Climatic niche shifts between species native and naturalized ranges raise concern for ecological forecasts during invasions and climate change

Early & Sax (in press). Global Ecology and Biogeography DOI: 10.1111/geb.12208. Climatic niche shifts between species native and naturalized ranges raise concern for ecological forecasts during invasions and climate change


Come to America – fame, fortune, and the chance for a fresh niche! Figure 3 from Regan & Sax.

Lynsey McInnes

Lynsey Bunnefeld

I like this paper a lot. It plays to all my new found interests in, what I like to call, intelligent macroecology. It takes a small subset of species (50 or so plants that are native to Europe and naturalised in the United States) and conducts a suite of well thought out analyses on them in order to ascertain if there are any general patterns in niche shifts following facilitated expansion of the native range. Sure, not all European plants naturalised in the US have been looked at, but that doesn’t matter one bit.

Early & Sax draw heavily on comparisons with another recent study by Petitpierre et al. who showed that multiple large range agricultural weed species show little evidence for niche shift following range expansion. In contrast, Early & Sax find plenty of evidence for major changes in niche position and niche breadth (alongside evidence of species with little or no niche shift).  They argue that Petitpierre’s dataset is unlikely to be representative of the majority of species which are range-restricted and have little history of human-assisted movement into a broad niche space. So, while this study does not refute their findings, it does expand them to provide a more nuanced picture of potential outcomes.

I am as guilty as the next person of glibly stating that range limits are mostly climatic at the macro scale and that, although biotic interactions probably play a role, its 100% fine to base conclusions and indeed policy on the idea that the niche that species are currently realising would be similar anywhere on the globe. This paper and a suite of others are rapidly kicking down this argument. Small scale transplant experiments and other comparative datasets are indicating time and again that a species’ realised niche is delimited by much more than just climate.

This is, of course, a problem for the massive field of predicting species’ responses to climate change.  Species are commonly expected to move, adapt or perish in the face of an altered climate regime. In fact, they might also be able to tap into an ability to occupy a wider or different niche without evolving new adaptations. Conversely, if other species ‘get there first’ they might lose niche space to a novel competitor.

These findings are both really interesting from a how do macro niches work in principle perspective as well as a what on earth is going to happen in the very near future perspective. I think they argue for an ecosystem or community (however you might choose to define either of those concepts) wide perspective. If biotic interactions or historical contingencies or even landscape barriers are really influencing the niche space that many species are currently occupying, we have no hope of predicting how species will respond to climate change (because don’t get me wrong, climate still has a major influence on occupiable niche) if we don’t also consider how species are influenced by the actions of others. Sounds simple, but it probably isn’t.

I don’t know much about network analysis at all, but it seems like this is the way forward (see last week’s post too). At the most broadest scale, one could look for the primary species with which the focal species interacts within its range, one can also focus on similar species found surrounding the focal one’s range that might be playing a similar role and thus excluding the focal one from range expansion. I think it is crucial to think more broadly than phylogenetically-close relatives but also look for functionally-similar species. And of course add in trait variation of all involved parties through space and time. Of course.

In conclusion, a great paper that adds to the growing voice among macroecologists that climate alone just won’t cut it. Not even for just understanding how spatial diversity patterns come about, let alone for conservation of these patterns into the future.

Will Pearse

Will Pearse

This paper deserves some attention. Using a quite amazing dataset, the authors (Regan has posted on PEGE!) looked at the native and introduced ranges of plants. They found that species’ introduced ranges often extend beyond the conditions of their native range (which they term niche shift).

Isolating when species distribution models fail because they don’t account for non-stationary processes like dispersal is a really, really important thing to do. Dispersal ability is the problem that a lot of people bring up time and time again – interestingly, dispersal ability had no correlation with species’ niche shift. However, time since introduction did, and the explanation given for that (time for humans to spread the species) is essentially dispersal limitation. I think another thing at work here is release from biotic controls – species evolve in a regional community, and when they get into an area with a radically different biotic community (…when they’re spread around more by us…) biotic limitations are relaxed and they can tolerate more novel environmental conditions. I think I just re-regurgitated some Ricklefs, but with maybe less of an emphasis on pathogens. The most range-restricted species seemed to show the greatest increase; assuming this isn’t some sort of artefact (lesser range –> more likely to detect increase) then I think these species should be more limited by species interactions.

Which brings me to what I think is the part of the paper most likely to annoy people – that we might have over-estimated species range change under climate change if we assume everything is niche limited. I think we almost certainly have, but I would caution that range expansion in a completely different continent is different from range change at home. Leaving aside the arguments above about species co-evolving, under climate change the entire community is being stressed, whereas in an introduction/invasion the new species is both the invader and the sole novel stressor. Moreover, there is a lot of variation in these results,: I find it quite harrowing that while the authors were able to explain some of the variation in niche shift they couldn’t explain it all. Put frankly, we still don’t know what’s going to happen to species’ ranges under climate change, and (to me) that’s terrifying.


Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory

JP Grime. The American Naturalist 111(982): 1169-1194. Evidence for the existence of three primary strategies in plants and its relevance to ecological and evolutionary theory

I find your lack of competitive ability disturbing. Grime's CSR triangle (main), with his estimates of where trees (top-left) and annual herbs (top-right) might lie within it.    Taken from figures 2 and 3 from Grime (1977).

I find your lack of competitive ability disturbing. Grime’s CSR triangle (main), with his estimates of where trees (top-left) and annual herbs (top-right) might lie within it. Taken from figures 2 and 3 from Grime (1977).

Will Pearse

Will Pearse

I’m a zoologist who somehow keeps looking at plants, and this paper is probably the best demonstration of how plant and animal ecologists really do seem to think differently (in my mind, at least). Grime has a section about animals and fungi at the end, but all of this is clearly written by a plant person – and is all the better for that.

There’s a growing feeling in plant trait ecology that plant traits can be grouped together along some economic spectrum (reflecting pay-back of nutritional investment), be that of leaves or wood. Recently, Peter Reich has argued that all of these can be considered as part of the same system, where plants are either fast-adapted or slow-adapted. I don’t want to point out connections between these ideas and Grime’s CSR strategies, but instead draw attention to how plant functional trait studies have somewhat lost their way. Grime makes it quite clear that position along each of his spectra depends on plant traits, but that there’s no one-to-one matching of trait onto CSR strategy. I feel as thought his general message has been lost among papers pointing out how different species have different adaptation to drought, and how SLA or xylem diameter has changed, as if those particular traits defined those species’ niche dimensions. Convergence means that there are many ways to skin a cat, and there are many ways to be ruderal – individual traits that we have decided to measure do not define species.

