Mark Lynas’s wonderful new book ‘The God Species’ attempts to put environmentalism back on track. Humankind, he says, will only be able to keep within natural boundaries by using science and technology to help minimise our growing impact on the planet. He looks at nine specific environmental indicators - the atmospheric concentration of CO2 is the best known – and offers a view of how close we are to the safe limit. One of these indicators is the loss of biodiversity.Humankind is presiding over an astonishingly rapid extinction of species and Lynas says that this loss ‘arguably forms humanity’s most urgent and critical environmental challenge’. He suggests that the rate of extinction is possibly two orders of magnitude greater than the world’s eco-systems can sustain. Put at its simplest, the justification for the concern over biodiversity loss (of which extinction is merely one facet, of course) is that species variety helps maintain stable natural environments. Extinguish all the predators and the prey can become dangerously dominant. Cultivate just one crop and nutrient loss into watercourses is far worse than if many types of plant are grown. But just how strong is the evidence that biodiversity loss is economically damaging? If we cannot show a financial calculation our chance of getting policy-makers to take the issue seriously is close to zero.

Many reasonable people bemoan the current mass extinction but don’t understand why Lynas and the ‘planetary boundaries’ group of scientists think it is so disastrous. What really suffered, they ask, after the last wolves were hunted to extinction in England in the early nineteenth century? Sheep could be more safely grazed and food production increased. Is there really a strong case that biodiversity is worth more than the economic benefits of reducing pests and predators? I think we are all very willing to be convinced that biodiversity is crucial but, to be frank, the evidence may not yet be powerful enough.

A new paper puts some interesting numbers into the debate.(1) It looks at whether the degree of diversity in land use in the agricultural heartlands of the United States affects the risk of severe crop damage from insects. The theory is this: in agricultural monocultures, insects can breed without predators whereas a mixed landscape, with woodland and multiple crops, provides the living space for birds and bats that can help control any infestations. So Timothy Meehan and his colleagues asked the obvious question: do diverse landscapes result in farmers having to use less insecticide? Assuming that farmers respond rationally to the beginnings of insect damage and spray the crops that are affected, the number of hectares receiving insecticide is a reasonable proxy for the threat from insects.

As we might expect, Meehan shows that diverse landscapes result in less insecticide use. In other words, biodiversity has direct economic value because spraying a crop costs money and time. What the research team calls ‘landscape simplification’ increases the likelihood that any particular hectare has to be sprayed. In what seems to me to be a heroic calculation, the scientists suggest that 1.4m more hectares need to receive insecticide each year as a result of the extensive use of monocultures of wheat, soya and maize across the Midwestern states. But the direct cost per hectare is assessed at only about $48. Compare this figure with, for example, the average yield of 20 tonnes of corn a hectare in good fields in Wisconsin, valued today (July 2011) at over $6,000. Put crudely, the value of the crop is more than two orders of magnitude more than the increased cost of insecticide on an affected hectare. And, equally powerfully, the research shows that only about 4% of total cropland needs insecticide application as a result of locally low levels of plant biodiversity. (Some crops will need pesticide protection even in the most diverse landscapes).

The lesson from the paper is therefore a simple one. In the specific case of the Midwest, landscape simplification is tending to push up insecticide use but the direct economic cost of this is trivial compared to the value of the crops. If the whole of this vast area were given over to a single crop, and every hectare had to be sprayed every year, farmers would still not be losing financially from the loss of biodiversity.

The response is to say that the costs to the farmer are only a small fraction of the total impact on society, now and in the future. High levels of pesticide use mean poorer water quality and air pollution, possibly affecting the health of people hundreds of miles away. Heavy insecticide use will eventually cause pest mutations that will require a new generation of chemicals. Applications of insecticide may cause the deaths of beneficial soil organisms. Nevertheless, Meehan’s paper does not immediately provide support for Mark Lynas’s conclusion that biodiversity loss is potentially the worst environmental problem the world faces.

Timothy Meehan et al., Agricultural landscape simplification and insecticide use in the Midwestern United States, PNAS (OPEN ACCESS) July 2011.