Competition for one resource may influence competition for another

Finally, it is worth noting that competition for one resource often affects the ability of an organism to exploit another resource. For example, Buss (1979) showed that in interactions between species of bryozoa (colonial, modular animals), there appears to be an interdependence between competition for space and for food. When a colony of one species contacts a colony of another species, it interferes with the self-generated feeding currents upon which bryozoans rely (competition for space affects feeding). But a colony short of food will, in turn, have a greatly reduced ability to compete for space (by overgrowth).

Comparable examples are found amongst rooted plants. If one species invades the canopy of another and deprives it of light, the suppressed species will suffer directly from the reduction in light energy that it obtains, but this will also reduce its rate of root growth, and it will therefore be less able to exploit the supply of water and nutrients in the soil. This in turn will reduce its rate of shoot and leaf growth. Thus, when plant species compete, repercussions flow backwards and forwards between roots and shoots (Wilson, 1988a). A number of workers have attempted to separate the effects of canopy and root competition by an experimental design in which two species are grown: (i) alone; (ii) together; (iii) in the same soil, but with their canopies separated; and (iv) in separate soil with their canopies intermingling. One example is a study of interspecific competition is frequently highly asymmetric root and shoot competition

Maize Shoot And Roots
Figure 8.6 Root and shoot competition between maize and pea plants. Above are the experimental plants used, below are the dry weights of pea plants after 46 days as a percentage of those achieved when grown alone. (Data from Semere & Froud-Williams, 2001.)

maize (Zea mays) and pea plants (Pisum sativum) (Semere & Froud-Williams, 2001). In full competition, with roots and shoots intermingling, the biomass production of maize and peas respectively (dry matter per plant, 46 days after sowing) was reduced to 59 and 53% of the 'control' biomass when the species were grown alone. When only the roots intermingled, pea plant biomass production was still reduced to 57% of the control value, but when just the shoots intermingled, biomass production was only reduced to 90% of the control (Figure 8.6). These results indicate, therefore, that soil resources (mineral nutrients and water) were more limiting than light, a common finding in the literature (Snaydon, 1996). They also support the idea of root and shoot competition combining to generate an overall effect, in that the overall reduction in plant biomass (to 53%) was close to the product of the root-only and shoot-only reductions (90% of 57% is 51.3%).

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