Sun Yat-sen University
Guangzhou, China
PhD Thesis
Title
Investigations on the invasiveness of Mikania micrantha Kunth (Asteraceae) with respect to its spread and distribution in Greater Kolkata
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 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. A baseline data created for the study area shows that the plant is mostly restricted to small patches. A follow-up survey revealed 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 factor for spread into new areas. Experimental evidence revealed 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 produce 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 modelling 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.
Overview of the thesis chapters
Tracing the introduction pathway
In this chapter, the most probable pathway of Mikania introduction of this species into India was reconstructed based on archival botanical records and herbarium data of Mikania. Evidence in support of multiple introductions could be found contrary to the popular belief that it was introduced to northeast India during the 2nd World War. It appears that the plant was introduced into India from multiple sources 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. The issue of nomenclatural ambiguity regarding the correct identification of the plant was also addressed and based on available evidence, the results indicate that the only species present in India is micrantha Kunth.
Mapping the present distribution
This chapter was conceptualised to map the present distribution of M. micrantha and assess its potential to spread within the city using RS and GIS technologies. A field survey was conducted during 2011-2012 and M. micrantha infestation was detected mostly as satellite populations in terrestrial habitats. The satellite 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. Its presence among all the representative land use classes makes it a prospective propagule source which can be easily transferred to new uninvaded localities.
Survival ability in terms of abundance and trait plasticity
This chapter deals with the survival ability of M. micrantha in terms of its fitness (as a measure of abundance and biomass) and phenotypic plasticity of some fitness related traits. In the first phase, the seasonal (summer, monsoon, and winter) and habitat (littoral, transitional terrestrial, and terrestrial) influence on the survival ability of M. micrantha was evaluated in a quadrat based study conducted in an experimental undisturbed plot during 2011-2012. Another three habitats (roadsides, natural areas and abandoned plots), representative of the disturbances prevalent in the study area, were similarly studied in the second phase during January - February, 2015 to assess the effect of disturbance on trait plasticity. Significantly higher growth of M. micrantha was observed during the monsoon season and in terrestrial habitats. In response to rise in water levels, the plant exhibited below ground trait plasticity by producing a significantly high amount of aerenchyma in its roots to survive the anoxic stress conditions. M. micrantha also demonstrated differential responses in terms of modulation of its plastic traits to different disturbance factors in order to survive and grow. For example, in case of reproductive traits, roadside populations allocated more resources to its reproductive biomass, the population in natural areas showed highest germination efficiency of seeds while the seeds produced in abandoned plots had a longer germination duration. Thus, M. micrantha was able to maintain its fitness and survival ability irrespective of season and habitat types.
Vegetative growth dynamics
This chapter focuses on regeneration experiments conducted in a disturbed and undisturbed plot, during the winter (October 2011 to February 2012) and monsoon period (April to August 2012) in order to study and compare the relative growth rate and productivity in terms of vegetative biomass of M. micrantha. The disturbed plot was studied during both seasons while the undisturbed plot was only studied during the winter season. The logistic growth model was found to be the best fit to explain the growth dynamics of this plant. In winter, the undisturbed site favored growth of M. micrantha, while the slow but steady RGR in the disturbed habitat helped M. micrantha to maintain its biomass and outcompete the native vegetation. During the monsoons, M. micrantha exhibited higher growth rate with low deviation of RGR enabling it to maintain maximum carrying capacity in the population. Thus, steady growth rate of M. micrantha in disturbed habitats gives it a competitive advantage to establish and achieve dominance over the native species in the community.
Sexual reproduction, seed germination and viability
In this chapter, the sexual reproduction phenology of micrantha was addressed both at a temporal and spatial scale. The disturbed plot used earlier was further studied at weekly intervals during November, 2011 to January, 2012 to record the flowering phenology, and seed viability after storing the seeds for specified periods of time (0, 60, 90, 120 and 150 days). The effect of disturbance on seed germination was also studied similarly across different sites. The flowering phase persisted for 60 days, and the seed formation continued for approximately for 45 days from mid-December. Temperature was found to influence the flowering phenology as well as production of viable seeds. The seeds produced by M. micrantha showed maximum viability when the seasonal temperature was the lowest. Irrespective of the sampling time, the seeds could remain viable for 90 days. Lighter seeds were produced in roadside populations while the most efficient germination and viability retention (150 days) were observed for those sites which were anthropogenically disturbed. Thus, M. micrantha was also able to maintain its population in different habitat types during its sexual reproductive phase.
Role in influencing community composition
The observational data from the survey field of 2011-12 was used in this chapter to study the community composition in terms of species associations, diversity indices and the native and exotic relationship. The study revealed that most sites had more than one exotic species and a non-random association between micrantha was observed using a null model. Exotic species richness did not turn out to be a good predictor for the native-exotic richness relationship. Analysis of the abundance data identified M. micrantha as the community-dominant which could be considered to be a ‘passenger’ of habitat alteration which suppressed native species, but was likely to become a ‘driver’ once it attained high covers due to its reported superior competitive abilities.
Temporal spread in the city and throughout the country
To detect the temporal spread of M. micrantha within the study area as well as throughout the country, a change detection survey was conducted in 2013-2014 after a gap of two years. The study revealed that new satellite population of M. micrantha had cropped up in nearly 50% of the sites which had no M. micrantha population in 2011-12, thereby denoting fresh infestations. To visualize the spread of M. micrantha on a larger spatial and a longer temporal scale, a biogeographical analysis was carried out to estimate its rate of invasion in India. 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 for the three regions which showed separate introductions. The rate of spread was found to be significantly higher with the shortest lag period in southwest India. The lack of attainment of the plateau region of the invasion curves in all the three regions indicated that the expansion process of the plant is still continuing in its introduced habitats within India.
Modeling the potential distribution
Due to its expanding range in India, this penultimate chapter attempted to map its potential distribution in India and identify the invasion hotspots. The expansion of its invasive range in terms of climatic niche and biome consistency of micrantha was also checked. The potential distribution map of M. micrantha in India was generated using the Maximum Entropy Species Distribution Model. In addition to the reported presence of the plant in northeast and southwest part of the country, the model identified new areas to be climatically suitable. Intersecting the potential distribution map with the terrestrial ecoregions and anthropogenic biomes layers of India, the invasion hotspot of this invasive weed were identified. High climatic suitability of some of the urban centers of India were detected along with its shift in climatic niche and towards a novel biome indicating a greater possibility of invading new habitats throughout the country.