The invasive alien species are introduced to other parts of the world from their native ranges either deliberately or accidentally (IUCN, 2017). They are one of the primary causes of biodiversity loss, leading to changes in the structure and composition of the ecosystems (IUCN, 2017), ecosystem services, human well-being, agriculture and economic growth (Simberloff et al., 2013). They also act as hosts to pathogens and spread disease to humans, crops and native biodiversity (Barker, 2002; Simberloff et al., 2013). In conjunction with habitat loss, invasive species have been associated with nearly 60 percent of species extinctions in the last century (Bellard et al., 2016). With ever-increasing international trade, human-induced habitat destruction and climate change, the pressure caused by invasive species is only likely to increase in the future (Hulme, 2014).
Niche conservatism is the extent to which ecological niches are conserved through time and space (Wiens et al., 2010). Many studies have looked into the niche conservatism perspective (Wiens et al., 2005) and change in the fundamental or realised niche or both (Pearman et al., 2008) of the invasive species to understand the speciation effect of climate change due to biological invasions (Hulme, 2017). Several studies have suggested that climatic niche occupied by invasive species including plants (Broennimann et al., 2007; Holt et al., 1990; Dietz and Edwards 2006), insects (Fitzpatrick et al., 2009; Medley et al., 2010), fishes (Lauzeral et al., 2011) and amphibians (Tingley et al., 2014) may not be conserved between their native and introduced regions (Goncalves et al., 2014). In the case of invasive alien species, niche differences between native and introduced ranges are of three types, a) invasive species colonising novel environmental conditions relative to their native range, i.e., niche expansion, b) partial filling of the native niche in the invaded range, i.e., niche unfilling and c) proportion of the invaded range overlapping with native niche, i.e., niche stability (Warren et al., 2008; Strubbe et al., 2015). One commonly discussed topic in ecology is niche shifts during biological invasions (Guisan et al., 2014; Broennimann et al., 2007). Generally, climatic niche shifts contradict the assumption of niche conservatism, which implies that species retain their niches in space and time (Wiens, 2005). This assumption underlies ecological niche modelling, the most commonly used approach for assessing the impact of climate change on biodiversity (Rodda, 2011). Comparisons between species' native and non-native climatic niches may identify species that have undergone adaptive evolutionary changes during the invasion process (e.g. change of the fundamental climatic niche) and lead to a better understanding of niche dynamics (Broennimann et al., 2007). The reproductive ability of the invasive species might influence the invasibility of the species. For example, asexual, parthenogenetic and hermaphroditic self-fertilising species might be more aggressive than sexually reproducing ones. However, a recent meta-analysis across taxa (both plants and animals) has shown no difference between them (Roman and Darling 2007).
The adverse effects of invasive species may be intensified by climate change (Pyke et al., 2008). Negative impacts are further increased as invasive species are typically generalists with broad climatic tolerances; they are considered likely to cope with change in the climate, enabling them to expand into new areas (Walther et al., 2009). In recent times, there is growing evidence for the effects of climate change on invasive species distribution and is considered one of the main drivers of future invasions (Bellard et al., 2013). Studies involving various taxa have shown that climate change has serious impacts on the niche expansion and niche shift in invasive species (Ahmed et al. 209; Atwater et al. 2018; Wan et al., 2016). Forecasting responses of invasive species to climate change using ecological modelling or species distribution modelling has been extremely useful (Dillon et al., 2010; Gilman et al., 2010; Pereira et al., 2010; Salamin et al., 2010; Beaumont et al., 2011; Dawson et al., 2011; McMahon et al., 2011; Sarma et al., 2015) and play a vital role in identifying potential future risk areas and help in developing sound strategies to reduce future impacts on native biodiversity (Pereira et al., 2010; Parmesan et al., 2008). The ecological niche modelling tool is widely used to predict the geographical distribution of a given species based on correlations of species occurrence and suites of environmental and climatic variables (Philips et al., 2006). The species distribution models are also used to select the areas to determine the risk of species invasions under current and future climate scenarios (Guisan et al., 2013).
Lissachatina fulica, commonly known as the Giant African Snail, is one of the 100 worst invasive species in the world (Lowe et al., 2000; Raut and Barker, 2002). The native range of L. fulica is Ethiopia, Kenya and the United Republic of Tanzania (Bequaert, 1950a, in Raut and Barker 2002) and was introduced to other tropical and subtropical regions with warm and mild temperatures and high humidity. Lissachatina fulica is well adapted to various land use and land cover types such as agricultural areas, coastal areas, wetlands, natural forests, riparian forests, scrublands, shrublands, forest edges, plantations and modified forests (Raut, 2002; Moore, 2005) and even urban areas. This species is found to transmit diseases through spores (found in faeces) and is known to infect a variety of agricultural plants such as pepper, coconut, papaya and vanilla (Mead, 1961, 1979; Turner, 1964, 1967; Muniappan, 1983; Schotman, 1989). The snail L. fulica has a tremendous impact on native biodiversity, agriculture and horticultural crops and is known to feed on more than 500 native plants and crop species (Raut and Baker 2002). It also competes with native snail species (Sharma et al., 2015). Thus, there is an urgent need to understand the relationship between climate change and L. fulica distributions since it has a significant impact on biodiversity and the livelihoods of farmers. A handful of studies have assessed invasion patterns of L. fulica using distribution models but are on a local or regional scale (Thiengo et al., 2007; Vogler et al., 2013; Sridhar et al., 2014; Sarma et al., 2015). A couple of recent studies at the global level shows that L. fulica has shown low haplotype diversity for mitochondrial markers in the invaded regions of the world (Fontanilla et al., 2014; Vijayan et al., 2020). In this study, we model the potential global distribution of L. fulica under present climatic conditions and future climate change scenarios using a maximum entropy algorithm. The following are the objectives of this study: (1) identify the potential distribution of L. fulica at a global scale (2) assess the impact of climate change on invasion patterns of L. fulica across continents, (3) test niche conservatism in invaded range and under climate change scenarios and (4) compare haplotype richness with habitat suitability derived from niche modelling in the invaded areas.