Biol Invasions (2019) 21:947–960 https://doi.org/10.1007/s10530-018-1874-1 (0123456789().,-volV)(0123456789().,-volV) ORIGINAL PAPER Linear and non-linear effects of goldenrod invasions on native pollinator and plant populations Dawid Moron´ . Piotr Sko´rka . Magdalena Lenda . Joanna Kajzer-Bonk . Łukasz Mielczarek . Elzbieta_ Rozej-Pabijan_ . Marta Wantuch Received: 28 August 2017 / Accepted: 7 November 2018 / Published online: 19 November 2018 Ó The Author(s) 2018 Abstract The increased introduction of non-native and native plants. The species richness of native plants species to habitats is a characteristic of globalisation. decreased linearly with goldenrod cover, whereas the The impact of invading species on communities may abundance and species richness of bees and butterflies be either linearly or non-linearly related to the decreased non-linearly with increasing goldenrod invaders’ abundance in a habitat. However, non-linear cover. However, no statistically significant changes relationships with a threshold point at which the across goldenrod cover were noted for the abundance community can no longer tolerate the invasive species and species richness of hover flies. Because of the non- without loss of ecosystem functions remains poorly linear response, goldenrod had no visible impact on studied. We selected 31 wet meadow sites that bees and butterflies until it reached cover in a habitat encompassed the entire coverage spectrum of invasive of about 50% and 30–40%, respectively. Moreover, goldenrods, and surveyed the abundance and diversity changes driven by goldenrod in the plant and of pollinating insects (bees, butterflies and hover flies) D. Moron´ (&) Ł. Mielczarek Institute of Systematics and Evolution of Animals, Polish Department of Forests and Nature, Krako´w Municipal Academy of Sciences, Sławkowska 17, 31-016 Krako´w, Greenspace Authority, Reymonta 20, 30-059 Krako´w, Poland Poland e-mail: [email protected] e-mail: [email protected] P. Sko´rka E. Rozej-Pabijan_ Institute of Nature Conservation, Polish Academy of Institute of Biology, Pedagogical University, Sciences, Mickiewicza 33, 31-120 Krako´w, Poland Podchora˛zych_ 2, 30-084 Krako´w, Poland e-mail: [email protected] e-mail: [email protected] M. Lenda M. Wantuch School of Biological Sciences, The University of Institute of Environmental Sciences, Jagiellonian Queensland, Qld 4072 Brisbane, Australia University, Gronostajowa 7, 30-387 Krako´w, Poland e-mail: [email protected] e-mail: [email protected] J. Kajzer-Bonk Institute of Zoology and Biomedical Research, Jagiellonian University, Gronostajowa 9, 30-387 Krako´w, Poland e-mail: [email protected] 123 948 D. Moron´ et al. pollinator communities were related to species loss species at which negative effects on native commu- rather than species replacement. We demonstrated that nities appear (Fig. 1). It is expected that, if the the impact of goldenrod cover on a habitat is not threshold of density is high (Fig. 1), the number of instantaneous. Habit management aimed at preventing species effectively endangered during the course of the invasion process and alleviating its impact should invasion tends to be underestimated and hence the take into account that, for the non-linear relationships, consequences of invasions on biodiversity might be the negative impact can appear rapidly after crossing undervalued. the threshold point. This study explores the pattern of the density- dependent impact of invaders using pollinating arthro- Keywords Bees Á Biodiversity Á Biological pods. Pollinators are key components of ecosystems conservation Á Butterflies Á Ecological invasions Á providing various ecosystem services important for Hover flies Á Invasive plants agriculture and the human economy (Mouquet et al. 2012). The effect of invasive plants on a pollinator community is rather equivocal (Bjerknes et al. 2007). The invasion of alien plants might affect pollinator Introduction populations by decreasing the diversity of native plants, and subsequently the pollinator food-base The introduction of non-native species to habitats (Moron´ et al. 2009). At the same time, alien flowering increases consistently as a result of globalisation (Amano crops might boost pollinator population sizes by et al. 2016). Established alien organisms—invasive increasing their resource availability (Tepedino et al. species—cause environmental changes that threaten 2008). Thus, a non-linear impact of invaders on native native biodiversity and the human economy (Pejchar pollinators may be caused by the buffering character- and Mooney 2009). The environmental changes induced istics of the environment (McCary et al. 2016). For by invasive species concerns the composition of invaded example, plant-pollinator food webs may exhibit