Small isolated populations are the history of living beings. Studies of demographic histories of non-model organisms reveal that almost all populations have faced severe bottlenecks at least once in the past. Additionally, speciation events are often correlated with founding bottlenecks. Classical population genetics theory suggests small isolated populations are faced with threats of extinction. This is generally visualized as an extinction vortex where, small populations lose diversity due to stochasticity and inbreeding leading to reduced fitness of the individuals accompanied with reduced survival and fewer offspring further leading to a smaller population spiralling to extinction. However, despite the severe bottlenecks faced we observe the present biodiversity. Genomics studies of wild populations threatened with extinction present insights into how populations may have persisted over time to escape the extinction vortex. Modern insights from studies of threatened populations highlight the importance of neutral genetic diversity along with the role of purging and geneflow in the persistence of populations. Here, using tigers, rhinocerous and elephant populations, I illuminate a potential model of survival and extinction in small populations that might explain the observed biodiversity. Additionally, I will explore some ways in which research on small populations can be promoted in the tropical countries generally underrepresented in the literature.

Mangrove forests inhabit the intertidal areas along the tropical and subtropical coastal regions and monitor the exchange of matter at the edges of terrestrial, marine and atmospheric ecosystem. This specialized ecosystem offers several ecosystem services, like alleviating coastal erosion by wind and waves, ensuring fishery resources and food security for coastal population, and protecting the coastal biodiversity. It plays a key role in carbon exchange cycle and is an important blue carbon sink that can aid in the mitigation of climate change. However, mangroves are highly vulnerable to climate change and fluctuations in relative sea level. The mangroves are harmed by low intensity precipitation due to less freshwater discharge, fluvial silt, and nutrient input into the mangrove habitats. High frequency winter cooling episodes can also hinder mangrove development due to variations in monsoon intensity. Hypersaline environments with high evaporation rates brought on by extreme warming episodes also cause mangrove forest degradation. Moreover, anthropogenic pressures, such as overexploitation of resources, environmental pollution, and landuse/landcover changes, have greatly impacted the mangrove ecosystem. Currently, the world is seeing rapid sea level rise, frequent extreme climatic events, and an ever-rising population. Hence, the reconstruction of past mangrove responses through climate indicators recorded in sediments provides a baseline data for species distribution models in predicting the fate of mangrove ecosystem under the influence of rapid environmental changes.

Fossil pollen plays a vital role as a direct proxy for estimating vegetation cover in the past and an indirect proxy for understanding past climate. Palynological assemblages from estuarine formations are constantly influenced by both marine and terrestrial factors such as coastal erosion, accretion by rivers or sea, tidal waves, high salinity, water-logged soils and other edaphic factors due to their location along coasts. This, along with the distance from shoreline, duration and frequency of tidal inundation govern the distribution of mangrove species and their succession. Hence, studies attempting to identify the modern pollen dispersion and deposition processes, test correlations between pollen, vegetation, and climate using models, and compare these models with fossil pollen databases for reconstructing the key plant species distribution at a continental spatial scale are much needed for climate modelling studies.

Conservation is fundamentally about human behavior. Protecting biodiversity and mitigating climate change requires that stakeholders across different sectors and societies cooperate and act. My work has used data science techniques ranging from the local scale, focused on interviewing resource users in tropical Asia, to the global scale of social media to elucidate the social, economic, and ecological factors that shape human interactions with nature. I will discuss several case studies that highlight how text analyses and online “Big Data” can generate powerful new insights for conservation and social-ecological systems research. I will discuss work that performs a systematic characterization of environmental personas on social media, using natural language processing to geolocate users and evaluate their views toward the environment. I will discuss how these findings can drive new approaches for conservation messaging and engagement. Finally, I will show how large language models can be used to synthesize and map evidence for natural climate solutions at a global scale.

Plant domestication, a process spanning about 10,000 years, involves the selective nurturing of wild species to meet human needs, leading to the domesticated crops we see today. Papaver somniferum, cultivated for food, ornamental, and medicinal purposes, serves as an ideal model to study trait changes under domestication. The first chapter of my thesis will compare morphological, phenological, and phytochemical traits across Papaveraceae varieties, focusing on P. somniferum domesticated for specific purposes versus related taxa unaffected by human intervention. This chapter will explore "domestication syndromes" (DS), such as changes in defensive chemicals, seed morphology, and biochemistry, to understand how these traits vary with different domestication goals. This analysis will use plant trait measurements taken from existing literature and from plants grown in a controlled environment.

The second chapter examines how plant traits associated with domestication in P. somniferum vary with the intensity of management and contrasting biogeographical contexts: Plains, Plateaus, and Mountains. Mountainous regions, with low human intervention, contrast with the more intensively managed farmlands of plateaus and plains. I expect to find that differences in human management practices, influenced by the constraints and opportunities of each landscape, along with cultural differences between farming communities affect the domestication intensity of this species. For this, I will use field observations, questionnaires, and surveys to identify practices impacting artificial selection in plants.

