Introduction Pathways of Invasive Alien Plants in the Lake Cluster of Pokhara Valley, Nepal

The Lake Cluster of the Pokhara Valley (LCPV), a Ramsar site, has been invaded by several invasive alien plant species (IAPS), yet the introduction pathways of these species remain unknown. In this paper, the possible introduction pathways and invasion history of five wetland IAPS of the LCPV have been identified and reported for analysis. Direct field observations were combined with data obtained from the questionnaire survey with the local residents to identify introduction pathways and invasion history. Among five invasive alien plants, Pontederia crassipes and Pistia stratiotes were introduced as ornamental plants that escaped to wetlands and established their populations during 1980s and 1990s respectively. Similarly, Ipomoea carnea subsp fistulosa was introduced during the 1990s for fencing and soil bioengineering purposes. It is highly likely that Leersia hexandra and Alternanthera philoxeroides were introduced accidentally as none of the local residents interviewed could explain the purpose of their introduction. The local residents have noticed the occurrence of L. hexandra for >50 years while A. philoxeroides might have reached to the LCPV sometime between 2015 and 2020. The results suggest the wetland IAPS have been introduced to the LCPV both intentionally as well as accidentally, and that the prioritizing pathway management can prevent their future introductions.


Introduction Pathways of Invasive Alien Plants in the Lake
Cluster of Pokhara Valley, Nepal

INTRODUCTION
Biological invasions are the result of human-mediated extra-range dispersal, which, unlike natural extra-range dispersal, is usually the result of many introductions from the multiple sources to the multiple locations (Wilson et al., 2009).The pace of invasion is affected by the introduction frequency of the propagules of alien species, the availability of suitable areas for their establishment, and population growth rates (Elliott, 2012).The entire process of biological invasion constitutes the four stages: transport, introduction, establishment, and spread (Blackburn et al., 2011).Understanding the extent of the alien plant introduction pathways is critical for species invasions (Pysek et al., 2011).An interception of the potentially invasive species in their way of 'transport' and 'introduction' is the most cost-effective measure for the management of invasive species.For such measures to be implemented at the local level, it is indispensable to identify the introduction pathway of invasive alien species.
Alien species can enter a new location through three basic introduction pathways: the importation of a commodity, the introduction of a transport vector, and/or natural spread from a neighboring region where the species is already alien (Hulme et al., 2008).These three systems provide six major pathways, depending on the level of human involvement: release, escape, contamination, stowaway, corridor, and unaided.A single species may utilize the multiple introduction pathways and vice versa.Similarly, the invasive alien plants are introduced to wetlands both intentionally (e.g. trading item) as well as accidentally (Azan et al., 2015).However, the information pertaining to the introduction pathways of invasive alien plants in most of the wetlands in Nepal including the Lake Cluster of the Pokhara Valley (LCPV) remains unknown.In this context, the present study was conducted with the aim of identifying the introduction pathways of wetland invasive alien plants in the LCPV and tracing back their invasion history.Almost five aquatic invasive alien plant species have been found, which invade the lakes (see Table 1).For their management including preventing them from the further invasion to neighboring freshwater bodies, the knowledge of their dispersal mechanism and invasion history is crucial.

MATERIALS AND METHODS
Pokhara Valley, the capital city of the Gandaki Province is the most visited tourist destination of Nepal.Almost a million tourists visit Pokhara every year (Pathak, 2023).The valley is located between 27.55º to 28.23º N latitude and 83.48º to 84.11º E in longitude within an elevation range of 580-900 m asl (Pathak, 2023).It receives an average rainfall of 3515 mm with a minimum temperature of 7ºC (esp. in January) and a maximum temperature of 31.4ºC(Pathak, 2023).The valley has wet seasons from mid-February to mid-November and dry periods from mid-November to mid-February.Its floor is bestowed with nine lakes of international importance, which are included in the Ramsar site with the name, Lake Cluster of Pokhara Valley (LCPV).Among them, Phewa, Begnas, and Rupa lakes are popular.Most probably with tourist activities, the invasive alien plant species are increasing, spreading, and causing social, economic, and ecological impacts (Pathak et al., 2021).
Three larger lakes: Phewa, Begnas, and Rupa lakes of the LCPV, were the study sites.The authors chose this site for the study because there were the increasing concerns about the detrimental impacts of the invasive alien plant species in the lakes of LCPV and the study of this kind may help the management of these invaders.The interview with the inhabitants with a semi-structured questionnaire was the technique of the primary data collection.The respondents were the stakeholders as well as beneficiaries of these lakes.

Table 1
List of aquatic invasive alien plant species present in each lake (Pathak et al., 2021) SN Name of the species Phewa Begnas Rupa Direct field observations by the first author were combined with data obtained from the questionnaire survey with the residents to identify the introduction pathways and their invasion history.The boaters, farmers, hoteliers, teachers, political activists, and fishermen (a total 56 persons) were interviewed for data collection.The total percentage was obtained by dividing the number of respondents for the particular response by the total number of respondents, multiplied by 100.The respondents were selected by snowball sampling method (Etikan et al., 2016) from the inhabitants and beneficiaries of three major lakes, i.e., Phewa, Begnas, and Rupa.The remaining six wetlands of the LCPV were excluded since they remained free from the major invasive alien plant species (however, the Leersia hexandra may present) at the time of the study.Similarly, the open-ended questionnaires, as well as the photographs of wetland IAPS (Pathak et al., 2021) were used during the interview.Personal interviews were conducted and recorded with the permission of elderly inhabitants of the study sites.The collected data were analyzed to identify the purpose (pathways) of introduction (if any) of a particular IAPS and their first date of introduction on a scale of decades.

