Grassland vegetation in urban habitats – testing ecological theories

Abstract

During the last millennium, urbanization has considerably changed natural ecosystems and formed new artificial habitats. Habitat loss and changes in the abiotic environment are seriously affecting urban biodiversity. We investigated the vegetation composition of three urban habitat types, vacant lots, urban parks, and peri-urban grasslands, which are characterised by species typical to semi-natural grasslands and ruderal assemblages in the city of Debrecen (East-Hungary). We used five spatial replicates of each habitat type and five random plots (5 m × 5 m) in every site for our analyses. We tested the following hypotheses: (1) lower species numbers and Shannon diversity, and a higher proportion of weeds and disturbance-tolerant species are present in the city centre (i.e. urban parks) compared to more peripheral habitats (vacant lots and peri-urban grasslands), (2) the proportion of warm- and nitrogen-demanding species increases, while the proportion of moisture-demanding species decreases in habitats typical to city centres (3) we also tested the increase in cosmopolitan and alien species and the decrease in species of the natural flora in habitats typical to city centres as predicted by the urban homogenisation hypothesis. We found that species composition of urban habitat types is considerably affected by the specific disturbances and site histories associated with the certain habitats. The most urbanised habitats, the urban parks harboured the lowest number of species and the lowest Shannon diversity. The ratio of weeds and disturbance-tolerants was the highest in the city centre likely due to the high-intensity trampling and soil disturbances. Plant species of city centre were more drought-tolerant compared to periurban grasslands, which is likely due to the increased level of drainage. The ratio of nitrogendemanding species was lower in urban parks and peri-urban grasslands than in vacant lots, likely due to the high level of recent soil disturbance in this habitat type. The proportion of alien species was high both in vacant lots and peri-urban grasslands, even though their disturbance regimes differed considerably. The proportion of cosmopolitan species was significantly higher in urban parks compared to vacant lots and peri-urban grasslands. The large proportion of alien and cosmopolitan species together with the continuous human disturbance put native species at a competitive disadvantage, and accordingly the proportion of these species was lowest in the city centre. Even though the studied urban habitat patches did not contribute considerably to the preservation of rare or endangered plant species, they have an essential role in preserving the last remnants of grasslands in intensively used landscapes, and can be a good basis for urban greening projects.