Climatic control of dispersal–ecological specialization trade-offs: a metacommunity process at the heart of the latitudinal diversity gradient?

Jocque et al.. Global Ecology and Biogeography 19(2): 244-252. DOI:10.1111/j.1466-8238.2009.00510.x. Climatic control of dispersal–ecological specialization trade-offs: a metacommunity process at the heart of the latitudinal diversity gradient?

Dispersal's important too, don't'cha'know. From

Dispersal’s important too, don’t’cha’know. From Jocque et al..

Yael Kisel

Yael Kisel

Though it was a nice bonus, I didn’t pick this paper because it says that dispersal (one of my pet topics) is a key process in the creation of global biodiversity patterns. I picked it because it presents an elegant central thesis that I haven’t heard before: that climate variability may modulate species richness indirectly, by deciding whether a region’s species pool will be biased towards ecological generalists that are good at dispersing (and thus have low speciation and extinction rates) or ecological specialists that are poor at dispersing (and thus have high speciation and extinction rates). To me, this idea gives me the “how intuitive and straightforward! why didn’t I ever think about it that way before?” feeling that I associate with true scientific advance and beauty. I just love how it ties together so many key factors – climate, ecological specialization, dispersal, speciation, extinction – and the authors even manage to tie in sex (well, asexual species)! From now on this idea will definitely be a part of my mental framework of how biodiversity probably works.

There are also a few smaller bits and pieces that I quite like in here. I am quite happy with the reasons the authors give for why climate variability should select for increased dispersal. Clif notes: 1) Seasonal weather with some very harsh seasons selects for seasonal migration, which involves a lot of movement and could thus lead to increased dispersal. 2) Environmental variability will likely lead to increased population extinctions, selecting for increased dispersal to recolonize those empty locations when they become habitable again. 3) Occasional harsh environmental conditions favor the evolution of dormant stages, which also make dispersal possible over longer distances. I also really like the idea that climate-driven extinction will disproportionately affect specialized, poor dispersing species. I hadn’t thought about extinction that way before; it makes sense; and it fits into my feeling that diversification over long time periods is characterized by cycles of wide-ranging generalist species budding off lots of small-ranged specialists that don’t do much speciating and eventually die out in big chunks, allowing for another burst of budding from the survivor generalists. Finally, I like how the authors put forward a lot of specific predictions that we should go out and test, like “are tropical species usually poorer dispersers than temperate/polar species?” and “are specialist/poor dispersing species less common during times of faster climate change?” (I wonder if that second question is testable with paleo data though?).

All that said, there’s also a lot that disappointed me in this paper, and I wouldn’t immediately recommend you to read it thoroughly. I felt the authors were trying too hard to sell their idea, I didn’t understand why they needed to discuss “metacommunities” and “continuity of habitat availability in time and space” so much instead of using simpler language, and there were many specific points in their reasoning that I didn’t agree with or couldn’t follow (for instance, I don’t agree that ecological specialization and competitive ability are interchangeable). I also think the central figure is a bit sloppy – it’s unclear to me why ecological specialization should itself limit gene flow, and it’s unclear whether “isolation” refers to reproductive isolation or geographic isolation – a big distinction. I also wish that they had used the latitudinal diversity gradient as one example of a possible application of their theory, rather than the main topic, as for me that focus both limited and confused the paper. Finally, just to vent for a second about typos, I found it lame that the annoying word eurytopic was spelled wrong the one time it was used!

Moving on, I think this theory deserves to be tested properly and I see some cool ways to do that. Of course, as I said, the authors lay out a number of rather specific predictions and those should be tackled (are any students reading this that need a research project for their degree?). I also had a few more ideas while reading through. First, assuming that invasive species are generally rather generalists that thrive in disturbed areas and disperse well (correct me if I’m wrong!), this would suggest that invasive species should generally come from more climatically variable regions. Is that true? Second, though the authors really focus on tropical vs. polar species/communities, what about other gradients in climate variability, for instance between coastal regions and continental interiors? Do these gradients also show the expected patterns of variation in dispersal propensity, species richness, speciation and extinction rates, etc?

I don’t have any other big thoughts about the paper to conclude with, so instead I’ll conclude with an appeal for PEGE readers to consider doing more research that would produce results useful to me. Study dispersal! Especially with comparative population genetics or new databases of dispersal related traits! It’s fascinating, I promise!

Will Pearse

Will Pearse

I’m not a dispersal person, and I’m not much of a macroecologist, so if I say something stupid below please correct me in the comments. I liked this paper; they put their heads above the parapet, whacked out some testable hypotheses, and that enables me to be constructive in my criticism (I hope) because they’ve given me something concrete to aim at.

