New dimensions in resource tradeoffs

Posted on September 24, 2012

The concept of resource competition can help to explain why the species composition of ecological communities varies across time and space.  For example, if plant species A is relatively efficient at nitrate uptake it may be dominant over a less efficient species (B) when the supply of nitrate is limited.  However, if species B is better at phosphate uptake than species A, then species B is more likely to dominate when phosphate supplies are restricted. And if there are constraints on the ability of an organism to compete for two or more resources at the same time (for instance, because of limitations on the amount of energy, material or cell surface available for the uptake of nitrate or phosphate) then one competitive trait must be traded off against another.  Given the complexity of interactions between organisms and their environment, many tradeoffs are likely to involve multiple traits, but there’s been almost no research to identify and explore these.  Workers at the University of Michigan drew on a physiological database for 69 freshwater and 60 marine species of plant plankton to reveal a three-way trade-off between cell size and the competitive abilities for nitrate and phosphate uptake.  They found that an increase in terms of one trait involves a decrease in the other two traits.  In other words, low-nutrient conditions favour small-celled species, probably because when nutrient uptake is limited by molecular diffusion, small cells have an advantage in terms of uptake rate relative to the amount of nutrient needed for growth.  And the other dimension of the three-way tradeoff means that different types of small-celled species are favoured depending on the relative scarcity of nitrate and phosphate.   Significantly, this finding answers a fundamental question that has been unresolved for both aquatic and terrestrial plants – namely whether competitive abilities for nitrate and phosphate uptake are correlated. 

Reference:  Edwards, K.F., Klausmeier, C.A. & Litchman, E.  2011.   Evidence for a three-way trade-off between nitrogen and phosphorus competitive abilities and cell size in phytoplankton.  Ecology, 92(11),  2085–2095.