodiversity loss is too much?
by Tom H. Oliver
How much of something do we need to keep people safe and well? This question is frequently asked by those working in risk management. Across diverse sectors from flood protection to health care, practitioners assess risk as the product of the impact of a given event and the probability of its occurrence. Although these estimates are often uncertain, policy-makers must ultimately make spending decisions aimed at averting these risks, because the costs of inaction to society can be substantial. Biodiversity loss is a similarly critical, yet uncertain, issue. On page 288 of this issue, Newbold et al. (1) quantify global biodiversity losses, providing much-needed information on the encroachment of proposed “safe limits.”
Economic analyses suggest that the total global value of ecosystem services is in the realm of tens of trillions of dollars (2). Many of these ecosystem services are underpinned by biodiversity. However, there is currently a lack of coordinated action to halt biodiversity declines, despite repeated setting of international targets (3). We know, broadly, the types of actions that are needed. They include habitat restoration as well as limiting human-derived pressures such as habitat loss, pollution, and invasive species. But the opportunity costs of these actions, in combination with the high levels of uncertainty around biodiversity change, appear to hamper commitment to action.
This uncertainty has multiple components. We must ascertain both the current extent of biodiversity losses and the effects of these losses on people's health and well-being. Newbold et al. report a crucial advance in tackling these issues. Their analysis is the most comprehensive quantification of global biodiversity change to date, considering over 1.8 million records of abundance from 39,123 species across 18,659 sites. Biodiversity losses vary widely across biomes. The authors find that, on average, the local abundance of each species has fallen to ~85% of its original value in the absence of human land use; that is, there is 85% “biodiversity intactness” (4). The authors then go further to relate these losses to a planetary safe limit of 90% biodiversity intactness, as proposed in a recent study (5). The hypothesis is that below the safe limit, the wide range of services provided by biodiversity that underpin human well-being—such as crop pollination, waste decomposition, regulation of the global carbon cycle, and cultural services that are central to emotional and spiritual health—are critically threatened (5). Newbold et al. find that ~58% of the world's land surface, and 9 out of 14 of the world's terrestrial biomes, have fallen below this safe threshold.
If such a large proportion of land has already passed the safe planetary boundary for biodiversity loss, why have we not already noticed more widespread negative effects on humans? Biodiversity loss can clearly lead to dramatic and rapid effects on ecosystem services. For example, invasion by the spiny water flea Bythotrephes longimanus in Lake Mendota in Madison, Wisconsin, USA, caused declines in key algal-grazing zooplankton species and consequent reductions in water quality, which will cost $86 million to $163 million to restore (6). In many other cases, however, effects may be delayed, with ecosystem services only lost after further perturbation (7). By analogy, cumulative structural damage to a bridge may only lead to sudden collapse after an extreme storm. Recovery from such catastrophic “tipping points” can be very costly if the replacement cost far exceeds ongoing repair costs. But the environment may be unique in that the extinction of species is essentially irreversible.
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http://science.sciencemag.org/content/353/6296/220.full
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