RESULTS
Pontederia crassipes was introduced first in Phewa Lake during the early 1980s (see Table 2).It might have been introduced accidentally through the fingerlings or intentionally as an ornamental plant from the cities like Bhairahawa or Chitwan in the southern part of Nepal (see Figure 1).It was brought to Begnas Lake and Rupa Lake in the late 1980s as an ornamental plant.Pistia stratiotes was first noticed in Phewa, Begnas, and Rupa during the 1990s and might have been introduced accidentally along with the fingerlings or introduced as an ornamental plant.The propagules of Pistia stratiotes might have also originated from Chitwan or Bhairahawa.Pistia stratiotes was noticed in these lakes nearly five years later than Pontederia crassipes.Similarly, Ipomoea carnea subsp.fistulosa was introduced to Phewa during the early 1980s and to Begnas during the late 1980s.Likewise, it was brought to Rupa during the 1990s.The introduction pathways and their invasion history of Alternanthera philoxeroides were not reported by any respondents but the weed was spotted by the authors in 2019 at a single location in Khahare of Phewa Lake.Given that the weed was confined at a single location with very low abundance, it might have arrived at the location after 2015.Another IAPS Leersia hexandra was commonly found in all three wetlands but the respondents could not inform the purpose and time of introduction; they informed that the species was there for >50 years.

DISCUSSION
Although Pontederia crassipes, one of the globally worst invasive species, was introduced in Phewa, Rupa, and Begnas during the 1980s, there is no introduction of these species in the other seven wetlands of the LCPV (Pathak et al., 2021).It is therefore of utmost importance to prevent the introduction of this species to these seven wetlands by making the different stakeholders aware of the introduction pathways and ecological consequences when introduced.The introduction pathway of P. crassipes in the present study area was similar to the report of several other previous studies in the different parts of the world.For example, the intentional introduction of P. crassipes as fish feed has been reported in western Nepal (Bhatta & Shrestha, 2021).An evidence that fishes feed on P. crassipes is also available from a study in Illinois River, USA (VonBank et al., 2014).In the inter-connected wetlands, the hydrological connectivity may also serve as the introduction pathways to the downstream water bodies (May, 2022).The weed was reported to be introduced to Asia at the end of the nineteenth century via Japan and Indonesia (Tellez et al., 2008).In some regions such as Iran, the introduction pathway remains unknown (Zarkami et al., 2021).
Although Pontederia crassipes, Pistia stratiotes, and Ipomoea carnea subsp.fistulosa have been introduced both intentionally as well as accidentally, the intentional introduction appears to be more important.For example, P. crassipes and P. stratiotes are commonly offered in the water garden and aquarium trade in Ontario, Canada (Adebayo et al., 2011).Ipomoea carnea subsp.fistulosa has been also cultivated extensively for ornamental purposes (Chen et al., 2018) as well as for soil bioengineering (Pathak et al., 2021a).Once it escapes to the wild, it can be problematic in wetlands.For example, in Koshi Tappu Wild Life Reserve (KTWR) area, a Ramsar site, Ipomoea carnea subsp.fistulosa is the most problematic along with the Pontederia crassipes (Pandey et al., 2020).
When the introduction pathways of three species, for which they are known, are assessed against a framework proposed by Hulme et al., (2008), the introduction pathways of all three species are associated with the importation of commodities.In particular, the introduction pathways of these species can be categorized as 'release' (e.g.Ipomoea carnea subsp.fistulosa for soil bioengineering), 'escape' (e.g. from ornamental Pontederia crassipes and Pistia stratiotes to their weedy populations) and 'contaminants' (e.g.P. crassipes as a contaminant of fingerlings transportation).Since the purpose of introduction of the remaining two species (Alternanthera philoxeroides and Leersia hexandra) is currently unknown, it is highly likely that they were also introduced as contaminants.But, further investigations are needed to support this notion.

CONCLUSION
Pontederia crassipes and Pistia stratiotes were introduced primarily as ornamental plants, which might have escaped to wetlands and established their populations during the 1980s and 1990s respectively.Ipomoea carnea subsp.fistulosa was introduced during the 1990s for fencing purposes.It is highly possible that Leersia hexandra and Alternanthera philoxeroides were introduced accidentally as none of the residents interviewed could explain the purpose of their introduction.Six of the nine wetlands of the LCPV are still free from noxious invasive weeds such as Pontederia crassipes and Pistia stratiotes.Similarly, Alternanthera philoxeroides have been reported only in Phewa Lake.The further expansion of these IAPS into other wetlands can be prevented when the introduction pathways of the IAPS are accounted for in the management of the LCPV.This study is unique to this type in LCPV and will help for more discussions in the further research of this kind.

RECOMMENDATION
It seems that the aquatic invasive alien plant species have been introduced in the recent past.Recognizing the negative impacts on various aspects of life, every year, the mechanical removal of water hyacinths was carried out after 1996 from Phewa Lake.However, the problem appears every year.Therefore, it is recommended for the complete removal from the source and lake shores without leaving any traces of the IAPS.The present study on the dispersal pathways of the IAPS may contribute to their management.