The question in the post title has bugged me for years. I’ve read and thought about it enough that I think it’s a good question, meaning a question without an obvious answer. But it’s on a topic on which I’m far from an expert, so there might well be some non-obvious answer that I’m unaware of. So I’m going to pose the question and look forward to learning from the comments.
I was reminded of this question most recently by Grady et al. (2019 Science). I emphasize that this paper is merely one among many that could be used to illustrate my question, and that I’m not criticizing this paper or its authors. This isn’t a post-publication review of Grady et al. (2019). This is me thinking out loud about a broad issue, and picking a recently-published paper as an illustration purely because it would be work to look up a bunch of additional illustrations and
I am lazy this is only a blog post.
Anyway, Grady et al. report a bunch of data establishing two main claims. First, species diversity of large-bodied ectothermic marine predators is highest in the tropics, but species diversity of large-bodied endothermic marine predators is highest in the subtropics and temperate zones. Second, endotherms outperform ectotherms in colder water–they can swim faster, their neurons fire faster, they can consume more food relative to their metabolic demands, etc.
Grady et al. then argue that the performance pattern explains the diversity pattern–that species diversity of ectotherms is maximized where it is because those are the places where ectotherm individual performance is maximized. And similarly for endotherms.
But why should that be the case? Why should correlation indicate causation here?
Grady et al. suggest that high individual performance causes high species diversity because “[H]igher abundances and foraging success reduce extinction rates and permit specialization, which promotes speciation” and “species with high abundances tend to have large ranges and subsequent fragmentation may promote allopatric speciation, particularly across ocean basins or hemispheres”. To which, maybe! But that strikes me more as plausible hand-waving than anything. Which is fine! It’s a starting point, and you have to start somewhere. But I dunno, I also feel like I could dream up other plausible hand-wavy explanations for the correlation between performance patterns and diversity patterns. And I bet I could dream up plausible scenarios in which performance and diversity are uncorrelated, or even negatively correlated, because high performance does not cause high species diversity, or even causes reduced species diversity.* The point is that I don’t trust my own intuitions. Especially not when I’ve already seen the data, because whatever data I’ve seen are going to make whatever post-hoc causal explanation I dream up seem totally plausible to me. Had the data come out differently, I’d have dreamed up a different plausible-seeming post-hoc explanation. Like Duncan Watts said, everything is “obvious” once you know the answer. Post-hoc intuition should either be the starting point for further research, or the summary of an already-established conclusion. Not a key plank in an argument aiming to establish a conclusion.
Returning to the question in the post title: I have the vague, admittedly-anecdotal sense that many biologists, especially those working in macroecology and macroevolution, are quick to make the leap from “organisms with trait(s) X perform well in environment(s) Y” to “there should be many species with trait(s) X in environment(s) Y”. But my brain stumbles at that leap. What apparently seems like an intuitive leap to many people strikes me as a gap in the argument, one that could use a lot more theoretical and empirical research to fill it in.**
And maybe that gap has been filled in in many cases, and I just don’t know about it! It certainly seems like a really interesting and important line of research to pursue, which probably means it has been pursued and I’m just ignorant of it. But if I’m ignorant of it, well, maybe others are too. So you tell me: what are the best worked-out case studies of high organismal performance causing high diversity? Looking forward to learning from your comments.
UPDATE: See the comments, where Mark Vellend reminds me that he made the same point in his very fine book. Apologies for having forgotten that, Mark! More broadly, I’m sure this point must’ve been made by others too.
UPDATE #2: John Grady was kind enough to take the time to comment. Thank you to John for contributing to the very thoughtful and interesting conversation we have going in the comments.
*And it’s sounds like Grady et al. could too. They also write “In addition, high metabolism may increase range size and reduce allopatric speciation, and respiratory constraints may limit utilization of benthic resources near coastlines.” That is, they suggest reasons why the high performance of endotherms might actually reduce rather than increase their propensity to speciate in subtropical and temperate waters.
**Part of why I stumble over this gap is that the theories of speciation I’m most familiar with don’t invoke high absolute individual performance. They invoke frequency-dependent individual performance (e.g., character displacement). They also involve mechanisms having nothing to do with individual performance, like different selection pressures in different locations plus barriers to gene flow between locations. Conversely, I tend to think of high performance as the culmination of adaptation, the achievement of a (frequency-independent) “peak” on the “fitness landscape”. Which leaves me wondering why achieving a frequency-independent fitness peak would promote diversification. But maybe all this just shows how ignorant I am of the enormous theoretical and empirical literature on speciation.