Introduction:

 Nepal, despite its small territory, is celebrated for its rich biodiversity and diverse ecosystems that support a wide array of plant and animal species. This vital natural diversity faces significant threats from anthropogenic activities, climate change, pollution, fragmentation and introduction of invasive alien plant species (IAPS). Invasive alien plant species (IAPS) is a species that has been deliberately or inadvertently introduced to a place, area, or region where it naturally does not occur [1]. They are endemic to one place or territory that affects biodiversity, ecology, and environments, and human safety and shows a tendency to spread out of control [2]. Invasion of exotic plant species are the second greatest threats to native forest ecosystems and species richness, after habitat fragmentation and loss [2].

These species exhibit several key attributes that possess threats to agrobiodiversity and ecosystem.

• Aggressive growth rate and high germination capacity.
• Ample seed production potential that are long-lived in nature.
• Higher ability to develop a wide range of phenotypic plasticity.
• Sustenance potential to a variety of environmental factors and types of food available.
• Release of chemical compounds having probability to change ecosystems. etc.

Overview of Invasive Species in Nepal:

The transportation of species started with colonization and exploration of past, where the history of IAS Chromolaena odorata is two centuries old in Nepal [3]. About 179 exotic species have been estimated to be naturalized in Nepalese forests [4]. More than 3/4^th of the naturalized plant species including IAPS are native to tropical and subtropical region of the world [3].so, the southern half of the country with tropical and sub-tropical climate has higher concentration of IAPS which includes Terai, Siwalik and Mid-hills running east-west [5]. This region includes most troublesome species such as Chromolaena odorata, Eichhornia crassipes, Lantana camara and Mikania micrantha, which are listed as the world’s 100 worst invasive species [6]. But Some species like Ageratina adenophora and Parthenium hysterophorus have spread their invasion up to northern border [5]. Ageratum conyzoides, Erigeron karvinskianus, Galinsoga quadriradiata and Spermacoce alata are found to be invasive in agro-ecosystems and rangelands [5]. E. karvinskianus is problematic mainly in mid hills and low mountains of western Nepal while Ageratum haustonianum are problematic in Terai, Siwalik and Mid hills [5]. Eichhornia crassipes is found most problematic in wetlands. In forest and shrublands, the major problematic species include Ageratina adenophora, Chromolaena odorata, Lantana camara and Mikania micrantha while the species like Parthenium hysterophorus is threatening grass/grazing lands [5].

The major reason for Nepal to host such species is the government’s concern to promote tourism and trade but without adequate policy, no proper quarantine facilities and second reason being the limited research on such topics hindering effective control measures [7]. Nepal’s diverse climate and geography serves the suitable habitats to such species. Climate change has further exacerbated this issue by altering habitats and potentiality to expand the range of invasive species. In addition, frequent availability of unused resources increases vulnerability of a habitat to invasion [8]while human actions raise invasive species proliferation pressure. [9]

Impacts:

The issue of invasive plant species is worldwide bearing high risk at social, health and environmental level [1]. But the impact is severe on developing countries like Nepal where livelihoods are resource based like agriculture, fisheries, aquaculture and forestry. Being inherently linked with human activities, IAPS are more common in anthropogenic landscapes [5] having impacts like decrease in agriculture production, increased labor to remove the weeds, suppression of useful species, and health hazard to human and livestock [1]. But some IAPS expand to natural landscape like forest, grassland and wetland even at national parks and conservation zones having alarming impacts of competing to native species, degrading their habitats and increased biodiversity loss. They even suppress the growth of grasses and regeneration of trees, reduce the carrying capacity of rangelands, increased the risk of fire damage [5]. The invasive species threaten biodiversity by the replacement of native species with the change in ecology of a given habitat by changing the nutrient cycle and soil pH [1]. Moreover, they bear an intimidating remark to biodiversity by causing diseases, acting as predators or parasites, homogenizing ecosystems, changing hydrological characteristics, acting as competitors or degrading gene pools hybridizing with native local species [1], [10]. Therefore, species extinction, habitat transition and development are the evolutionary and biological consequences to be paid. [1]

Such invasive plant species are endangering and disrupting the natural environment and affecting agriculture in Nepal as well. Farmers have suffered major declination in crop yields and seed production due to the invasive nature of IAPS [1]. They often have to incur additional cost along with reduced crop value. Because, unwanted plants act as a new or additional hosts to new or existing crop diseases and pests that requires increased use of pesticides to control them. They even invade agricultural land with severely reduce productivity and land values [1]. On top of that, such plants are poisonous to domestic animals triggering several health risks. For example, Ageratum houstonianum is poisonous to grazing livestock, triggering liver lesions. [11] They even compromise the pollination services essential in agriculture, as the dominance of invasive species reduce the natural plant diversity disrupting the mutualistic relationship of native plants and their pollinators. This diminishes the food sources for pollinators. Similarly, their difference in flowering pattern compared to native can further disrupt the pollinators populations.

Climate Change and Invasive Species Dynamics:

Climate change is a significant driver of ecological shifts worldwide and with this change the distribution and prevalence of IAPS is expected to pose further threats to Nepal’s biodiversity and ecosystem services. Climate change and biological invasion interact, producing a multitude of effects that are more severe than their individual impacts combined. The disruption of climate factors of ecosystem creates more accommodating habitat for invasive species and allowing them to spread beyond their original geographic locations [12]. Climate change broadens the pathway for biological invasion. Changing climatic conditions can reduce native species potential to strive against the non- native species and hence invasive species have advantage over them. [13] The attributes of such invasive species support them to succeed over them even under climate change. [13] 

Management Strategies:

• Focus on Identifying and monitoring the common invasive pathways and mapping the hotspots of invasion. [7]
• Establishment of environmental biosecurity departments to carryout search procedure at first points of entry with proper quarantine measures. [7]
• Integrate invasive species management into existing environmental and biodiversity policies.
• Implement regular monitoring programs to track the spread and effects of invasive species.
• Involve local communities in management efforts through awareness campaigns and participatory action research
• Strengthen the capacity of forest technicians and local stakeholders through training programs on invasive species management
• Most of the IAS is light demander, therefore, it is essential to understand where to perform heavy thinning and where to keep canopy intact. Closed canopy discourages the growth of invasive plant species. [7]
• Utilization of the removed biomass may reduce the management cost. Therefore, manual/mechanical cutting followed by utilization such as composting or bio-briquette or pellet would be the appropriate strategy. [7]

Conclusion:

Spreading throughout Nepal, the invasive plant species exert an intense and complex impact on its ecosystems as well as agrobiodiversity. Some of their attributes- fast-growing, adaptive, allelopathic, disruptors of ecological functions, and efficient modes of propagation-make them a serious threat.  This condition demands a holistic approach by means of collaboration among government agencies, researchers, and the general community. Thus, effective management practices can be applied in Nepal to preserve its rich biodiversity for sustainable agriculture meant to benefit future generations.

References