The ecological concerns mainly relate to pesticide use and the eradication of heritage or native species. Because a lot of GMO plants are designed to be pesticide and herbicide resistant, farmers using GMOs can use chemical pesticides on their crops as often as they like. This puts heritage or native species who are not resistant to pesticides at a disadvantage.
Furthermore, GMOs can contaminate native and heritage plants by cross-pollination. In the long-term, this sharing of genetically modified DNA could lead to a lack of natural plant varieties and a change in ecological systems due to the unnatural advantage of GMOs. In the US 93% of current soy crops, 90% of current canola crops, and 86% of current corn crops are GMO. This level of competition between native and GMO plants makes the possibility of native plant destruction very real.