I fee Grime implies that there’s very little difference between being in a nutrient-poor or stressful environment based on the species around you or because the environment is inherently that way. He argues an area can be nutrient-poor because all of your neighbours have soaked up all the nutrients and won’t let go. If (as he argues) some species have evolved to maximise nutrient retention, not rate of absorption, this becomes doubly important. I’m constantly referring to abiotic and biotic drivers, and on second thought there’s really very little reason to distinguish between them if they have the same effect – shade is shade, no matter what the cause.

At times I felt like Grime was implying that much of plant traits come from the environment they’re exposed to – I’m not sure things are always as plastic as some would believe, but I certainly agree plants can change. Most importantly, we shouldn’t be teleological in assuming that observed shifts in species’ traits reflect the thing we’re interested in – what may seem like an adaptation to drought may just be a consequence of altered nutrient turnover rates, and without some kind of breeding experiment I don’t think we could ever disentangle the two.

Lynsey McInnes

Lynsey McInnes

I let Will pick the paper this week, forgetting that he was trying to teach himself about plants. He might be a zoologist learning about plants now and again, but I’m just a bad mathematician/pattern seeker masquerading as a biologist. I found this paper tough going, mostly because I haven’t really thought all that much about the ins and outs of actual individual plants making it in their stressful /disturbed/competitive environments. So, I learnt a lot while reading this and would certainly recommend the paper to budding ecologists interested in a framework for classifying their different plant species based on how they might do in different environments.

Once I got to the end of the paper, I realised it, in fact, did fit into my pattern seeking world view (Grime was trying to squeeze plants into a triangle with three distinct lifestyles at its vertices, after all). But only if you are willing to really work for your patterns. Counting species is easy, but disentangling how many of each kind of species is already one step harder. Especially when, of course, the three strategies form a triangle rather than three distinct boxes.

I am typing as I think here, but I just wonder how macroecology, and its resulting insights, would change if we stopped counting species and really tried to count function. Sure, there have been macroecological studies of functional diversity, genetic diversity, body size, but all still very much tied to species counts. I wonder if we will ever let go of the notion of a species as this magic unit. But perhaps it is a magic unit and function/type/etc. still ultimately harks back to species.

I really don’t know if broad-scale analyses need finer divisions than species counts, but, as Will states above, macro people obsess over the relative importance of biotic and abiotic drivers of diversity patterns and thinking in terms of Grime’s classification, or some other similarly nuanced one,might actually help us spend work out what the biotic drivers might be or rather how they might work.

Ok, this is a real ramble now. In short, I appreciated all that I learnt in this paper and it underlined that we macro people have to get more smart in what we measure if we really mean it when we say we want to know how diversity is ‘generated & maintained’ (& turns over).

The ecological dynamics of clade diversification and community assembly

Mark McPeek. The American Naturalist 172: E270-E284. The ecological dynamics of clade diversification and community assembly

Blue damselfly; McPeek has studied damselflies extensively. Taken by Umberto Salvagnin (via Wikimedia).

Blue damselfly; McPeek has studied damselflies extensively. Taken by Umberto Salvagnin (via Wikimedia).

Lynsey McInnes

Lynsey McInnes

I picked this paper because I remember reading it when it came out and being taken in by McPeek’s approach. It felt like he had built up a pile of phylogenies with negative gammas (a metric mired in controversy, but that’s for another post) and then he decided he should find out whether more or less feasible processes of ecological or non-ecological divergence generate similar distributions of gamma values.

A short background note: Pybus & Harvey’s gamma purports to detect ‘slowdowns’ in diversification by quantifying the pattern of nodes in a reconstructed phylogeny. In brief, if nodes cluster near the root of the tree, gamma is negative and this indicates that diversification has slowed as we approach the present. Positive gammas could suggest that diversification has sped up as we approach the present, but inference is complicated because of the signal left by extinction, such that a positive gamma does not unambiguously support a particular pattern. Since publication of McPeek’s paper there has been a swath of further papers documenting problems with the metric, probably the no.1 being that you get one gamma value per tree so the results will be dependent on what scope of clade you have chosen as to whether you can detect a clean slowdown or not. Again, this is not a post about the ins and outs of gamma.

Rolling with the assumption that gamma can provide a useful summary of the distribution of nodes in your tree, it can be used, as here, to quantify the different branch lengths attainable under different diversification scenarios. Regular readers will know I have a soft spot for simulation studies, so could anticipate that I liked McPeek’s setup here. Basically, he wanted to find out whether if, he enforced ecological divergence upon speciation, trees are produced that show signs of slowdowns in diversification (as ecological gradients are filled in). And this was indeed what he found. Some might argue that you get out what you put in, and presumably this is true to some extent. But I still found it a neat and tidy finding.

Now its time to go off on a tangent. Don’t get me wrong, I like this paper and appreciate this kind of study. I really believe that many clades are probably subject to slowdowns as they diversify and this might often be because they have filled some limited set of ecological niches and additional species are formed by geographic isolation without ecological divergence or some other mechanism like sexual selection or filling the niche of a species that has gone extinct. I just wonder how easy or indeed, possible, it is to detect the signal of this diversification trajectory on phylogenies. Perhaps the pattern emergent at the ‘evolutionary’ timescale is rough enough that slowdowns or equilibrial diversity dynamics are detectable and ‘real’ and the more haphazard activities occurring at ecological timescales will always be evened out and undetectable and this is OK. I’m also intrigued by methods that work ‘the other way’ and take massive phylogenies and try to delimit more restricted clades that somehow obey these slowdown patterns. It often seems like taxonomists’ brains have worked in similar ways and genera and families conform to these delimitations. Which is actually pretty cool.

In short, I am mesmerised by broad-scale patterns in phylogenies and often buy into the current trend of assigning ecological explanations to them. In some sense, ecological and evolutionary processes are all part of one continuum so must impact each other, but how often have we made ourselves believe that something more than just random spliting processes are at play. I’ve currently turned my back on macro-scale analyses, but will always have a soft spot for finding out how these crazy, very clearly real, patterns are generated and why.

