The metacommunity concept: a framework for multi-scale community ecology

Leibold et al. (2004) Ecology Letters 7: 601-612. DOI:10.1111/j.1461-0248.2004.00608.x. The metacommunity concept: a framework for multi-scale community ecology

metacommunity_types

Naughtily, this is a diagram of (roughly) the same concepts as discussed in this paper but from Logue et al. (2011). NM: Neutral Model, PD: Patch Dynamics, ME: Mass Effect, and SS: Species-Sorting.


Will Pearse

Will Pearse

What surprised me most about this paper was how much of it I feel I have absorbed, and yet I can’t consciously recall reading it. It’s a classic in the field, and I think either influenced or consolidated a lot of what people thought about metacommunity structure. It’s a great paper, and if you can’t recall reading it I suggest you go ahead and do so.

I don’t want to dig up old ground, but I was pleased to read the authors making explicit claims about how different processes would be picked up depending on the evolutionary history of the system. It’s great to see an attempt at integrating fields (when was the last time you heard someone call Neutral Theory a metacommunity model?) that doesn’t just stop at the line of ecology. Last time we discussed whether species truly neutrally dispersed, and how dispersal traits can interact with traits that we consider in a classic ‘here’s my quadrat what’s growing in it’ ecology. Metacommunity dynamics open up a whole range of additional processes and evolutionary interactions that can be simulated and estimated using empirical data – although whether we actually do that is a different question.

The authors claiming not to have covered spatially explicit models got me thinking. When we say ‘spatially explicit’, we typically mean ‘each individual has an x,y(,z) co-ordinate which we model’, and these models can be very difficult to fit. I think the authors are right that we don’t always have to use such models to capture interesting dynamics – three levels of hierarchical spatial nesting are often enough for me! However, if we were to fit a spatially explicit model over a large enough area, with different habitat types and dispersal across the entire space (perhaps separating between long-distance and short-distance dispersal), we should essentially be able to replicate metacommunity dynamics. I don’t think I’m alone in saying that, while there is a metacommunity, there’s no real such thing a community – it’s just what individuals happen to be in the unit that we’ve defined at that point in time to be useful for us to study something of interest (here’s some Vellend). It’s communities all the way down, each capturing a different scale of interactions or species, and perhaps we would have a better chance of capturing such dynamics if we examined whether we can get meta-community-like behaviour emerging natural from spatially explicit models. In passing, for every person who emails/comments screaming about how communities are real, I will donate $1 to the ‘I made a sweeping statement sorry everyone’ fund.


Lynsey McInnes

Lynsey McInnes

Contrary to Will, I found this paper tough-going. Not because it was bad, uninteresting or poorly written, probably just because it was extremely dense. And my mind constantly kept wandering and wondering – was this really published 10 years ago? How have we moved on from here?

I’ve always had a soft spot for meta- type models while never knowing many of the details. But from my ill-informed sideline position, I don’t really feel like we have moved on much from this landmark paper. Have we? Correct me if I’m wrong.

So, that nagging feeling led me to wonder why we might not have moved on much? Is it a data availability thing? A model availability thing? A every collection of ‘communities’ is different thing? Or what? Ja, ja, it’s probably just a combination of all three and more.

So, where would I like to see things go? Well, unlike me, I think we need to spend more time working out what makes a metacommunity ‘real’ before we can really tackle how it fluctuates through space and time. Maybe a good place to focus would be working out what populations within a ‘community’ interact, how stable or transient these interactions are and then add in links to neighbouring communities and quantify how strongly connected they are. I say – use genetics! Use the genome. Let the populations tell you how they are related to each other. Fit admixture models. Fit migration models. See how congruent models are among populations. Sure, this perspective is limited to a distinct time band, it won’t work for really transient metacommunities, but it will work for established ones and could help identify which populations are stable within (meta)communities and which fluctuate in importance and could lead to more informed models for faster-turnover metacommunities. If we use genetics to let populations speak for themselves, we also won’t go wrong if we add another layer of complexity and incorporate trait variation. We might be considering six communities, each with an overlapping set of species, but spatially-distributed populations of the same species will not have the same trait complement. Recognise this! Quantify it! Find out how it happens and why it matters!

No doubt these models are already been fitted, but how much crosstalk is there between pure ecologists, metacommunity ecologists and population biologists on the one hand and geneticists on the other hand. Let’s integrate!

My big dream is for us to one day understand how diversity gets organised from the scale of individual interactions through community dynamics to shifting ranges and ultimately species’ turnover. We will not get there without more communication from the people best placed to understand the processes occurring at each scale. The metacommunity concept is a great place to start as it links individuals, populations, trophic interactions and communities. We just need to use the best data to make inferences about all of these.
*Apologies for the rushed, overly exclamation-marked rant… Metacommunities are a great concept, let’s see how far we can push them. (And apologies if all this integration has happened and just passed me by…).

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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!

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