Barnacles are much like trees

I am not a forest ecologist. OK, that’s not entirely true, as demonstrated by the strapline of this blog and the evidence on my research page. Nevertheless, having published papers on entomology, theoretical ecology and snail behaviour (that’s completely true), I’m not just a forest ecologist. Having now published a paper on barnacles, one could suspect that I’m having an identity crisis.

When a biologist is asked what they work on, the answer often depends on the audience. On the corridor that hosts my office, neighbouring colleagues might tell a generally-interested party that they work on spiders, snails, hoverflies or stickleback. Likewise, I usually tell people that I work on forests. When talking to a fellow ecologist, however, the answer is completely different, as it would be for every one of the colleagues mentioned above*.

If you walked up to me at a conference, or met me at a seminar, I would probably say that I work on spatial self-organisation in natural systems. If you were likely to be a mathematician or physicist** then I’d probably claim to study the emergent properties of spatially-structured systems. I might follow this up by saying that I’m mostly concerned with trees, but that would be a secondary point.

What I and all my colleagues have in common is that we are primarily interested in a question. The study organism is a means to an end. We might love the organism in question, rear them in our labs, grow them in our glasshouses, spend weeks catching or watching them in the field, learn the fine details of their taxonomy, or even collect them as a hobby… but in the end it is the fundamental question that drives our work. The general field of study always takes priority when describing your work to a fellow scientist.


Behold the high-tech equipment used to survey barnacles. This is the kind of methodology a forest ecologist can really get behind.

The work on barnacles was done by a brilliant undergraduate student, Beki Hooper, for her final-year project***. The starting point was the theory of spatial interactions among organisms most clearly set out by Iain Couzin in this paper****. His basic argument is that organisms often interact negatively at short distances: they compete for food, or territorial space, or just bump into one another. On the other hand, interactions at longer ranges are often positive: organisms are better protected against predators, able to communicate with one another, and can receive all the benefits of being in a herd. Individuals that get too close to one another will move apart, but isolated individuals will move closer to their nearest neighbour. At some distance the trade-off between these forces will result in the maximum benefit.

Iain’s paper was all about vertebrates, and his main interest has been in the formation of shoals of fish or herds of animals (including humans). I’m interested in sessile species, in other words those that don’t move. Can we apply the same principles? I would argue that we can, and in fact, I’ve already applied the same ideas to trees.

What about barnacles? They’re interesting organisms because, although they don’t move as adults, to some extent they get to choose where they settle. Their larvae drift in ocean currents until they reach a suitable rock surface to which they can cling. They then crawl around and decide whether they can find a good spot to fix themselves. It’s a commitment that lasts a lifetime; get it wrong, and that might not be a long life.

If you know one thing about barnacles, it’s probably that they have enormously long penises for their size. Many species, including acorn barnacles, require physical contact with another individual to reproduce. This places an immediate spatial constraint on their settlement behaviour. More than 2.5 cm from another individual and they can’t mate; this is potentially disastrous. Previous studies have focussed on settling rules based on this proximity principle. They will also benefit from protection from exposure or predators.  On the other hand, settle too close to another barnacle and you run the risk of being crushed, pushed off the rock, or having to compete for other resources.


Barnacles can be expected to interact negatively at short distances, but positively at slightly longer distances. This disparity in the ranges of interactions gives rise to the observed patterning of barnacles in nature.


What Beki found was that barnacles are most commonly found just beyond the point at which two barnacles would come into direct contact. They cluster as close as they possibly can, even to the point of touching, and even though this will have the side effect of restricting their growth.

Furthermore, Beki found that dead barnacles had more neighbours at that distance than would be expected by chance, and that particularly crowded patches had more dead barnacles in them. There is evidence that this pattern is structured by a trade-off between barnacles wanting to be close together, but not too close.


On the left, the pattern of barnacles in a 20 cm quadrat. On the right, the weighted probability of finding another barnacle at increasing distance from any individual. A random pattern would have a value of 1. This shows that at short distances (less than 0.30 cm) you’re very unlikely to find another barnacle, but the most frequent distance is 0.36 cm. Where it crosses the line at 1 is where the benefits of being close exceed the costs.

Hence the title of our paper: too close for comfort. Barnacles deliberately choose to settle near to neighbours, even though this carries risks of being crowded out. The pattern we found was exactly that which would be expected if Iain Couzin’s model of interaction zones were determining the choices made by barnacles.

When trees disperse their seeds, they don’t get to decide where they land, they just have to put up with it. The patterns we see in tree distributions therefore reflect the mortality that takes place as they grow and compete with one another. This is also likely to take place in barnacles, but the interesting difference lies in the early decision by the larvae about where they settle.

Where do we go from here? I’m now developing barnacles as an alternative to trees for studying self-organisation in nature. The main benefit is that their life cycles are much shorter than trees, which means we can track the dynamics year-by-year. For trees this might take lifetimes. We can also scrape barnacles off rocks and see how the patterns actually assemble in real time. Clearing patches of forests for ecological research is generally frowned upon. The next step, working with Maria Dornelas at St. Andrews, will be to look at what happens when you have more than one species of barnacle. Ultimately we’re hoping to test these models of how spatial interactions can allow species to coexist. Cool, right?

The final message though is that as an ecologist you are defined by the question you work on rather than the study organism. If barnacles turn out to be a better study system for experimental tests then I can learn from them, and ultimately they might teach me to understand my forests a little bit better.


* Respectively: Sara Goodacre studies the effects of long-range dispersal on population genetics; Angus Davison the genetic mechanisms underpinning snail chirality; Francis Gilbert the evolution of imperfect mimicry; Andrew MacColl works on host-parasite coevolution. I have awesome colleagues.

** I’ve just had an abstract accepted for a maths conference, which will be a first for me, and slightly terrifying. I’ve given talks in mathematics departments before but this is an entirely new experience.

*** Beki is now an MSc student on the Erasmus+ program in Evolutionary Biology (MEME). Look out for her name, she’s going to have a great research career. Although I suspect that it won’t involve barnacles again.

**** Iain and I once shared a department at Leeds, many years ago. He’s now at Princeton. I’m in the East Midlands. I’m not complaining…


1 thought on “Barnacles are much like trees

  1. Pingback: How to avoid the Allee effect, assuming that you’re a tree. Or a barnacle. | Trees In Space

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