Will Pearse

Will Pearse

This paper is the scientific equivalent of Samuel L  Jackson in Snakes on a Plane; an old-school “I just sat down and thought” kind of paper, and I like it. It’s probably the best example of how a simulation study can be insightful, starting with some fundamental observations from empirical data, and making a model just complicated enough to derive insight.

McPeek’s reviewer raises a very good point: what does it mean for the fossil record if his simulations shows we get species appearing and then rapidly disappearing? This reminds me strongly of raceme phylogenies, although I think it’s an open question whether these short-lived species would ever be so abundant as to swamp out the main ‘trunk’ of the tree of life. That said, most people would agree speciation is rarely instantaneous, and the shape of the simulated phylogenies would probably be different if a species only became a true species after a delay (à la protracted speciation), even if those proto-species were still ecologically different from one-another.  It makes me wonder the extent to which the diversity on Earth right now is comprised of these side-shoots, and how many of these species (ignore our influence for a moment) will be around in a few million years.

I also enjoyed the discussion of variation in structure among clades. There is no reason to assume that models of evolution are constant across a clade simply because, in the present, a researcher has decided that a particular group forms coherent ecological assemblages. As always, I’m convinced that different ecological processes should be operating in clades that have undergone different kinds of evolutionary processes, but drawing a distinction between ecology and evolution is somewhat arbitrary. The power of McPeek’s approach is that there is no disconnect between the two: in his models, evolution is just ecology over-and-over-again, and examining the two simultaneously must ultimately be the best approach.

There is one caveat to this. Many (and McPeek notes this) are quick to point out that we may never be able to estimate extinction and speciation rates using phylogenies. There are very, very few cases where the fossil record is as rich as we would like, and a molecular phylogeny necessarily misses extinct species. While modelling exercises like this are fantastically useful, I am somewhat skeptical that we can ever reliably fit models this complex to real data; of course that doesn’t mean we shouldn’t try!

Linking Life-History Traits, Ecology, and Niche Breadth Evolution in North American Eriogonoids (Polygonaceae)

Pink (Japanese) knotweed Persicaria capitata from Wikipedia

Lynsey McInnes

Lynsey McInnes

Another micro/macro choice from me. Surprise!

I liked this paper. I thought it was a thoughtful attempt to address a specific question that is bypassed by many a macroevolutionary biologist. The authors want to tease apart whether they can explain patterns across species in niche breadth and rate of niche breadth evolution by looking below the species level (given that niche breadth is really not a good ‘species-level’ trait).

Now, bridging the micro to macroevolutionary divide might be expected to require multiple data points per species to get a measure of intraspecific variation. The authors sidestep this by using measures of climatic tolerance to characterise niche breadth, reasoning that evidence for local adaptation in plants is common and a broad niche necessarily comes about through intraspecific variation. I kind of agree, but do wonder what other ways intraspecific variability could be/might need to be quantified.

They find that perennials have higher rates of niche breadth evolution while annuals have higher rates of niche position evolution. In essence, they put this down to perennials inhabiting more variable, colder and higher environments and annuals specialising in more stable environments. Perennial species thus do better with a broad niche as there are always some adaptations floating around the species to deal with the different conditions thrown on them. Annuals on the other hand have evolutionary speed on their side to pop out a new species when needs be (so that across species niche position change is more rapid). The authors concede that some of their reasoning is (very thoughtful) conjecture although the story they build up is a convincing one.

The authors state themselves that to understand what mechanisms underlie these variations across life-history strategies needs more evidence. One place to start would be looking for confirmation of local adaptation across populations of each species and to find out the genetic mechanisms underlying this. Another would be to fill out the phylogeny to see if sampling gaps are biasing the story. From the phylogeny it looks like perenniality is pretty clustered on the tree, how clustered? And what is missing? Do the independent origins of perenniality have the same kind of niche breadth? What are the problems with using this kind of niche modelling approach? How much broader a niche could these species persist in if they had the opportunity?

I wonder what other systems this type of approach would be good for? Is variance in climate experienced within the range a good measure of intraspecific variability? It obviously captures something but how correlated is it with range size?  Have we just shifted the question of how life history strategies affect range size to how do they affect a convoluted measure of range size? This might be a good thing, but it might also be dangerous if niche breadth/climatic tolerance is not the main driver of range size variation (saying that, it probably often is).

A lot to think about.

Will Pearse

Will Pearse

This is an interesting paper; the methods are very through, and I enjoyed the focus on the evolution of intraspecific variation.

I like the idea that annuals, by hiding in the seed bank, can avoid adverse environmental conditions and this have a narrower niche breadth. The pedant in me is tempted to argue that this dependence on the seed bank will require specific adaptations and an extension of the niche, but I don’t think that’s tremendously important. If true, this hypothesis suggests that climatically stable regions (the tropics) should show less annual vs. perennial variation. I also wonder whether an annual spending longer underground has fewer effective generations and so a reduced capacity for evolution.

The intrinsic dimensionality of plant traits and its relevance to community assembly

Daniel Laughlin. Journal of Ecology (early view). DOI: 10.1111/1365-2745.12187. The intrinsic dimensionality of plant traits and its relevance to community assembly

Plant traits, plant traits, everywhere, and plenty of root traits with which to absorb water. From Laughlin's paper.

Plant traits, plant traits, everywhere, and plenty of root traits with which to absorb water. From Laughlin’s paper.

Will Pearse

Will Pearse

I think I should read more essays, because I really enjoyed this one. I’ve been searching for good recovering-zoologist-friendly summaries of the plant trait literature and this is one. The methods were also nice, and it’s cool to see how machine learning is really finding its place as a useful tool and not just something to scare reviewers with. I’m going to talk a little bit about plant traits in general (which I’m not qualified to do), and then have a brief digression on dimensionality (which I’m also not qualified to do).

Gathering plant trait data is hard, and while Laughlin isn’t the first to point out that our view of the botanical world is biased by what we can easily gather data on, he does it very eloquently. I was at the TRY working group meeting a few months ago, and I was very struck by how hard everyone is scrambling to fill in the gaps in our trait knowledge. We know remarkably little about plant roots, and I can’t help but wonder if knowing more might get people thinking a little more about facilitation. I’ve heard all kinds of weird things about facilitation (whatever it is, I’m not certain it’s sufficient to just call it “negative competition”), and I feel the roots, and the mycorrhiza they harbour, are the key to understanding it. I’m a great fan of the leaf economic spectrum (even if the details are sparking some debate), and given the work on above-ground vs. below-ground allocation of biomass I think it’s only a short time until we start seeing root-above-ground economic spectra.

