Biol Invasions https://doi.org/10.1007/s10530-020-02452-w (0123456789().,-volV)(0123456789().,-volV) ORIGINAL PAPER Differing impacts of two major plant invaders on urban plant-dwelling spiders (Araneae) during flowering season Tobias Bauer . Daria Alison Ba¨te . Fabian Kempfer . Jens Schirmel Received: 15 May 2020 / Accepted: 21 December 2020 Ó The Author(s) 2021 Abstract Plant invasions can have major impacts on goldenrods further had no effect on total spider ecosystems and influence global species diversity. In abundance and potential prey item abundance. In Central Europe, Himalayan balsam (Impatiens glan- contrast, goldenrods showed a significantly increased dulifera) and American goldenrods (Solidago crab spider (Thomisidae) abundance while being less canadensis and S. gigantea) are important invaders inhabited by web builders. Himalayan balsam nega- often establishing dense and homogeneous stands, tively influenced free hunters and running crab spider especially in urban and other disturbed habitats. We (Philodromidae) abundance, while we found no investigated their impacts on plant-dwelling spiders effects on other groups and total spider abundance. (abundance, family structure, guild structure) and For Himalayan balsam, potential prey item abundance potential spider prey items during flowering season was higher than in native vegetation stands. Notwith- within an urbanized landscape using a paired design standing that our results only represent a snapshot of comparing invaded and native reference vegetation the system, they suggest that large-scale removal of plots. In general, flowering American goldenrods and urban goldenrod stands during flowering season might Himalayan balsam had no significant impacts on the negatively influence local spider abundance, espe- spider family composition. Invasion of American cially of crab spiders. Management efforts should therefore be accompanied by compensation measures to avoid disruptive effects on local plant-dwelling Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/ spider communities. s10530-020-02452-w. Keywords Impatiens glandulifera Á Novel & T. Bauer ( ) ecosystems Philodromidae Solidago canadensis State Museum of Natural History Karlsruhe, Á Á Á Erbprinzenstr. 13, 76133 Karlsruhe, Germany Solidago gigantea Á Thomisidae Á Urban management e-mail: [email protected] T. Bauer Á J. Schirmel Institute for Environmental Sciences, iES Landau, University of Koblenz-Landau, Fortstraße 7, Introduction 76829 Landau, Germany Biological invasions can have tremendous ecological ¨ D. A. Bate Á F. Kempfer and socio-economic consequences (Sala et al. 2000; Institute of Geography and Geoecology, Karlsruhe Institute of Technology, Kaiserstraße 12, Nentwig et al. 2018). Invasive alien plant species are 76131 Karlsruhe, Germany 123 T. Bauer et al. one of the major drivers of current global biodiversity with fast dieback after initial frost events in autumn erosion (Bellard et al. 2016; Simberloff et al. 2013), (Beerling and Perrins 1993). Its relatively large seeds yet their eradication can result in enormous costs are released by ballistochory and are able to germinate (Hoffmann and Broadhurst 2016; Pimentel et al. synchronously in spring. Today, the conspicuous 2005). purple or pink flowers are a typical aspect of many In general, invasive plants have negative effects on riparian and forest edge habitats in Central Europe. In animal diversity, fitness and abundance (Schirmel the European Union, Himalayan balsam is considered et al. 2016). However, the consequences and severity an invasive plant species (Reg. 1143/2014; European of invasions can vary over time and among different Union 2014). However, reported effects of Himalayan spatial scales, ecosystems and taxa (Dosta´l et al. 2013; balsam on local plant diversity are partly contradic- Hulme et al. 2013; Schirmel et al. 2016). Herbivore tory. While Hulme and Bremner (2006) showed a communities are often directly affected by alien plant reduction in local plant diversity due to the replace- invasions due to loss of indigenous vegetation (Gerber ment of widespread, but native ruderal species by et al. 2008; Proches¸ et al. 2008) and respond with a Himalayan balsam, no significant effects on plant significant reduction of biomass and species diversity diversity were found by Hejda and Pysˇek (2006) and (Schirmel et al. 2016). Invasive plants can also Cˇ uda et al. (2017). American goldenrods are rhizoma- negatively affect predator communities by altering tous perennial plants with a shoot height of up to 2 m, habitat conditions (e.g., Balkenhol et al. 2018; Gerber typically invading disturbed sites where they form et al. 2008) and might lower foraging success of some dense stands with rich, yellow flowers