Published online August 9, 2023
Journal of Ecology and Environment (2023) 47:08
Faculty of Forestry, Agriculture and Forestry University, Hetauda 44107, Nepal
Correspondence to:Balkrishna Ghimire
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Background: Knowledge of the spatial trends of plant invasions in different habitats is essential for a better understanding of the process of these invasions. We examined the variation in invasive alien plant species (IAS) richness and composition at two spatial scales defined by elevation and habitat types (roadside, forest, and cultivated lands) in the Makawanpur district of Nepal. Following an elevation gradient ranging from 500 to 2,400 m asl along a mountain road, plant species cover was recorded within sample plots of size 10 m × 5 m. Systematic random sampling was adopted in every 100 m elevation intervals on three habitat types.
Results: Altogether 18 invasive alien plants belonging to eight families were recorded within 60 plots, of which 14 species (representing 80%) were from tropical North and South America. The most common plants by their frequency were
Conclusions: The species richness and density of IAS were higher in the road site followed by the cultivated land and forest sites. This pattern occurred throughout the elevation range and habitats. IAS were found mostly in the open land with high sunlight availability. Information from such scientific assessment of invasive alien plants will assist in developing appropriate management plans in the Makawanpur district.
Keywords: altitudinal gradient, habitat types, plant invasion, specie richness
After being introduced outside of their natural distribution area, invasive alien plant species (IAS) invade and threaten biological diversity, ecosystem, and human well-being in another region and the processes of this invasion are known as biological invasions (UN Environment Programme 2014). The rising biological invasion has a higher influence on biodiversity (Kohli et al. 2004), ecosystem functioning (Pejchar and Mooney 2009), agricultural productivity, health, and socioeconomic stability globally (Paini et al. 2016; Seebens et al. 2017). IAS are spreading into new areas that are not native to them due to human-induced factors, either purposely or accidentally (Ehrenfeld 2010), and this is happening more frequently in the changing world, having an impact on the environment, agriculture, and livelihoods (Paini et al. 2016; Pejchar and Mooney 2009; Simberloff et al. 2013). As a result, it is seen as a major contributor to environmental degradation which threats biological diversity and ecosystem services (Simoncini et al. 2019).
Along with climate change, several changes such as a rapid change in land use and land cover including the construction of roads, prolific growth in international trade, a steady increase of labor migration, and a flourishing tourism sector facilitate an increase in human mobility (Ghimire 2016), which helps in the spread of invasive species directly and indirectly (Pyšek and Richardson 2010). Short generation times, greater environmental tolerances, high fecundity, and strong dispersion abilities make invasive species highly suited to change (Bradley et al. 2009). Similarly, the trade of ornamental plants and seeds facilitates biological invasions (Pyšek and Richardson 2010). However, the intensity of biological invasions is likely to increase in the future with changing climate and increasing anthropogenic disturbances in the mountains (Petitpierre et al. 2016). Biological invasions and climate change act simultaneously, and the relationship between these two parameters in this changing world is considered a serious threat to biodiversity (Mainka and Howard 2010; Walther et al. 2009). The change in the climatic conditions and ecosystem services regulate the regional shifts, species distribution, and composition across landscapes (Intergovernmental Panel on Climate Change 2022). Similarly, humans also play a significant role in reshaping socio-ecological systems in desirable ways, altering the species composition and diversity (Biggs et al. 2022).
