Ecological fitness and the decline of resistance to the herbicide MCPA in a population of Ranunculus acris.
Experiments were conducted (1) to determine whether resistance to MCPA would subside in a population of Ranunculus acris subsp. acris after ending a 30-year regime of annual and biennial applications of MCPA, and (1) to test the hypothesis that MCPA-resistant R. acris plants are less competitive than MCPA-susceptible plants when grown together. Experiment 1 was conducted in a dairy pasture population of R. acris that had developed resistance to MCPA after repeated exposure to this herbicide over a period of 30 years. Application of the herbicide was either stopped or continued on plots from 1984/85 until 1988/89. The LD50 value for the seedling progeny of the 1988/89 plants from 'stopped' plots was two-thirds the LD50 for the progeny from 'continued' plots. Previous experiments had shown the LD50 for a susceptible biotype to be one quarter that of the resistant biotype, and so it was estimated that 28 years (90% confidence limits 11 and 84 years) of discontinued treatment would be required for the LD50 of the resistant population to fall to a level 10% above that of the susceptible biotype (which had never been exposed to the herbicide). In Experiment 2, the seedling progeny of a susceptible (S) and resistant (R) field population were grown in monocultures and in 1:1 mixtures under glass over a wide range of densities. A two-species competition model fitted to the plant dry weights indicated that S and R plants were similar in yield at low densities, but that S plants were higher yielding at high densities than R plants. It is suggested that MCPA-resistant plants of R. acris are less fit ecologically than susceptible plants, but the exact mechanisms have not been clearly established.