ecology - Is there a hypothesis that attempts to explain patterns of species richness along all three energy-related environmental gradients?

My question is related to one of the oldest question in ecology: "What determines global patterns of species richness?". However, I want to focus on one particular part of this question, which has been bothering me for a long time.

Background information

One of the most widely recognized ecological patters on Earth, which is found at most scales and in most biological taxa, is the latitudinal diversity gradient (LDG) -- there are more species in the tropics than in the temperate regions, and the further away you move from the tropics, the fewer species you encounter. Furthermore, such pattern exists not only along the latitudinal gradient, but species richness also covaries with altitude in terrestrial environments and depth in marine environments, showing the same diversity gradient!

It seems to be fair to suggest that energy should somehow underline all these diversity gradients and create some sort of universal mechanism that would ultimately affect all species richness patterns on Earth. Unfortunately, there is no consensus on questions as grand as this one, but I'm looking for hypothesis that would specifically attempt to explain all three gradients together.

Question

Is there a hypothesis that attempts to explain patterns of species richness along all three energy-related environmental gradients together: latitude, altitude and depth? If there is, what it's weakness? If there's no such hypothesis, do we have reasons to believe that such a broad link across the three gradients can exist?

Please note how I'm trying to emphasize that I don't want you to list all the hypotheses that describe LDG only, but rather the three gradients together.

Answer

This is a big question and a very active field of research. I'm not deeply into this litterature, but you should look into the the different scaling relationships (often power laws) that have been described on metabolism vs body size, species-area relationships and species richness vs biomass. Also consider that energy-use by species in a community is considered a zero-sum game both in the neutral model (Hubbell, 2001) and in Red Queen models of evolution. You will probably not find a definite answer to your question though, but there are some interesting intersections of ideas out there. As others have suggested you also need to consider and take into account differential historical extinction rates, and how this will influence current patterns (Mittelbach et al. 2007).

Points of entry could be:

A couple of caveats/ideas though; first of all, it has been argued that the latitudinal gradient of species richness is likely to be due to many different mechanisms in different taxa (see Gaston, 2000). Here you are looking for a hypothesis to explain not only this single pattern, but all three of them, which makes a single explanation even less likely. Second, many non-exclusive explanations have been put forward for the latitudinal gradient. Even if we don't know which ones of these that are the true mechanisms, some are clearly incompatible with being explanations for all the three patterns you mention, while others might be applicable to all of them. For instance, environmental stability in the tropics is one suggested mechanism for the latitudinal gradient, but this doesn't make much sense for a gradient along ocean depth (deep sea has arguably been a more stable environment both over shorter and longer time-frames). It might be relevant for an altitude gradient though. Going through different hypothesised explanations for the latitudinal gradient in this way (from what I know, the best studied of these gradients) could give you a list of ideas that are most interesting to target (maybe somebody has already done this though).

I hope this helps.