Organic Farms Not Always Best for Butterflies

The most devastating impact on biodiversity is caused by agriculture. Farming is already the greatest extinction threat to birds, and its adverse impacts look set to increase, especially in developing countries (Green et al. 2005).

Thus one of the global challenges for the next century is the need to develop high-yielding varieties that require minimal inputs, so that impacts on biodiversity can be minimized.

An alternative to the “high-input” approach is to expand the number of organic
farms. Because organic farmers do not use synthetic pesticides, their farms support
higher levels of biodiversity than conventional farms. Some organic farms can yield as much, for some crops, as conventional agriculture (Reganold et al. 2001; Maeder et al., 2002), although in some cases the yields of organic cropping systems is considerably lower than that of conventional or integrated cropping systems (Bruulsema TW et al. 2003;
Maeder et al., 2002)

In cases of low yield, organic practices would require more land be farmed to maintain and increase output levels, thus potentially leading to reduced overall biodiversity. What is the net effect on wildlife when the land being converted to wildlife-friendly farming has a lower yield, and so more land, somewhere, must be farmed to provide the same harvest?

A new study this week published in Ecology Letters seeks to answer this question. The research indicates that when the organic yield per hectare falls below 87% of conventional yield, wildlife does not benefit.

The researchers surveyed Sixteen 10 × 10 km landscapes in the Central South West and North Midlands of England. Within each landscape they surveyed one organic farm, one conventional farm, and one grassland SSSI (Site of Special Scientific Interest: a UK conservation designation). They then counted butterflies in each area.

A few of the butterflies surveyed are shown here:

Thymelicus syvestris, Ochlodes venata, Erynnis tages, and Pieris napi (my favorite).





They found that, for the type of fields and farms investigated, organic farms support a higher density of butterflies than conventional farms, but a lower density than grassland reserves. Organic farms support more butterflies than conventional farms, so if there were no difference in yield it would always be better to farm organically. However, the lower the organic:conventional yield ratio, the more advantageous an alternative land sparing strategy would be.

What this means is that even if we convert ALL of agriculture to organic farms (now only ca. 2% in the United States), we still need to increase yield on these farms if we want to spare land and protect wildlife. The study also suggests that if our goal is a sustainable farming system, we may not be investing wisely. The authors indicate that in the UK alone, £435 m was spent on agri-environment schemes in 2008, as compared to a budget for all other nature conservation of c.£80 m. More interdisciplinary research is urgently needed on how the net benefits of different farming methods compare, so that agricultural policy can be as environmentally sustainable as possible. And, as readers of this blog will not be surprised to hear me say, we need to use the most effective modern genetic methods to increase crop yields on ecologically managed farms.

Hat tip to Stephen Daubert, author of “Threads in the Web of LIfe” for alerting me to this paper.