Ecology and impact of an invasive weed Ageratina Adnophora in central Nepal

Abstract

Invasive alien species (IAS) have been increasing at an alarming rate all around the globe with significant negative impacts on the environment, ecosystem services, human health and socio-economy. Therefore, understanding the ecological features of IAS invasion mechanisms, potential shift in climatically suitable regions of IAS under future climate change scenarios, and factors influencing abundance of their biological control agents is an urgent need to better inform the management policies and practices. In this study, ecological features such as distribution (current and future), functional traits variation along the elevation gradient, ecological impacts, and influence of abiotic factor (elevation) on the abundance of a biocontrol agent of a noxious invasive weed Ageratina adenophora (Sprengel) R. King & H. Robinson were analyzed in central Nepal. Maxent model was used to predict the suitable areas of A. adenophora in Chitwan Annapurna Landscape (CHAL), Nepal under current and three future climate change trajectories based on three representative concentration pathways (RCPs 2.6, 4.5 and 8.5) in two different time periods (2050 and 2070) using occurrence data of the species in the study region, climatic variables, and topographic features. To analyze the change in area invaded by A. adenophora from the year 1990-2018 in CHAL, Landsat images were used and knowledge-based image classification technique of remote sensing was applied assigning seven rules (elevation, landuse, Normalised Difference Vegetation Index (NDVI), digital number, maximum temperature, minimum temperature and rainfall). Furthermore, spatial distribution of A. adenophora was assessed in small area of interests viz. Mahadevbesi (Dhading district), Sarangkot (Kaski district) and Ghasikuwa (Tanahun district) using both low spatial and spectral resolution (Landsat) and high spatial and spectral resolution (Worldview-2) images to find out the more accurate multispectral sensors. Six functional traits namely Specific leaf area (SLA), Leaf nitrogen content (LNC), number of floral heads per ramet, total seeds per floral head, seed size and seed mass of A. adenophora were taken into consideration for analyzing their variation along the elevation gradient of 240 2965 m asl in central Nepal. Changes in species richness, species diversity, species composition and soil chemical characteristics (Organic carbon, Total nitrogen, phosphorus and potassium) due to A. adenophora invasion was studied at two sites (Chandragiri and Simbhanjyang) in Middle Mountain region. Role of elevation on the abundance of a biocontrol agent, i.e., Procecidochares utilis Stone and on size of gall formed by this gall fly was explored. Furthermore, the effect of this agent on the stem diameter of the weed, and the capacity of the gall to act as nutrient sink was further analysed. Minimum temperature in the coldest month (bio 6) was the most important variable affecting the distribution of A. adenophora. About 38% of the CHAL area is currently climatically suitable for the weed, Middle Mountain being the most suitable one. A vii

predicted increase in suitable area under future climate scenarios ranged from 1-2 % from the current suitable areas. All protected areas and three physiographic regions (Siwaliks, High Mountains and High Himalaya) are likely to gain climatically suitable areas in future climate scenarios. The upper elevational distribution limit of the weed is expected to expand by 31-48 m in future climate scenarios. The area invaded by A. adeophora increased from 0.22% in 1990 to 4.55 % in 2018. Multispectral sensors, i.e., Worldview-2 images had higher accuracy in comparison to Landsat images and provided a better scenario of species distribution. SLA, LNC, number of floral heads per ramet and total seeds per floral head decreased with the increasing elevation along the elevation gradient of 500 2500 m asl. However, different trend in functional traits was observed along elevation gradient of 1500-2530 m asl. Seed size and seed mass increased with the increasing elevation at all sites along the elevation gradients. A trade-off was noticed between seed size and seed number indicating the strategy of the weed to adapt to low temperatures at high elevations. Ageratina adenophora reduced the species richness as well as species diversity in the invaded sites. Shannons diversity index in non-invaded plots was 1.9 times higher than in invaded plots. Similarly, diversity index in non-invaded plots was 1.6 times higher than in invaded plots. Invasion by A. adenophora also caused significant change in species composition. difference in species composition between invaded and noninvaded plots. Furthermore, A. adenophora invasion increased the carbon and nitrogen content in the invaded plots. However, A. adenophora invasion had no effect on phosphorus and potassium concentration. Site-specific variation in the impact of A. adenophora was noticed. Elevation had a significant effect on the gall abundance as well as gall size. Both gall abundance and size peaked at mid elevation (1950-2000 m asl). Stem galling by the fly reduced the stem diameter of the weed and all the three nutrients (N, P and K) were higher in galls than in ungalled stems. This study indicates that the elevation is an important abiotic factor that influences the P. utilis stem gall abundance and gall size. Furthermore, the galls formed by P. utilis act as nutrient sinks. In conclusion, probability of range expansion and upslope movement as revealed by niche models and functional trait studies suggests for a need to prevent further spread of the weed in the study area. Significant negative impact on plant community and soil chemical characteristics reported in this study also emphasizes for a prompt action and management interventions to be adopted by scientific community and resource managers. As the study revealed that insect gall abundance is affected by elevation and peaks at mid-elevation range, this knowledge on gall dynamics is central to the successful management of the weed through the use of biological control agents. Keywords: Invasive species, Distribution, Functional traits, Impact, Biocontrol