How many species of tropical trees are there?

June 16, 2015 • 2:19 pm

I’m not going to get into the long-debated issue of why the tropics are so much richer in species than the temperate zones (theories include physical disturbance, coevolutionary pressures, higher temperature that accelerates evolution, and so on). There is no consensus, but let me just present some data showing the huge difference, data collected in a new paper,”An estimate of the number of tropical tree species,” in the Proceedings of the National Academy of Sciences by J. W. Ferry Silk et al. (and the et. al., as you’ll see, has a gazillion authors). Here are the data from the “nontechnical” abstract (my emphasis)

People are fascinated by the amazing diversity of tropical forests and will be surprised to learn that robust estimates of the number of tropical tree species are lacking. We show that there are at least 40,000, but possibly more than 53,000, tree species in the tropics, in contrast to only 124 across temperate Europe. Almost all tropical tree species are restricted to their respective continents, and the Indo-Pacific region appears to be as species-rich as tropical America, with each of these two regions being almost five times as rich in tree species as African tropical forests. Our study shows that most tree species are extremely rare, meaning that they may be under serious risk of extinction at current deforestation rates.

The technical abstract says the same thing more verbosely (why do scientists feel they have to write in turgid, third-person prose?), and adds this:

Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼19,000–25,000 tree species. Continental Africa is relatively depauperate with a minimum of ∼4,500–6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions.

This pattern holds for most taxa, and knowledgeable readers can discuss the theories below. I just wanted to show you the huge difference, one that has now been well quantified for tropical trees, which are often hard to identify.

And here are the authors:

Screen Shot 2015-06-16 at 2.18.12 PMWell, someone had to count all those species!

21 thoughts on “How many species of tropical trees are there?

  1. Makes me wonder how many species humanity has lost by burning the tropical forests. Along, of course, with the species of animal, insect and so on that lived there.

    1. Beat me to it…we’ve got ~50k left, but how many did we have a century before, a millennium ago, ten or a thousand millennia ago…and how many will remain a century from now?

      b&

      1. Somewhere I read that early homonins – probably as early as Homo erectus – set fires as a hunting method. If true, I suspect they did away with many species before the agricultural era.

    1. The counter-counters. Not to be confused, of course, with the contra-counters, the counter-revolutionaries, the bean-counters, or the counter-tops.

      b&

  2. I’m not going to get into the long-debated issue of why the tropics are so much richer in species than the temperate zones (theories include physical disturbance, coevolutionary pressures, higher temperature that accelerates evolution, and so on).

    I read an account of this debate recently, and it didn’t mention what I’d have thought was an obvious candidate explanation. Just wondering whether the following has been considered and discounted:

    In the temperate zones the differences between summer and winter are much greater. Therefore species have to cope with a much wider range of conditions. Thus they have to be “generalists”. If they are generalists then their ecological niches are quite broad, and thus there are fewer such niches.

    In the tropics the differences between summer and winter are much less. Species can be much more specialised. Thus they can occupy much narrower ecological niches. Thus there are many more such niches.

    1. The sheer abundance of solar energy presumably also plays a role. We’d scarcely expect the same level of species diversity at the poles as at the equator, simply because there isn’t enough sunlight there to support it. So it makes sense that the temperate zones should be intermediate in diversity.

      That said, why diversity declines as quickly as it does with increasing latitude is an interesting question, to which your hypothesis seems a plausible answer.

    2. I was thinking along the lines of ancient-ness of the habitat. Equatorial zones where most rain forests reside do not get buried under glaciers every once in a while, so these environments had time to form many species.

  3. I guess this richness applies to the gymnosperms as well as the flowering plants. The tropical New Caledonia has 44 species of conifers that occur no where else.

    One of these species (Austrataxus spicata) is descended from Northern Hemisphere conifers! How did that happen?

    1. They are in alphabetical order, except for the first name and the last couple. This is pretty standard, as those spots (first and last) are typically reserved for the primary author and their advisors/mentors/bosses, respectively.

  4. I didn’t check the Supplemental material, but from the map provided in the paper it looks as if one or two of their sites in eastern Australia might correspond to the Gondwana Rainforest. I listened to Germaine Greer’s White Beech as an audiobook (can’t recommend it in that format, tbh) earlier this year – she describes her experiences attempting to rehabilitate an old dairy farm. That’s right, a dairy farm in the middle of the rainforest, in southeastern Queensland (almost in NSW). It’s in the Numinbah Valley, to be precise, and Greer called the project the Cave Creek Rainforest Rehabilitation Scheme. Anyway, the diversity of trees alone that she describes for her 60 hectares pretty much blew my mind – and I’m a person who pays attention to tree species wherever I go.

  5. Makes me wonder how many species haven’t been discovered yet, and the role of isolation, niche isolation, temperature ranges, radiation differences, mutation rates, ad infinitum in terms of variables, and how all of this should be integrated/synthesized.

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