I agree entirely that we can and should reduce the dimensionality of the plant trait literature; TRY has 682 different traits when I last checked and I fail to believe they’re all completely independent. We know that niche convergence need not be convergence of traits, and finding the principle axes of variation should help us better understand the evolution of trait trade-offs. This many niche axes are also sufficient to allow co-existence of large numbers of species because there’s a lot of niche space to split among many species; this is the complex (and often chaotic) situation that allowed us to ‘solve’ the Paradox of the Plankton. Thinking in terms of a smaller number of niche axes does make things more tractable, and I do happen to think that Laughlin is right, but there is a sting in the tail of even a ‘simple’ six dimensional description of plants. If we (conservatively) assume there are only six niche axes and that species can either have low, medium, or high values along that trait axis, we have 729 (3^6) different kinds of species – or 2187 species with the 7 axes indicated on the figure at the top of this post! Discretising continuous variation like this also has its problems, but I still have a feeling that plant traits are going to remain complex for some time yet!

Lynsey McInnes

Lynsey McInnes

I wanted to read this paper this week simply because I was curious what the intrinsic dimensionality of plant traits was…I didn’t really have a feeling whether it would 3 traits or 10 or what those traits would be. So, I learnt quite a bit through reading this essay!

It looks like intrinsic dimensionality is about 5ish depending on what dataset (i.e., what traits) you look at. The general (n = 3 anyway) conclusion is you need a trait from each of the major bits of a plant (leaf, stem, root, etc.) and that within major organs, traits are correlated, bordering on redundant. This all sounds reasonable, but I’m pleased to see it documented here.

Of course, it would have been great if n could have been bigger to explore the wrapper relationship of the effect on dimensionality of the traits and species included in the tests. Presumably you might get an extra dimension if you ramp up either no. of traits or no. of species, but this could probably be anticipated and dealt with.

I know it wasn’t the subject of the essay, but how easy is it to measure these traits? The author mentions the lack of root measurements as these are (obviously) harder to measure than leaf traits for example. Is it realistic to suggest targetting them? Are there any non-root traits that could cover them (if I remember correctly, I think not really). What is the relationship of dimensionality with richness? Could species numbers cover for trait diversity? (Maybe both have asymptotic effects?). I’ve no real idea if that is a reasonable suggestion or not, I am a big fan of the idea that trait, or functional, diversity captures ecosystem processes better than species identity, but I’ve not actually thought before whether there is a simple relationship between the two (wouldn’t that be nice?).

The author alludes often to this exercise being useful for community assembly studies, especially in this time of rapid environmental change. Although (I think) we still don’t really know how stable communities are through time (transient mixes of species that happen to come together in one place vs. tightly co-evolved units), understanding how the mix of traits changes in a community through time could help reveal the answer (contingent on a ton of data being available). For instance, if just a small number of trait dimensions needs to be studied, perhaps one could extract how their trait values change through time (or through space). Does a community retain the same trait values over time for ‘optimal’ functioning? Or is a lot of change apparent? Is there turnover of species identity, but retention of trait diversity? Are there datasets available to use for these types of studies?

Exciting times.

Climate envelope modelling reveals intraspecific relationships among flowering phenology, niche breadth and potential range size in Arabidopsis thaliana

Banta et al. Ecology Letters 15(8): 769-777. Climate envelope modelling reveals intraspecific relationships among flowering phenology, niche breadth and potential range size in Arabidopsis thaliana

Easy to forget that Arabidopsis thaliana doesn't just live in laboratories! Via Wikimedia

Easy to forget that Arabidopsis thaliana doesn’t just live in laboratories! Via Wikimedia

Lynsey McInnes

Lynsey McInnes

You guessed it, this was another of my choices. I found it a while back and was intrigued over the approach the authors would take. In brief, the paper looks at whether genotypes underlying (or rather, linked to) flowering time generate significantly different niche models (and by extensions niches) and thus can they (the authors) provide evidence that there are intraspecific differences in niche and niche breadth. A. thaliana is a great species to use to ask this question, it is an exceedingly well-known model organism and has a wide distribution.

The conclusions of the paper are far from surprising, with such a wide distribution and with known flowering time variation, it was inevitable that the authors would find evidence of intraspecific variation in niche dimensions. Nevertheless, this is really important to show! There is a whole market devoted to niche models and projections of range change due to climate change that almost exclusively treat species as a single entity. Oops. This is clearly too simplistic for species beyond a certain range size (and yes people kind of know this already). Papers like this (and the Razgour et al. paper we covered earlier) demonstrate this well. Phew.

Now here comes the issue. This paper was only possible because it builds on a ton of A. thaliana research. For instance, genetic variation in flowering time was already known and the underlying loci already characterised. The authors state themselves that defining coherent populations that might be expected to have significantly different niches to each other is really difficult (they probably could not have emphasised this enough!). Populations are rarely closed entities. They circumvent this problem here by going straight for a crude single locus genotype definition. How would you make your population buckets without this additional data? Can you? Is movement among populations (with their specific local adaptations) something that might save chunks of a species range? Probably.

If niche models don’t die a death completely, their next incarnation is going to be models which incorporate not only intraspecific variation, but also connectivity among these chunks (this is going to have to involve dispersal among spatially discrete chunks and degree of genetic exchange among co-occurring genotypes). For such models to be successful and have ‘conservation relevance’ a lot more crosstalk is going to be needed among (macro)ecologists and landscape geneticists/phylogeographers (yay, that’s me!).

This paper is a great start and I look forward to seeing developments in the field that enable it to be useful for non-model organisms (perhaps with no genetics), using multi-locus genotypes, integrating additional ecological traits, adding depth to our understanding of how populations interact to make up a range and ultimately, one day, far into the future, what a realised niche is relative to a fundamental one?

Will Pearse

Will Pearse

A lot is said about model systems in evolution and ecology, and I think papers like this, where model systems for which we have a lot of information are used to answer questions in related fields, are great.

I wear my love of phylogeny and niche conservatism on my sleeve, but that doesn’t mean I don’t appreciate a good demonstration of intraspecific variation when I see one. Flowering time should be variable in a species that spans so much of Europe, because it’s exposed to such different environmental conditions. Whether ability to adapt to novel conditions, or the general constraints that mean the species has to vary its flowering time, are as variable within the species is a slightly different question. I’ve been spending some time thinking about the inheritance of species’ potential for intraspecific variation, and I’m not entirely sure I can come up with a bullet-proof way of deciding what should, or shouldn’t, show such variation. I’d be interested to know if you can!

