Survivorship and fecundity of the radiate and non-radiate morphs of Groundsel,Senecio vulgaris L., raised in pure stand and mixture

British populations of Senecio vulgaris frequently contain two common capitulum morphs (radiate and non-radiate) and one rare intermediate morph. The radiate morph shows a higher maternal rate of intermorph outcrossing than the non-radiate morph and due to the ‘cost of outcrossing’ should decline in frequency, ultimately to be lost from a population. To determine whether the radiate morph exhibits some inherent advantage in fitness to offset the ‘cost of outcrossing’, a comparison was made of the survivorship and fecundity of the radiate and non-radiate morphs raised in pure stands and 1 : 1 mixture at three planting dates (autumn 1983, and spring and autumn 1984). Plants in stands established in spring 1984 were harvested in late August 1984, while plants in stands established in autumn overwintered before being harvested the following summer. In spring planted stands, the two morphs exhibited equivalent survivorships, while the fecundity of the non-radiate morph tended to be greater than the radiate morph. In autumn planted stands, survivorship and Net Reproductive Output (survivorship × fecundity) of the non-radiate morph was greater than that of the radiate morph in mixture, and also in pure stands established in 1983. In no instance was the Net Reproductive Output of either morph significantly greater in mixture than in pure stand. Density had a contrasting effect on morph survivorship and fecundity in the spring and autumn 1984 planted stands. Whereas, in spring stands, fecundity was subject to compensating density dependent regulation while survivorship was density independent, the opposite trend was observed in autumn planted stands. It is concluded that under the conditions of the experiments, the radiate morph exhibited no fitness advantage which might offset the inherent disadvantage it suffers in natural polymorphic populations due to the ‘cost of outcrossing’.