In Nepal, the changing climate has increased the invasion risk at a higher level. Since the majority of invasive species are native to tropical America, their distribution is also expanding in the southern lowland with tropical to subtropical climates (Shrestha 2016; Tiwari et al. 2005). Many of the species are introduced by gardeners, horticulturists, foresters and the returned British Gorkha soldiers which has become a severe problem since they are invading Nepal’s lands (Kunwar 2003). Many of these species spread rapidly, invading native plants and altering the biological community and ecological systems, such as nutrient cycling, energy flow, or the hydro-dynamic features of its recipient ecosystem (Huston 2004). The topography of Nepal is characterized by its extreme altitudinal variation, which leads to the formation of diverse climatic zones (Bhattarai et al. 2004), making it susceptible to the establishment of IAS from a wide range of their origins. Altogether, 183 species of naturalized flowering plants have been reported from Nepal and 28 of them are considered invasive (Shrestha et al. 2021; Shrestha and Shrestha 2021). The majority of these IAS are herbs (including herbaceous climbers) or shrubs; none of the alien tree species in Nepal is invasive (Shrestha 2016). Among these 28 IAS, five species are most common and troublesome in Nepal’s protected areas:
The distribution patterns of invasive species in Nepal and their colonization shows that it may have a serious impact on the biodiversity, particularly in the tropical and subtropical region, if steps are not taken to control it immediately. In addition, altitude significantly influences the composition of plants inhabiting a specific ecosystem due to the variation in biotic and abiotic factors that occur along with elevation. As altitude rises, plants encounter drastic abiotic transformations including temperature reduction, oxygen depletion, atmospheric pressure drop, intensified irradiation, accelerated wind speeds, increased snow accumulation, dry and frozen soils, and others (Körner 2003). It has been suggested that the high elevation presents a diverse range of abrupt environmental transitions within relative proximity (Körner 2003; Peterson et al. 1997), and many non-native species tend to establish their distribution boundaries along these elevational gradients. Despite the growing concern about plant invasions, little research has been conducted on the invasion of high mountain systems. Moreover, the limited number of studies that have been published demonstrated the significant variability between mountain areas regarding the altitudinal distribution of alien plants. For instance, Western and Juvik (1983) reported a less number of both introduced and native plant species along altitudinal gradients in roadside habitats in Hawaii. Similarly, Tassin and Riviere (2003) observed a comparable pattern for introduced plant species in the higher regions of an altitudinal transect on the tropical island of La Rѐunion. On the other hand, Johnston and Pickering (2001) documented a substantial presence of non-native plant species at subalpine elevations exceeding 1,500 m asl in the Australian Alps. Unfortunately, there is a lack of studies on the local and regional level about the distribution of invasive species along the altitudinal gradient in the mountain areas of Nepal.
Precise and reliable information concerning the presence and distribution of IAS is of utmost importance, as it provides a fundamental basis for evaluating the potential risks associated with them and facilitating the implementation of appropriate response strategies. This study has been conceptualized to attain the distribution of IAS along with elevation in Makawanpur district, central Nepal. We examined the diversity and distribution patterns of IAS across various elevation levels and habitat types. The objective of this study was to understand the diversity and distribution trends of IAS along the elevation gradient. Furthermore, we sought to identify the preferred habitat types for IAS distribution along the elevation gradient, specifically focusing on roadside areas, forests, and cultivated lands.
The study was conducted on roadsides along an elevation gradient from Hetauda (500 m asl) to Simbhangyang (2,400 m asl) (27°27’57.89”N 85° 2’34.22” E–27°36’11.35”N 85° 4’19.15” E) along the Tribhuvan highway of Nepal (Fig. 1). It is the oldest and the first highway in Nepal which was the first serviceable road connection with India operated in 1959. This route is the arterial road from the lowlands to the highland of the Makawanpur district. This study area comprises tropical, subtropical, and temperate climatic zones (District Development Committee 2015). The tropical area stretches up to an elevation of 1,000 m asl, characterized by hot and humid summers and mild winters. On the other hand, the subtropical region spans from 1,000 to 2,000 m asl, featuring hot and humid summers and mild to cool winters. Similarly, above 2,000 m asl, the climate is characterized by mild to warm summers and cool to cold winters. Our study sites represent a diverse vegetation composition concerning elevation gradient. The tropical zone is characterized by Sal and riverine forests at the southern lower belts where
Study plots were systematically located at 100 m elevation intervals from 500–2,400 m asl. In each step of 100 m elevation, we placed three plots, one on the roadside, one in the forest, and one on cultivated land. On the roadside, starting from the end of the paved or other hard artificial surfaces of the road, plots were established. In the forest, sample plots were located 100 m horizontally away from the roadside, and nearby cultivated land (between 100–150 m from the road) was selected for sampling on cultivated land. A total of 60 rectangular plots, each measuring 10 m × 5 m, were sampled across 20 altitudinal points in the study area. The rectangle plots were fixed such that the long axis was placed parallel to the road. In each plot, we recorded all IAS. Sampling was conducted between October 2022 and December 2022. Identification of invasive species was carried out by following Adhikari et al. (2022) and also by comparing specimens deposited at National Herbarium and Plant Laboratories (KATH), Godawari Lalitpur. The elevation of each plot was determined and the slope was measured with a clinometer. The percent tree canopy cover was estimated by using a densitometer (Lemmon 1957).