When I saw figure 4, which shows that earlier-flowering genotypes have larger potential ranges, I was very happily sold. I’m often somewhat nervous when I read a paper involving niche modelling or species distribution models, simply because the things seem so damn hard to get right. Therefore that the authors found any kind of relationship (albeit one with some scatter) is extremely impressive, and indicates they’ve really found something cool. The extremely tight relationship between niche breadth and potential range size is less surprising to me, both from existing theory (that the paper cites), and from the definition of niche breadth itself. A species has a higher niche breadth if “different habitats are equally suitable” (p. 772) based on the MaxEnt suitability scores for each cell, and the potential range is also defined from the MaxEnt predictions which, presumably, incorporate habitat suitability. Thus I’m not so surprised that the relationship between these two things are so strong, because I think they’re related to one-another; that doesn’t make the conclusions any less valid, but it did leave me hoping (as the authors themselves mention) that someone will be able to quantify observed range occupancy for these genotypes. If I’ve missed something obvious about the above, please do let me know – I’m no niche modeller for sure!

Mycorrhizas in the Central European flora: relationships with plant life history traits and ecology

Stefan Hempel et al., 2013. Ecology 94(6): 1389-1399. DOI:10.1890/12-1700.1. Mycorrhizas in the Central European flora: relationships with plant life history traits and ecology

I'm reliably informed there are some mycorrhizae in this photo...

I’m reliably informed there are some mycorrhizae in this photo…

Will Pearse

Aaron David

One of the overarching goals in ecology is to understand the distributions of species and how interacting species shape these distributions. In the world of plant-symbiont interactions, one ongoing question is when is it advantageous to have a symbiont? Hempel et al. compile a dataset of Central European plants and their mycorrhizal associations in order to address several ecological hypotheses. Hempel et al. classify plants as obligate (OM), facultative (FM), or non-mycorrhizal (NM), and ask whether certain types of plants are more associated with various life-history traits (ie. life-form, life-span, pollination, etc.) or environments. The authors find support that different mycorrhizal statuses are over/underrepresented in with different traits and environments. The paper provides strong evidence that mycorrhizal associations may influence plant distributions, and that the benefit of such associations is very much environment dependent. Those such patterns have been shown for individual plant species, Hempel et al. show the generality of this pattern using a large set of plant species.

The authors tackle the exciting question of when it is advantageous to form mycorrhizal associations. For instance, they report that OM plants are found in higher than expected abundance with low soil acidity, while FM plants are found in higher abundance with high soil acidity (NM abundance wasn’t affected by acidity). This result was somewhat puzzling to me, as I would have expected the NM plants to be found with the high soil acidity and the FM to be unaffected. This could suggest that while some plants are able to adjust their associations in different environments, this may not be a general rule for FM plants. The authors define FM plants as those that can form a mycorrhizal association but are not always found with one. Therefore it’s possible that the FM plants as a whole might be composed of NM plants with the occasional mycorrhizal association. It would be interesting to see the FM group split into more definitive categories.

As a fungal ecologist, I found this paper provided interesting insights towards questions of fungal distributions. One of the burgeoning areas in fungal ecology is understanding the distributions of mycorrhizal associations (see the work of Peter Kennedy). Hempel et al.’s results suggest to me that these limitations to mycorrhizae distributions could arise from local environmental conditions or host plant distributions. Of course as the author’s note, it’s not necessarily clear which symbiont is limiting the other. One way to test this idea would be to overlay maps of mycorrhizae distributions with those of plant distributions. Environmental sampling of soil can be used to get a broader picture of mycorrhizae distributions, since many may live as saprobes in addition to being symbionts, though it’s likely the authors could generate a similar distribution map using their available data. Understanding both sides of symbiont distribution would more fully address how species are limited.

Will Pearse

Will Pearse

I’m no mycologist (for what it’s worth I love eating them), but I still thought this was an interesting paper. Literature reviews like this, where massive databases that are going to be of use to future scientists are just thrown out for all to enjoy, are exactly what science should be about. Hurrah.

I think the conclusions of this paper are sound, but I’m going to draw attention to two things just to be picky. Firstly, I have been brought up to think that the phylogenetic corrections employed here should be avoided (listen to Rob Freckleton please). This is a very touchy subject (I’ve heard of people bursting into tears over it!), but in brief the authors create eigenvectors that represent the phylogeny, and then by including them in their analyses hope to correct for phylogeny. A similar approach is used in spatial analyses, but for both it’s hard to know how many eigenvectors is ‘sufficient’, and it’s always unclear to me why you wouldn’t just other methods that directly incorporate the phylogenetic variance-covariance matrix you’re making eigenvectors to describe. Phew. Got that off my chest. Secondly, I wonder what effect the publication bias (that the authors find) will have on these results, particularly as the results are in agreement with what we might expect.

However, as I say, I think the results are pretty sound, and so I wonder whether we could model the co-evolution of plant and fungi. Indeed, there are some very neat new methods (we covered one recently) that examine these questions. More specifically, I wonder if the evolution of a tight association with mycorrhizae would allow a clade to break away from its close relatives and suddenly radiate out into as-yet unexplored habitats and niches. Equally, there could be links between mycorrhizal diversity and plant associations, although I’m almost certain this has already been looked at, and defining fungal species is hard (I think!). I’d quite like to hear from more fungus people!

Phylogenetic niche conservatism: what are the underlying evolutionary and ecological causes?

Michael Crisp and Lyn Cook, 2012. New Phytologist 196(3): 681-694. DOI:10.1111/j.1469-8137.2012.04298.x. Phylogenetic niche conservatism: what are the underlying evolutionary and ecological causes?

The multi-coloured world of phylogenetic niche conservatism (from Crisp and Cook)

The multi-coloured world of phylogenetic niche conservatism (from Crisp and Cook)

Jan Schnitzler

Jan Schnitzler

Much has been written about phylogenetic niche conservatism (PNC) over the past few years (e.g. Revell 2008, Losos 2008, Cooper et al. 2010, Wiens et al. 2010), so one might wonder what another review can add? Given that PNC still seems to be both a ‘hot’ topic, but also one of considerable disagreement, a conceptual paper might ideally help to clarify some open questions and suggest directions that research should take in the future. In my opinion, this is exactly what Crisp and Cook have done here.

