Understanding the causal processes that have generated the stunning biodiversity in tropical forests has been fundamental to ecological and evolutionary research. In this talk, I will explore the role of the geographical, geological and ecological processes on biogeography and diversification among two arthropod groups (centipedes and butterflies) at different spatial scales, the Western Ghats in peninsular India and the South and Southeast Asia. To this end, I have integrated multiple lines of evidence, including DNA sequences, morphological traits, and data from geology, climate, and ecology, finding that geographic and geo-climatic processes have played an important role in diversification in both centipedes and butterflies. While some of the molecularly-delineated centipede species did not exhibit morphological divergence, they occupied distinct climatic niches across the Western Ghats, suggesting ecological speciation. Butterflies showed morphological variation in wing patterns and mimicry, which are thought to be involved in ecological adaptation. However, there was no significant effect of wing pattern diversity, and mimicry detected on diversification rates, indicating that geographic factors played an important role in their diversification. These two case studies highlight the need to assess multiple ecological and evolutionary axes when examining diversification patterns and processes. I will follow this by demonstrating the use of molecular phylogenies and biogeography to understand community assembly of species, which has traditionally been studied from under an ecological lens. I will show that evolutionary history, biogeographic isolation, and stochastic colonisation influence assembly of mimetic butterfly communities.
Towards the end, I will talk about my future research, where I plan to focus on exploring processes that influence extant diversity patterns using multiple soil arthropods taxa that vary across evolutionary and ecological gradients. Currently, such multi-taxa and multi-disciplinary studies are lacking in India and are limited to few studies in the world. I hope insights from these studies will contribute towards our understanding to broader questions in evolutionary biology such as: How did tropical biodiversity originate and how is it maintained? Why are the tropics so species rich? Why are some lineages more diverse than others? Also, I hope to contribute substantially towards understanding arthropod evolution and systematics in one of the most biodiverse and less explored regions in the world.

Exploring drives of ecological communities has been central to community ecology. Multiple factors have thought to influence community structure, including geographic isolation and area, niche-based, neutral, and historical processes. In the last two decades, community phylogenetic framework has emerged to integrate both ecological and evolutionary processes and their role in community structure.
Community phylogenetic method explicitly incorporates species’ evolutionary relationships, a proxy for how species are similar to each other based on DNA data. It helps to understand the relative role of different factors such as phylogenetic inertia, biogeography, and more contemporary ecological processes such as competition, habitat filtering, and predation simultaneously. It also helps community ecologists to link short‐term local processes to continental and global processes that occur over deep evolutionary time scales. In this lecture, I will discuss the merging of community ecology and phylogenetic biology, highlighting the challenges, debates, and new areas of enquiry in this field.

Thesis defense

Thesis Colloquium

Thesis Colloquium

Recent advances in technology and quantitative methods have led to a growth in our ability to study mobile animal groups in their natural environments. Understanding the movement patterns of these groups requires the study of individual movement behaviour and the interactions between leadership, imitation, and environmental drivers that influence movement decisions. In this talk I will also discuss the methods we're using to investigate these questions in the field, including tools to collect video footage, computational methods to locate animals within images, and statistical techniques to infer behavioral rules from movement data for both individuals (GPS collar data) and social groups (from UAV footage).

Many regions are experiencing unprecedented change in their climatic conditions. Temperature and precipitation are the major determinants of vegetation structure and biomass. Understanding how vegetation respond to change in temperature and precipitation will be key for predicting the status of the terrestrial Carbon sink and many interconnected ecosystem responses under future climate scenarios. In my study, I analyze long-term data sets at different spatial scales to identify patterns and processes of interactions between climate and vegetation.

In my first chapter, I address growing concerns of vegetation degradation in the Trans-Himalayas. This cold-arid ecosystem is a climate-change hotspot because it is experiencing rapid rise in temperature, and is also getting progressively wetter. While warmer and wetter conditions may favor plant growth, yet, there are widespread concerns of degradation in this ecosystem. I evaluated whether long-term trends in vegetation change, i.e., greening or browning, can inform management concerns over degradation. I analyzed satellite-derived vegetation datasets (NDVI) at six spatial scales: MODIS (250 m, 500 m, 1 km, and 5.5 km), SPOT (1 km), and GIMMS (8 km). Results indicate browning (degradation) in the spring and greening in late summer. This pattern was consistent across all spatial scales. The timing and location of degradation did not coincide with human land-use (livestock grazing), suggesting vegetation trends may be more strongly related to climate than to human land use. Overall, the results show the importance of evaluating the consistency of inter- and intra-annual vegetation trends across different spatial/temporal scales for interpreting degradation.

