What are the biggest puzzles in ecology?

Evolutionary biology is full of puzzles, most of which have the form “Evolution by natural selection should produce X but yet we see Y. How come?” Examples include the surprisingly high frequency of sterile males, individuals that help unrelated individuals reproduce, and senescence. Resolving the puzzle usually involves figuring out why trait or behavior X actually is adaptive despite appearances to the contrary, as with individuals that help non-relatives reproduce.

What are the biggest puzzles in ecology? Does ecology have as many puzzles as evolutionary biology? And if not, does that indicate a failing of ecology?

Here’s a classic ecological puzzle: Hairston, Smith, and Slobodkin’s question, why is the world green? That is, why is the world covered with plants, given that there are lots of herbivores around that you’d think would eat all the plants?

Interested? Read more about the puzzles in Ecology and which ones are global and which ones are local, here!