Starting with the more general part, the paper provides a nice discussion of PNC, how different researchers have defined it, and how it compares to ‘niche conservatism’ and ‘phylogenetic signal’ (and continuing the discussion of whether it is a pattern or process – I admit that I tend to agree with Crisp and Cook here…). I get the impression that there is still quite a bit of uncertainty (understandably) regarding the use of these concepts in the scientific literature, so I believe this is a very good overview.

In the next part of the paper, they highlight a number of key processes and discuss how these may lead to PNC. One that caught my attention in particular was extinction, which that could lead to a pattern of PNC as an artefact. Even if the evolution of a niche-related trait is not constrained in the first place, higher extinction rates in a particular state (rainforest vs. scleophyll biomes in their example, but it could of course also be a continuous trait like body size) may result in a pattern of PNC. I think indirect processes (like extinction) have not really received much attention in the past. Also, this is a reminder again that molecular phylogenies of extant species might give a somewhat biased picture of the evolutionary history of a clade. The growing number of ‘total-evidence’ phylogenies will hopefully contribute to an improved (unbiased) understanding of trait evolution.

Another interesting section highlights the different tests that could be used to evaluate the degree of PNC. Blomberg’s K and Pagel’s λ are well known and widely used tests for phylogenetic signal, but as other studies have shown before, Crisp and Cook point out that the relationship between phylogenetic signal and phylogenetic niche conservatism is not always straightforward (especially if evolutionary dynamics diverge from the simple Brownian motion model).

Towards the end, the authors bring up some intriguing challenges for studying patterns of PNC. For example, we need to consider that transition rates between traits might be unequal, and that different traits might be linked to differential rates of speciation and/or extinction. Finally, I did like the outlook on the possibilities that incorporating genomics offers (yes, everything nowadays is done using genomics). If we do get a better understanding of how genomic processes influence phenotypic evolution, we will be a lot closer to understanding why some specific niche-related traits are conserved in some groups, but not in others.

In summary, I really enjoyed the paper, in particular the focus on identifying the underlying processes rather than just documenting the patterns of PNC. However, given the uncertainty about the best way to quantify PNC and the potentially confounding effects of different processes, I wonder how close we really are to achieving this.

Will Pearse

Will Pearse

Crisp and Cook have written a very thorough review of what can cause different levels of phylogenetic niche conservatism (PNC), and I find it hard to think of anything they haven’t covered. So, seeing as how I work on eco-phylogenetics and am always being accused of blindly accepting PNC without giving it any thought, I’m going to play devil’s advocate and try to argue that PNC isn’t that interesting, in the hope that someone will take issue with everything below and put me in my place!

The authors go to some pains to point out that PNC is both a pattern and a process, because while some processes generate PNC (and thus it is a pattern), PNC itself generates other patterns (and thus it is a process). I don’t like this argument; increased algae in a pond is caused by putting fertiliser in that pond (and thus it is a pattern), but increased algae has implications for other species in the pond (and thus it is a process). Making predictions using algae is probably fine, but if we want to understand the system we should model the cause of algae population levels – the fertiliser. In the same way, to understand the patterns generated by the PNC, I think it makes more sense to skip the middle-man and model the process that generated the PNC itself. Perhaps the only situation in which you would care only about observed PNC is when inferring something about the present-day ecology of those species, when past evolutionary dynamics matter only in the sense that they affect species today. However, in such cases why not just use the trait data used to derive PNC and cut out the phylogenetic middle-man (regular readers know I’ve been repeating this idea like a worn-out record).

To my mind, PNC is useful to evolutionary biologists in exactly the same way that diversity measures are useful to ecologists. Diversity measures are something we can measure about a system, and help us understand the mechanisms driving that system. The authors describe how PNC has helped us understand Darwin’s ‘abominable mystery’ (the sudden radiation of the angiosperms), but in reality it is only by making models to explain PNC that we have understood it. That’s not to say that measuring PNC is not important, but understanding the origin of what we have measured is also key!

Lynsey McInnes

Lynsey McInnes

Phylogenetic niche conservatism has come up in a bunch of our posts so far, and I’m glad Jan chose this paper this week so we could tackle PNC head on. I really enjoyed reading this paper, I thought it was a well-written, balanced, but still clearly an opinionated piece that does make a useful contribution to the already overflowing literature on PNC. I thought the authors managed to cut through a lot of the confusion and controversy, but still did not sit on the fence regarding their own stance. They unreservedly come down on the side of PNC is a pattern caused by a set of processes, and the interest lies in determining what these processes are and how they do or do not generate PNC. I also appreciated their repeated emphasis that the most fruitful avenue of research is a relative approach (e.g., is this niche-related trait more conserved than this one?) rather than an absolute one.

The authors also emphasise that niche conservatism is intimately related to spatial patterns of diversity and community assembly. I feel that it is often overlooked that niches, more or less, are inherently spatial entities (this is probably debatable but most papers that purport to have looked at PNC so far are looking at conservatism in traits that have a spatial dimension like maximum climate found within a species’ range, rather than the physiological traits that actually mediate an organism being able to cope with such a temperature). Until it is easier to measure physiological traits across broad sets of taxa, these spatial proxies for niche-related traits will remain popular (and useful) so (I think) its good to explicitly realise their geographic dimension.

Clearly, you can’t cover everything in a single article, but I was surprised by some omissions/elements that were skimmed over. First, what is a niche? This was restricted to boxed text and I think the paper could have been stronger with a lengthier introduction into what a niche is, especially to get straight a definition that has relevance across clades. But perhaps this discussion has been done to death, so it was fine to keep it short and sweet. I also wonder what the authors’ views are on the difference between phylogenetic niche conservatism and niche conservatism (without the phylogenetic bit). Is there a difference? Does the concept only have meaning in the context of a phylogeny? I’m really not sure.

The authors were quite concerned with temporal scale, and the idea that some niche traits are conserved over very very long timescales and broad swaths of taxa (all angiosperms for example). There was less focus on spatial scale. I do wonder if PNC might also be interesting to study at very limited spatial scales…we often talk about tropical niche conservatism and the inability of tropical lineages to colonise temperate latitudes. But what about within tropical or temperate latitudes? There are quite some niches in both – how are they divided/shared among lineages? Are the processes that determine PNC patterns at these scales the same as those are broader spatial scales?

