Size Asymmetry of Resource Competition and the Structure of Plant Communities: Commentary on DeMalach et~al 2016. Herben, T. 104(4):911–912.
Size Asymmetry of Resource Competition and the Structure of Plant Communities: Commentary on DeMalach et~al 2016 [link]Paper  doi  abstract   bibtex   
[Excerpt] The hump-back relationship between diversity and productivity is one of the well-known patterns in ecology that have defied unequivocal explanation (Mittelbach et al. 2001; Š́ımová, Li & Storch 2013). While it has often been argued that the decline of species richness under high productivity is due to more intense competition, it has never been made fully clear why extinction under high productivity should be more likely compared to low productivity. DeMalach et al. (2016) present a simple and elegant explanation: it is due to asymmetry in competition for light. Competitive asymmetry means that resources (light in this case) are not divided among competing individuals in proportions corresponding to the size of their resource-acquiring organs, but a bigger individual gets disproportionately more. It means that the bigger individual gets more also in relative terms, that is per unit biomass. [] Competitive asymmetry has been a standard concept in plant competition for light (which is a resource with a clear tendency to asymmetric competition) for decades. It has been shown to explain size distribution in monospecific stands (Weiner & Thomas 1986), but has almost never been invoked to play a role in the productivity-diversity relationship. DeMalach et al. (2016) show in a convincing way that it can yield unimodal patterns of species richness along the productivity gradient. Increasing productivity means (as all of us have been taught) bigger individuals and shift from competition for nutrients, which is more or less symmetric, to competition for light, which is asymmetric. Their model thus links two major concepts in plant ecology, viz. diversity-productivity relationship and change in plant size along the gradient of productivity. The role of plant size in richness-productivity relationship was pointed out already in a conceptual paper by Oksanen (1996), but they go much further by linking plant size distribution with competitive asymmetry using a multispecies dynamic model. They point out the simple fact that size differences make a difference only when competition is asymmetric - the very same idea that led to identification of size asymmetry as the likely driver of skewed size distribution of individuals in plant populations (Weiner & Thomas 1986; Hara 1993). [] [...] [] Their model also casts new light on data where relationship between richness and productivity is monotonic (i.e. no hump back occurring), which are found namely at larger and/or biogeographic scales. While it can always be argued that this is due to sampling of a short range of the underlying productivity gradient, in many cases it is fairly unlikely. While their model does predict monotonic relationship in a perfectly symmetric case, even very small deviations from the perfect symmetry immediately lead to hump-back patterns. Such extreme sensitivity means that the size symmetric processes that the model is capable of showing will almost never occur in the field. It is hard to argue that any competition is perfectly size symmetric and the authors themselves authored papers (Schwinning & Weiner 1998 for example) and cite references showing that. Therefore, if we take the model seriously, a positive productivity-richness relationship must mean that it is driven by processes other than resource competition (Xiao et al. 2010; Grace et al. 2016). [] [...]
@article{herbenSizeAsymmetryResource2016,
  title = {Size Asymmetry of Resource Competition and the Structure of Plant Communities: Commentary on {{DeMalach}} et~al 2016},
  author = {Herben, Tomáš},
  date = {2016-07},
  journaltitle = {Journal of Ecology},
  volume = {104},
  pages = {911--912},
  issn = {0022-0477},
  doi = {10.1111/1365-2745.12591},
  url = {https://doi.org/10.1111/1365-2745.12591},
  abstract = {[Excerpt]

The hump-back relationship between diversity and productivity is one of the well-known patterns in ecology that have defied unequivocal explanation (Mittelbach et al. 2001; Š́ımová, Li \& Storch 2013). While it has often been argued that the decline of species richness under high productivity is due to more intense competition, it has never been made fully clear why extinction under high productivity should be more likely compared to low productivity. DeMalach et al. (2016) present a simple and elegant explanation: it is due to asymmetry in competition for light. Competitive asymmetry means that resources (light in this case) are not divided among competing individuals in proportions corresponding to the size of their resource-acquiring organs, but a bigger individual gets disproportionately more. It means that the bigger individual gets more also in relative terms, that is per unit biomass.

[] Competitive asymmetry has been a standard concept in plant competition for light (which is a resource with a clear tendency to asymmetric competition) for decades. It has been shown to explain size distribution in monospecific stands (Weiner \& Thomas 1986), but has almost never been invoked to play a role in the productivity-diversity relationship. DeMalach et al. (2016) show in a convincing way that it can yield unimodal patterns of species richness along the productivity gradient. Increasing productivity means (as all of us have been taught) bigger individuals and shift from competition for nutrients, which is more or less symmetric, to competition for light, which is asymmetric. Their model thus links two major concepts in plant ecology, viz. diversity-productivity relationship and change in plant size along the gradient of productivity. The role of plant size in richness-productivity relationship was pointed out already in a conceptual paper by Oksanen (1996), but they go much further by linking plant size distribution with competitive asymmetry using a multispecies dynamic model. They point out the simple fact that size differences make a difference only when competition is asymmetric - the very same idea that led to identification of size asymmetry as the likely driver of skewed size distribution of individuals in plant populations (Weiner \& Thomas 1986; Hara 1993).

[] [...]

[] Their model also casts new light on data where relationship between richness and productivity is monotonic (i.e. no hump back occurring), which are found namely at larger and/or biogeographic scales. While it can always be argued that this is due to sampling of a short range of the underlying productivity gradient, in many cases it is fairly unlikely. While their model does predict monotonic relationship in a perfectly symmetric case, even very small deviations from the perfect symmetry immediately lead to hump-back patterns. Such extreme sensitivity means that the size symmetric processes that the model is capable of showing will almost never occur in the field. It is hard to argue that any competition is perfectly size symmetric and the authors themselves authored papers (Schwinning \& Weiner 1998 for example) and cite references showing that. Therefore, if we take the model seriously, a positive productivity-richness relationship must mean that it is driven by processes other than resource competition (Xiao et al. 2010; Grace et al. 2016).

[] [...]},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-14128475,competition,complexity,ecology,environmental-modelling,feedback,limiting-factor,non-linearity,nutrients,primary-productivity,size-asymmetry,solar-energy,species-richness,trade-offs,vegetation},
  number = {4}
}
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