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Tropical forests may be particularly vulnerable to extreme climatic conditions. These forests contribute disproportionately to global ecosystem services, and have dominant effects on global land-atmosphere interactions. Thus, understanding the responses of tropical forests to extreme temperatures and drought remain a major limitation in predicting global vegetation responses to future climate change. Work in my group examines how integration of anatomical, morphological and physiological traits translate to plant performance in changing environmental conditions particularly when experiencing extreme temperature and drought. Can this understanding help predict behaviour of tropical trees in the field; growth, mortality and regeneration over longer time scales; and ultimately, to distribution of species over environmental gradients, and responses of tropical forests to global warming? I highlight this work with two studies. The first, investigated the upper temperature limits of photosynthetic function. We asked how high temperature tolerance was related to morphological and physiological traits, and examined the consequences of this in the context of global warming. Our results show that tropical trees are precariously close to their upper thermal limits, and likely going to be severely affected by future warming. Importantly, thermotolerance differed between species and was related to leaf functional traits and photosynthetic rates. In the second study, we investigated water-use strategies in tropical trees, examining water uptake under well watered condition, and drought tolerance when water was limited. We asked if water uptake and drought tolerance were related to stem xylem anatomical traits, and tested the relationship between water transport efficiency and safety. Xylem size was positively related to water uptake, but negatively related to drought tolerance, resulting in a tradeoff where water uptake and growth under well watered conditions was negatively related to drought tolerance when water was limiting. These results suggest that tropical trees with acquisitive resource use strategies may be more negatively affected by increased temperatures and drought, and future climates may favour slower growing species with conservative resource use strategies.
Spiders, one of the top invertebrate predators in the terrestrial ecosystems are an ideal system to study ecological patterns and processes. Their hidden, but fascinating lives is full of drama: they are voracious predators, but get eaten by their own kind; they are also masters of trickery. In this talk, I will narrate two spider stories. One is of deception in the spider world: how ant mimicking spiders doubly deceive both visual predators such as jumping spiders and chemical predators such as mud-dauber wasps. Second story is about cooperation in hunting and web-building in social spiders. I will specifically talk about some recent work on how group size and hunger can influence the web architecture of social spiders.
Animal groups exhibit many emergent properties that are a consequence of local interactions. Linking individual-level behaviour to group-level dynamics has been a question of fundamental interest from both biological and mathematical perspectives. However, most empirical studies have focussed on average behaviours ignoring stochasticity at the level of individuals. On the other hand conclusions from theoretical models are often derived in the limit of infinite systems, in turn neglecting stochastic effects due to finite group sizes. In our study, we use a stochastic framework that accounts for intrinsic-noise in collective dynamics arising due to (a) inherently probabilistic interactions and (b) finite number of group members. We derive equations of group dynamics starting from individual-level probabilistic rules as well as from real data.
First, using the chemical Langevin method, we analytically derive models (stochastic differential equations) for group dynamics for a variable m that describes the order/consensus within a group. We assume that organisms stochastically interact and choose between two/four directions. We find that simple pairwise interactions between individuals lead to intrinsic-noise that depends on the current state of the system (i.e. a multiplicative or state dependent noise). Surprisingly, this noise creates a new ordered state that is absent in the deterministic analogue.
Next, we develop a method to derive the group-level dynamical equation directly from the data of collective dynamics. We assert that such an equation extracted from the data encodes important information about the underlying interactions. Therefore, we derive this equation describing the dynamics of order in two real systems- Fish (Etroplus suratensis) and Whirligig Beetles (Gyrinidae dineutes) in my next two chapters.
Focussing on small-to-intermediate sized groups (10-100), we demonstrate that intrinsic-noise induces schooling (polarized or highly coherent motion) in fish groups. The fewer the fish, the greater the intrinsic-noise and therefore the likelihood of alignment. Such empirical evidence is rare, and tightly constrains the possible underlying interactions between fish. Our model simulations indicate that E. suratensis align with each other one at a time (positive-pairwise), ruling out other complex interactions.
Finally, we apply the same method to swarms of Whirligig Beetles which shows contrasting rotational order. We find that a different set of interactions - negative-pairwise and positive-three-body interactions between individuals are required to explain the observed group dynamics. Whilst the three-body interactions can explain the structure of the deterministic part of the equation, negative-pairwise explains the stochastic counterpart.
Broadly, our results demonstrate that rather than simply obscuring otherwise deterministic dynamics, intrinsic-noise is fundamental to the characterisation of emergent collective behaviours, suggesting a need to re-appraise aspects of both collective motion and behavioural inference.
