Diversity of native and alien plant species on rubbish dumps:: effects of dump age, environmental factors and toxicity

The flora of 96 rubbish dumps consisting of organic, inorganic and industrial wastes was studied in the Czech Republic. Some dumps contained toxic substances (heavy metals, chlorethylenes, phenols, polychlorinated biphenyls, oil hydrocarbons and biogas). Statistically significant factors explaining the number and proportional representation of native plant species, archaeophytes (introduced before 1500) and neophytes (introduced later) were determined. In total, 588 species of vascular plants were recorded, with archaeophytes (133 species) over-represented and native species (322 species) and neophytes (133 species) under-represented compared to their proportions in the national flora. Minimum adequate models were used to determine the effects of several factors on species numbers and proportions, independent of other factors. Dump area, human density in the region and altitude (non-significant only in archaeophytes) were correlated positively with species numbers. Dump age, expressed as time since dump establishment, interacted with the dump toxicity; species numbers increased with dump age on non-toxic dumps, whereas on toxic dumps no increase in numbers was noted. For neophytes, dump toxicity also interacted with human density; the increase in numbers of neophytes with human density is more pronounced on toxic than on non-toxic dumps. The variables measured failed to explain observed differences in proportional representation of native species, archaeophytes and neophytes. This suggests that the occurrence of species growing in such extreme habitats is driven overwhelmingly by factors such as anthropogenic disturbance. A possible explanation for the positive effect of altitude on species numbers on dumps is that the effect of heating of the deposited substrate by microbiological processes, documented by previous studies, overrides the effect of altitude which was shown repeatedly to have a negative effect on species richness. Neophyte distribution is driven by an interplay of factors distinct from those influencing the distribution of native species, namely toxicity and human density (the latter we interpret as a surrogate for propagule pressure). Their distribution on studied dumps is more restricted than that of native taxa and archaeophytes, and they are more limited by toxic substrata; more intensive propagule pressure is required for their establishment at dumps with higher toxicity levels.