"Knowledge gained over the past 15 years that GM crops have been grown commercially indicates that the impacts on biodiversity are positive on balance. By increasing yields, decreasing insecticide use, increasing the use of more environmentally friendly herbicides and facilitating the adoption of conservation tillage, GM crops have already contributed to increasing agricultural sustainability.

Overall, the review finds that currently commercialized GM crops have reduced the impacts of agriculture on biodiversity, through enhanced adoption of conservation tillage practices, reduction of insecticide use and use of more environmentally benign herbicides and increasing yields to alleviate pressure to convert additional land into agricultural use.

The key findings of the review are: The impact of the introduction of GM crops on crop diversity has not been thoroughly studied. However, the small number of studies that have been done (cotton in the US and India, soybeans in the US) find that the introduction of GM crops has not decreased crop diversity"
https://www.landesbioscience.com/journals/gmcrops/CarpenterGMC2-1.pdf