Animals across taxa and habitats are known to use available space non-randomly. They are known to concentrate their space use around locations rich in food, mates or refuges. There could also be cascading effects of such disproportionate use for the individual itself, its conspecifics or even the landscape it inhabits. In addition to using their habitats non-randomly for foraging, avoiding predators and optimizing homing routes; some social insects were also discovered to use their nest space non-randomly. We tested if the primitively eusocial paper wasp Ropalidia marginata used its nest space non-randomly and indeed found a majority of individuals using parts of the nest more intensively than expected by chance (spatial fidelity). We tested several hypotheses that were primarily based on studies on ants, to understand the relationship between the social and spatial organization of individuals in social insect colonies. We found that the non-random space use by adults within R. marginata nests is a result of maximizing nutritional exchange and minimizing disease spread in the densely populated colonies. In addition, in order to understand the role of non-random space use by adults on task performance, we tracked individuals while they performed the task of food distribution, as it is the most conspicuous and important task in social insect colonies. We found that wasps within a feeding bout cooperatively (and often repeatedly) fed the randomly distributed larvae, thus minimizing the chances of any larvae going hungry. Each wasp that fed larvae in a feeding bout optimized its feeding route by minimizing the distance per unit larvae it fed. We conclude that understanding the spatial organization of adults might help us better understand the mechanism of efficient division of labour on social insect nests.
The evolution of flamboyant traits in animals is typically attributed to the selective force of sexual selection. However, natural selection can constrain the degree of elaboration of such traits. Therefore, animal signals reflect a balance between natural and sexual selection. I examined the role of these forces in the maintenance of a complex visual signal: dynamic colour change. Males of the Indian rock agama (*Psammophilus dorsalis*) exhibit rapid dynamic colour changes on their dorsal and lateral body regions during social interactions. The costs, benefits and adaptive significance of this relatively rare signal type is yet unknown.
Using a combination of visual modelling and field experiments, I first examined the predation risk on social colours and found that the courtship signal of males is costlier than the aggression signal. I then tested whether male colours expressed during aggression convey information about individual physiology and performance measures. Apart from a negative association between testosterone levels and the yellow colour expressed during aggression, body size and bite force were correlated, suggesting that body size could be an honest predictor of fighting ability. In the third chapter, I examined differences in health parameters of males and females that occupy dramatically different habitats as a consequence of urbanization. Our results suggest that lizards in urban areas appear to have shifted their innate physiology in order to cope with urban stressors. Finally, I examined the response of receivers to different components of the male colour signals by assessing attention paid by conspecific receivers to each signal component independently and together. Both males and females responded equally to all male social colours although females showed difference in response to achromatic signals. Overall, we conclude that dynamic colour change may have evolved in this species to actively balance the costs of predation risk with the benefits of social signalling.
The Western Ghats (WG) is one of the major global biodiversity hotspots, harbouring a rich diversity of flora and fauna many of which are endemic to the WG. The current understanding of the biogeographic history of WG comes from paleo-floral records and taxonomic diversity studies, but hasn’t been explored from a phylogenetic perspective. This was the inception of my study with the main aim to understand the imprints of biogeographic history on the phylogenetic diversity (PD) of the flora of Western Ghats. I first studied the PD patterns of local deciduous forest patches (Nandi Hills, Savandurga, and Devrayanadurga) to evaluate the usefulness of PD in the Indian context. Whereas other studies have shown that PD can be decoupled from taxon richness in biodiversity hotspots, my results showed this decoupling even in regions of low diversity. I then used these tools of community phylogenetics to analyze the patterns of PD across the WG. My premise was that if the deciduous forests of the WG are indeed more recently established than the evergreen forests (as literature suggests), then evergreen PD would be high and deciduous PD would be low. My results indeed show this pattern, corroborating this hypothesis. Within the evergreen belt, I found PD patterns that corroborate the southern refuge hypothesis, with higher PD in south compared to north. I also analyzed the phylogenetic turnover between these forests and showed that whereas the deciduous and evergreen taxa have shared evolutionary histories, the evergreen taxa from different forest types have quite disparate evolutionary histories. Phylogenetic endemism (PE) analysis (analyzing ranges of clades rather than taxa) showed that most paleoendemic plots are found south of 12-degree latitude indicative of refugial regions as postulated by the southern refuge hypothesis. Toward the north and south are clusters of neo- and paleo- endemism, which indicate that clades are restricted in distribution mostly in south, but also in north, with the central WG being a region of overlap of these ranges. My study is the first to provide a phylogenetic perspective toward understanding the biogeographic history of Western Ghats. It provides a fresh line of evidence corroborating current hypotheses and uncovered many interesting patterns which need further exploration, integrating tools from both community ecology and biogeography.
Dispersal has important ecological and evolutionary consequences for a species. Marine dispersal is unique because of facilitation by ocean currents, where oceanography interacts with species traits and environmental heterogeneity to determine connectivity between populations. However, marine dispersal research has largely focused on coral reefs and temperate shores, while tropical coastlines remain poorly studied. To address this gap in knowledge, we studied dispersal patterns and processes along the Indian coastline using two genera of intertidal littorinid snails (Littoraria and Echinolittorina) as a model system. We used a comparative framework to study the influence of species traits on patterns of mitochondrial phylogeography and deduced environmental correlates of range using species distribution models. We also performed larval dispersal simulations to derive estimates of coastal connectivity.
