Tradeoffs among functional traits are essential for explaining community structure and species coexistence. While two-way tradeoffs have been investigated in many systems, higher-dimensional tradeoffs remain largely hypothetical. Here we use a literature compilation of phytoplankton traits, in combination with a population dynamic model, to characterize the multivariate relationships among competitive ability for nitrogen, competitive ability for phosphorus, and cell size.
We demonstrate a three-way tradeoff between cell size and competitive abilities for nitrogen and phosphorus, in marine and freshwater phytoplankton. At a given cell size, competitive ability for N and P are negatively correlated, but as cell size increases, competitive ability decreases for both nutrients. The relative importance of the two trade-off axes appears to be environment-dependent, suggesting different selective pressures: freshwater phytoplankton separate more along the N vs. P competition axis, and marine phytoplankton separate more along the nutrient competition vs. cell size axis.