Plant Ecol https://doi.org/10.1007/s11258-019-00964-7 (0123456789().,-volV)( 0123456789().,-volV) Predicting the distribution and abundance of invasive plant species in a sub-tropical woodland-grassland ecosystem in northeastern India Dhritiman Das . Subham Banerjee . Robert John Received: 30 November 2018 / Accepted: 20 July 2019 Ó Springer Nature B.V. 2019 Abstract Invasive plant species have become km2 wildlife reserve that has sub-tropical woodland increasingly problematic in tropical and sub-tropical and grasslands. We sampled 134 plots of size 30 9 30 ecosystems, with the potential to decrease native plant m2 and found that Mikania micrantha (a climber) and diversity, increase fire occurrence, and cause ecosys- Chromolaena odorata (a shrub) were the most tem degradation. Numerous factors including distur- prominent invasive plants. We then tested the influ- bance due to fire, grazing, roads, human activities, ence of eleven environmental variables that are either reduction of native diversity, and soil fertility are direct measures or proxies of resource availability, known to influence invasibility of a habitat and/or vegetation density, disturbance, and moisture stress. promote the spread of invasive species. We studied Using these predictors, we performed a decision-tree- invasive species distribution and abundance in a 519 based regression and prediction to test the influence of these variables on invasive species abundance and to generate distribution maps. The model had significant Communicated by Scott J Meiners. predictive power in the case of Mikania (R2 = 0.469) 2 Electronic supplementary material The online version of but was poor for Chromolaena (R = 0.056). Annual this article (doi:https://doi.org/10.1007/s11258-019-00964-7) precipitation, soil phosphorus, and vegetation attri- contains supplementary material, which is available to autho- butes had a significant influence in Mikania, and fire rized users. frequency had the strongest influence on Chromo- D. Das laena. We could not quantify direct disturbance such Academy of Conservation Science and Sustainability as cattle grazing and resource extraction, which could Studies, Ashoka Trust for Research in Ecology and the add to the predictive power for these species. Given Environment (ATREE), Royal Enclave, Sriramapura, that invasive species continue to expand in range and Jakkur Post, Bangalore 560 064, India abundance, more directed ecological monitoring and D. Das analyses are needed to manage ecosystems under the Manipal Academy of Higher Education, Manipal, threat of invasions. Karnataka 576104, India S. Banerjee Á R. John (&) Keywords Invasive species Á Woodland–grassland Á Department of Biological Sciences, Indian Institute of Mikania micrantha Á Chromolaena odorata Á Manas Science Education and Research, Kolkata, Room S-235, National Park Research Complex, Mohanpur, West Bengal 741246, India e-mail: [email protected] URL: http://www.iiserkol.ac.in/ 123 Plant Ecol Introduction species or relative success of exotics appears to vary with scale. The observed negative relationships at The rapid spread of invasive alien plant species fine scale and positive relationship at large/regional (hereafter invasive species) in recent decades has scales (Kennedy et al. 2002) have been termed the become a major concern in many sub-tropical and ‘invasion paradox’ (Fridley et al. 2007). On one ` tropical ecosystems (Foxcroft et al. 2010; Vila et al. hand, the factors that favour plant growth at large 2011; Foxcroft et al. 2017). Invasive species have the scales in species-rich regions may help both native potential to disrupt native plant community structure, and exotic species alike, with the added risk that any decrease forage availability for herbivores, reduce reduction in native species richness may benefit species diversity, and degrade ecosystem function invading species. At small scales, however, local (Mack et al. 2000; Hejda et al. 2009; Powell et al. species richness appears to restrict both the numbers 2011; Sankaran et al. 2014; Joshi et al. 2015; Thapa of invasive species and their proportions in the et al. 2016; Schirmel et al. 2016; Early et al. 2016; community (Kennedy et al. 2002). Bellard et al. 2016). Although there were reports that There is more compelling evidence for the role of rates of new introductions of alien species may have disturbance, and observations show that open and peaked decades ago (Tye 2001), recent analyses show disturbed forests are more susceptible to invasion, no evidence for saturation in the accumulation of particularly with the altered vegetation and microhab- invasive species worldwide (van Kleunen et al. 2015; itat characteristics that accompany forest degradation Seebens et al. 2017). Dramatic increases in