Founder takes all: density-dependent processes structure biodiversity

Waters et al. 2013. Trends in Ecology and Evolution 28(2) 78-85. DOI:10.1016/j.tree.2012.08.024. Founder takes all: density-dependent processes structure biodiversity

Below, we give our first impressions of this article. Please comment below, or tweet Will or Lynsey (maybe use #pegejc). Think of this as a journal club discussion group!

Will Pearse

Will Pearse

This week’s paper is a whistle-stop tour of how diversity-dependence drives a lot of ecological patterns, and I found it pretty damn hard to disagree with anything they wrote. Essentially, the authors argue that once a species has established in a particular area, it stops other (similar) species invading by virtue of numerical superiority – density dependence drives everything.

I’m not a bacterial ecologist, so ‘microbial sectoring’ was entirely new to me. In it, bacteria spread through an agar plate and competitively exclude different genotypes as they do so, creating wedge-shaped patterns of genetic diversity once the colonies have matured. The authors (rightly, I think) view this as a sort of postglacial colonisation in miniature, and suggest that ecologically equivalent marine species are excluded through similar processes, even in the absence of dispersal limitation. However, I’m not sure I agree with their definition of ecologically equivalent; in most neutral models, ecologically equivalent species intermingle and successfully coexist because there’s no way to tell those species apart. I think density-dependent processes like allelopathy might drive these kinds of patterns, since without some kind of selectivity conspecifics wouldn’t be able to recruit either. There are (a number!) of counter-arguments to what I’ve just said, and I’m just pedantically splitting hairs since I’m invoking a different kind of density-dependence to explain these patterns!

I think invasive species are another exciting area where we can see differences among advancing species. There’re a few examples of invading populations that have different traits and genetic compositions to native populations, and it makes good intuitive sense that individuals able to survive dispersal by humans should be well-adapted to slightly different conditions to the rest of their source populations. Equally, individuals on the leading edge of an expanding range might be better-adapted to dispersal, or have higher reproductive rates to enable rapid colonisation, like human colonists expanding along a river in Quebec seemed to bring a number of genes for female fertility with them. However, before I get too teleological, the authors stress that sometimes genes are just piggybacking on advancing waves – they’re just allelic surfers.

Lynsey McInnes

Lynsey McInnes

Hm. This was a weird article! I started off thinking – wow, profound – and quickly segued into – wow, trivial? I’m basically not sure I got the point. I certainly don’t disagree that density-dependent processes are important and that they operate at a variety of temporal and spatial scales. But I don’t think I’ve gained any deeper understanding of general biological processes by having these across-scale processes highlighted to me.

The authors also skirt around the genetic underpinnings of the processes they talk about, making it unclear whether they are actually the same at microcosm to continental scales. I guess computer simulations have shown, e.g., how deleterious mutations can surf on an invasion front, and how newly-established populations (e.g., following postglacial reconstruction) can exhibit less genetic diversity than “older” populations.  I’m biased because I’ve just started thinking of the underlying genetics of macro-scale processes myself, but the paper did make me wonder what’s going on with the genetics of all these events.

So far, I’ve mostly thought of density-dependent processes in the context of cladogenesis (following the past couple of years flurry of publications reporting evidence of declines in diversification rates widely thought to be due to the operation of ecological limits/filling of niche space), i.e., the density- (or diversity-) dependence of cladogenesis. The authors touch on this idea, but don’t go into much detail. On the one hand, it’s perhaps a little off-topic, the authors seem most pre-occupied by density-dependent processes operating as a kind of barrier to the influx of further genetic diversity after an initial colonisation event, while, within the diversification/cladogenesis literature, density-dependence has largely been invoked in relation to something like a closed system where all members (lineages) are equally affected and the niche/range/genetic diversity of the initial colonizer would be similarly reduced to accommodate additional lineages. But perhaps this outcome is just one step further along from what the authors concern themselves with and so is relevant to the discussion. I.e., does the founder advantage (or our ability to detect it) drop off through time?

Data. I know this is a review, but it would have been great to see some kind of more or less formal meta-analysis of the existing data across scales to innumerate instances of founder takes all events (versus instances where there wasn’t evidence for this – stronger competitors arriving later? This must happen sometimes – surely?). Alternatively, some kind of ‘simple’ simulations exploring a broad parameter space to see when founder takes all is expected versus when it might break down (A REALLY strong competitor coming later? Some kinds of poor trait x environment combinations? Incredibly slow dispersal rates? A highly stochastic environment?)