Species richness was calculated using the total number of shrubs and herbs recorded in each 100 m wide elevation band. The classification of species as native or alien was based on the literature by Adhikari et al. (2022). All the alien plants mentioned in the above literature are categorized as IAS. We determined the elevational range for species present in different habitats. For each species, we plotted the elevation of the lowest and highest plot where the species occurred. To explore the altitudinal richness patterns of invasive alien plants in different land-use sites, we performed linear regression analyses to detect the relationships between species richness and elevation for IAS. Also, we performed linear regression analyses to detect the relationships between the density of IAS on different sites and elevations. Additionally, linear regression plots have been utilized to demonstrate the correlations between tree canopy cover and the richness and density of IAS. All the statistical analyses were accomplished using the Statistical Package for Social Sciences version 16.0 (SPSS Inc., Chicago, IL, USA) and graphs were prepared in OriginPro.
A total of 18 invasive plant species belonging to eight families have been recorded at 53 sampling plots (20 roadside, 17 forests, and 16 cultivated lands). Seven sampling plots, comprising three within the forest and four within the cultivated lands, were free from alien invasive species. Among eight families Asteraceae with nine species was found to be dominant, followed by Fabaceae with three species; while the remaining 6 families were represented by single species (Table 1). The life form pattern distribution showed that 67% (12 species) of the plant species were herbs, followed by 28% (5 species) shrubs and 5% (1 species) of climbers. Concerning the longevity of the plants, 61% of the plant species were annuals while 39 % were perennials. In terms of families, Asteraceae exhibited a higher number of alien species richness along the altitudinal gradient compared to other families, which were predominantly confined to elevations below 1,600 m (Table 1).
The most widespread alien species was
IAS were distributed all along the elevation gradient in the study area i.e. from 500 m to 2,400 m. The maximum richness of IAS was observed at 500 m elevation. After this, a gradual decline in their richness was observed. Among the 18 species, only 3 species such as
The distribution of IAS richness showed statistically significant with elevation. It decreased with increasing elevation (
The density of IAS at different habitat sites also showed a statistically significant relationship with elevation. It decreased with increasing elevation (
The density and species richness of IAS showed a significant negative correlation with tree crown cover (Fig. 5A, B). IAS were found mostly in the open canopy, degraded forest, and open land where light penetration is high. The species richness and abundance of IAS were found higher in open canopy sites. Similarly, the species richness of IAS showed a significant positive correlation with density.
The present study area was found to be inhabited by a total of 18 IAS belonging to eight families. Among them, Asteraceae, with nine species, is the most dominant family followed by Fabaceae (3 species). Some previous studies on the floristic composition and diversity of IAS in tropical and Himalaya regions of India have also described Asteraceae as a dominant family (Kosaka et al. 2010; Reshi et al. 2017; Sekar 2012). In our case, it would be the obvious reason that Asteraceae is the most species-rich family with 10 IAS in Nepal (Shrestha and Shrestha 2021). Alternatively, the plant species belonging to this family can tolerate the anthropic disturbance regimes and also have the potential nitrogen-fixing capacity that would help to colonize easily in vulnerable ecosystems (Sekar 2012).
Among 18 IAS in the study sites, the highest frequency and wider distribution range of
With increasing elevation in the study area, there was a decline of IAS richness and coverage in all habitats, for instance, roadside, forest areas, and cultivated land. Studies around the world consistently demonstrate a pattern of declining non-native species richness from a maximum at the lowest elevation, regardless of the specific broad bioclimatic region. This trend has been observed in all biomes of the world (Alexander et al. 2011; Seipel et al. 2012) and regional studies from cold arctic systems (Lembrechts et al. 2014), temperate mountains in North America (Andersen et al. 2015), Europe (Medvecká et al. 2014) and China (Zhang et al. 2015), Mediterranean mountains (Barros and Pickering 2014), and Himalaya region of India (Ahmad et al. 2018; Kosaka et al. 2010; Manish 2021). In low-elevation regions, more invasive species richness is expected due to favourable climates for colonization, low geographic and climate isolation, high human disturbance, easy road/transportation access, human population, less environmental heterogeneity, and less extreme habitats (Becker et al. 2005). Because the roads and trails are regarded as key pathways for invasion into mountains (Fuentes et al. 2010; Lembrechts et al. 2017), they can provide opportunities for non-native species establishment and dispersion (Becker et al. 2005). The decline of exotic species richness at high elevations can be further explained by the lack of pre-adaptations to survive in extreme high-altitude conditions, along with limited colonization of high-elevation regions (Alexander et al. 2016). Mountains serve as environmental filters, where non-native species initially take root in the lowlands and the only species adapted to lowland conditions can expand their presence to higher elevations (Haider et al. 2010). Environmental constraints (Alexander et al. 2011; Seipel et al. 2012), a lower propagule pressure (Quiroz et al. 2011), or decreased human land use (hence lower disturbance, Seipel et al. 2012), all contribute to an alien species’ gradual drop-out with increasing elevation. Further limiting the spread of invasive species in the highland include low temperatures, poor ecosystem productivity, short growing seasons, and low precipitation at high elevations (Manish et al. 2017). Our study area is no different, as the higher elevation range of the sampling site is situated in a temperate climatic zone and experiences extremely cold (even snowfalls) during winter. Furthermore, higher elevational sites exhibit limited human settlements and transportation infrastructure within the area which limit the occurrence of non-adapted plant species from tropical regions.
