PhD: a plant chose me
Thesis title: Investigations on the invasiveness of Mikania micrantha Kunth (Asteraceae) with respect to its spread and distribution in Greater Kolkata
Worked at
Indian Statistical Institute, Kolkata
Time spent
6 years 2010-2016 (August)
Guided by
Prof. Anjana Dewanji
Registered at
University of Calcutta (Botany)
Thesis abstract
Plant invasions are considered to be one of the major threats to biodiversity. Mikania micrantha Kunth, of Latin American origin, considered to be one of the top invaders in moist tropical zones of Asia-Pacific, has been widely studied, though mostly in forest and plantation areas. This thesis attempts to highlight the ability of M. micrantha to overcome the barriers of the invasion process with special reference to its encroachment in urban areas, together with trying to understand its pathway of introduction, the rate of spread, and the status of present distribution both within the Greater Kolkata region as well as for the entire country.
The first evidence of multiple introductions of M. micrantha into India, with documentary evidence of Calcutta being the region of first introduction, could be ascertained from this study. Baseline data created for the study area shows that the plant is mostly restricted to small patches. A follow-up survey revealed an increase in the number of small patches within a two-year period, denoting quick spread. Anthropogenic activities and expanding transport networks within the city appear to a contributing factors to the spread into new areas. Experimental evidence revealed the strategic adaptability of M. micrantha to seasonal changes as well as to different types of disturbances prevalent in urban areas by modulating its fitness-related traits. The plant could modulate its growth around its carrying capacity, thereby maintaining a high vegetative relative growth rate (RGR) and also producing lightweight seeds with extended viability. M. micrantha appeared to be a “passenger” of change, taking advantage of disturbance in urban habitats, which native species are unable to cope with. Regular monitoring together with the ‘early detection and rapid response’ approach in urban fragmented landscapes may be the most pertinent control option to prevent further spread. However, if growth is left unchecked, M. micrantha may become the community-dominant or “driver” of change by competing with the resident species. The niche modeling approach suggests that the plant is still spreading within the country and revealed additional areas to be climatically suitable, probably due to niche and biome inconsistencies of the plant.
Chapters
Introduction pathway reconstructed (published)
Chapter 1
Highlights
Based on archival botanical records and herbarium data.
Evidence in support of multiple introductions was found.
The plant was introduced into India from multiple places in Latin America to multiple locations and multiple times by multiple routes.
The first introduction was probably into Calcutta via the Calcutta Botanical Garden during the late 19th century, followed by its introduction into northeast India and finally into the southwest part of the country.
Distribution in a city mapped (published)
Chapter 2
Highlights
Using remote sensing and GIS technologies.
Image classification – land use and land cover.
Soil sample analysis.
A field survey was conducted following a proper sampling design.
Small populations were restricted to the ‘built up’ land use class within the city, and along minor roads, whereas most of the large infestations occurred in the ‘sparse vegetation’ land use class and along major roads.
Survival ability investigated (published)
Chapter 3
Highlights
In terms of its fitness (abundance and biomass) and phenotypic plasticity of selected fitness-related traits.
Quadrat-based study.
Soil sample analysis.
Seasonal (summer, monsoon, and winter) and habitat (littoral, transitional terrestrial, and terrestrial) influence on the survival ability.
Three habitats (roadsides, natural areas, and abandoned plots), representative of the disturbances prevalent in the study area, were studied to assess the effect of disturbance on trait plasticity.
Vegetative growth dynamics mapped
Chapter 4
Highlights
Regeneration experiments in winter and monsoon
Quadrat-based study
To compare the relative growth rate and carrying capacity in terms of vegetative biomass.
The logistic growth model was the best fit to explain the growth dynamics of this plant.
In winter, the undisturbed site favored growth. The slow but steady RGR in the disturbed habitat helped to maintain its biomass and outcompete the native vegetation.
During the monsoons, it exhibited a higher growth rate and maintained maximum carrying capacity in the population.
Plasticity in sexual traits found
Chapter 5
Highlights
Addressed both at a temporal and spatial scale
Quadrat-based study at weekly intervals
Record the flowering phenology, and seed viability after storing the seeds for specified periods of time
The effect of disturbance on seed germination was also studied similarly across different sites.
Temperature influenced the flowering phenology and production of viable seeds. The seeds could remain viable for 90 days.
Lighter seeds were produced in roadside populations while the most efficient germination and viability retention were observed for those disturbed sites.
Role in community decoded (published)
Chapter 6
Highlights
Community composition in terms of species associations, diversity indices, and the native and exotic relationship.
Based on field survey data
Most sites had more than one non-native species.
Mikania micrantha was the community-dominant which could be considered to be a ‘passenger’ of habitat alteration that suppressed native species, but was likely to become a ‘driver’ once it attained high covers due to its reported superior competitive abilities.
Spatial and temporal spread detected (published)
Chapter 7
Highlights
New satellite populations were found in nearly 50% of the sites that had no M. micrantha population 2 years ago.
Using the location data gathered from herbarium sheets as well as from literature records from the time since its introduction into India, invasion curves were constructed. The rate of spread was found to be significantly higher with the shortest lag period in southwest India.
