On Cinco de Mayo Darwin reflected on the diversity of species in the tropics, writing:
“I find one hours collecting keeps me in full employment for the rest of the day. — The naturalist in England enjoys in his walks a great advantage over others in frequently meeting with something worthy of attention; here he suffers a pleasant nuisance in not being able to walk a hundred yards without being fairly tied to the spot by some new & wondrous creature.” (May 5/6)
Of course, part of this statement stems from the fact that many more European naturalists had walked among and written about the species of England than the species of South America. So there was a lot more new things to discover.
But, as naturalists since the later 1700’s (including Darwin’s hero Humboldt) also recognized, species diversity (or what is more accurately called species richness) increase as one moves towards the tropics. A couple of examples:
Notice that there are more plant species in the tropics than the temperate regions –”warmer” colors represent higher diversity (click on the image for a larger version)
(from Barthlott, W., Biedinger, N., Braun, G., Feig, F., Kier, G., and Mutke, J. (1999))
Richness of bird species drops dramatically with and increase in latitude (from Dobzhansky (1950) via Ecology by M. Molles)
Although there are always exceptions, you’ll find that this pattern is true for most major types of organisms.
One of the most intreiging thing about this pattern of species richness with latitude, is that, although we’ve known about it for well over 200 years, we still don’t exactly know what causes it. There are several hypotheses and each one has its strengths and weaknesses. Without getting into a lot of details today – here are a few ideas (though by no means an extensive list):
(1) The tropics are just more favorable to species – there are more species that can adapt to the warmer conditions than in the harsh conditions of the poles.
(2) There is more energy to go around in the tropics. Life is ultimately based on photosynthesis (well, most of it is anyway) – living things require energy to live. There is a gradation in the amount of energy the Earth receives, with the tropics receiving far more than the higher latitudes. More energy in the tropics = more species.
(3) There is more landmass in the tropics than at higher latitudes. More land = more room for a variety of species. If you don’t believe me check out this graphic showing the distribution of land on the Earth by ecological (essentially latitudinal) zones (from Rosenzweig (1992) via Ecology by M. Molles)
(4) The tropics are more stable. Stability tends to reduce the number of extinctions allowing species to “accumulate” to higher numbers. The northern latitudes, for example, were covered with ice sheets about 20,000 years ago – so species have only been repopulating that area for a relatively short time.
(5) There is something about the tropics that allows for the faster formation of new species (speciation) or, alternatively, the slower extinction of species. (The number of species in any area over time is a balance between the number of new ones being added and the number going extinct.) Hypotheses #3 and #4 above may be related to this one, and so would the idea that there are more mutations in the tropics (leading to more rapid speciation).
(6) Species in the tropics are more specialized (they have what ecologists call a narrow niche), providing more potential for variety. For example, rather than having one type of beetle that eats any kind of flower, you may have a different beetle species that feeds on each type of flower (leading to hundreds or thousands of different beetles, rather than one).
For now – it is something to ponder (and comment on, if you have some ideas). I’ll come back to this again some time, but for now, let me just say that I just love the opportunity of unanswered questions. It is one of those things that makes science great! (RJV)