In my second chapter, I address how spatial and temporal variation in climatic factors can influence vegetation phenology across the greater Trans-Himalayan landscape. Global warming has caused relaxation of thermal constraints for plant growth in cold regions, which include both high-altitude and high-latitude landscapes. However, this hypothesis has found poor support globally, including the Trans-Himalayas where temperature alone could not explain the long-term greening/browning patterns. An alternate hypothesis is to examine the effect of water limitation across the Trans-Himalayan landscape. I investigated this by analyzing the plant phenological response in terms of change in shape of the annual growth pattern – a unimodal curve that is represented mathematically by a double-logistic function. I analyzed long-term changes in geometric properties of vegetation phenology, e.g., skewness and kurtosis of the double-logistic curve. This long-term phenological analysis showed that the plants are reaching their peak biomass earlier in the growth season, and also attaining a higher peak biomass. These changes were explained by variation in snowfall and rainfall. This study shows the differential effect of snow and rain in determining the phenological trends in mountain ecosystems.

In my last chapter, I explore whether long-term trends in global vegetation change is linked to thermal constraints on the biochemical steps in photosynthesis. A key aspect is temperature-sensitivity of net photosynthesis, which declines above 32 oC due to thermal sensitivity of participant enzymes, particularly Rubisco Activase. I find that warming over the past four decades has altered the window of thermally suitable days for photosynthesis and plant growth, through effects attributable to this enzyme’s thermal-sensitivity. This explains satellite-records of long-term vegetation trends during 1982-2015 (greening/browning) for nearly 80-million km2 across the world Change in temperature was more successful in explaining vegetation trends than simultaneous change in water-stress over the same period. This match (Bayesian probability) between thermal-sensitivity of enzymes and vegetation response could not be achieved by random chance. Comparatively, match against browning was lower than for greening, due to confounding effects (pests, fire, logging). However, I found that the water-stress could reverse the trends expected from temperature, especially in the warm regions. These results can help improve our understanding and prediction of future changes in terrestrial productivity, and the fate of the terrestrial carbon-sink, as they link processes occurring at molecular and planetary scales.

Many eukaryotic hosts are associated with microbes that enhance their fitness. The establishment and maintenance of such relationships likely depends on the faithful transmission of beneficial microbes across generations. However, in some cases the microbes are not maternally transmitted, but are environmentally acquired. Thus, when hosts disperse to new habitats or switch to a new diet, they may not have access to the appropriate microbes, leading to reduced fitness. In such cases, can the host establish new microbial partnerships, and are they specific? More generally, how are microbial communities assembled, and (how) does host association influence this process? In this talk, I will discuss our efforts to address these questions in different natural and laboratory insect populations.

Life history evolution is central to evolutionary biology. It deals with the schedule of reproduction and mortality over lifetime and attempts to understand the variation in the timing and frequency of reproduction and allocation of energy to reproduction, growth and maintenance. In this teaching lecture, I will cover the fundamental aspects of life history theory, place it in the context of natural selection and discuss its scope. I will then consider the major life history traits: age and size at maturity, number and size of offspring, and reproductive life span and ageing. I will then explore life history strategies using insects as an example. Finally, I will discuss limitations and paradoxes associated with these concepts. This talk will aim to give a broad and conceptual introduction to the theoretical body of work dealing with the evolution of life histories.

Resident microbes and the microbiome (collective microbial genomes associated with individuals) can provide novel functional benefits to their hosts. In insects, microbial associations mediate host adaptation to abiotic and biotic stresses and are important contributors to the insects’ remarkable ecological success. This talk will highlight the microbiome’s role in insect dietary specialization and in their utilization of challenging breeding resources. I will present my research on microbiome-mediated usage of unconventional and potentially noxious diets such as carrion and dung that are ephemeral and select for rapid development. Despite being susceptible to microbial spoilage, carrion beetles and dung beetles prolong the palatability of these diets to support larval development and are immune to any ill effects of feeding on decomposing matter. I will discuss the importance of the microbiome in the nutritional ecology of these insects, its implications on host diversification, behavior, and regulation of ecological processes.