A potential distribution map was generated using the Maximum Entropy Species Distribution Model. In addition to the reported presence of the plant in the northeast and southwest parts of the country, the model identified new areas to be climatically suitable
Postdoc: expanded my horizon
Thesis title: Past, Present, and Future – Introduction history, current population structure, and potential distribution of an invasive plant species, Mikania micrantha, in South and Southeast Asia
Worked at
Sun Yat-sen University, Guangzhou, China
Time spent
3 years (+3 as a Research Personnel)
Guided by
Prof. Yelin Huang
Number of papers
36 (invasives and mangroves)
Thesis abstract
Biological invasions are now considered a major global challenge for the conservation of natural resources and biodiversity. Mikania micrantha Kunth, a plant species of Latin American origin, has been considered one of the top invasive species of the tropical and subtropical belt of the world. The invasive ability and management implications of the species have been widely studied at regional and national scales. This thesis focuses on the species’ dispersal pathways outside its native range, genetic diversity, population structure, and potential distribution under current and future climate scenarios covering a large area of its invasive range in South and Southeast Asia.
Reconstruction of the introduction pathways of M. micrantha revealed evidence of at least six introductions in this region. The species is not at equilibrium with the current environmental conditions of its invasive range, thereby indicating ongoing range expansion of the species. Genotyping 1052 individuals from 46 populations at 12 nuclear microsatellite loci, persistent gene flow throughout the invasive range was found to provide beneficial genetic variation to the marginal populations, thereby facilitating range expansion of the species. High genetic diversity and strong population structure characterized the current genetic structure of M. micrantha in this region. Combining the genetic lineage information in an environmental niche modeling framework, a large area of South and Southeast Asia was found to be climatically suitable for both lineages, although significant differences between the climatic niches of the two invasive lineages were found. Finally, taking clues from the studies conducted so far, a unified framework of future experimental approaches was proposed to understand the molecular mechanisms of phenotypic variation of an invasive species and to disentangle the effects of standing genetic variation and epigenetic regulation of phenotypic variation.
The findings of this thesis can be used for continuous monitoring of the predicted suitable areas, devising site-specific management options, implementing quarantine measures to safeguard this region from novel introductions, and prioritizing areas for sampling classical biological control agents for M. micrantha.
Mangrove phylogeography
Mangroves comprise tropical and subtropical intertidal plant communities and are valuable for providing a range of essential ecosystem services. Due to the foreseeable future of mangrove extinction through human exploitation, climate change, natural geographic events, and cryptic ecological degradation, it is important to understand their evolutionary history and present distribution. Unlike terrestrial plants, most mangroves have buoyant propagules capable of long-distance dispersal through ocean currents for an extended period of time, which plays a pivotal role in shaping the population structure of mangroves by maintaining gene flow between geographically distant populations. Multiple studies have identified multiple factors, including glacial vicariance and contemporary oceanic currents, which act as barriers to propagule dispersal and thereby can influence the geographic distribution of genetic variation within the mangrove species. However, the influence of these phylogeographic factors driving population structure is not consistent across mangrove taxa and geographic regions. Therefore, it is important to study the taxon-specific genetic structure of major mangrove lineages through regional sampling design across their distribution range for a better understanding of the phylogeographic factors shaping the geographic distribution of genetic variation.
My research interest is to assess the genetic diversity and population structure of the mangrove and mangrove-associated taxa across their distribution range and to identify the phylogeographic forces that would influence the present distribution of the species. I have studied the phylogeographic pattern of the following species using chloroplast DNA, nuclear DNA, and nuclear microsatellite markers.
- River mangrove [Aegiceras corniculatum]
- Holy mangrove [Acanthus ilicifolius]
- Beach bean [Canavalia rosea]
- Common derris [Derris trifoliata]
- Looking glass mangrove [Heritiera littoralis]
- Black mangrove [Lumnitzera racemosa]
- Indian fleabane [Pluchea indica]
- Beach naupaka [Scaevola taccada]
Chapters
Climatic niche dynamics (published)
Chapter 1
Highlights
Most probable pathways of introduction into South and Southeast Asia were reconstructed based on literature reports, archival botanical records, and herbarium data.
Evidence in support of at least six introductions was found.
Species distribution modelling.
Although the climate conditions at the presumed introduction sites matched with the realised climate niche limit of the species in its native range, the species is not at equilibrium with the current environmental conditions of its invasive range.
Genetic diversity and population structure (published)
Chapter 2
Highlights
Analysis of genetic diversity and structure of 1052 individuals from 46 populations for 12 nuclear microsatellite loci.
High genetic diversity, the presence of multiple population clusters, and admixture between populations were found.
Persistent gene flow throughout the invasive range was found to cause low genetic differentiation among populations and provided beneficial genetic variation to the marginal populations in heterogeneous environments.
Genetically-informed SDM (published)
Chapter 3
Highlights
The genetic lineage information of M. micrantha in South and Southeast Asia was integrated into an environmental niche modelling framework.
A large area of South and Southeast Asia was found to be climatically suitable for both lineages, although significant differences between the climatic niches of the two invasive lineages were found.
Highlights the importance of integrating genetic data in future modelling studies to have finer-scale information on species’ distribution.
Genetic and epigenetic regulation of phenotypic variation (published)
Chapter 4
Highlights
Review work aimed at summarizing and synthesizing information on the genetic and epigenetic basis of phenotypic variation of alien invasive plants.
Taking clues from the studies conducted, a unified framework of future experimental approaches was proposed to understand the molecular mechanisms of phenotypic variation of an invasive species in its introduced range and to disentangle the effects of standing genetic variation and epigenetic regulation of phenotypic variation.