Expansion risk of invasive plants in regions of high plant diversity: A global assessment using 36 species

Highlights

• We evaluated the expansion risk of invasive plant species (IPS) in global regions of high plant diversity.

• IPS may expand across the vulnerable biome areas, including nature reserves.

• Conservation management should be enhanced, to reduce the expansion risk of IPS across different biomes.

Abstract

Invasive plant species (IPS) have a high potential for expanding within biodiversity hotspots and threatening global plant diversity. Hence, it is urgent to assess the expansion risk of IPS in regions of high plant diversity and their potentially negative effects throughout the world. We used the world's 36 worst IPS as focal study species and applied species distribution modeling to project the suitable habitat distributions of these IPS in centers of plant diversity at the global scale. Subsequently, we assessed the expansion risk of IPS based on habitat suitability across biomes and nature reserves. We found that IPS, particularly Chromolaena odorata, Eichhornia crassipes, Leucaena leucocephala, and Mimosa pigra, have high expansion potential in Tropical & Subtropical Moist Broadleaf Forests, Tropical & Subtropical Dry Broadleaf Forests, and Mangroves within global regions of high plant diversity. Furthermore, IPS such as Imperata cylindrica and L. leucocephala may expand dramatically into Vulnerable and Critical or Endangered biome areas, including nature reserves across Tropical & Subtropical Moist Broadleaf Forests and Tropical & Subtropical Coniferous Forests. We suggest that effective conservation management should be enhanced in order to reduce the expansion risk of IPS in the Vulnerable and Critical or Endangered biome areas, that temperature changes should be carefully monitored, and that conservation policies of nature reserves should be reviewed, particularly regarding IPS with high potential to naturally disperse into the nature reserves.

Introduction

With increasing trade and tourism associated with globalization and the expansion of human populations, invasive plant species (IPS) have a high potential to expand widely outside of their natural boundaries (Kalusová et al., 2013; Rejmánek, 2015; van Kleunen et al., 2015). Previous studies have shown that the expansion of species could become apparent later in invasion events and consequently have extensively negative effects on native species and the overall stability of native ecosystems (Adams et al., 2015; Mainali et al., 2015; Pyšek et al., 2012; Roger et al., 2015; Vicente et al., 2013). The International Union for the Conservation of Nature's (IUCN's) Invasive Species Specialist Group (ISSG; http://www.issg.org/) list has shown that of the world's 100 most invasive species, 36 are plant species, and these IPS may seriously affect biodiversity worldwide (Lowe et al., 2000). IPS represent introduced plant species with generally broad physiological niches and/or special functional traits and may respond quickly to changing environmental conditions. Thus, IPS can quickly adapt to new habitats with climates that resemble those of their native habitats and can compress the living spaces of native species, thereby enhancing their invasiveness in non-native habitats (Catford et al., 2012; Hellmann et al., 2008; Lowe et al., 2000; Pyšek et al., 2012). Pyšek et al. (2012) have shown that the negative effects of IPS expansion include reduced survival of resident biota and of resident community productivity, lower mineral and nutrient content in plant tissues, and reduced plant diversity at the community level. Additionally, IPS have a high probability of expanding into areas with high protection values, particularly the hotspots of plant diversity and nature reserves. This has negative economic and ecological effects, for example, deterioration of soil quality and diminished biological diversity (Catford et al., 2012; Foxcroft et al., 2011; Hellmann et al., 2008; Stohlgren et al., 1999; Thalmann et al., 2015; Vicente et al., 2013; Wakie et al., 2016). These factors make it necessary to assess the expansion risk of IPS in regions of high plant diversity.

Previous studies have shown that risk maps based on potential distributions for IPS can be useful tools for anticipating species invasions and controlling species expansion in regions of high plant diversity (O'Donnell et al., 2012; Roger et al., 2015; Stohlgren et al., 2010; Thuiller et al., 2005; Vicente et al., 2013). Conservation ecologists mapped the expansion of risk hotspots of plant invasion for Australia, India, and the United States, based on the potential distributions produced by species distribution modeling (SDMs), and suggested effective control and prevention methods for the spread of IPS in invaded ranges (Adhikari et al., 2015; Bradley et al., 2010; Garcia et al., 2013). These studies examined regional expansion risks of IPS invasion for areas of high plant diversity (Adhikari et al., 2015; Bradley et al., 2010; O'Donnell et al., 2012; Pěknicová and Berchová-Bímová, 2016). However, it is still a challenge to evaluate the expansion risk of IPS at global scales owing to a lack of distribution data and modeling robustness. SDMs are good tools for assessing the expansion risk of IPS in global regions of high plant diversity. For example, Bellard et al., 2013, Bellard et al., 2014 used SDMs to assess the expansion risk of the world's 100 most invasive species, including 36 IPS, for 14 biomes and 34 biodiversity hotspots at a global scale. Those results give us new insights into the assessment of plant invasions in hotspots of plant diversity based on the SDMs.

The IUCN and the World Wide Fund for Nature (WWF) have identified the regions with the highest plant diversity worldwide (http://www.biodiversitya-z.org/). These regions play an important role in biodiversity conservation and social-cultural values. However, there is no specific management prescribed for those regions with the highest plant diversity, although many are represented, at least in part, in existing nature reserves or have been proposed for inclusion (http://www.biodiversitya-z.org/). IPS have a high potential for threatening the plant diversity of nature reserves, and even these nature reserves could lose conservation functions if invaded (Hiley et al., 2013; Thalmann et al., 2015; Vicente et al., 2013). Hence, it is urgent to prevent and control the expansion of IPS in these nature reserves. Different biomes may contain habitats with varying degrees of likely expansion for IPS (Donoghue and Edwards, 2014; González-Moreno et al., 2014). Some studies have developed model-based methods to evaluate the expansion risk of IPS in invaded regions across different biomes worldwide (Adhikari et al., 2015; Catford et al., 2012; Pěknicová and Berchová-Bímová, 2016; Thuiller et al., 2005). Considering the great negative effects of plant invasions on nature, society, and economic activities, two tasks need to be conducted to enable effective control and prevention of plant invasions: (1) assess the expansion risk of IPS in regions with high plant diversity, particularly in nature reserves stratified by biomes and (2) assess the potential effect of IPS expansion on the conservation of plant diversity.

To address these practical issues, we examined the expansion risk of IPS through three complementary objectives: (1) identifying the biomes with the highest expansion risk of IPS; (2) exploring the IPS with the highest expansion risk; and (3) assessing IPS expansions in nature reserves. In this study, we examined 36 IPS from the list of the world's 100 most invasive species as the focal study species, and Maxent modeling, a common SDM, was used as the assessment tool to model suitable habitats for IPS.