We analyse demographic data from a seven-year study of the annual crucifer Erophila verna which showed two-year cycles of abundance in the field. The unusual behaviour of this population is shown to be the consequence of overcompensating density-dependence. Very local differences in germination succes between plots can account for observed differences in their population dynamics over the long term.

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Impatiens parviflora DC. was brought to European botanic gardens in 1837, from which it escaped and spread over almost the entire continent colonising predominantly synanthropic habitats. However, in the second half of the 20th century it also became permanently naturalised in natural deciduous forests. The causes of small balsam's success in colonisation of new habitats are not known. They may include either the continuous degradation of forest phytocoenoses or those properties of the alien species that allow it to colonise degraded habitats more effectively as compared to the native species. The aim of this paper was to verify the hypothesis on the relationship between the floristic and structural degradation of the herbaceous or groundlayer and its resistance to the invasion of I. parviflora. The investigations were carried out in the Forest Reserve "Las Bielański" in Warsaw using 30 transects that crossed natural floodplain forests and natural and degraded lime-hornbeam forests. The study has shown that: (1)there is a significant negative correlation between the species richness and cover of the groundlayer and the frequency, cover, and density of the population of I. parviflora; (2) small balsam easily penetrates into degraded and floristically impoverished communities; (3) dense groundlayer of the natural communities provides an effective barrier to the expansion of I. parviflora. This expansion is promoted by such disturbances of the groundlayer that induce formation of gaps providing safe microsites for seed germination and further growth of seedlings

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This paper is concerned with the effect of disturbance on some crucial characteristics of annual plants. The theoretically optimal life-history traits that maximize individual fitness in disturbed environments are described and critically evaluated. It seems that none of them holds for all annual species.Self-pollination and especially seed polymorphism are considered important adaptations to life in unpredictable environments. The thesis is put forward that amphicarpic annuals, which exhibit both self-pollination and extreme seed polymorphism, are best adapted to life in hazardous habitats. The hypothetical course of the evolution of amphicarpy is demonstrated on the grounds of the comparison of contemporary annual species producing chasmogamous and cleistogamous flowers on a single individual.Nomenclature: follows Flora Europaea (Tutin et al. 1964(Tutin et al. -1980, except for non-European species where the nomenclature used in the papers cited has been followed.