Alien and native plant richness and abundance respond to different environmental drivers across multiple gravel floodplain ecosystems

Abstract

Aim To assess whether native and alien plant cover and richness respond similarly to flow regime, propagule pressure, climate and floodplain characteristics in highly dynamic braided river ecosystems.Location Canterbury, New Zealand. Methods A regional, multiscale survey was conducted across 19 braided river floodplains in multiple catchments. We measured alien and native cover and richness across gradients of flow regime (flow magnitude, variability and high/ low-flow events), propagule pressure (inferred from land cover), climate and local-scale floodplain substrate and topography. Boosted regression trees were used to determine the relative and absolute importance of these variables on plant cover and richness. ResultsThe floodplain ecosystems were highly invaded with 154 alien species and only 31 natives. Alien cover was higher in rivers with larger maximum flows, in plots with fine substrate texture, and at higher local river bed elevations. Alien richness increased as the variability of winter flows increased and followed a hump-shaped relationship with river bed elevation. In contrast, native species richness and cover were both shaped primarily by climate and by land cover, higher in cooler and wetter areas with more adjacent native vegetation.Main conclusions Alien and native richness and cover were shaped by different variables, so managing the ecosystem (e.g. flow regime) to mitigate aliens would not necessarily promote natives. Promotion of natives will require considerations of propagule supply and whether extant native species are suited to low-elevation climatic conditions. Aliens were associated with predictors that approximate disturbance processes. Increased flow variability in winter could lead to an increase in the number of aliens; conversely, flow stabilization is likely to allow problematic invaders to increase in cover locally.

Correlation of native and exotic species richness: a global meta‐analysis finds no invasion paradox across scales

“…Subsequently, theoretical, experimental, and observational studies have reported negative relationships between diversity and invasibility in many ecosystem types including riparian corridors (Brown and Peet 2003), tall grass prairie and sagebrush (Stohlgren et al 2006b), forest (Byrne et al 2010), subtropical wetlands (Boughton et al 2011), grasslands (Harrison et al 2015), and agricultural sites (Peltzer and MacLeod 2014). However, positive native-exotic relationships (NER, a general term for any measure on which the diversity-invasibility relationship is based, including, e.g., species richness, biomass, density, and dominance) have also been reported in many field studies, suggesting the opposite pattern: habitats with more native species may be more readily invaded by exotic species (Stohlgren et al 2003, Brummer et al 2016, Hui and Richardson 2017). These studies are generally based on observational data that span large spatial scales such as nature reserves and islands (Lonsdale 1999, Stohlgren et al 2003, Fridley et al 2004).…”

Clearing islands as refugia for black-fronted tern (Chlidonias albostriatus) breeding colonies in braided rivers

“…For example, lower river flows stabilise islands and increase channelisation, facilitating alien plant species encroachment and establishment, as well as increasing mammalian access to areas where black-fronted terns nest (Hicks et al 2008;Pickerell 2015;Brummer et al 2016). Furthermore, introduced vegetation provides habitat and cover for mammals (Norbury & Heyward 2008;Pickerell 2015).…”

“…We tested whether clearing islands of vegetation improves the nesting success of black-fronted terns, particularly for lowland rivers which are the most invaded by alien plant species, but also still harbour significant breeding populations of black-fronted terns (O'Donnell & Hoare 2011;Brummer et al 2016;O'Donnell et al 2016;Schlesselmann et al 2017). Using the lower Waitaki River as a case study, we addressed the following questions: (1) Which mammalian species are present on vegetated and cleared islands, and adjacent riverbanks?…”

Relationships between Plant Species Richness and Terrain in Middle Sub-Tropical Eastern China

“…Topography was regarded as the important factor for governing the biodiversity pattern at landscape and regional scales [3][4][5]. Topographic variables, such as altitude, slope and aspect, were often used as predictors to analyze the biodiversity-topography association [6][7][8][9][10]. In recent years, instead of incorporating topographic metrics as predictors to analyze the relationship between species richness and topography, geodiversity was used as a comprehensive indicator to represent environmental heterogeneity [11][12][13].…”

“…However, there still exist conflicting views on the relations between species richness and topographic variables. For example, at least three patterns of species richness along altitude were reported: negative [6,15], positive [16,17] and hump-shaped [7][8][9][10]18]. Recently, some research works provided quantitative proof to underpin positive Environmental Heterogeneity (EH) -species richness relationships across EH subject areas, habitat types, taxonomic groups and …”