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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.

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Neat question! I imagine any responsible theory would have to take into account the associated gradients of total area (that is, there are few square meters of space to inhabit as you go higher in elevation and as you go up in latitude). I haven't thought about depth, but I guess it might be a skewed hump– there is some area at low depths, much more at mid-depths, and then declining area at lower and lower depths. –  Oreotrephes Jan 14 at 12:50
    
Are you only concerned with richness, or with biomass too? –  Oreotrephes Jan 14 at 12:51
    
What did you mean "energy related gradient"? Can you give more words about that? Are you talking about temperature? If so, can your question be reduced to "How does temperature influence species diversity"? –  Remi.b Jan 14 at 17:02
    
@Remi.b, not exactly temperature, however the guy who first discovered LDG (von Humboldt) related it to mean annual temperature. Energy is different from temperature, not only from a purely physical point of view, but also biologically available energy explains why there is not many species in deserts, where temperature can be very high. –  Th334 Jan 14 at 22:44
    
Well, I think for a start, except for thermal vents, energy comes from the sun. So more sun = more energy captured by plants, which can support more non-plant life. At higher altitudes, the limiting factor is the thin air, which limits plant life, which limits everything else. –  swbarnes2 Jan 14 at 22:45

3 Answers 3

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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.

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Thanks, it does. I have a simple question that you should be able to answer without any research (maybe add it to your post?), and which I tried to include in my original question too: "What's wrong with just saying that all these gradients correlate with the amount of solar energy available for primary producers?" I can't seem to answer it myself. Is the difference in solar radiation really significant for plants in tropics and temperate regions? Is there really less available solar energy at higher altitudes? Does it fail to explain some biodiversity hot-spots outside tropics? Cheers. –  Th334 Jan 17 at 4:34
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I tried to squeeze everything in one comment, but what the heck. I actually remember doing a presentation for an ecology seminar at uni on this very Mittelbach paper :-) I absolutely agree that there might be no actual answer to this question (but that's why I agreed to accept even flawed hypothesis that attempt to explain the three gradients). I also agree that the best way to answer it is to read everything you cited... but to be honest, I was looking for a shortcut specifically on the three gradients together :-D Just saying, I'm still grateful for your answer. –  Th334 Jan 17 at 5:36
    
Well, for one, the amount of solar energy doesn't explain differences in species richness between hemispheres, and I doubt that it can explain gradients over altitude. –  fileunderwater Jan 21 at 13:25

Nice question!

For the concern of plant diversity over altitudinal gradient it has been shown in several studies in the alps and in Andes that the greatest abundance and diversity is not found in high altitude neither in low altitude but in middle altitude. Some argue that the abundance and diversity depends on temperature and human impact that is what explain this pattern. There is a theoretical model that cut the mountains into three patches along the altitudinal gradient and assume that plants - when they reproduce - either send seeds in the same patch or in a nearby patch. Because, the patch in the middle can receive gene flow from both "sides" while the "extremities" can receive gene flow only from one "side", the diversity is greater in the middle patch.

In aquatic communities, there is very few physical barrier to gene flow and therefore, speciation often occur through sympatry (I think!). Depth has an important role in isolating population (allowing allopatric speciation) because of the sensory drive evolution. Some fishes (to talk only about fishes) have opsins (proteins involved in vision) that are adapted to deep water while other fishes have opsins adapted to shallow water. This divergence, allows to push fishes to chose a preferential habitat and prevent gene flow.

High temperatures lead to high mutation rate and high mutation rate leads to high speciation rate (I think).

Because of past glaciation, it has been found that species in southern hemisphere are younger in average than those in northern hemisphere. Therefore, past ecology has to be taken in account when talking about present diversity. Land mass of different size might also impact species diversity differently.

I don't think a theory that explain species diversity along these three gradients might be easily found. I would tend to think these three gradients are important because they are correlated with parameters that influence species diversity in several ways. Those correlated variables are temperature, humidity, light, time of daylight, land mass, connectivity (rivers that increase gene from mountain to valley or surrounding land mass for example), human impact, …

What did you mean "energy related gradient"? Are you talking about temperature? If so, can your question be reduced to "How does temperature influence species diversity"?

Sorry for the missing references!


This wiki article lists pretty well (better than what I did in my above answer!) different hypothesis for explaining latitudinal gradient in species diversity. Some of these hypotheses can be used to describe effect of depth or altitude.

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Cheers, @Remi.b. As to your paragraph 1, if it's temperature, than the foot of the mountain should have the highest biodiversity, no? Next issue, due to the gravity, shouldn't the seeds move much more easily down the gradient (which, again, would contribute to the biodiversity of the lowest region). Finally, even if the middle patch receives more gene flow, it should reduce species richness, because gene flow acts against speciation, no? –  Th334 Jan 17 at 5:12
    
Yes, even though both you and fileunderwater wrote great answers, I must admit that I did know about the Wiki article you cited, and I read some literature on LDG too... but I thought I should ask about the three gradients combined, not just LDG, partly because I have no idea where to look for an answer to this phrasing, partly because I'm curious, and partly because I didn't want to make you write me a book with all these theories listed :-) Thanks anyway. I still hope someone else might chew this up for me so I have only to swallow, but if not, I'm happy to dig deeper into this myself too. –  Th334 Jan 17 at 6:02
    
Now let's get back to our temperature/energy thing, maybe I do need to paraphrase the question a little. I was mainly referring to the amount of solar radiation available for photosynthesis. You can't really call it temperature, can you? Does it answer your question or not really? But with this definition, I'm not too sure whether the amount of solar energy decreases with increasing altitude... If it doesn't, than maybe "energy" doesn't correlate with all three gradients either. Also see my question under the fileunderwater's post, maybe we should answer it first. –  Th334 Jan 17 at 6:11

I've studied diversity of euglossine bees along altitudinal gradients in amazonian mountains. The references I've read showed no consensus regarding altitude. For some groups, diversity were higher at low altitudes; for other groups, it was in the middle. I've found the biggest diversity in the middle altitudes (the mountains were around Equator line). One hypothesis was about randomness: throw a lot of species inside any gradient, and most of them will overlap in the center by chance alone. Other hypothesis said about chance of recolonization. You can have extinctions anywhere, but close to Equator (low latitude) and close to sea level (low altitude) you can have species recolonizing faster, because there is more land around. After all, I don't think one simple rule will explain each and every case. And I have never thought about that below sea level. Aren't marine vulcanoes a rich environment?

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Well, I guess they are biologically rich environments, but they would be an exception to the general rule where the deeper you go, the less sunlight penetrates the water, the less plankton to eat there are, the colder it gets, and even the pressure is higher (which is not related to energy). So I guess overall there is such a gradient. –  Th334 Jan 17 at 4:47
    
What about the plants that your bees ate, did they exhibit the same trend? I also don't get your randomness explanation: if it is a uniform distribution, by chance alone the bees should be distributed, well, uniformly, no? –  Th334 Jan 17 at 4:55
    
In the sea, temperature isn't linearly related with profundity. Look the second image here: marinebio.org/oceans/temperature.asp –  Rodrigo Jan 17 at 12:13
    
I see the plants changed with altitude, but didn't measure their richness or diversity. Imagine a gradient from 100 to 3000 meters. Some bees will have a height span of 200m, others of 2000m. Imagine this is random, too. When you put them all together, most will overlap in the center, not in the borders. Look: sciencedirect.com/science/article/pii/S016953479901767X –  Rodrigo Jan 17 at 12:19

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