Niche syndromes, species extinction risks, and management under climate change

DF Sax, R Early, and J Bellamare. Trends in Ecology and Evolution 28(9): 517-523. doi:10.1016/j.tree.2013.05.010. Niche syndromes, species extinction risks, and management under climate change

Do you have a niche syndrome? You can get a cream for that you know... From Sax et al. (joke from Sarah!)

Do you have a niche syndrome? You can get a cream for that you know… From Sax et al. (joke from Sarah!)

Sarah Whitmee

Sarah Whitmee

I chose this paper before I knew that Lynsey and Will were to review a very similar one just two weeks earlier, coincidence or something more sinister? Actually, I don’t think either of those things, but is in fact due to a new phase in the field of species distribution modelling (SDM), one in which concepts are clarified and the assumptions of models are questioned and tested. Well that’s my hope anyway…. Nevertheless, it is becoming increasingly clear that a big assumption made by those practicing the dark art of SDM, namely a linear relationship between the realised niche (the area of environmental space actually occupied by a species) and the true environmental tolerance limits of a species does not exist, at least not for most species. This was illustrated in the Araujo et al study discussed here, where it was neatly demonstrated that lower thermal tolerances varied widely across species while upper tolerances were much more conserved. Happily, I think this study complements the earlier paper, phew!

The authors start in true TREE style with a nice overview of the current state of the field and the definitions of key terms. While often this is just restating the obvious it’s actually a pretty useful exercise for anyone working in SDM to go through, varying definitions of the fundamental, realized and potential niche plague the discipline and formally stating them for yourself can help later down the line when trying to interpret and understand models.

They then get down to the business end of the paper, the introduction of a new concept: the ‘tolerance niche’. The tolerance niche is defined as “the set of physical conditions and resources that allow individuals to live and grow, but preclude a species from establishing self-sustaining populations”. While I’m not convinced that the field of species distribution modelling needs more jargon I can see the usefulness of this concept in theory, specifically in relation to predicting the impacts of climate change on species persistence. The idea is that while a species cannot thrive in these tolerance zones they can persist temporarily in them, for example during dispersal to reach new suitable climates or to outlast temporary climate fluctuations. The introduced concept is then used to illustrate a number of niche syndromes, or ways we might want to think about species responses under climate change. They choose horticultural plants as a key group for illustrating how you could formulate hypotheses for these niche syndromes and also how you might work out a species tolerance niche, from evidence of specimens in botanic gardens outside a species native distribution.

Overall I like this paper, its clear and well thought through. I buy into the idea of the tolerance niche and think it’s a good step towards more dynamic species distribution models, rather than the time-slice approach employed in earlier analyses. Being a macroecologist I have a problem with this paper that I often encounter with the more fine scale SDM work. While the concept or model works well for a single species or a particularly well studied ecosystem (its no coincidence that a large proportion of SDM studies are all about the South African fynbos you know) these concepts fall down due to a lack of data when you try to scale up for multi-species predictions. The example species given in the paper works beautifully for the concept they are proposing but I wonder how many other species show such clear relationships. So I would have like to have seen more real world examples to convince me that the tolerance niche can truly be estimated. I know very little about plants (outside the ones in my garden) but I’m nervous about the idea of inferring tolerance simply from presence in a botanic garden outside the native distribution with a better understanding of how the plant is managed in situ for example it might be protected against winter frost or supplemented with food or key nutrients. I guess for species of key conservation concern such an approach might pay dividends.

I did like the idea of managed relocation for slow growing species, putting them in an area that they can tolerate in the short-term but which will eventually become climatically suitable for growth and reproduction. It’s a risky business though and I would want a much higher confidence in my climate models before attempting such a bold step.

To sum up the tolerance niche is a neat concept but how applicable it will be in the long term, I’m not sure. I’m a fan of these authors though so perhaps other will have a different view.

Will Pearse

Will Pearse

Like Sarah, I like these authors (and I know Lynsey does too!), so perhaps this is going to be a bit of a biased PEGE. I view the essential idea of this paper as defining the idea of a tolerance niche: conditions where a species can just-about survive, but can’t form a self-sustaining population. I buy it, and think it’s a nice idea and a great paper.

