According to Mayfield and Levine (2010), when two competing species differ primarily in their niches, competition causes trait overdispersion (divergence), but when they differ primarily in their competitive abilities (fitness), competition causes trait clustering (convergence). Therefore, two co-occurring species will be competing if they have different values of a plant functional trait which reflects niche differences, but also if they have similar values of a plant functional that reflects fitness differences!!. Thereby, to test hypotheses on species coexistence and community assembly by means of trait-based studies, it is crucial to know if certain PFTs contribute to variation in niche or fitness differences among species. For some traits it is very clear, but not for others. I hypothesize the following traits to contribute more to niche or fitness differences.
- Relative growth rate (mm/yr) Fitness
- Mean wood specific gravity (g*cm-3) Niche
- Shade-tolerance index (Comita et al. 2010) Niche
- Drought sensitivity index (Engelbrecht et al. 2007) Niche
- Mean seed dry mass (g) Fitness
- Maximum tree height (m) Fitness
- Mean leaf mass per area (g*cm-2) Fitness
My QUESTION is, does anyboby disagree with this classification? Why?
References
Comita L.S., Muller-Landau H.C., Aguilar S. & Hubbell S.P. 2010. Asymmetric density dependence shapes species abundances in a tropical tree community. Science 329: 330–332.
Engelbrecht B.M.J., Comita L.S., Condit R., Kursar T.A., Tyree M.T., Turner B.L. & Hubbell, S.P. 2007. Drought sen- sitivity shapes species distribution patterns in tropical forests. Nature 447: 80–82.
Mayfield M.M. and Levine J.M. 2010. Opposing effects of competitive exclusion on the phylogenetic structure of communities. Ecology letters 13: 1085-1093.
I though a lot about this interesting question...
I believe that generally each of the traits you mention can influence both "niche" and "fitness" differences. It is all context dependent since traits usually involve trade-off. Let's assume that high values of trait X increase fitness under condition A but decrease it under condition B. So obviously it affect fitness but if condition vary among time in would be regarded as "storage effect"(Sensu Chesson 2000) which is a "niche" difference, I think that in order to raise hypothesis one should know his specific system well to suggest a set of processes in which specific trait affect "niche" differences more than "fitness" or vice versa... Alternatively one can find empirically that some traits affect "fitness" more than "niche" and suggest a model\hypothesis for explaining it.
I though a lot about this interesting question...
I believe that generally each of the traits you mention can influence both "niche" and "fitness" differences. It is all context dependent since traits usually involve trade-off. Let's assume that high values of trait X increase fitness under condition A but decrease it under condition B. So obviously it affect fitness but if condition vary among time in would be regarded as "storage effect"(Sensu Chesson 2000) which is a "niche" difference, I think that in order to raise hypothesis one should know his specific system well to suggest a set of processes in which specific trait affect "niche" differences more than "fitness" or vice versa... Alternatively one can find empirically that some traits affect "fitness" more than "niche" and suggest a model\hypothesis for explaining it.
First, the traits you use are going to be specific to an ecosystem type, which is important, and the ones you define are forest based. Traits in a grassland would be very different. Second, the traits you suggest as defining fitness versus niche are not independent, as pointed out by Niv. For example, Relative Growth Rate (fitness trait) will strongly influence Mean Specific Wood Gravity (niche trait).
Dear Niv and Patti, many thanks for your feedback. It is true, Niv, that almost any of the traits I mentioned can influence both, "niche" and "fitness", and that it is context dependent. As you highlight, it is very important to know the system in which you are working very well (and to read a lot) in order to raise hypotheses about which specific traits affect niche and/or fitness differences on it, and under which circumstances!. Regarding the relationships between "niche" and "fitness" traits, Patti, it was also mentioned by Mayfield and Levine (2010) and Adler et al. (2013). Niche and fitness traits are dependent, and many traits can influence both, niche and fitness, but some will influence more niche than fitnesss differences and viceversa. I obviously recognised that all traits I mentioned are forest-based, in the case of grasslands, ¿may you please tell us which traits do you think have the strongest influence for niche and fitness differences?
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