Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a fascinating example of ecological adaptation: the saguaro cactus, native to the Sonoran Desert. The saguaro can grow up to 40 feet tall and live over 150 years. But what's truly remarkable is how it survives extreme heat and drought. Its shallow root system spreads out widely to capture rainfall quickly, while its pleated stem expands like an accordion to store water. Each saguaro can hold up to 200 gallons of water after a heavy rain. However, the cactus faces a threat from climate change. Warmer temperatures and prolonged dry spells reduce the frequency of summer rains, which are crucial for seedling survival. Also, an invasive grass species, buffelgrass, introduced for cattle grazing, now fuels wildfires that can kill mature saguaros. In the past, fires were rare in this desert because natural vegetation was sparse. Now, with buffelgrass filling gaps, fires spread rapidly, endangering not just saguaros but the entire ecosystem that depends on them, including birds like the Gila woodpecker that nest in their arms. So, conservation efforts are focusing on controlling buffelgrass and protecting the remaining saguaro populations.
❓ 题目 What is the main threat to saguaro cacti mentioned by the professor, beyond climate change?
A. A. Overgrazing by cattle that damages the cactus roots
B. B. Invasive buffelgrass that increases the risk of wildfires
C. C. A decrease in the population of Gila woodpeckers
D. D. The expansion of the cactus's shallow root system
✅ 正确答案
B. Invasive buffelgrass that increases the risk of wildfires
