How does genetic engineering affect the environment?
There are at least two ways we can approach this question: by asking how the GMOs themselves affect the environment, and evaluating how their use affects human practices that impact the environment.
One of the most common targets for criticism and application of GMOs is the agriculture industry. Companies are successfully engineering new types of crops, sold in seed form, that offer a variety of benefits, such as resistance to pesticides or higher crop yields. One of the downsides of this, from the consumer perspective, is that farmers are sometimes not allowed to keep seeds from one year's harvest to plant in the next; the seeds and their genetic content are technically the property of the company that creates them. Further, it's entirely possible the seeds and crops could be intentionally or accidentally transported beyond the areas they are intended to be planted in, displacing native species due to superior traits. This would be another form of human-caused invasive species potentially leading to an ecological collapse.
On the other hand, the use of modified organisms may have positive effects on the environment due to factors such as the aforementioned built-in resistances, or they could be modified to require less water, less nutrients in the soil, or resistance to infection. All of these factors would reduce the ecological impacts demanded in order to care for them. Increased crop yields would also reduce the amount of land necessary to grow food, or increase the yield of the land used, which is a necessary consideration in light of the expanding world population and the limited space available for agriculture.