What is a primary concern regarding the use of GMOs?

The primary concern regarding the use of genetically modified organisms (GMOs) is their potential to disrupt natural ecosystems. This stems from the fact that GMOs can introduce new genes into wild populations and impact local biodiversity. For instance, if a GMO crop exhibits traits such as herbicide resistance or pest resistance, it may lead to unintended consequences like the decimation of non-target species or the development of resistant pest populations. Furthermore, the introduction of GMOs can alter nutrient cycles and habitat structures, which can significantly impact the interactions among various species within ecosystems.

Considering other options, while water use could vary based on the specific GMO and its environment, it's not a universal concern for all GMOs. The reduction of pests is not entirely guaranteed, as resistance can develop over time. Lastly, the economic aspect of GMO production can vary widely and does not represent a primary ecological concern.