This study indicated that throughout the elevation range, the density of IAS was higher at the roadside followed by cultivated land and forest area. Studies revealed that the degree of anthropogenic disturbance is a key predictor of IAS richness (Jauni et al. 2015; Liu et al. 2005). Studies suggested that anthropogenic factors can influence the time, amount, and pathways of IAS, and environmental factors can affect how many species can persist when introduced to new locations (Dyer et al. 2017; Wang et al. 2020, 2022). We found that the distribution rate of IAS on roadsides and cultivated land was high, and presented a pattern of spreading from road to cultivated land. The areas of lower elevation and proximity to roads are highly populated, resulting in the most significant disruptions to cultivated land, road construction, and urban development. Furthermore, roads significantly contribute to the spread of non-native species by creating corridors for dispersal providing suitable habitats, and containing reservoirs of propagules for future episodes of invasion (Arianoutsou et al. 2013; Dimitrakopoulos et al. 2022; McDougall et al. 2018; Pauchard and Alaback 2004). Shrestha et al. (2019) provided evidence that road verges not only serve as suitable microhabitats for colonization but also function as dispersal corridors for IAS like
IAS are predominantly found in the open canopy, degraded forest, and open land where light penetration is high (Ghimire et al. 2020). In this study, we also recorded higher species richness and abundance of IAS in open canopy sites. Forest areas with dense canopies serve as physical obstacles that impede the dispersal of IAS, while the prevailing light and moisture conditions within these forests create environmental barriers that hinder the establishment of such species (Mavimbela et al. 2018). A closed canopy restricts seed dispersal and hampers the growth of early successional species due to limited light penetration within the forest (Lawes et al. 2004). Our result agreed with previous research indicating that roads serve as conduits for IAS, facilitating their spread from disturbed areas to cultivated lands in temperate forests (Pauchard and Alaback 2004; Trombulak and Frissell 2000). Despite harsh environmental conditions for growth in higher elevations, some IAS can establish themselves if there is an ample supply of seeds. Furthermore, our findings align with existing evidence highlighting roadsides as the initial sites of colonization for IAS, offering insights into potential invasion ranges (Parendes and Jones 2000; Pauchard and Alaback 2004; Trombulak and Frissell 2000).
The study revealed that species richness and density of IAS declined along with increasing elevation. The majority of IAS are introduced at low elevations, and as climatic conditions worsen and anthropogenic pressure decreases along the elevation gradient, they are gradually filtered out. IAS were found mostly on the roadside than in other habitats, open canopy, degraded forest, and open land with abundant sunlight penetration. Roads significantly contribute to the spread of non-native species by creating corridors for dispersal and disturbed areas to infiltrate adjacent natural vegetation. Asteraceae had the higher IAS richness as compared to other families which were found restricted below 1,600 m. This study revealed that only
IAS: Invasive alien plant species
We are grateful to Professor Dr. Bharat Babu Shrestha, Central Department of Botany Tribhivan University, Nepal for his constructive comments in the early version of this manuscript.
BG and DK conceptualized the research. DK and BP collected and analyzed the data and wrote the manuscript. BG critically commented and approved the final version of the manuscript. The authors read and approved the final version of the manuscript.
The data that supports the findings of this study will be made available upon reasonable request.
The authors declare that they have no competing interests.