We found that all species showed signatures of demographic expansion in the Pleistocene, and only two species with wide environmental tolerance showed population genetic structure. Gradients in sea surface salinity and tidal range along the coastline were important in predicting distribution patterns across species and scales. There were seasonal differences in oceanic connectivity patterns, where certain sections remained isolated from the rest of the coastline. Regions predicted to have poor connectivity overlapped with observed species turnover for a range of marine taxa. These multiple lines of evidence suggest that variation in environment and oceanographic connectivity can influence dispersal patterns along tropical coastlines. This study presents hypotheses related to species-environment relationships and population genetic connectivity, which can be tested in other taxa to arrive at a unified framework of coastal biogeography for this region.
In the tree cricket, Oecanthus henryi, males invest in long-distance acoustic signals which the females use to identify and localize conspecific mates. Males that live and call longer or louder are expected to have higher mate attraction opportunities. O. henryi males with softer calls also exhibit an alternate signalling strategy called baffling. Baffling increases the sound pressure level (SPL) of the signal by 8-12 dB, thereby increasing their mate attraction potential. Interestingly, most call features in O. henryi, including SPL, show a low across-night repeatability, indicating immediate condition-dependence. Nutrition and age can contribute to immediate male-condition. Therefore, in this study we investigated the effect of diet on male longevity and the interaction of diet and age in determining lifetime male calling propensity & calling duration in a semi-natural set-up. In concordance with our expectation, males on high-nutrition lived longer, had a higher lifetime signalling and had longer within-night calling duration. We, moreover, studied diet-dependent phenotypic senescence of male acoustic signals. Males on high dietary-condition sustained their high signaling activity while males on poor dietary-condition suffered a sharp decline in signaling. We also examined how nutrition & age affect non-baffling SPL and consequently, baffling behaviour. We hypothesized males on poor diet-condition to have lower non-baffling SPL and hence, higher baffling probability. We also expected to find similar pattern of age-dependent decline in non-baffling SPL and consequent increase in baffling probability. We, however, did not find any effect of diet and age on signal SPL and baffling probability.
How does our brain enable us to pay attention selectively to important events in the world and ignore irrelevant events? While neuroscientists have largely studied how evolutionarily newer "forebrain" regions (e.g. neocortex) contribute to attention, comparatively little is known about how evolutionarily older "midbrain" regions control attention.
In this talk, I will describe our recent work seeking to understand the role of the superior colliculus (SC) -- an evolutionarily conserved midbrain structure -- in attention. The SC can be found in all vertebrates -- from fish to mammals -- and is known to be involved in controlling eye movements in all of these species. I will describe recent experiments in birds, monkeys and humans that show that the SC is also involved in controlling attention. Specifically our experiments show that the SC controls a particular component of attention called "spatial choice bias". Understanding how the SC controls attention will be critical for understanding how evolutionarily conserved mechanisms of attention operate in the brain.
A central question in ecology involves understanding the processes underlying patterns in population abundance and the distribution of species at small and large spatial scales. The distribution of individuals of a species across a landscape may be influenced by both local factors, such as resource abundance; and by landscape-level factors, such as the size of habitat patches, connectivity between patches and the permeability of the matrix surrounding habitat patches, all of which influence the colonisation and extinction of local populations and the movement of individuals between populations. How these local and landscape-level factors affect the distribution of a species may vary widely between species, because the response of species to these ecological conditions may depend on species-specific traits, such as body size, behaviour and other functional traits. There is relatively little known about how ecological factors interact with functional traits to influence species distribution in a landscape. I investigated the ecological processes at local and landscape levels influencing population densities by taking a behavioural ecological approach and using butterflies as a model system. I also examined how functional traits affect the relationships between ecological factors and species distribution in a landscape. I first examined how resource dispersion, an important ecological condition affecting butterfly populations, affects key behavioural decisions of butterflies. Studying the behaviour of individuals allows us to link population patterns with underlying ecological and evolutionary processes. I describe how butterflies appear to respond to resource dispersion at both small and large spatial scales and to balance acquiring two distinct types of resources when making foraging and habitat-use decisions. I then examined how landscape-level factors, specifically patch size, connectivity and matrix permeability, affect butterfly populations. I tested whether the apparent response of a species to landscape-level factors was affected by species-specific traits, specifically whether it was a habitat generalist or specialist and how permeable the matrix was to it. Finally, I test and describe how diverse functional traits, including morphological, life-history and behavioural traits, affect relationships between landscape composition and population density patterns of butterflies.