Beta diversity of urban floras among European and non‐European cities

Abstract

Aim Cities represent an ideal study system for assessing how intensive land-use change and biotic interchange have altered beta diversity at broad geographic extents. Here we test the hypothesis that floras in cities located in disparate regions of the globe are being homogenized by species classified as invasive (naturalized species that have spread over a large area) or as a European archaeophyte (species introduced into Europe before ad 1500 from the Mediterranean Basin). We also test the prediction that the global influences of European activities (colonization, agriculture, commerce) have supported this outcome.Location One hundred and ten cities world-wide. MethodsWe examined the richness and composition of urban floras among European (n = 85) and non-European cities (n = 25) for species classified as native or non-native, or further classified as European archaeophyte or invasive. We modelled how geographic, climatic and anthropogenic factors were related to compositional similarity between European and non-European cities. ResultsWe found that most plants in the cities we examined, particularly nonEuropean cities, were native and unique to each city. Non-native species were similarly unique, but occurred in much lower proportions relative to natives. Although European archaeophytes and invasive species also occurred in lower proportions, they had similar compositions among cities. European archaeophytes were most prevalent in European cities, but were most similar among nonEuropean cities. Contrasting European and non-European cities, geography and climate were most relevant for native and invasive species, whereas climate and agriculture were most relevant for European archaeophytes.Main conclusions Cities in disparate regions of the globe retain regionally distinct native and non-native plant assemblages, while invasive species, and especially European archaeophytes, were associated with lower beta diversity among cities. These findings suggest that intensive land-use change and biotic interchange, shaped through European influences, have had a world-wide effect on the beta diversity of urban plant assemblages.

How Should Beta-Diversity Inform Biodiversity Conservation?

“…In southern California, urbanization increases soil moisture, which permits the 190 establishment of an invasive ant that homogenizes native ant communities by excluding all but a few species [61]. Native plant assemblages tend to become more homogeneous with urbanization [62], and cities often support numerous invasive species, which tend to have lower turnover than natives [63]. Recently established exotics, however, can 195 show higher beta-diversity than natives [62,64], suggesting a short-term heterogenizing process prior to the more widespread establishment of invasives ( Figure 1).…”

“…Both processes can initially heterogenize communities as the invader spreads, but can ultimately result in biotic 210 homogenization once the invader is ubiquitous (additive homogenization due to the invader's ubiquity, and subtractive homogenization if natives are extirpated; figure 1) [63,64,66]. Conservationists are mainly concerned with minimizing the impacts of invasives on native species, rather than maximizing the total diversity of invaded communities (i.e.…”

Impacts of urban sprawl on species richness of plants, butterflies, gastropods and birds: not only built-up area matters

“…As a result, at high levels of urbanization species richness generally decreases and urban biotas tend to become more and more similar -also called biotic homogenization -dominated by a few common native species and some ubiquitous nonnative species (McKinney 2002(McKinney , 2006Clergeau et al 2006;Lososová et al 2012a, b;Le Viol et al 2012;Aronson et al 2014;La Sorte et al 2014).…”

“…The spatial scale at which effects of urbanization on biodiversity are analyzed has also been found to be relevant, with impacts like biotic homogenization being more evident at larger spatial scales, both in terms of the extent of the study area and in terms of grain size (Deutschewitz et al 2003;Kühn and Klotz 2006;La Sorte et al 2014). However, studies have traditionally focused on particular urban areas, and although some of them have compared urban impacts in different cities across regions, countries, or even continents (see e.g., Pyšek 1993;Pyšek 1998;Aronson et al 2014;La Sorte et al 2014), large-scale analyses along broad urbanization gradients are still scarce (Devictor et al 2007;Lososová et al 2012a, b;Le Viol et al 2012).…”

“…However, studies have traditionally focused on particular urban areas, and although some of them have compared urban impacts in different cities across regions, countries, or even continents (see e.g., Pyšek 1993;Pyšek 1998;Aronson et al 2014;La Sorte et al 2014), large-scale analyses along broad urbanization gradients are still scarce (Devictor et al 2007;Lososová et al 2012a, b;Le Viol et al 2012).…”