What is one of the oldest ecological observations?

One of the oldest is the latitudinal diversity gradient: biodiversity increases from the poles toward the equator. Documented since the 1700s (e.g., by Linnaeus) and formalized by Alfred Russel Wallace in 1878.

• a. Biodiversity typically increases toward the equator. → Matches the latitudinal diversity gradient.
• b. Biodiversity loss is slowing down. → Modern claim (disputed; rates are high). Not historical.
• c. The extinction of one species is not likely to cause the extinction of another. → Relates to coextinction (modern ecology).
• d. There are low numbers of endemic species in the tropics. → False; tropics have high endemism.

Option a is the oldest ecological observation.

Final answer
a