16
$\begingroup$

Being this my first questions on this .SE I hope that the topic is appropriate.

I've been told that 99% of the living species on earth live in between -200m and 800m above sea level, this made me just wonder how much of earth surface lies in this elevation span, but I couldn't find the answer googling.

1) Does anybody have a source to check what percentage of dry land has sea level elevation lower than 1 km (or 800m, or in between -200m and 800m...)?

2) How should I interpret the information given to me about the number of living species? I would just say for the 99% of the living species the intersection between the "natural habitat" and the altitude "-200m - 800m" is non empty, is it correct? Is there a source where I can check this information?

Thanks in advance!!

$\endgroup$
  • 2
    $\begingroup$ It would be good for the question to cite exactly what has been claimed, because i don't see how "dry land between -200m and 800m" would have anything to do with living species in that range. In that range probably live a lot of fishes, a lot of insects and bacteria IN the soil and so forth, not just land animals. My guess is all those also count towards 'living species'. $\endgroup$ – kutschkem Jun 24 '14 at 11:09
  • 1
    $\begingroup$ We also have the question whether it really is a count of species - as suggested by the question or a count of individuals or - perhaps more likely - a sum of biomass. $\endgroup$ – Oddthinking Jun 24 '14 at 13:50
  • 1
    $\begingroup$ It would be nice to see a citation. However, that figure is plausible because it captures essentially all of the productive part of the oceans and well over half of the Earth's land area. $\endgroup$ – David Hammen Jun 25 '14 at 0:25
  • 4
    $\begingroup$ Saying with precision where 99% of the species live is a bit dubious given that "some 86% of existing species on Earth and 91% of species in the ocean still await description." Reference: Mora, Camilo, Derek P. Tittensor, Sina Adl, Alastair GB Simpson, and Boris Worm. "How many species are there on Earth and in the ocean?." PLoS biology 9, no. 8 (2011): e1001127. $\endgroup$ – David Hammen Jun 25 '14 at 0:42
  • 1
    $\begingroup$ Thank you all for the useful comments! In specific, regarding question 1) I'll have to give it a try, but it's probably going to take some time. And about question 2) I'm unfortunately unable to provide any written source, since I heard the information in a seminar. But am I wrong in thinking that if we consider it as a sum of biomass then the lack of description of species is irrelevant? At least one could try to check the information by this point of view, or is it completely non-sense? (My field of specialization is unluckily quite far from biology.) $\endgroup$ – Giovanni De Gaetano Jun 25 '14 at 6:43
11
$\begingroup$

enter image description here

(Fig. 2.4 of Deep-Sea Biology: A Natural History of Organisms at the Deep-Sea Floor)

8.3% of the Earth's surface is above 1 kilometer.

This corresponds to about 29% of the land area.

So 71% of the land area is below 1 km.

$\endgroup$
  • 1
    $\begingroup$ It would be great if you could give a reference for these charts. $\endgroup$ – mwengler Dec 11 '14 at 15:31
2
$\begingroup$

For question 1), the exact number will vary depending on the elevation model you use for the calculation. High resolution elevation models are available for land, but for sea the resolution is much lower. The best global elevation model covering both land and seafloor is arguable ETOPO1, with a resolution of about 2 km. And if we mask all areas outside of the range -200 to 800 m it turns into this

enter image description here

And you can see that most tropical forest and large areas of boreal forest are included, so I wouldn't be surprised if 99% of living land species are within those ranges.

Now, the total area within those limits tuns out to be $1.286 \times 10^8 {km}^2$ of a total of $5.095 \times 10^8 {km}^2$ (A good match for the figure of $5.101 \times 10^8 {km}^2$ listed by wikipedia). Giving a total of 25.2% of Earth's surface.

From that total, a 5.3% is contributed by the terrain between -200 m and sea level ($0.270 \times 10^8 {km}^2$).

Note that so far we have NOT made any distinction between water covered and dry surface. Now, that distinction seem weird to me, as it rules out massive areas of shallow seas full of life. The oceans are know to contain 15% of the known species, therefore, ruling them out immediately render the "99% claim" invalid. Instead, that distinction seem to be there to include the minuscule amount of dry land between 0 and -200 m, that in turn is in general very dry (otherwise they would become lakes) something that doesn't makes any sense to me. If the sentence you heard really refers to "dry land", I would think it must then refer to land species only. Anyway, it is not difficult to properly answer question 1 by quantify the relevance of those below-sea-level dry areas. That can be accomplished using the MODIS global water mask, a very useful dataset at 250m resolution that allows to distinguish between dry land and water covered areas. It looks like this:

enter image description here

(It doesn't cover Antarctica, but that continent doesn't contain any below sea level interior lands as seen in the first figure, so it doesn't really matter)

Using the two datasets, I've calculated that the surface of dry land between 0 and -200 m of elevation correspond to $0.0065 \times 10^8 {km}^2$ (a 0.1% of Earth's surface).

Taking that into account, the percentage of dry land between -200 and 800 m is a 69.1% of Earth's dry surface, or a 20.1% of the total Earth's surface.


For question 2, I think your proposed interpretation is valid, and reasonable. But also other interpretations (like the total biomass mentioned in the comments) are valid. Nevertheless, as mentioned above it must refer to that "99% of land species live in dry land between -200 and 800 m" or that "99% of all species live between -200 and 800 m". Otherwise the claim is incompatible with the above mentioned fact that 15% of species live on the oceans.

A second caviat is that a large amount of species are yet to be discovered, perhaps up to the 86% of them, and those will arguably be found preferentially in the less explored areas of the oceans.

I haven't seen quantitative biodiversity maps considering estimated for all known species. You will be able to validate that claim if you consider one special well studied group, like vertebrates or vascular plants. Tor those two groups, comparing the following two figures with the top one makes the "99% claim" perhaps reasonable, but an overwhelming majority of species don't belong to those groups, and I don't know if the biodiversity distribution would be similar.

enter image description here (vertebrates biodiversity map from savingspecies.org)

enter image description here (vascular plants biodiversity map from Hamburg University)

If an actual number wants to be calculated based on actual data, the GIS datasets at biodiversitymapping.org are a great resource.

Perhaps one argument in favour of the biomass interpretation of the claim, is that it would be a much easier and better constraint calculation.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.