Macroecology: Does it ignore or can it encourage further ecological syntheses based on spatially local experimental manipulations?

Macroecology: Does it ignore or can it encourage further ecological syntheses based on spatially local experimental manipulations?

"Macroecology... is not a fruitful path for thoe of us seeking to understand how ecosystems are sutrcutred and function" - Paine (2010). Image from seaotters.com

Macroecology… is not a fruitful path for those of us seeking to understand how ecosystems are structured and function” – Paine (2010). Image from seaotters.com


Will Pearse

Will Pearse

This paper made  a real impression on me; I also think it wins the ‘most polite knock-down of a field’ award, in that the first two paragraphs make it clear he doesn’t like the field but somehow he does it so politely I want to read on.

Paine is right; marine ecosystems are different, and since they’re the largest part of our planet we shouldn’t be so terrestrially-focused. I feel that much of what a terrestrial ecologist means by ‘history’ is really ‘dispersal limitation’; at evolutionary timescales vicariance and/or distance leads to speciation where we wouldn’t otherwise expect it, and in ecology it leads to species with identical environmental tolerances not co-existing. Thus when Paine identifies marine ecosystems as having huge dispersal distances (particularly in the larval stages), he’s essentially pointing us towards how terrestrial and marine ecologists are always going to view history differently. If everything truly can be everywhere, then historical contingency is a very different thing in water.

Paine is also right when he says that a species list is no end-goal for a field. Yet, for all the importance of local-scale contingency (Paine did invent the keystone species concept!), it’s worth remembering that species distribution models (SDMs) do actually work (some/most of the time). That’s not to say that they always work – trophic cascades and other biotic interactions are always mentioned but seldom modelled in SDMs – but if everything can be everywhere in water, can that change how they should be modelled? Can trophic cascades be captured by understanding the environmental conditions that enable those keystone species to survive there?

I think Paine is missing an important trick when he discusses comparative experiments. He wants comparative experiments on taxonomically similar species to identify underlying rules, and macroecology is the search for those same principles that determine a system’s rules of engagement. We study the species pool because, particularly on land, that determines what smaller-scale communities look like; maybe fundamental differences in dispersal mean the kind of macroecology Paine discusses is more appropriate for terrestrial systems. I’m always harping on about the importance of changing the phylogenetic, spatial, and temporal scales at which we examine an assemblage to help us better understand ecology. Perhaps terrestrial ecology has shifted too far in favour of laptop-jockeys like me who re-analyse datasets, and maybe we do need more local-scale experiments where we can test ecological mechanisms. Yet if broad brush-strokes without detail will never help us understand mechanism, detailed work without a context will never help us predict.


Lynsey McInnes

Lynsey McInnes

This paper is great. I think my fellow commuters were perplexed by what I was reading that was making me smile so much. As a macroecologist, I’d never heard of this man Paine until this paper came out in Am Nat a couple of years ago. But Paine is no macroecologist, so perhaps that’s OK. In fact, he is a vocal marine microecologist (I think) and is attempting in this address to argue the dual points that ‘micro’ ecology has a lot of insights to give that are impossible from macroecological techniques (fair enough, really) and then a slightly weirder, this might be my fault because I know more about terrestrial than marine systems, but not really that much about either, that macroecology is better suited to terrestrial than marine systems?

Well, we picked a nothing if not provocative paper to relaunch PEGE with.

Paine’s address is packed with home truths about macroecological approaches, but regularly (at least to me) jumps off the deep end into ridiculous. Macroecology (‘the study of relationships between organisms and their environment at large spatial scales to characterise and explain statistical patterns of abundance, distribution and diversity’) is often berated for being overly concerned with documenting pattern without a thought to processing generating patterns, and I think this criticism does stand true. However, there is a. plenty of work that tries much harder to understand process and b. more fundamentally (and this is where my opinion differs from Paine’s) I think there are processes operating at these broad scales that are of interest to identify and understand (a factor that probably contributes to so many people attempting to document the patterns these processes produce).

I’d argue that an across scales approach is the most valuable. Are local scale happenings relevant to emergent broader scale patterns? Are they divergent? How do ecological responses at one trophic level affect others? How do responses on evolutionary timescales affect things? What governs turnover through time and across space? What allows invasion from one site into another? And so on. Sure, documenting yet another latitudinal diversity gradient, or an exception to it, doesn’t get us much further in any endeavour, but a comparative analysis of food web structure across continents does.

I am not sure I followed Paine’s terrestrial vs. marine arguments, but I think the crux of it might have been that ‘local’ or ‘micro’ in marine systems is already way broader than in terrestrial systems where people can more easily summarise whatever they wish in 100km grid cells (oh the horror) and go to town with their pattern documentation. I would argue that both systems have interesting and important ‘micro’ and ‘macro’ ecology and that perhaps the easier route to deeper understanding of ecology or ecological responses comes from an objective comparison of the two (like here and here).

Lastly, Paine laments the ‘niche craze era.’ What a great phrase. And so true. I, and many others, jumped on the – somewhat specialised – niche conservatism bandwagon and went a little crazy. We documented crude patterns using deficient taxonomies and didn’t get very far in working out what drives change in some dimensions’ of a species niche and not others. This was perhaps because we didn’t know what a niche was or how to quantify as we embarked on this endeavour or perhaps because we didn’t care. No doubt about it, understanding species’ roles in ecosystems is more vital than quantifying variation along some orthogonalised niche axis, but once robust methods are devised (and I don’t think they have been yet) to quantify ‘role’ (and I mean on a ‘micro’ or ‘macro’ scale), I imagine they will simply be some axis of the (again not properly defined yet) elusive niche. I see these advances as an exciting challenge for the pretty near future than a reason for contention.

