Final answer:
Human visitors pose the greatest threat to the Great Barrier Reef due to their detrimental impact on the reef ecosystem.
Explanation:
The greatest threat to the Great Barrier Reef is human visitors. Human activities such as pollution, overfishing, coastal development, and climate change have negative impacts on the reef ecosystem, leading to coral bleaching and decline in species diversity. These threats endanger the health and sustainability of the Great Barrier Reef, a vital marine habitat.
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