high communities (Moron´ et al. 2009). The effects may move functional redundancy (Memmott et al. 2006). Thus, through food webs to influence other trophic levels and even though an invasive plant at lower densities may different habitats (Simao et al. 2010; Lenda et al. 2018). eliminate several native plant species, high functional In particular, the loss of plant species, primary producers, redundancy of the food web reduces the probability may impact key ecological processes such as ecosystem that this will lead to a decline in pollinators. Moreover, productivity (Pysˇek et al. 2012), decomposition (van der a novel invader might not substantially reduce forag- Putten et al. 2013), or ecosystem resilience to distur- ing options for arthropods (McCary et al. 2016). Less bances (Richardson et al. 2007). As a result, the diversity food-specialised pollinators may be able to find their decline at the level of the primary producers may impact preferred resource while ignoring the presence of the associated arthropod species (Haddad et al. 2009). plant invader, resulting in zero or minimal changes to a However, whether plants and arthropod communities pollinator community in response to plant invasions at change at all points along the invasion pathway (i.e. a low densities. Additionally, the non-linear impact of linear response to invasion), or if there is rather a invasive species can be generated by the neutral or threshold beyond which the communities change as the partially positive effects of invaders on natives (Hejda invasive plant becomes dominant (non-linear response to and Pysˇek 2006; Hulme et al. 2012), for example if an invasion), remains unclear. invader constitutes a novel, collateral food-source at It is frequently assumed that the impact of an low abundance and density (Salisbury et al. 2015). invading species is proportional (linearly related) to its Regardless of the direction of the invasive species density or abundance in a habitat (Yokomizo et al. impact on pollinators, a full understanding will require 2009; Elgersma and Ehrenfeld 2011; Panetta and knowing whether this impact may be linear or non- Gooden 2017). However, one may hypothesise that the linear as the invader becomes more abundant. density-dependent impact of invader populations may As a model invasive alien plant we used North also elicit a non-linear response from native species American goldenrods (Solidago canadensis and S. (Crooks 2005; Panetta and Gooden 2017). Thus, there gigantea), among the most invasive species in Europe might be a threshold of density of invasive alien and Asia (Weber 2001). Flowers of this species are 123 Linear and non-linear effects of goldenrod invasions 949 Fig. 1 Conceptual model of density-dependent impact of invasive species on native communities. With the increasing density of an invasive species, the richness of native communities decreases. Decrease of native communities could be non- linear (community 1 and 2) or linear (community 3) dependent on the invaders’ density. Arrows at the top indicate the threshold points for community 1 and 2 rich in pollen and nectar (Kabuce 2006), and are native plants and pollinators start to show a decrease in visited by a spectrum of pollinating insects in their abundance and species richness? And, (3) Are plant native areas (Pavek 2012) and in invaded ones and pollinator community changes caused by species (Kabuce 2006). Earlier findings showed that, in replacement (i.e., turnover) or elimination (i.e., nest- meadow habitats invaded by non-native goldenrods, edness) as goldenrod cover increases? there is a significant decline of native plant diversity (Moron´ et al. 2009; Pal et al. 2015). The goldenrod invasions also cause communities of native pollinators Materials and methods (Moron´ et al. 2009; Fenesi et al. 2015), ants (Lenda et al. 2013; Kajzer-Bonk et al. 2016), beetles (de Groot Study area et al. 2007; Baranova´ et al. 2014) or birds (Sko´rka et al. 2010) to disappear. The study was carried out in the meadow landscape We selected sites ranging from 0 to 100% of located in the valley of the Vistula river near the city of invasive goldenrod cover, and surveyed the abundance Krako´w, southern Poland (Fig. 2). Using present and and diversity of the most important pollinator com- historical vegetation data, we mapped the meadows munities of the temperate zone (bees, butterflies and (Kornas´ and Medwecka-Kornas´ 1974; Dubiel 1995; hover flies). For better mechanistic understanding of Pepkowska 2002; Sko´rka et al. 2007; Moron´ et al. the potential linear or non-linear pollinators’ response 2009) where soil is periodically saturated with water. to the invasive goldenrods, we also recorded the The meadows have similar geological history, climate number