Predicting ecosystem stability from community composition and biodiversity

de Mazancourt et al. (2013). Ecology Letters 16(5): 617-625. DOI:10.1111/ele.12088. Predicting ecosystem stability from community composition and biodiversity

Decomposing variation in community structure is… exactly as difficult as you would expect (taken from de Mazancourt et al.)

Will Pearse

Will Pearse

I’ve lost track of how many papers have tried to put forward a new way to understand ecosystem stability. I was drawn to this paper because it develops a novel conceptual framework that requires no more data than we already have, yet has greater explanatory power than other methods. The math is better, and so the model is better.

de Mazancourt et al. use data on individual species to predict what might seem like an abstract component of ecosystems – the coefficient of variation of community biomass. They’re not just predicting biomass or community composition, rather the stability of that composition over time. You’ve probably noticed I’m always desperate to link species’ ecology with how those species evolved; I wonder what the evolution of synchrony of environmental responses looks like. Do species that have coexisted for millions of years tend to be more synchronised? Or do they respond differently, and by responding differently ensure stable coexistence because they are occupying different niches (which reminds me of last week’s diversification limits paper)?

A fair bit of space is taken up with mechanisms by which observational error (which is an important component of their model) could have a biological interpretation. I’m not sure I quite follow, but I would be interested to know what effect intraspecific variation (which might be viewed as ‘error’) could have on all this. Intraspecific variation is a real (if, in my opinion, small) source of variation, and we might expect it to play a greater role in species that are more prevalent within a community (there are more of them, and so more opportunity for variation).

Finally, it is almost unbelievable that they were able to explain more than 70% of variation in the Texas dataset. So, seeing as how they’ve done unbelievably well in some datasets, and just plain-old-fashioned-very-well in others, why is there this variation? What is it about Texas that is so amenable to this method, and what makes Jena so different? I have literally no idea, and would be very grateful for ideas!

Lynsey McInnes

Lynsey McInnes

I told Will the paper I had chosen was too hard (what do you think – check it out here) and he came back with this one! Much harder! Although, so neat. I think I more or less get it. The authors set out to develop new theory as to why we ecosystems are so often more stable when they are more species rich. They neatly set out the conundrum of increasing richness stabilizing total community biomass, but at the same time destabilizing individual species abundances. Why does stabilization win out in most ecosystems?

The author list comprises a distinguished group of researchers working in this field and they bring together theory with four amazing time-series datasets. They are all set to make progress. And they do!  Basically (I believe) they find that stability is obtained through three distinct, but of course interacting, mechanisms: and I paraphrase, in more diverse communities you get a nicely complementary set of species that response asynchronously to environmental fluctuations so there is less chance of community collapse; in more diverse communities demographic stochasticity is weakened so you don’t get crashes of individual species; and finally more diverse communities, in effect, homogenize the intrinsic heterogeneity of an area through the provision of a set of species that altogether occupy all available ‘niches.’

The neat thing about this paper is that all of these mechanisms have been floating around in the literature already and have now been brought together into a single model and that model is verified with four independent empirical studies. The authors provide visually satisfying path diagrams to show how one gets from species richness to observed coefficient of variation of community biomass through each of the above mechanisms and they show the strength and direction of each effect.

Although this paper was a bit of dive back in time to one or two of my undergraduate lectures on overyielding and the insurance hypothesis, I did appreciate this paper and was thankful that it was well-written and well-explained. So many people work on elements of this puzzle, particularly motivated by curbing biodiversity loss into the future, but I think it’s grand projects like these – that set the situation up in a coherent framework – that might be most helpful in really demonstrating why and how diversity is beneficial to ecosystem stability. I imagine the authors’ heads are already teeming with next steps: climate change, evolutionary responses, invasive species, etc. One might want to know both how perturbing a system affects stability, but also if the relationship between diversity and stability stays the same during and post perturbation.


About will.pearse
Ecology / evolutionary biologist

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