in late summer predators (Maerz et al. 2005). and autumn (Weber 2000). American goldenrod On the other hand, there are well-known examples species are considered invasive in Central Europe where invasive plants can facilitate some native (Nehring et al. 2013) due to their known negative species (Rodriguez 2006). Invasive plants may offer effects on local plant diversity (Hejda et al. 2009; suitable resources and habitat requisites for native Weber 2000). animals, such as web attachments for spiders (Pearson Impacts of both Himalayan balsam and American 2009), habitat analogues for small mammals (Packer goldenrods on native animal diversity are still not fully et al. 2016) or pollen and nectar supplies for pollina- understood, and past research mainly focused on their tors (Davis et al. 2018; Russo et al. 2016). Especially effects on pollinators and ground-dwelling arthropods. in landscapes with large proportions of urban and Himalayan balsam offers extensive floral resources for novel habitats, alien plant species are abundant pollinators and can facilitate native pollinators to some (Kowarik 1995) and can contribute to local biodiver- extent (Davis et al. 2018; Lopezaraiza-Mikel et al. sity conservation by facilitation of native animal 2007). On the other hand, Tanner et al. (2013) species (Buchholz et al. 2015; Hausmann et al. 2016; demonstrated strong negative effects on foliage- Packer et al. 2016; Rodriguez 2006). Urban green- dwelling arthropod diversity and abundance. The spaces, gardens, wasteland and parks might even be invasion of American goldenrods can negatively important habitat analogues for rare or endangered affect pollinator communities in protected areas and arthropod species (e.g., Buchholz et al. 2018; Eckert in abandoned arable fields due to its dense stands, et al. 2017) as well as apex predators like birds of prey competitive advantage and subsequent simplification (Boal and Dykstra 2018). of floral resources (Fenesi et al. 2015; Moro´n et al. In Europe, both Himalayan balsam (Impatiens 2009). In contrast, it was shown that a relatively high glandulifera Royle) and American goldenrods (Sol- number of native pollinator insects, such as wild bees idago canadensis L., Solidago gigantea Ait.) have (Weber 2000; Westrich 2019), visit flowering Amer- been actively introduced during the last few centuries ican goldenrods. However, there is a clear research as ornamental and nectar plants (Beerling and Perrins gap in analyzing the impact of these major plant 1993; Weber 2000); nowadays, these species are very invaders on higher trophic levels (White et al. 2006), widespread and common in a wide variety of habitats such as plant-dwelling predatory arthropods like in many regions of Germany (Nehring et al. 2013). spiders (Araneae). Especially during flowering season, Himalayan balsam is a tall, annual herb with a height invasion by Himalayan balsam and goldenrods may of up to 2.5 m that flowers in late summer and reacts 123 Differing impacts of two major plant invaders on urban plant-dwelling spiders (Araneae) affect spiders via influences on phytophagous and goldenrod species (Solidago canadensis L., S. gigan- flower-visiting insect prey items. tea Ait.; Asteraceae) are widespread in numerous Crab spiders (Thomisidae) are particularly well disturbed urban habitats such as parks, areas along known ambush hunters, as they lurk on flowers for artificial channels and in abandoned industrial waste- visiting pollinators. Because both plant invaders lands. Major areas of Karlsruhe are located in the produce very conspicuous flowers or inflorescences, Upper Rhine valley on Pleistocene sand and gravel they provide potential hunting grounds especially for (Karlsruhe 1999). The urban vegetation in open green crab spiders. Many spiders are habitat specialists with spaces of Karlsruhe is characterized by a mosaic of a fast reaction to environmental changes and stress extensively managed meadows with 1–2 cuts and (Ha¨nggi et al. 1995; Buchholz et al. 2015, 2018; intensively managed lawns (from 3–5 up to 12 cuts per Entling et al. 2007). Additionally, spider diversity is year). An establishment of goldenrod and Himalayan usually not related to plant species numbers (Buchholz balsam on these sites is prevented by a first cut in 2010; Buchholz et al. 2018; Harry et al. 2019), but spring (intensively managed lawns) or summer (June/ rather to the structure and microclimate of the habitat July; extensively managed meadows). Goldenrod and (Clausen 1986). There is also evidence that spiders can Himalayan balsam stands are therefore restricted to react to higher prey availability with increased ruderalized areas with no regular management, like production of offspring (Wise 1979) and survival anthropogenic