Indeed, I think similar ideas have been floating around in population biology for some time. We have source populations, that fire off propagules into the meta-community, and sink populations, where propagules arrive, individuals persist, but there’s a net loss of individuals and so that population can’t sustain itself. These concepts have really helped population biologists think about meta-community dynamics, and as we try to link macroecology more intimately with local-scale abundance changes and interactions, it seems sensible to conceptually link these concepts.

I wonder if we can go a step further, and stop treating different parts of the niche (fundamental, realised, tolerance, etc.) as discrete boundaries, and instead be up-front in acknowledging that we know species do better in certain parts of their niche than others, and maybe try to quantify that. Essentially, just have some value (why not fitness) that changes throughout niche space, and acknowledge that, in some parts of niche space, fitness will be so low that a population won’t be sustaining. Indeed, such an approach (borrowing heavily from Chesson) would let us handle realised vs. fundamental in a much more intuitive way, because we could distinguish between niche differences and fitness differences when trying to understand whether species can coexist. Because, as I’m sure we can all agree, parameterising a fitness function across niche space would be really, really tractable and easy to do :p

Hopefully the above makes sense; I have a very serious case of man flu!

Lynsey McInnes

Lynsey McInnes

This paper touches on plenty of topics that we have covered in different guises here at PEGE, with a perhaps more applied angle than most. In essence, the authors seem to be drawing attention to the potential importance of a kind of buffer zone of conditions that species could survive in beyond their current distributional limits (the tolerance niche) and how this zone might be extremely important in mitigating climate change induced species’ disasters.

The idea seems pretty reasonable and meshes well with experimental studies that find that translated populations can make it in conditions not found anywhere within their range. The authors also showcase a brilliant dataset of naturalised and garden centre/botanical garden populations of a plant species in the eastern US. The inclusion of this data lifts the paper from one based on loose concepts to one with real results; that was nice!

However, I do still worry how ‘useful’ the concept of a tolerance niche is going to be beyond these plant examples. In effect, the authors are creating an additional category of niche that in most cases is going to be quite difficult to identify? I would have liked to have seen one additional step to go the figure one and that would be to investigate traits that predict the extent and/or location of this tolerance niche. For instance, the authors draw attention to the distinction between short- and long-lived species, but are there any other distinguishing features? These could be along the lines perhaps of large range (probably also has a bit of tolerance niche), restricted range endemic (probably doesn’t), is part of a complicated food web (probably doesn’t have much of a tolerance niche), most kinds of generalist (probably do have tolerance niches). The next step, as the authors emphasise, is where is the tolerance niche located in relation to the realised niche and how easy is it to get to?

The above just gave me the nagging feeling that the tolerance niche concept might be quite difficult to implement as, like everything macro- it seems (I’m having a down on general patterns week), these things depend on so many other things: landscape structure, biotic interactions, the usual suspects. So, while the concept of the tolerance niche could provide US with a kind of buffer so that we can worry less about species’ survival (they can probably tolerate a bit more heat, drought, what have you) than they currently do, it seems like a difficult concept to draw strong or helpful conclusions from across broad taxonomic or spatial scales.

In conclusion, this was a well-written, thoughtful paper, but I am not convinced that the new concept and piece of jargon are robust or flexible (can something actually ever be robust and flexible?) enough to be rolled out very widely. As always, its a data problem…


About will.pearse
Ecology / evolutionary biologist

One Response to Niche syndromes, species extinction risks, and management under climate change

  1. Reblogged this on Dr Regan Early and commented:
    Here are some opinions on a recent article I published with Dov Sax and Jesse Bellemare. These bloggers have had the clever idea of setting up an online journal club – wonderful for all those who need a little stimulating discussion outside of their immediate research group. The blog was quite nice to us, but as you can see some doubts were raised about the everyday usefulness of the ‘tolerance niche’ that we identify, and methods that could be used to investigate it. Hopefully our ongoing research will demonstrate that this is a useful and mensurable concept.

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