Do communities exist? Complex patterns of overlapping marine species distributions

Leaper et al. 2014 Do communities exist? Complex patterns of overlapping marine species distributions. Ecology 95:2016–2025

Jan_van_Kessel_004

Benthic flatfish and benthopelagic cod on a shore – Jan van Kessel senior, 1626–1679 (Or: what you get when you google ‘demersal fish australia’)


Will Pearse

Will Pearse

I picked this paper because I felt it was a more stats-y/empirical-y way at getting at some old debates in ecology. The methods seem both very complex and very simple at the same time; if you’re interested, skim the Dunstan paper(s) they reference to get a handle on what’s going on. In essence, they’re simultaneously binning species intoarchetypes (groups) and figuring out what predicts where those archetypes are. It’s a binned species distribution model. Speaking as someone who tried (and failed) to do this, I can assure you it’s a hard thing they’ve done a good job of 😀

The authors find archetypes, but do a good job of stressing that this doesn’t mean they’re present all together in the same place all the time. For me, this variation is almost the most interesting thing, because it could reveal competition. Under the archetype approach, things become simpler because you’ve restricted yourself to the species you most expect to be closely-interacting, and so hard-to-find processes should be easier to detect. I’ve spent a lot of time bleating on about how competition can take place within the context of habitat filtering, and how I think phylogeny is a great way to get at this – I think this kind of approach is even better. There should be different traits that determine species ‘ distances from the centroid of their archetype than those that determine how distant each archetype is, because these two things should reflect fundamentally different processes. Goodness only knows what space you’d calculate these distances in, though!

There may be a further temporal and spatial component to these communities. Fish don’t exactly stay still, and a diver conducting a transect is likely to encounter a number of ‘swim-by’ species that are hovering around the area. For species to interact they have to spatially and temporally overlap, and I wonder to what extent all of these species were doing that. I don’t mean this as a criticism: by treating these species as an archetype, we can drill down and explore what each species is doing. How many predators are in a particular archetype, and how many prey species? How frequently were two predators within an archetype seen at the same time (–> whether they compete for prey). Could the method be extended to deal with fuzzy archetypes, such that commonly-encountered predators could affect multiple archetypes? Why is it that, for the whole of this article, I kept wanting to type “habitat type” instead of archetype? Is there a difference? I’m quite excited to find out!


Lynsey McInnes

Lynsey McInnes

This was a funny paper on the back of a suite of PEGE posts commenting on rather grand visions of the future of various subdisciplines of ecology. I think it might have done us good to remember that applying swanky new concepts and frameworks to actual datasets in the first instance involves a lot of work collecting a lot of data and then a degree of frustration interpreting results that don’t fall neatly in one neat quarter of some kind of hypervolume.

So, kudos to Leaper et al. for a well-executed study apparently conducted by people that know their study system well alongside people developing new methodologies. The authors are interested in the nature of communities, whether they are discrete and easily circumscribed or more fluid, with species composition merging into one another across neighbouring communities.

They use the idea of a species ‘archetype’ to get at these questions, this being the quantification of similarity in species’ responses to environment. I.e. rather than seeing which species co-occur in space, the method clusters species based on results of species distribution modelling (i.e. this set of species likes it hot and wet, and so on).

I am on the fence as to whether this idea is a good one. On the one hand I like to cluster and to generalise, on the other I wonder about circularity and the proliferation of too elaborate data transformations. Let alone all the dodginess that comes with SDMs.

The authors find that although they can identify different clusters of species, these species don’t correspond to distinct spatial entities (‘communities’) suggesting that communities, at least of demersal fish or macroinvertebrates off the southern coast of Australia and northern coast of Tasmania, are not highly structured units. Are we surprised?

I think that I, ever the macroecologist, would have liked to have gone one step further and investigated whether there are traits associated with archetype strength (probably things like dispersal ability, philopatry, niche breadth, species richness, the usual) and by extension how ‘closed’ communities generally are. To do this properly, you would of course need a broader dataset spanning a wider range of communities and species. And you get back to the mismatch between burgeoning numbers of frameworks and a limited amount of appropriate data.

The authors note that communities appear to be made up of a selection of species from multiple archetypes that all match bits of the environment found at a site, but that environmentally similar sites are likely to have a different set of species (albeit chosen from the same archetypes). I think this is a neat, but also intuitive, observation that there are assembly rules, that all bases need to be covered, but exact species can be subbed in and out. I like the idea of substitutable species and wonder how true it is in terms of functional redundancy. My bet is species are not really substitutable, but wonder how much variation in this ‘trait’ there might be.

In short, I think we still have a long way before we have a good grasp of how communities are structured and how that structuring changes through time and across space. My hunch is that careful studies such as this one will be important in building enough real world examples of how things work, whether or not the idea of archetypes is crucial or superfluous to the debate.

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One Response to Do communities exist? Complex patterns of overlapping marine species distributions

  1. Do communities exist? This is a fantastic ecological question and, perhaps the greatest philosophical question our science faces in ‘new-age’ ecology. This is also precisely the question the Edwin James Society has addressed via its research programs since 2009. We have developed and implemented an innovative and revolutionary array of statistical approaches to address this question on the basis of quantum and Newtonian physics. We published a seminal manuscript on the topic in 2013: The Theory of Quantum Microbiogeography: Mechanisms of the Priority Site Determination.

    Soon, we shall release its companion publication: Life Under the Big Top: Quantum & Classical Mechanics of the Ecosphere.

    We conclude communities do in fact exist, but behave in ways that are independent and unique from classical population biology. Our president, Dr. David Sabaj-Stahl, has established the fundamental, philosophical principles we believe that will enable ecologists to at long last examine nature within quantitative community contexts. You can learn more at ejsociety.org.

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