Plants, animals, and universal foraging strategies: Plant root foraging supports the predictions of marginal value theorem

Background/Question/Methods: Heterogeneous resource distributions are a ubiquitous problem for all organisms. When fitness can be correlated with resource uptake, evolution favors adaptations which optimize resource uptake in this heterogeneous world. Marginal Value Theorem (MVT) has been one of the most successful models of optimal animal foraging behavior. MVT predicts how much foraging effort should be allocated to patches of differing quality. In general, organisms should spend more time in higher quality patches relative to lower quality patches. MVT has traditionally only been applied to animal behavior, however plants are similarly faced with heterogeneity in the distribution of soil resources. We designed an experiment to test whether the root foraging behavior of Achillea millefolium conformed to the predictions of optimal foraging theory. Rectangular pots were designed where plants would be forced to grow through patches of differing quality, or through homogeneous soil. We measured the maximum lateral breadth of roots grown in these heterogeneous and homogeneous soils with a mini-rhizotron camera. We also measured total plant biomass and root proliferation in each patch. Based on the predictions of MVT we developed three hypotheses; 1) roots would remain in higher quality patches for longer periods of time before growing beyond the borders of the patch; 2) roots proliferation should occur in proportion to patch quality; 3) this behavior should lead to higher benefits from higher quality patches.
Results/Conclusions: The foraging behavior of A. millefolium supported the predictions of MVT. Specifically; 1) roots remained in the high quality patches longer than low quality patches before growing beyond the borders of the patch; 2) root proliferation was correlated with patch quality; 3) plants foraging in the highest quality patches received the highest benefits from foraging measured as total plant biomass. In the past researchers have been content use the idea of plant behavior as a mere metaphor. This is the first study to actually link the root foraging behavior of plants to models of optimal foraging developed from animal behavior.