Precociously developing individuals in populations of chickweed [Stellaria media (L.) Vill.] from different habitat types, with special reference to the effects of weed control measures

Abstract

summaryLong‐term weed control in gardens and arable areas is likely to be a potent selective force favouring precocious development of annual weed species. In an experiment with Stellaria media (L.) Vill., we grew progenies from open‐pollinated seed of a number of seed‐parents collected from 12 British populations of five habitat types. We raised plants in a temperate glasshouse and a garden plot, to see if plants from areas subject to weed control ‐gardens and arable fields – had a quicker rate of development from germination to flowering and fruiting than plants from habitats subject to less weed control (waysides) or no weeding pressure (seasonally exposed river sediments and bird colonies).Comparison of the behaviour of plants in the glasshouse and out‐of‐doors revealed a broadly similar pattern of behaviour. However, for some, but not all seed‐parents, rate of development was different in the two sets of conditions.In conformity with hypothesis, plants from one of the gardens were quick to reproduce, but plants from Ness Botanic Garden and arable fields were more variable than expected. Plants from less well weeded and non‐weeded sites did not conform to expectation. Plants from waysides were quick to reproduce, so too was much of the material from a bird colony on the island of Skomer. While some late‐developing plants were detected from a second bird colony (Fame) and from exposed river sediments by the Tweed and Manifold, we found a small number of precocious and some intermediate individuals in the progeny of some seed‐parents.It is possible that bird colonies and exposed river sediments are heterogeneous habitats, in which there are niches where ‘native’ variants with different rates of development are at a selective advantage.However, there is another explanation of our results. First, we discovered that, while some of the seed‐parents were probably behaving as homogeneous lines for rate of development, others show ed segregation, providing evidence that a degree of outcrossing is possible in populations of Stellaria. Secondly, there is evidence that human activities have led to habitat disturbance in and around the bird colonies and riverside vegetation we have studied. Our findings are consistent with the hypothesis that, gene flow by pollen and/or seed of precocious weedy plants has occurred in these man‐disturbed sites, leading to different patterns of variation involving hybridization between ‘introduced’ precocious plants and ‘native’ slow‐developing variants.