Urban Rivers as Dispersal Corridors: Which Factors Are Important for the Spread of Alien Woody Species along the Danube?

Dietmar Moser,Franz Essl,Sabrina Wagner

doi:10.3390/su12062185

Abstract

Cities are hotspots of invasions, and this is particularly the case for urban rivers, which are known to serve as corridors for the spread of alien plant species to floodplain forests. Here, we present a case study on woody (shrubs, trees) species invasions across a gradient from a metropolis (Vienna) to rural regions along the Danube River in eastern Austria. In total, we identified 44 native and 25 alien woody species in 75 plots. Five alien woody species occur in at least 10 plots. The most wide-spread ones were species of floodplain forests (Acer negundo, Fraxinus pennsylvanica, and Populus x canadensis), while Ailanthus altissima and Robinia pseudoacacia—which prefer dry sites—were recorded substantially less often. The average level of invasion—i.e., the relative proportion of alien to native woody species in plots—was high across all three study regions. Still, there was a moderate decline of alien woody species richness along the urban—peri-urban—rural gradient. Generalized Linear Mixed Models showed that population density and the proportion of urban habitats in the environs of the plots is significantly positively correlated with the presence of Acer negundo and Ailanthus altissima. Conversely, the occurrence of Robinia pseudoacacia is negatively correlated with surrounding population density and urban habitats. Occurrence of Acer negundo is positively correlated with urban habitats. For Fraxinus pennsylvanica, we found no significant relationships. Our results confirm that gallery forests at river banks are highly susceptible to invasions. We argue that managing alien woody species in urban and peri-urban sites is not appropriate and useful, given that re-invasion is likely in most cases (from adjacent urban green spaces). We acknowledge that this recommendation entails the implicit recognition that gallery forests along urban sections of the Danube will contain a substantial—and likely further increasing—proportion of alien woody species.

Highlights

Biological invasions have become a significant component of global environmental change [1]
We addressed the following research questions: (1) What is the proportion of alien woody species in different sections of the Danube River in eastern Austria? (2) Which alien woody species are the most common ones? (3) How does alien woody species abundance change along the urban-rural gradient? (4) Which factors explain the distribution of alien woody species? We hypothesized that richness of alien woody species decreases from urban to rural rivers sections and that this decline in alien woody species richness is related to anthropogenic habitat modification that affect and propagule pressure and invasibility [5,7]
Implications for Management and Conclusions. Rivers and their accompanying habitats are highly important—yet increasingly threatened—corridors for biodiversity conservation. Additional factors such as recreation further add to the high importance of rivers for urban ecology and human livelihoods

Summary

Introduction

Biological invasions have become a significant component of global environmental change [1]. While levels of invasions are highly uneven among habitats in Central Europe [2], they are high in urban-industrial habitats, gardens, and public green spaces [3]. Due to high levels of anthropogenic disturbance and propagule pressure, urban environments have become a hotspot of biological invasions [4,5]. Humans are altering the biophysical world with unprecedented velocity, and one of the eminent drivers is increasing urbanization. Cities have grown rapidly due to immigration from rural areas, and due to endogenous population growth. 25% of the global population has moved from rural into urban settlements during the past six decades, and this trend is expected to continue [5]. The number of alien plants and animals in urban regions has increased [5,6]

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Conclusion