Which of the following is NOT a main threat that plagues grasslands today?

The lasting effects of the Dust Bowl, while historically significant, do not represent a primary ongoing threat to grasslands today. The Dust Bowl occurred in the 1930s, resulting from a combination of severe drought and poor agricultural practices that led to soil erosion and the degradation of grassland ecosystems. Although its consequences were profound and led to changes in land management and conservation efforts, today's grasslands face more immediate threats.

In contrast, urban development is actively encroaching on grassland areas, leading to habitat loss and fragmentation. Overgrazing by livestock remains a critical issue, as it can lead to soil compaction, reduced plant diversity, and erosion. Additionally, the introduction of invasive species disrupts native grassland ecosystems, often outcompeting local flora and altering habitat structures. These factors continue to have a significant, detrimental impact on grassland health and biodiversity. Therefore, while the Dust Bowl has had long-term implications, it is not a direct threat to grasslands in the current context.