Shyam Thomas

Mercury in fish is a major environmental concern as it is a direct threat to human health and well-being. Organic methylmercury in fish bioaccumulates, such that larger individuals tend to have higher amounts of mercury. Fish mercury levels are thus predicted as a function of body size. Additional factors like climate, surrounding landscape conditions, lake water chemistry, trophic position, species composition can further modulate mercury bioaccumulation. Inter-and within-lake variations are thus expected. However, these variations are not well quantified, and more importantly, the significance of the various multi-scale drivers and their interactions are poorly understood.

At Ryerson University, I collaborated with the Ontario Ministry of Environment & Climate Change to study mercury bioaccumulation in Walleye and Northern Pike. Using mixed-effects models, spatiotemporal variations in mercury bioaccumulation were quantified at fine and broad scales. Fine-scale models at the lake and year level explained most of the variations. Broad-scale models at 3 latitudinal degrees and 5-year time periods provided a useful index that summarized mercury trends in a climate change context. For an improved understanding of the role of various drivers and processes operating across multiple scales (climate-, landscape-, lake-, & fish- levels), structural equation models were used. Climate and landscape conditions were found to impact fish mercury levels, but indirectly by altering lake water chemistry and fish size.

Biological invasions are characterized by dynamic spatiotemporal patterns and associated processes that are most evident at the landscape level. However, landscape-level distribution patterns of invasive species are rarely leveraged to comprehensively understand the process of invasion. Here I address the potential of spatiotemporal distribution patterns in understanding the process of invasion by considering a stage-based approach for an invasive insect. In short, the recent invasion of Asian Citrus Psyllid (a destructive insect pest) in southern California and the associated spatiotemporal patterns were analyzed to characterize three key stages of invasion.

As part of this (postdoc) research, I worked with a large monitoring database of ACP collected across urban South California. The extensively collected spatiotemporal data on ACP invasion since 2008 provides a unique opportunity to understand an active ongoing invasion process and characterize key stages of invasion - introduction, establishment, spread, and impact. Using a suite of spatiotemporal analyses and habitat suitability models, the study addresses three key questions: (i) Do major roads serve as long-distance dispersal corridors of ACP introduction in an urban landscape?, (ii) Can habitat suitability models based on presence and abundance information capture ACP's establishment and impact risks, respectively?, and finally (iii) Do the locations predicted as high establishment and high impact risk overlap?

The study of species distribution is one of the oldest and central premises in ecology that strives to explain biodiversity patterns in space and time. Earlier, biogeographical studies considered mostly factors such as climate that operate at large spatial scales. Species distributions are now increasingly viewed as an 'emergent' outcome of multiple factors (biotic and abiotic) operating hierarchically in space. Not surprisingly, species distribution models (aka habitat suitability/niche models) now explicitly incorporate ecological factors in a hierarchical framework, wherein effects of various factors are scale-dependent (see above figure).

My Ph.D. dissertation project explored the spatial distribution of an invasive wetland plant - purple loosestrife (inset picture above) by taking a hierarchical approach. My research highlighted that loosestrife invasion across an urban landscape is a complex process wherein three key spatially nested ecological factors - availability of suitable habitat (i.e. wetlands), its spatial neighborhood, and propagule pressure, together explain loosestrife distribution. Predictive invasive species models based on surrounding land-use conditions and propagule pressure further vindicated the significance of incorporating fine-scale processes such as dispersal, since models that included propagule pressure as the eventual nested factor yielded accurate predictions and also minimized the area predicted as high risk of invasion.

Grasslands are interesting ecosystems in that they are largely maintained by a suite of natural disturbances like fire & grazing. Not surprisingly, grasslands have provided a 'natural' platform to study and understand how various disturbances, both natural & anthropogenic, modulate ecological systems. Given the increasing trend in anthropogenic modifications of natural habitats and disturbance-driven establishment of exotic species, understanding the 'ecology of disturbances' is today all the more significant. And, grassland systems continue to provide an ideal ecological setting for this purpose.

My masters research project explored how species diversity and composition respond to disturbances in an Oklahoman tallgrass prairie grassland, through a long-term experimental mowing study. The study highlighted the usefulness of mowing as a restoration tool as it enhances species richness by creating microsites or gaps with high light availability. The study also revealed the confounding effects invasive plant species can have on grassland conservation as disturbances that enhance plant diversity can also facilitate the establishment of invasive plants (Dee et al. 2016). Alongside, I also reviewed the literature on the lesser-known montane shola-grasslands of Western Ghats, India in an effort to understand the community ecology and conservation lessons learned and identify key gaps that remain (Thomas & Palmer 2007).