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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The latitudinal species richness gradient in New World woody angiosperms is consistent with the tropical conservatism hypothesis

Kerkhoff, Moriarty & Weiser  (2014) The latitudinal species richness gradient in New World woody angiosperms is consistent with the tropical conservatism hypothesis. PNAS 111:81258130

angiosperm_trait_transitions

Figure 4 from Kerkhoff et al. Diagram of ancestral–descendent transitions among different latitudinal zones


Lynsey McInnes

Lynsey McInnes

Yet another latitudinal diversity gradient/phylogenetic niche conservatism paper I hear you cry? Oh yes! I was deeply sceptical upon opening up this paper: more LDG/PNC, patchy datasets only including woody angiosperms, a dodgy family-level phylogeny. But somehow it almost won me over. If nothing else is was pretty brave.

The paper starts with a really clear introduction to the background to the question of what drives latitudinal diversity gradients, how the tropical conservatism hypothesis (lineages originate in the tropics and very few evolve the necessary adaptations to life outside the tropics, those that do then forming a temperate flora with tropical ancestry) is nested within other hypotheses such as ‘differential diversification rates’ or ‘out of the tropics.’ The paper goes on to summarise recent analyses finding evidence for or against tropical conservatism.

I also really appreciated that the authors were open as to the limitations of their dataset (patchy dataset, dodgy phylogeny, geographic restrictions). They conclude that these limitations do not prevent them concluding that they find robust evidence for tropical conservatism: most lineages are tropical, those that are temperate often have tropical ancestors. Done deal.

I’m trying really hard to distinguish between my hangups based on the dataset (e.g., is it ok to look at tropical-to-temperate transitions when the group you are looking at is not monophyletic (i.e., what about all the herbaceous lineages); is it ok to use an incomplete and family-level phylogeny (and only one at that)) and on the methodology (e.g., I did not understand their rerooting method for inferring ancestral states). I would have liked to have seen more extensive analyses that filled in the missing lineages or added ambiguity to their tropicality index and looked at any biases their restricted datasets might introduce. My gut feeling is that when we obtain the holy grail of ‘global, taxonomically comprehensive distributional, phylogenetic, climatic, and ecophysiological data resources ‘ (in the words of the authors), evidence for tropical conservatism will remain or will become stronger, but at the moment perhaps it is still presumptive to barge on pretending the data is adequate (at least without more comprehensive bias testing). Just so you know I also belong to the club of people that have barged on regardless. Maybe we all need to spend more time generating these better resources. Think of the questions we could ask when we get them.

Grumble, grumble.

Why is it so popular to investigate niche conservatism at the moment? Really, its a relatively new bandwagon which, if my rant above is anything to go by, is already ‘going out of fashion.’ What is going on? As the authors mention, the hypothesis has appeal because it integrates ecological and evolutionary drivers of diversity patterns and incorporates elements from a bunch of the most popular hypotheses for explanations behind the latitudinal diversity gradient. It’s delightfully vague in terms of what a niche is, falling back mostly on co-linear climatic factors and it can probably be adapted endlessly to include additional niche elements. All species have a niche, some aspects of which are likely to be conserved among closely-related species, the whole package is very satisfying.

Beyond getting complete data resources, a conclusive ‘yes’ to the tropical conservatism hypothesis also needs more sophisticated methods for ancestral state reconstruction. We get into a circular situation if we are looking for conservatism, but assumes minimal change across the phylogeny. Can fossils help? Can microevolutionary studies help? Can we ever really know?

Final note: extinction. What about extinction?


Will Pearse

Will Pearse

I’m not quite as jaded as Lynsey, but I admit I was somewhat skeptical about this paper; I’ve read so many diversity gradient papers that they’ve started merging in my head (like James Bond films). However, I liked this paper (like James Bond films): they bring their limitations out right away (of course it’s not a perfect dataset, they’re examining the world!) and by sticking to their hypotheses they are able to push the field forward.

Lynsey mentions the woody bias, but woodiness may not be as labile as I used to think. This either means that it’s not a ‘quick-fix’ trait that fluctuates to ‘allow’ species to move quickly (good for this study), or it means that it constrains species’ thermal tolerances and thus can affect long-term evolutionary dynamics (a bad thing for this study). Evolutionary biology is, to an extent, a historical science, and so whatever choices an investigator makes are always going to be pulled apart by someone being awkward and announcing that there was some other factor they didn’t take into account. Equally, there’s a real trend at the moment to be snarky about methods – my snarky comment is that attempting to reconstruct ancestral states over a tree of this size using either a Brownian motion or OU model is going to lead to problems. I’m not sure how much we can trust these ancestral states, but then again I’m not really sure how better they could have been done, because frankly I’m always concerned about ancestral state reconstruction.

However, I buy their hypothesis that younger lineages tend to be temperate. The question now is whether this is because of the general cooling of the Earth over the last 34 million years (as the authors seem to think), or whether it’s because it takes ~34 million years for clades to die out once they move out of the tropics. Paleo-ecologists have been asking questions like this for some time now, but perhaps now that they have a specific time-frame within which to look they have more hope of finding an answer. Fingers crossed!

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