Diversity, Distribution, and Impacts of Invasive Alien Plant Species in Lake Cluster and Associated Landscape in Pokhara Valley, Central Nepal

Abstract

Wetlands including lakes support high biodiversity providing essential ecosystem services, yet they are among the most vulnerable habitats worldwide. Several invasive alien plant species (IAPS) have invaded Nepal’s wetlands. However, the available data are insufficient to inform policy and management decisions. This research was carried out in the Lake Cluster of Pokhara Valley (LCPV), a Ramsar site in central Nepal, with the objectives to 1) analyze the diversity of naturalized plant species in the LCPV, 2) identify IAPS and their spatial distribution in Pokhara valley, 3) assess the level of eutrophication in the LCPV, 4) identify the pathways of introduction and dispersal of the IAPS, and 5) assess the impacts of IAPS on ecosystem services in the study area. Altogether 89 plots (20×5 m2) were sampled to record flowering plants and their cover along the shoreline of the nine wetlands that constitute the LCPV. The species were identified, classified based on their life forms (Raunkiaer’s life forms), and categorized into native and naturalized species. The variation of naturalized species richness with native species richness was analyzed by a generalized linear model. The relations of environmental variables with species composition were analyzed by ordination. A survey of roadside vegetation (201 5m  5m plots) was carried out to record the diversity and distribution of the IAPS in Pokhara valley. To assess the eutrophication status, altogether 89 water samples were collected from all nine lakes, and estimated nitrate nitrogen and total phosphorus. To analyze the impacts of nutrient contents on species richness and cover of native and naturalized species, multiple regression was used. To trace the sources and dispersal pathways of the IAPS, residents (n=56, age >37 yrs) living around the wetlands with a high level of plant invasions (Begnas, Phewa, and Rupa) were interviewed. To assess the impacts of IAPS on ecosystem services, household surveys (n=273), focus group discussions (n=19) and key informants’ interviews (n=23) were conducted among the beneficiaries of the LCPV such as the farmers, boaters, hoteliers, and fishermen. The study recorded a total of 258 species of flowering plants (73 families and 191 genera). The most species-rich family was Asteraceae (27 spp), followed by Poaceae (26 spp), Fabaceae (19 spp), and Cyperaceae (19 spp). Therophytes (29%) and Hemicryptophytes (27%) were the most common among the life forms. The richness of naturalized species increased with increasing native richness whereas the nutrients had no significant effect on the species richness of naturalized plants. The study found a total of 22 IAPS in Pokhara valley. Four of the recorded IAPS, viz. Chromolaena odorata, Lantana camara, Mikania micrantha, and Pontederia crassipes were among the 100 of the world’s worst invasive species. The most frequently encountered species were Bidens pilosa (63%), Ageratum houstonianum (61%), and L. camara (61%). In water samples of the LCPV, the phosphorus level was 0.29-1.25 mg/L and the nitrate nitrogen 0.04-0.17 mg/L. Based on these two nutrients, the wetlands of the LCPV were classified as eutrophic. According to the residents, most of the aquatic IAPS found in the LCPV were brought from the southern Tarai region of Nepal and northern India between the 1970s and 1990s. Among the IAPS, P. crassipes and Pistia stratiotes were brought for their ornamental values and Ipomoea carnea subsp. fistulosa was introduced for fencing purposes. It appears that Alternanthera philoxeroides was introduced accidentally to Phewa lake recently (observed for the first time in 2019). Local respondents had been familiar with Leersia hexandra for a long period (>50 years) and were not aware of the time and mode of its introduction, suggesting that this species could have the longest history of introduction to the LCPV. The study enlisted 18 ecosystem services obtained from the LCPV. The most cited services were water for irrigation, fish farming, livestock fodder, traditional mat materials (Machaerina rubiginosa), habitat protection, and natural beauty. The total economic benefit generated by the provisioning services of the LCPV in 2019 was estimated to be International $ 0.10 million per household per year. Before the proliferation of the IAPS (1989), the estimated economic value was almost triple (International $ 0.29 million per household per year). The decline in economic value was the most pronounced in fish production (International $ 151.1 thousand) and supply of lotus (International $ 24.1 thousand). The study revealed that the Ramsar enlisted wetlands of the Pokhara valley and the surrounding landscape have been invaded by several IAPS including some of the globally worst invasive species with significant negative impacts on wetland ecosystem services. As a recommendation, this study highlights science-based policy management for preventing the further spread of IAPS in the LCPV. It is suggested to prevention of the IAPS spread to non-invaded wetlands through public awareness and regular monitoring, and their control in the invaded wetlands through participatory approaches. Nonetheless, a well-managed IAPS in wetlands such as the LCPV not only supports the livelihood of the local people but also helps to meet national and global targets to conserve wetland biodiversity and habitats from further degradation.