The increasing use of chemical herbicides, both synthetic and organic kinds, is often blamed for the declining plant biodiversity in farms, but it is simplistic to think herbicide exposure is solely to blame.

The science doesn't add up. If herbicides are a key factor in declining diversity, then thriving species would be more tolerant to widely used herbicides than rare or declining species, according to J. Franklin Egan, research ecologist, USDA-Agricultural Research Service. But that isn't the case.

"Many ecotoxicology studies have tested the response of various wild plant species to low dose herbicide exposures, but it is difficult to put these findings in context," said Egan. "Our approach was to compare the herbicide tolerances of plant species that are common and plant species that are rare in an intensively farmed region. We found that rare and common plant species had roughly similar tolerances to three commonly used herbicides."

This could mean that herbicides may not have a persistent effect in shaping plant communities.

Writing in Environmental Toxicology and Chemistry, the researchers that over the past several decades, while the use of herbicides was on the rise, other factors such as the simplification of crop rotations, segregation of crop and livestock and increasing mechanization have also been rapidly evolving. In addition, the clearing of woodlots, hedgerows, pastures and wetlands to make way for bigger fields has continued apace and resulted in habitat loss. 

That's a lot of knobs turning ecologically. People focusing on one are often agenda-based rather than evidence-based. 

"These findings are not an invitation to use herbicides recklessly," he said. "There are many good reasons to reduce agriculture's reliance on chemical weed control. But, for the objective of plant species conservation, other strategies like preserving farmland habitats including woodlots, pastures and riparian buffers may be more effective than trying to reduce herbicide use."