The authors do highlight, as did Jan, that the advent of genomic datasets might be helpful in this regard. What genes/mutations/phenotypes/selection pressures/genetic backgrounds are responsible for the patterns that we see? How does the genetic basis differ depending on the process that produces the pattern? Perhaps the only way we are going to clear up the confusion and controversy surrounding PNC is to get down to the genetic basis of the ACTUAL traits that produce these patterns? Perhaps not…?

And, I have to say it, I am really interested in the insights we might gain from looking at niche conservatism below the species level. Niche conservatism is often looked at in traits emergent at the species’ level (e.g., mean temperature across the species’ range). What can we learn if we look at geographical variation in temperatures found within in the range? Are populations within the range located adapted to temperatures they are exposed to? This is directly related to the recent paper we discussed on cosmopolitan taxa – how do they get to be/stay cosmopolitan? But probably also has relevance for species with even moderate range sizes. How does niche variation/conservatism within a species relate to conservatism among species?

I concede that this has become a bit of a ramble on thoughts in my head about PNC in general rather than related to the paper itself. Sorry about that. But thank you to paper for provoking all these, perhaps tangential, things to think about. I do wonder quite why the study of PNC has taken off in quite the way it has. It’s related to data availability for sure and bandwagons, is there anything else? The authors note that the concept was already thought about by Tansley, I wonder where its next steps are?

Is regional species diversity bounded or unbounded?

Howard V. Cornell, 2013. Biological Reviews 88(1): 140-165. DOI:10.1111/j.1469-185X.2012.00245.x. Is regional species diversity bounded or unbounded?

This post is PEGE’s contribution to the first PEGE/EvoBio journal clubs crossover. Add your comments to the bottom of this post and then come and join us with the guys over at EB-JC ( next Monday (May 13th, 4:30p ET) to discuss things further over video chat.

EB-JC works a bit differently from us so we are keen to join forces and see what happens.

Is speciation rate bounded, unbounded, or... sort of both?

Is speciation rate bounded, unbounded, or… sort of both? (from Cornell 2013)

Will Pearse

Will Pearse

This paper is a brave attempt to reconcile the debate over what limits diversity through evolutionary time. Cornell’s ‘damped increase hypothesis’ is a compromise between a constant (unbounded) rate of speciation and one where clades have a (bounded) carrying capacity determined by ecological interactions. He acknowledges that diversity tends to increase through time, but this increase can be tempered by ecology; those looking for fireworks should look elsewhere (go read ‘that’ Wiens paper), because this is an attempt to reconcile and move forward.

It’s easy(ish) to derive models of evolutionary diversification, the problem is finding reliable data to validate them. You can’t sequence what isn’t here, so molecular phylogenies can’t incorporate extinct species, while the fossil record has biased sampling and makes it difficult to distinguish among species and higher taxonomic groups. Moreover, distinguishing between a clade becoming more diverse because of increased speciation or decreased extinction would be hard even with perfect data. Despite a number of really cool methods (I like these), I sometimes worry we may never be able to sort this mess out.

Much of this paper hinges on whether niche space ever gets filled. If species can fill up niche space, then it’s reasonable to expect that competitive effects would limit diversification, and Cornell’s first conclusion is that we need more experimental tests of whether these kinds of competitive effects exist. That’s not to say we need more research on competition – we have decades of that in ecology – but we need more explicit tests in extant clades that biogeographers are examining. We can’t turn back the clock, but we can validate the assumptions of our models in the present where we’re not data-limited. Now that is eco-evolutionary modelling!

Cornell spends a good deal of time discussing the importance of migration on clade dynamics, and species moving into an area and occupying niches is a rather thorny problem. However, I’m just not sure we’re ever going to be able to handle this; after decades of work, we’re only just beginning to understand how we can model species’ distributions in the present day, and attempting to do that using only biased fossil distributions for all of evolutionary time sounds like an incredibly tall order. Perhaps one thing we could do is look for correlated extinctions/speciations in the fossil record. Imagine a new species has just evolved that occupies a new kind of niche; if we assume it spreads essentially instantaneously in evolutionary time, any slow-down in other clades’ diversification rates should be immediately detectable.

Lynsey McInnes

Lynsey McInnes

This paper has been in my to read pile since I spotted it sometime last year. It probably remained there because it’s long, dense and comprehensive and takes more than a commuter train ride to get through. So when Rafael Maia over at Evo Bio journal club challenged Will and I to find a super cool paper on the geography of speciation, we decided to go with this one as it more or less covers ALL super cool papers on the geography of speciation ever published. So, thanks Howard Cornell for summarized this research field for us and providing us with plenty of food for thought for our first EB/PEGE journal club crossover.

For those of you who know me/have read my posts here at the site, you can guess that I chose this one and that it appeals to my interest in understanding the spatial nature of diversification. First, a note about the style of the paper. I really enjoyed reading it, and managed the whole thing in a single sitting and I think that is down to Cornell’s extremely clear writing style and his total command of the subject he is writing about. Wow. I was also impressed with his ability to navigate through a field that has become quite contentious in recent years; he managed to extract relevant points from a suite of Rabosky & Wiens papers without explicitly acknowledging the animosity apparent between these two camps. In fact, he elegantly concludes that regional diversity is, in effect, both bounded and unbounded and puts forward the damped increase hypothesis to cover this inclusive idea. Hoorah.

Enough rambling, what has Cornell left us to talk about?

First, I really liked his repeated emphasis on taxonomic scale. By this, he meant the conclusions we reach on the diversification of a clade will dependent on the breadth of diversity found within the clade (i.e., a subclade of mammals, a clade restricted to a single region, a clade found in a region with ecologically-similar species from different clades, a clade that has newly-colonised an area, and vice versa). I think this is an underappreciated point within macroevolution and one that deserves more explicit treatment in the future. If we are using a phylogeny to understand diversification of a group, we want to use a well-defined monophyletic clade, maybe we even want to compare two or more than two monophyletic clades. This is fine and admirable, but when doing so, we have to remember that the clade does not exist in isolation (ecologically-(dis)similar species probably occupy the same area, the clade probably included additional taxa that have now gone extinct, the size and nature of the area it currently occupies have probably changed substantially since the origin of the clade, and so and so on). If we really, really want to understand how clades diversify, we have to try really, really hard to account for at least some of the above. At the very least we have to circumscribe what we are trying to understand (How an area became populated? How a clade diversified? How extrinsic/intrinsic traits shape diversity/diversification?)