range and (Cadenasso and Pickett 2001; Pysˇek et al. 2002;te abundance of invasive species were reported long ago Beest et al. 2015b). However disturbance typically (Everett 2000; Mack and Lonsdale 2001), leading to also affects diversity, so the relationships between extensive worldwide research on the factors that invasibility on one hand and disturbance and diversity facilitate expansions of invasive species (Stohlgren on the other need to be carefully examined (Clark and et al. 1998; Fridley et al. 2007; Barbosa et al. 2010; Johnston 2011). Established native plants may provide Spear et al. 2013; te Beest et al. 2015b; Diekmann et al. invasion resistance even under disturbance and high 2016; Seebens et al. 2018), and also in predicting their propagule pressure (McGlone et al. 2011), but such a future distributions (Gallien et al. 2010; Adhikari et al. relationship may vary temporally with disturbance 2015; Mainali et al. 2015). (Clark and Johnston 2011). Nevertheless, the decline Although humans have been largely responsible for in tree density by fire and clearing of vegetation leads introductions of non-native species in different parts to the opening of habitats and facilitates invasions (te of the world (Cassey et al. 2005; van Kleunen et al. Beest et al. 2012). Such changes in vegetation 2015), the invasibility of different types of ecosystems structure may also lead to the increased availability to invasive species is less certain. Theoretical studies of light and soil resources, which invasive species may on invasibility have examined a wide range of exploit with greater abilities for light capture (te Beest processes from fluctuating resources, tolerance- and et al. 2015a) or nutrient acquisition (Sardans et al. suppression-based competition, and the availability 2017). Persistent disturbance to natural ecosystems is invading propagules or propagule pressure (Davis usually associated with increased human activities and et al. 2000; MacDougall and Turkington 2005). appears to have a strong influence on both plant and Empirical studies on invasibility of different ecosys- animal invasions (Spear et al. 2013). tems have implicated riparian habitats as havens or In addition to habitat factors, the specific life corridors of exotic invasions (Stohlgren et al. 1998; historical attributes that may give invasive species a Truscott et al. 2007), but other research has shown that competitive edge and determine their success have quantifying propagule pressure is critical for assessing also been widely investigated. Photosynthetic rate, the role of habitat (Eschtruth and Battles 2011). water- and nutrient-use efficiency, resistance or toler- Whether native species diversity itself influences ance to herbivory, disease, and fire are physiological invasibility has long been debated, and although it is attributes where non-native species may score over appealing to think that high native species diversity resident species (van Kleunen et al. 2010). Studies, confers resistance to invasion, the relationships however, do not provide unequivocal support for between native species numbers and the number of superior life history strategies of invading species but 123 Plant Ecol point to how these species exploit changed conditions. nutrient levels for the entire park, we also predicted For example, in Chromolaena odorata (L.) R. M. King invasive species abundance for the entire landscape. and H. Robinson (Asteraceae), a noxious non-woody sub-shrub widely prevalent in the tropical and sub- tropical ecosystems, photosynthetic traits indicate Materials and methods moderate shade-tolerance and ability to grow in moderate shade, but its expansion is only rapid in Study area open and disturbed habitats (Quan et al. 2015), with modified resource allocation under disturbance (te Manas National Park (MNP) is a 519 km2 protected Beest et al. 2015b). Therefore, ecosystems that are area located at the foothills of the Bhutan Himalayas heavily impacted by disturbance tend to be affected in (26°350–26°500N, 90°450–91°150E) under Bodoland numerous ways, and invading species may simply Territorial Council of Assam, India (Fig. 1). It is exploit the changed conditions that may be more bounded to the north by dense sub-tropical forest in the limiting to native flora (Didham et al. 2005; MacDou- contiguous Royal Manas National Park in Bhutan. To gall and Turkington 2005). In the short term, the the south of MNP are thickly populated villages and release of nutrients and space due to disturbance may agricultural land, while the eastern and western facilitate invasion under high propagule pressure boundaries are contiguous with reserve forests that (Catford et al. 2011) by providing a ‘niche opportu- are largely fragmented and subject to different