DENSITY-DEPENDENT HABITAT SELECTION IN PLANTS

Pea plants exhibit density-dependent habitat selection as they grow. W e split the root of a young pea (Pisum sativum L.) so that half grew i n one pot and half in an adjacent pot. The rest of the plant remained intact. This is a 'fence-sitter plant'. Each root-half was exposed eit her to no competition in its pot or to competitor plants sharing its p ot. There were one, two, three or five competitor plants. The total ro ot biomass and the fitness (= dry weight of fruit) of the fence-sitter decreased only slightly and insignificantly in response to increased density of the competitor plants. The fitness of the competing plants decreased with density. The fence-sitter shifted its root system from the pot with competition to that free of competition in proportion to the number of competitors. The fence-sitter apparently invested in eac h of its two roots so that the ratio between the roots was similar to the ratio between the resources in the pots. This result is analogous to the habitat-matching rule of the ideal free distribution of populat ions (Fretwell, 1972). We suggest that plants invest in each of their roots until the uptake rate per unit root biomass is equal for all roo ts.