A scientific puzzle is anything that’s true without any obvious reason for it to be true, especially if there’s some obvious reason for it not to be true. Resolving scientific puzzles often leads to deep and important insights.
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. Or else why natural selection can’t purge it, as with senescence. Other classic puzzles in evolutionary biology include how complex adaptations like the human eye could evolve via small, piecemeal steps, and why there are usually only two sexes.
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?
Here’s a recent ecological puzzle I’m intrigued by: why is the productivity of an assemblage (i.e. the amount of new biomass it produces per unit time) a power law function of its biomass with a slope of 0.75? (Hatton et al. 2015) Why should it be a power law relationship as opposed to a straight line or an exponential or whatever? And why should the exponent usually be 0.75 rather than some other number? This one’s especially puzzling because the most obvious explanation–“it has to do with the 3/4 power law scaling of metabolic rate and body size”–can be ruled out.
Those two ecological puzzles are what Mark Vellend calls “global” puzzles: puzzles about a common feature of many different systems. Here’s a more “local” ecological puzzle. I recently published a paper that solves a puzzle that had been bugging me. The puzzle is the apparent contradiction between two facts about metapopulation dynamics in the case where population dynamics are cyclic, as with outbreaking diseases. On the one hand, metapopulation persistence is maximized by intermediate rates of dispersal among populations. Intermediate rates of dispersal provide rapid recolonization of local populations that go extinct, but not so much dispersal that population cycles in different patches synchronize, creating the risk of all populations crashing to extinction at once. On the other hand, when local population dynamics are cyclic, we know that even very low rates of dispersal among populations synchronize their dynamics. So how is it possible that there could be intermediate rates of dispersal that aren’t synchronizing? The resolution of the puzzle is that local extinctions are desynchronizing. You need them to happen often enough keep the population cycles from syncing up, but don’t want them to happen too often because then recolonization wouldn’t be able to keep up. That is, what maximizes metapopulation persistence in the cases where local population dynamics are cyclic isn’t an intermediate rate of dispersal, but an intermediate ratio of dispersal rate to local extinction rate. Very low rates of dispersal are synchronizing only when local extinction rate is zero or extremely low. As I said, this is only a “local” puzzle, but I think it was big enough to be worth thinking about, and it’s satisfying to have solved it.
So, what do you think are the biggest puzzles in ecology? Is there a local puzzle about your own system that you want to solve? Is ecology short on puzzles compared to evolutionary biology, and if so, is that a problem? Tell us in the comments!