Dispersal is complex, and I’m pretty sure it’s not just one thing. Long-distance dispersal, in my mind, is this rare process that moves individuals very long distances. Rafts carrying seeds or stems of plants across oceans are an example of it. I don’t disagree with much of what these authors say, but I think they need to be more clear about the kind of dispersal they’re considering, and I can’t actually find much of a definition of dispersal in the paper. Is long-distance dispersal relevant when talking about regional co-existence? Probably not. Is long-distance dispersal relevant when talking about the latitudinal diversity gradient and whether the tropics are cradles or graves of diversity? Probably. I think mixing in community ecological definitions of dispersal and then using them to explore long-term evolutionary trends is a bit iffy, and (I never thought I’d say this) I’d almost like to see some kind of theoretical analysis of how some of this might work. More explicit and complex incorporation of dispersal into evolutionary processes is a good thing, but we need to know what we’re putting in.

Much of what the authors suggest comes from an intrinsic trade-off between ecological specialisation and dispersal ability. As the authors acknowledge, community ecologists have known about these sorts of trade-offs for a while, and have made them more complicated, but I buy the concept for a regional approach with the authors’ proviso that suitable habitat has to be hard to find. If you’re specialised, and your habitat is hard to find, it makes little sense to move. But that also means it makes no sense whatsoever to move, which means you’re going to be stuck as a very local-scale endemic species, unless there’s some king of long-distance dispersal process (…) that occasionally shunts you out of your local area. So, if there are rare, hard-to-find habitats, why is it that such small-ranged endemics are so rare, perhaps except for the tropics where many invoke Neutral Theory to explain how so many similar (and so not really specialised!) things are able to coexist?

Much of the above rests on my whole ‘different kinds of dispersal’ argument, and I’d be interested to hear what you all think about that. I sense I could be missing something very important!


About will.pearse
Ecology / evolutionary biologist

5 Responses to Climatic control of dispersal–ecological specialization trade-offs: a metacommunity process at the heart of the latitudinal diversity gradient?

  1. jeffollerton says:

    I’ve not read the paper in detail, only the abstract, but it seems that a central plank in the authors’ argument is that tropical species tend to be “more specialized” than species at higher latitudes. That’s a hugely contentious issue which is often taken for granted by many ecologists – see my recent commentary on this from Current Biology:

  2. Yael Kisel says:

    I’ve been meaning to reply to you for weeks, Will, oops. anyway, some thoughts:

    – I agree with you 100% that it is unsatisfactory to discuss “dispersal” when we expect long-distance and short-distance (? or normal? I have yet to find a good term for this) dispersal to have different causes, frequencies, effects, etc. I can’t believe I did not rant about this in my original write up! Thanks for bringing it up.
    Along those lines, I have been thinking that perhaps a better way to think about dispersal is to distinguish between dispersal that establishes new populations, ie colonization, and dispersal between existing populations, ie gene flow from an evolutionary perspective. I think this relates more to the different effects that dispersal can have, and takes away the problem of drawing a line between “long distance” and “short distance” dispersal.

    – Eventually I would like to do some simulation studies looking at the connections between dispersal, speciation, and maybe adaptation. Check back with me in 2 years 😉

    – Are there really so few small-ranged endemics outside of the tropics? At least for plants I feel like I could come up with many examples of very local temperate endemics.
    And anyway, if there are much fewer small-ranged endemics outside of the tropics, doesn’t this go along with the central thesis of the article? Outside of the tropics -> more seasonality, possibly more climate change in the past -> species have either adapted to be more dispersive/less specialized, or they have gone extinct -> fewer small range endemics …

    – I find it interesting that you say that species with similar niches are thus not specialized … as I had been reading “specialized” to mean “having a narrow/small niche” rather than “having a different niche to other species.” Hm, another word with multiple conflicting definitions!

    Thanks for the opportunity to discuss dispersal!

    • will.pearse says:

      I’m looking forward to the simulation results :p

      There are restricted-range endemics in many places, but I have a feeling they’re more common in the tropics. I’ve not tried even Googling this, though, so I’m very likely to be wrong. Perhaps their existence does back-up the theory though; so perhaps I should shut up!

      I think I fuddled my words with the specialisation comment: I don’t think it makes sense to talk about species specialisation if all species are ecologically equivalent (even though they could well be very ‘specialised’), because they’re all on the same playing field. So, what I meant, is that if we invoke neutral theory to explain something, we have to throw out any discussion of specialisation because everything is equivalent, and so equally specialised.

      • Yael Kisel says:

        Oh now I see what you were getting at with “specialisation” … I didn’t realize you only meant that bit in the context of Neutral Theory. what you were saying now makes sense.

        I do wonder, though, how many “ecologically equivalent” species *really* coexist … or if we are just not good enough at figuring out their requirements/life history to see that they really differ in some crucial way.

  3. will.pearse says:

    Hopefully I’m replying on the right thread…

    I have no idea, Yael. I went to a talk where Nathan Kraft (quite convincingly, I think) argued that the only appropriate niche axis in a plant community involved four different functional traits measures and their interactions with one-another. I don’t really think we have a hope of quantifying most communities in that level of detail!

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