Population ecology of Chrysanthemoides monilifera in South Africa: implications for its control in Australia.
Abstract
The population ecology of single species stands of Chrysanthemoides incana, C. monilifera monilifera [C. moniliferum subsp. monilifera], C. moniliferum subsp. pisifera and C. moniliferum subsp. rotundata was examined at 12 study sites established during 1987-88 in South Africa. Comparisons are made with populations of C. moniliferum subsp. monilifera and C. moniliferum subsp. rotundata, which are introduced invasive weeds of native vegetation in Australia. In South Africa, adult plant density averaged 0.2 plants m-2. Plants in the sample were 6 to 35-years-old as determined by growth rings. Recruitment in Cape Province sites appeared to be restricted to a few years, probably following major disturbance such as fire. In Natal, recruitment declined as the population aged. Seedlings were rare, being present at only 4 sites and at densities lower than that of adult plants. Insects and unidentified causes destroyed 10-58% of ovules before the flowers senesced. Larvae of seed-feeding flies (Mesoclanis spp.) destroyed a small fraction of the seeds (0.2-9.0% per site) and damage to seeds by other insects (chiefly larvae of Lepidoptera) accounted for 4-25% of ovules. Between 0.4 and 32.0% of ovules became viable seeds. Over all sites, the seed bank under the canopy of plants consisted of 410-4500 seeds m-2, of which 0-6% were viable. There was a mean of 26 viable seeds m-2 over all sites. Outside the canopy, total seed density ranged from 7 to 660 m-2, of which 0-6% were viable. Identifiable post-dispersal damage was mainly due to rodents (up to 62% of dispersed seed) and was higher in Cape Province than in Natal. Between 5 and 20% of the leaf surface area was consumed by herbivores. Chrysanthemoides populations in South Africa attain similar adult plant densities to Australian populations of Chrysanthemoides, but have lower percentage ground cover and much reduced recruitment once a population is established. It is likely that biological control agents causing similar damage in Australia will need the help of other management techniques such as fire to reduce weed density and persistence.