Years ago, I remember Joaquin Hortal – community ecologist extraordinaire – scoff in the face of the idea of ecological limits to diversity. His reasoning was that he knew of no system that was totally full, there was always scope to squeeze in new species if only the new species could be transported to the area in question. I kinda believed him, but somehow decided that he was talking at some kind of narrower scale than me, that at the continental scale or global scale of entire clade (what is an entire clade?!?!), limits could be reached. I think Cornell’s paper is the first I have read that both implicitly recognises and reconciles these multiple scales that I had in mind. I do wonder if we will ever work out a way to identify the optimal spatial and taxonomic scale to look at how diversity is generated or even if such an optimum exists?

To conclude a bit of a rambley post, I really appreciated this paper. It summarized the weight of evidence for two hypotheses that had somehow recently become quite polarized in the literature and emerged with a happier medium incorporating the best bits of both. Cornell also sets out his vision for making progress in this field and this involves taking a more thoughtful approach to improving our understanding of regional diversity patterns: collect more and better data, incorporate multiple types of data in any analysis, analyse it properly, and perhaps most importantly think about the SCALE of your analysis before you set out and as you interpret your results. Onwards and upwards!

Niche incumbency, dispersal limitation and climate shape geographical distributions in a species-rich island adaptive radiation

Algar, A.C. et al., 2012. Global Ecology and Biogeography 22(4): 391-402. DOI:10.1111/geb.12003. Niche incumbency, dispersal limitation and climate shape geographical distributions in a species-rich island adaptive radiation

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

I saw this work presented at Imperial College London, and I remember being impressed with Algar’s honesty. I can’t remember when I last saw someone actively drawing attention to the potential pitfalls in an analysis, and it  made me trust him all the more. This paper is a neat, self-contained look at what controls Anolis distributions, and concludes (quite rightly) that a number of factors play a role. I really like the approach they take, and I think it has potential to answer a lot of ecological and evolutionary questions.

The setup of niche incumbency as whether species tend to co-exist with things that are similar to them is nice and general, and I like the creation of these ‘morphological landscapes’ of how similar species are to one another. I think that’s a great way to visualise things across a landscape, and fits in quite nicely with previous posts about using phylogenetic similarity to improve the fit of species distribution models. Perhaps the only (likely unfair) criticism I’d make is the use of phylogenetically-constrained measures of species’ traits to make this landscape. We constrain according to phylogeny when we’re trying to ask questions about evolution; I think these processes of incumbency play out in ecological time, and so I don’t think it matters if species are similar because of niche conservatism. Put another way, I don’t care how or why species are similar, I care whether they are similar, and so I don’t see the need to control for phylogeny. Following on from this, I would be interested to ask what aspects of niche we can ascribe to the traits they measure, what aspects we can ascribe to phylogenetic similarity, and then what aspects are non-overlapping between the two. This would help us understand whether phylogeny is capturing something different to the traits they used – sort of the phylogenetic middleman problem turned around the other way.

I wonder what the effect of different spatial scales of action are on these results. Plotting maps like those in figure 2 implies that we can link these processess at the same spatial scale; lizards compete with one-another on smaller scales than climate variables, and this might complicate matters. Equally, small-scale environmental variation would mess around with this even further, although I’m not sure how I would model that (anyone?). If we really think blue-sky, perhaps agent-based modeling could be used to get at this sort of thing, although given current computer constraints we’d probably end up being limited to single species-pair interactions, and one of the great strengths of this paper is that it isn’t so limited.

Lynsey McInnes

Lynsey McInnes

Algar et al. generate measures of morphological similarity of co-occurring congeners and of dispersal cost to determine whether niche incumbency (i.e., a morphological (and thus ecologically) similar species is already there) or dispersal limitation (i.e, some factor prevents a species reaching ecologically suitable habitat) contribute in determining distribution patterns of Anolis lizards in Hispaniola or whether they are totally determined by climate (or by nothing in particular (in their null model)). They find some signal strength for niche incumbency and dispersal limitation, although measures of climate are still overwhelmingly the most explanatory factors.

I liked this paper a lot. It felt like the authors had spent a great deal of effort thinking about how best to quantify some notoriously tricky factors in a bid to unpick what really underlies the distribution of a diverse group in a restricted area. They also admirably do not harp on about the stacks of papers that generate climate-only SDMs with a biotic interaction/dispersal caveat stuck in at the end (I include my own papers here!).

I’ve been wondering for ages how to go about getting a better understanding of how the ranges of members of a clade are determined in a certain area (in my head I populate a 100×100 grid with a single species in a single cell and see the lineage diversify into x number of species each with abutting ranges vying for occupancy – in a heavy-handed dismissal of climate, I over-emphasise the role of biotic interactions perhaps). I imagine you can get quite far with (much better developed) simulations of range dynamics along the lines of Pigot et al. But this paper represents an impressive attempt to get at that type of question with empirical data (sure, Hispaniolan anoles are probably the system with the most comprehensive data ever – but why not start high?). I do wonder what other systems this approach could be used in (maybe the non-adaptive radiation of salamanders where expectations might be different if many species are ecologically equivalent? A bigger effect of phylogeny perhaps?). Amassing all the necessary data is always going to be a problem when you try and look at more explanatory factors, right?

This setup might also be useful to bring more clarity to the ‘ecological limits’ explanation for the prevalence of slowdowns in diversification. The idea has taken off in recent years and is extremely intuitively appealing, but the actual mechanisms by which a large clade in a large area (whatever large might mean) actually experience ecological limits remains, perhaps surprisingly, unclear. Do members of the clade prevent further diversification through their occupation of all available niches? What determines the niche breadth of the constituent species? Does niche breadth of the constituent species change as a clade becomes more diverse? Aspects of these questions have been tackled before, but, not to my knowledge, all in one go. Perhaps these questions are relevant to a broader time-scale than what is the focus of this paper, but they run along the same lines.

The result – that there is often a significant effect of niche incumbency and dispersal limitation, but climate still matters is not at all surprising, but it is nice to see it quantified. I wonder at the usefulness of their null model – all non-null models were so obviously going to be better – perhaps comparing climate-only versus climate + extras would have been sufficient? But maybe this is a more philosophical question on the relevance of null models.

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