I recommend this paper to all brands of ecologist. It helped me realise I did still find macroecology and the insights it seeks to identify interesting and important, while feeling chasistised that macroecologists can sometimes, in fact often, be lax in defining relevant and appropriate research goals.

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Mammal predator and prey species richness are strongly linked at macroscales

Sandom et al.. Ecology 94: 1112-1122. DOI:10.1890/12-1342.1. Mammal predator and prey species richness are strongly linked at macroscales

mammal_map

Mammals, mammals, everywhere, where there’s plenty of prey to eat. Taken from Sandom et al


Lynsey McInnes

Lynsey McInnes

Will picked this paper out of a choice of five I gave him as he wanted to diversify away from community phylogenetics. For me, I liked the safe return to pure macroecology and I liked the similarity in approach to a recent paper I published looking at whether global diversity patterns of vertebrates reflect those of monocots? (Blatant self-promotion). I also used structural equation modelling in that paper (requested by a reviewer, I will admit) but remain dubious over their power to extract relative strengths of direct and indirect effects (more on this in a second).So, Sandom et al. look to see whether there is evidence of top down or bottom up effects of predator richness on prey richness (and vice versa) using mammals as their study taxon. They find a significant effect of prey richness on predators but little evidence for an effect in the other direction. This is a nice result and I was quite surprised by the strength of the signal. Like Jetz et al.’s, my own paper and a few others, at these macro-scales there has only been limited evidence for biotic effects on diversity patterns – collinear diversity apparently being mostly the result of similar responses to environmental gradients.  And therein lies the problem – when can we ever know we have included the right variables in our model to cover the myriad environmental effects such that the variable ‘prey richness’ is not just filling into for some omitted environmental variable? This is macrocological madness strike 2 (strike 1 was when ‘latitude’ always came out as a significant predictor of richness gradients as it stood in for some immeasurable combination of abiotic variables). OK, while there was no mechanistic explanation for ‘latitude’ explaining richness patterns and there are strong mechanistic explanations for an expected association among trophic levels, I am still dubious…I have another couple of methodological worries, although I totally admit I don’t see any easy fix for them. My first worry is that there are so few predator species (125) vs. prey (3966) meaning that grid cell richness has a much lower range and maximum for predators (max: 19) than prey (186). I feel uncomfortable treating these groupings as equivalent, presumably the spatial autocorrelation among grid cells in terms of species present must extend over longer distances for predators than prey? Maybe this is not strictly an issue with the questions being asked here and may only be a helpful explanatory reason why there was not much evidence for a top down control of predator richness on prey richness? I also had a similar problem with my own data on monocots vs. vertebrates (there are a magnitude more monocot species than vertebrates…).

Two more queries: why such big grid cells? Why not 100km x 100km like most other macroecological studies on mammals so far? Why PCA climatic variables rather than make informed choices on the variables you want to include (I don’t think PCA is strictly a verb, sorry). This is, admittedly, a pet-hate of mine, but it just strikes me as an unnecessary extra step to remove the reader from the data. In this instance, I also wonder how each axis changed from region to region in the region-specific SEMs?

Finally, will the diet database be made openly accessible? I really hope so, it sounds like a great resource for a bunch of further questions. For instance I would really have just liked to have seen more ‘basic’ analyses of the data: average range sizes for the two groups? Body sizes?

That was a really grumbley post and very unfair given each and every point could have been applied to my own work. I suppose one is always harshest on what one knows best…I found this paper interesting to read, thoughtful on the different relationships possible and a great attempt to incorporate biotic effects into macroecological studies. More please!


Will Pearse

Will Pearse

This is an extremely far-reaching paper; the authors link predator and prey diversity at the macro-scale, and show that there are more predator species where there are more prey species.

My initial nit-picking niggle was wondering what exactly we can infer about such an inherently local-scale process as predation at the macro-scale – what relevance do predators and prey hundreds of kilometers away from one-another have? However, I think this link has got to be real – the authors control for spatial auto-correlation, they control for habitat (in the best way we can), and they still find this relationship. In fact, they find differences among predators and preys in their dependence on environmental conditions, which makes me think they’re picking up something real.

Maybe predators specialise on particular prey species, and so prey diversity begets predator diversity. We kind of know that to be the case, although we also know some species are generalists. An alternative, that I find more interesting, is that maybe (at a coarse scale) having more prey species makes a system more robust, and so able to support a wider variety of predators. Indeed, if prey species are more dependent on environmental factors than predators (as this paper finds), maybe having a wider variety of prey species gives the predators a more reliable food supply, and so a greater diversity of species can be supported. This is essentially a species-level re-working of the insurance hypothesis in the ecosystem function literature.

Which, essentially, brings me right back to the nit-picking question I had to begin with. Why is it that every explanation I think of for these patterns involves local-scale patterns, when this is a global-scale pattern? Can my thinking scale up to two degree grid cells? I’d now like to see how we can relate these coarser grid cell patterns to what’s going on inside the grid cells. What happens inside a grid cell that has an unusually high diversity of predators or prey?

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