In the third chapter, I investigate how seed trade and spatial connectivity affect the spatial genetic population structure of P. Somniferum. By comparing poppies from fields in the highly connected Gangetic plains, moderately connected Marwar plateau, and the least connected Mishmi hills – using microsatellite-neutral markers – the aim is to reveal how landscape connectivity and human-mediated gene flow through seed trades and cultural connectivity shape the genetic diversity and structure of these geographically varied subpopulations.

Once every 13 or 17 years within eastern North American deciduous forests, billions of periodical cicadas concurrently emerge from the soil and briefly satiate a diverse array of naive consumers, offering a rare opportunity to assess the cascading impacts of an ecosystem-wide resource pulse on a complex food web. We quantified the effects of the 2021 Brood X emergence and report that more than 80 bird species opportunistically switched their foraging to include cicadas, releasing herbivorous insects from predation and essentially doubling both caterpillar densities and accumulated herbivory levels on host oak trees. These short-lived but massive emergence events help us to understand how resource pulses can rewire interaction webs and disrupt energy flows in ecosystems, with potentially long-lasting effects. (And India has the world’s only other reported species of periodical cicadas!)

Once every 13 or 17 years within eastern North American deciduous forests, billions of periodical cicadas concurrently emerge from the soil and briefly satiate a diverse array of naive consumers, offering a rare opportunity to assess the cascading impacts of an ecosystem-wide resource pulse on a complex food web. We quantified the effects of the 2021 Brood X emergence and report that more than 80 bird species opportunistically switched their foraging to include cicadas, releasing herbivorous insects from predation and essentially doubling both caterpillar densities and accumulated herbivory levels on host oak trees. These short-lived but massive emergence events help us to understand how resource pulses can rewire interaction webs and disrupt energy flows in ecosystems, with potentially long-lasting effects. (And India has the world’s only other reported species of periodical cicadas!)

Hybridization is thought to have played an important role in shaping the evolutionary history of diverse island taxa. Yet secondary contact doesn’t always result in introgressive hybridization, and it is important to understand what determines the ecological and evolutionary outcomes of secondary contact, particularly in the face of widespread secondary contact between island endemics and recently introduced species. While there are reasons that secondary contact commonly leads to heterospecific mating on islands, the consequences of secondary contact will differ depending on the nature of the species involved. I will present the results of our field work on introduced and endemic species of the plant Psidium (guava) on the Galápagos Islands of Ecuador, exploring the role of secondary contact in the ongoing extirpation of the endemic species. I will then discuss the results from a quantitative analysis of published empirical research on secondary contact among vertebrate, invertebrate, and plant species on the five most well-studied remote oceanic archipelagoes. This analysis suggests the relative importance of different factors driving secondary contact and heterospecific mating, such as disturbance, inter-island dispersal, and compromised assortative mating. I will discuss the hypothesis that introgression is a more common outcome between island endemic species while reproductive interference is a more common outcome between endemic and introduced species, as well as the contention that reproductive interference between endemic and introduced species is a cryptic threat to the conservation of island endemics.

The terrestrial carbon (C) cycle involves fluxes between multiple pools that determine ecosystem functions and regulate global climate. These fluxes and pools are influenced by changes in abiotic (temperature, precipitation etc.) and biotic (animals, microbes, etc.) factors. In this thesis, I address three questions on how these abiotic and biotic drivers influence the size and stability of these fluxes and pools.    

            In the first chapter, I investigate how covariation between decadal trends (2001-2019) in temperature and precipitation influence the two opposing C-fluxes in the soil-C pool – (1) C-influx through primary production (NPP), and (2) C-efflux through soil heterotrophic respiration (Rh). I estimate how any imbalance between these opposing fluxes affects the vulnerability of soil-C across the globe. I find that changes in C-influx may not compensate for rising C-efflux, under wetter and warmer conditions. Soil-C loss can occur in both tropics and at high latitudes, and precipitation emerged as the key determinant of soil-C vulnerability in a warmer world. This implies that hotspots for soil-C loss/gain can shift in the coming decades to make the soil-C pool vulnerable to climate change despite widespread increase in NPP across the world. 

            In the second chapter, I explored the influence of climate on long-term correlations in vegetation fluctuations (i.e., persistence, measured by the Hurst exponent from time-series data). I found evidence for stronger persistence in warm and dry regions of the world, and there were non-linear relationships between persistence and two key climate variables (i.e., temperature and precipitation). While average temperature and precipitation together explained nearly three-fourths of the spatial variation in vegetation persistence, they had limited ability to explain the observed temporal changes in persistence. We find evidence for change in vegetation persistence across the globe driven by background change in climate. This provides some new insights into the resistance/resilience of vegetation in different ecosystems.  

In the third chapter, I investigated how animals – large mammalian herbivores – influence the terrestrial carbon cycle. Their influence on the size of the soil-C pool is well known, but how herbivores control the temporal stability of soil-C has remained largely unknown. I used a long-term field experiment in the Trans-Himalaya (2005-present) to estimate the consequences of herbivore-exclusion on interannual fluctuations in soil-C. I found high interannual variability in soil-C, and herbivores promote temporal stability of soil-C. Grazing by herbivores also mediated the influence of nitrogen on the stability on soil-C. Therefore, conserving large mammalian herbivores in grazing ecosystems can help achieve nature-based climate solutions. 