One other thing, there is not much discussion on what makes a founder? Are there traits associated with being first in line? Are these shared/analogous across taxa?

But perhaps I’m trying to sketch out a set of companion papers, and I should be less demanding!

To end on a more positive note – first, if nothing else, the paper has put me a bit on edge – why is it bothering me so much? So, in the end, it has ticked the thought-provoking box. Second, I did appreciate the breadth of examples the authors drew upon (simulations, microcosms, terrestrial and marine ecosystems, human dispersal & human impact) and I will go away now and think some more on the connections they have highlighted.


About will.pearse
Ecology / evolutionary biologist

One Response to Founder takes all: density-dependent processes structure biodiversity

  1. yaelsnail says:

    I really liked this paper and found it very useful and directly related to the thinking I have been doing about dispersal, species distributions, and diversification … so I am amused that you had such a hard time with it Lyns! Perhaps this is the successor to our “does Area have any direct effects” debate?
    Anyway, I really liked the idea itself for its simplicity and yet its applicability to patterns/processes at many different scales – big kudos to the authors for pulling off one of those “that seems obvious” papers that is obvious only after reading it. And, though I crankily ask for more proof in a minute, I was impressed by the range of examples the authors pulled together that all seem to clearly illustrate the same idea, yet at very different spatial scales and in very different contexts.

    I only have two critiques to offer, and the first might be a key to Lynsey liking the paper more: I think it was a poor choice to summarize what they’re talking about under “density-dependent processes.” As you said Lynsey, it’s hard for us macroevolutionary people to hear this and not think about density-dependent diversification. I would have preferred talking about “founder-exclusion effects” or “latecomer-exclusion” or something like that, since I think the whole point was that founders arriving in an empty region can exclude latecomers … a very different situation/process than what happens in a community already containing multiple species/lineages/clades, and one that doesn’t really depend on density of other species/lineages so much as their binary presence/absence.
    I think there are still interesting analogies to be made to density-dependent diversification scenarios though. For one, this founder effect ties in to the idea of adaptive radiations triggered by the colonization of an empty niche or region – in which case only the first colonizer is expected to radiate, not subsequent ones. And going the other way, perhaps if we loosen the requirement to consider “ecologically equivalent” (whatever that means) species/lineages/etc, we should expect the rate of successful colonization to slow down as more species become established in a new region (this is already in the original island biogeography theory), just as we expect the diversification rate to slow down as the number of species in a niche/region increases. Hm! I like these back and forth analogies. fun.

    Second critique – while the idea of founders excluding later arrivals is intuitively pleasing to me and seems like a great explanation of all the patterns they show, I would have liked to have more support for this theory other than “it fits the pattern seen.” I was particularly unconvinced by some of the examples in the “Biogeographic shifts following human-mediated extinction,” which could alternatively be explained by humans bringing invasive species with them directly (although perhaps this possibility was ruled out in the original papers, I didn’t check). I know that sometimes this is the best we can do … but still! The big exception here is the microbial work, where it seems to me that simulations etc. by Hallatschek and his coauthors have already done a great job of demonstrating the causal link.

    In response to Will: I suspect that the level of “ecological equality” needed for this founder-exclusion type process to occur is much less than the perfect ecological equality of neutral models. It seems to me that species should only need to block each other at one step – for example larval-attachment sites for marine inverts/algae – for exclusion to occur, and otherwise species could be very different. Perhaps breeding-blocking could also be important … for example if wind-pollinated plant A colonizes an island and spreads around happily, and then closely-related wind-pollinated plant B colonizes the same island, it might be likely that there would be so much A pollen floating around that individuals of species B would not get enough B pollen to get fertilized. Maybe that’s too far-fetched though, and it’s all about ecology! (ps. thanks for the impetus to look up allelopathy on wikipedia. Learned some cool new plant facts!) (pps. nice one integrating that cool Quebec study into your comments)

    In response to Lynsey, re: genetics: I think this pattern/process is more about the colonization process underpinning genetic patterns than the other way around (hope that makes sense). So this idea is basically a possible cause of genetic patterns (I guess usually a spatial patchiness, where each haplotype/genotype/species has its own patch of turf, rather than mixing). Hope that applies to what you were saying …

    Also in response to Lynsey: I think the additional questions you raise are excellent, but ideas for future work as you say, since I don’t think the data for any of these questions exists yet, apart from some work on how variation in dispersal ability within species covaries with other traits (ie. behavior). The simulations you suggest sound especially cool and doable … well I am going to have a meeting with Oskar Hallatschek in a few weeks, I’ll try to remember to suggest them!

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