18 January 2007

Does organic farming benefit local biodiversity?

A study by Fuller et al. published in Biology Letters1, evaluated habitat and biodiversity differences between 89 pairs of organic and non-organic farms growing cereal crops in England.

The hedges on organic farms were higher and denser than those on non-organic farms, but the numbers of trees or the numbers of tree and shrub species in hedges were not significantly different between organic and non-organic farms. Hedges provide habitat for plants and animals. That and the exclusion of synthetic pesticides from organic farms would be expected to create a higher biodiversity on them. The findings were, however, mixed.

The mean plant species densities were significantly higher on organic than on non-organic farms. The mean species densities of spiders (but only on pre-harvest crop areas) and bats were also significantly higher on organic than on non-organic farms. Mean species densities of birds and carabid beetles, on the other hand, were not significantly different between farm types, although the mean abundances of birds and carabids (on pre-harvest crop areas) were significantly higher on organic than on non-organic farms. One exception to these trends was the mean species density of carabids (on post-harvest field boundaries) that was significantly higher on non-organic than on organic farms2.

Many other factors beside farming practices probably influence the biodiversity on a given farm. The authors suggest that the proximity of a farm to population sources, that is wild(er) areas may be one important factor: "Many organic farms are isolated units, embedded in non-organic farmland managed with conventional levels of pesticide and fertilizer inputs, often coupled with relatively low levels of habitat heterogeneity, which inevitably affects species colonization."

1R.J. Fuller et al. 2005. Benefits of organic farming to biodiversity vary among taxa, Biology Letters, Volume 1, pages 431–434. Full text
2The mean species density of carabids on pre-harvest field boundaries was also higher on non-organic than on organic farms, but the difference was not statistically significant.


bev said...

Three years ago, I did a lot of frog pond surveys as a volunteer for a couple of grad students at the nearby university. The best pond of all was on an organic vegetable farm (teeming with aquatic life and there were many birds in the trees all around it). Next best ponds were on land that was retired from farming a number of years ago. Active farms were so-so. Worst of all were some storm water reservoirs in suburban areas. I disliked visiting those as they were depressing places - reeking water, mats of floating algae, and just the odd frog.


You should see the storm water reservoirs around here (or, maybe you'd better not see them). All the garbage on the streets eventually ends up in them. In fact, today I passed by one: there were 4 grocery store carts in it.

bev said...

Sounds delightful (not!). I always wish politicians had to put in some time doing habitat surveys. It might give them a better appreciation for some of the messes that are being created in the name of progress.