Final answer:
Humans have altered the environment in several ways, including turning prairies into farmland, overfishing species like cod, and introducing invasive ornamental plants. These activities, along with pollution and greenhouse gas emissions, have dramatically transformed ecosystems.
Step-by-step explanation:
One of the three ways humans have altered the environment and ecosystems during the past 100 years is the conversion of a prairie into farmland. This agricultural expansion often leads to the clearing of natural habitats, which in turn disrupts existing ecosystems and biodiversity. Another significant alteration is the overfishing of species like cod, which can exceed their natural replacement rate, causing a ripple effect throughout marine ecosystems. Additionally, the introduction of invasive species, such as ornamental plants, into new environments can outcompete native species and lead to ecological imbalance.
Human activities have also led to unprecedented levels of pollution, with the introduction of chemicals like CFCs and PCBs that have wide-ranging and sometimes unpredictable effects on both the environment and human health. The emission of greenhouse gases has been one of the most discussed changes, leading to an increase in average temperatures, affecting weather patterns, and contributing to global climate change. These actions have in some cases decreased the carrying capacity of the environment, and in others increased it, but the overall impact is significant alteration and often degradation of ecosystems globally.