            Overall, this thesis explores the linkages between three aspects of the terrestrial carbon cycle and climate, and herbivores. It highlights the non-linearities in vegetation-soil-animal interactions which are important for the stability of the terrestrial carbon cycle.

The rise in anthropogenic CO2 emissions has led to a rise in global temperatures which consequently affects global nutrient cycling, including in the largest terrestrial carbon pool in soil. Soil organic carbon is metabolized primarily by micro-organisms, and their activity determines if the soil carbon pool remains a sink or source. Soil microbial respiration accounts for ten folds higher CO2 emissions than anthropogenic sources. Hence soil microbes are crucial for maintaining ecosystem functions such as nutrient cycling and decomposition at proper levels. For decomposition, microbes secrete extracellular enzymes (EEs) in the soil matrix. EEs break down macromolecules into microbially assimilable compounds, which can further be used for their metabolism and growth. Since EEs are protein molecules, their activity changes with temperature making substrate decomposition temperature sensitive. The increase in the activity of EEs under warming can translate to a higher turnover of organic molecules to CO2, which in turn can increase the greenhouse effect creating a net positive feedback loop due to climate change. Previous studies have shown that the rise in soil respiration rate in warming conditions could be attributed to the change in community composition and soil EE activity. Through my thesis, I aim to understand the dependence of soil enzyme activity and microbial community on temperature.  Enzyme activity in soil also changes with space and time. The changes in enzyme activity have been attributed in different studies to climatic, edaphic, and microbial factors. For my first objective, I will do a meta-analysis of the published literature on EE activity in unamended soil to understand the global patterns and drivers of EE activity. But the in-situ measurements of enzyme activity have many limitations; one of the major drawbacks is that they do not take the temperature dependence of enzymes into account. Models of enzyme activities show that the enzyme activity increases with temperature till a critical point. The mechanisms driving temperature dependence in soil EE differ in long-term and short-term warming conditions. In short term it is the physical changes in enzyme kinetics, whereas in the long term it is that biological changes in quantity and quality of enzymes that drive the temperature dependent changes. Thus, my second objective is to understand temperature dependence of soil EEs and identify the possible mechanisms underlying it. Researchers have shown that change in temperature results in the alteration of soil microbial community structure. But the microbial community is known to posses’ high degree of functional redundancy. Hence for my third objective I will evaluate if the changes in soil microbial community due to warming could affect its functionality.  These objectives can improve our understanding of warming on soil microbes and their ecosystem functions. It can help develop better climate change mitigation strategies and pave the way for nature-based climate solutions.

Soil contains 2500 Petagrams of Carbon, which is two to three times as much as the Carbon in the atmosphere, or in land vegetation. The soil carbon pool receives organic matter from the land vegetation pool in the form of leaf litter, dead organisms and animal excreta. In turn, it loses carbon dioxide to the atmosphere through microbial respiration. Since the Industrial Revolution, atmospheric carbon dioxide levels have risen by 50% (from 280 to 421 parts per million), and global temperature has risen by about 1.1 degrees. Further, there is concern that rising temperatures may increase soil microbial respiration and hasten the release of carbon to the atmosphere, creating a positive feedback loop. We therefore need to study the rate at which carbon is lost from soil, and how human activities - such as land use patterns and agricultural practices - may alter it. In this thesis, I propose to study how much carbon is present, and how much is respired, from soil in the Spiti valley region of Himachal Pradesh, which is a part of the carbon-rich Trans-Himalayan rangelands. I will also look at how land use, fertilisation and warming affect soil respiration. The first chapter of this thesis will look at what climatic, ecological and anthropogenic factors influence soil respiration. Since soil respiration in different environments depends also on the substrates available, I will review published literature on catabolic response profiling, which measures soil respiration against various metabolic substrates. The second chapter will focus on understanding the spatial and chemical structure of soil pools and fluxes in Spiti. Current soil Carbon models, constructed based on principles of reaction kinetics, do not account for spatial heterogeneity. We plan to measure the amount of Carbon in various soil pools, and to perform lab incubations at different temperatures to understand the sensitivity of their fluxes to temperature. We will also distinguish between the two main soil carbon pools - mineral-associated and particulate organic matter. These pools have different flux rates and spatial distributions, and so would respond differently to changes in land use or temperature. The final chapter will look at how land use affects these various carbon pools. Land grazed by native wildlife in Spiti has been converted to livestock pasture and agricultural fields, and the global fertilisation experiment NutNet maintains plots with various types of fertiliser application. Differences in the magnitude and flux of each carbon pool will be measured for each treatment. This thesis is expected to improve our current understanding of the soil carbon loss by respiration, and to predict how land use change and fertilisation will influence the rate of this loss. This information will be useful in modelling climate change and soil quality, as well as in the making of policies related to land conversion and agricultural practices.