Many alien invasive plants disperse against the direction of stream flow in riparian areas Takeshi Osawa a, *, Hiromune Mitsuhashi b , Hideyuki Niwa b a National Institute for Agro-Environmental Sciences, 3-1-3, Kannondai, Tsukuba, Ibaraki Pref., 305-8604, Japan b The Museum of Nature and Human Activities Hyogo, 6, Yayoigaoka, Sanda, Hyogo Pref., 669-1546, Japan 1. Introduction Natural riparian zones constitute an interface between aquatic and terrestrial ecosystems and contain diverse habitats that help to conserve high biodiversity (Burkart, 2001; Naiman et al., 1993; Whited et al., 2007). At the same time, riparian zone biodiversity is strongly threatened by alien invasive species (AIS; Birken and Cooper, 2006; Hood and Naiman, 2000; Richardson et al., 2007). AIS negatively affect native species through predation, competi- tion, and the spread of pathogens (Cameron et al., 2011) and can modify ecosystem functioning and abiotic features of the environment (Ricciardi, 2007; Strayer et al., 2006). Successful invasion of AIS into a particular location requires dispersal, colonization, and establishment (Ficetola et al., 2009; Puth and Post, 2005; Richardson et al., 2000). Dispersal success varies with the quantity of dispersing propagules (Holle and Simberloff, 2005) and the landscape being invaded (With, 2002). In particular, propagule pressure, that is, the rate of spread of alien plant propagules into new areas (Chytry et al., 2009; Holle and Simberloff, 2005; Lockwood et al., 2005), often plays a much more important role in determining the distribution of species than environmental conditions in several ecosystems worldwide (Holle and Simberloff, 2005; Lockwood et al., 2005). Riparian ecosystems exhibit downstream connectivity via their natural flow regime. Hydrological connectivity in river–floodplain ecosystems strongly disperses propagules downstream, based on hydrochory (dispersal by water flow and/or rain; Pysek and Prach, 1993; Richardson et al., 2007). Thus, hydrochory is likely to serve as one of the most important dispersal methods for invasion success in riparian areas. However, plant species exhibit several dispersal methods other than hydrochory, such as anemochory (dispersal by wind) and zoochory (dispersal by humans and/or other animals). These dispersal mechanisms are not likely to be Ecological Complexity 15 (2013) 26–32 A R T I C L E I N F O Article history: Received 24 July 2012 Received in revised form 28 January 2013 Accepted 29 January 2013 Available online 5 March 2013 Keywords: Alien invader plant Invasion success Neighboring effect Propagule pressure River managements Spreading A B S T R A C T Propagule pressure plays an important role in the invasion of alien plants into riparian areas. In this study, we focused on propagule pressure from both neighboring riparian areas and anthropogenic land- use areas because propagules are likely to originate from both sources. We tested the effects of whether neighboring units contained the alien plant species, focusing on the direction of invasion by alien plant species into the focal unit, and how much anthropogenic land was contained within the unit, focusing on both farmland and urbanized areas, on alien plant occurrences in Hyogo Prefecture, Japan. We modeled the occurrence of 10 alien plants using generalized linear models to evaluate species invasions by both propagules from both neighboring units and anthropogenic land within a unit. We also investigated the biological and ecological plant attributes that are likely related to invasion success, such as seed dispersal methods, seed size, and clonality, and tested the relationships between the model results and each species’ attributes. Results showed that the occurrence of an affected neighboring unit was positively associated with the occurrence of all 10 alien plants. Note that two alien invasive species were influenced by upstream flow direction, six species by downstream flow direction, and in two species, propagule supply was not distinguished by direction. In short, the dominant direction of dispersal was against the stream current, while dispersal in the downstream direction was less common. Species attributes were associated with these directions of dispersal. In addition, anthropogenic land was positively associated with the occurrence of most alien plants, although this effect was weaker than the neighbor unit effects. These results indicate that alien plants spreading into riparian areas do not always follow the natural flow regime; rather, they spread against the flow regime in some cases. We discuss an ecological explanation for these results and provide perspectives for future river management of alien plants that invade the riparian zone. ß 2013 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +81 29 838 8148; fax: +81 29 838 8199. E-mail addresses: arosawa@gmail.com (T. Osawa), hiromune@hitohaku.jp (H. Mitsuhashi), h_niwa@nifty.com (H. Niwa). Contents lists available at SciVerse ScienceDirect Ecological Complexity jo ur n al ho mep ag e: www .elsevier .c om /lo cate/ec o co m 1476-945X/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecocom.2013.01.009