An overarching goal of my research program is to understand the mechanisms driving the temporal variation and covariation of biologicalpopulations.  This goal addresses an unresolved disparity between mathematical models that universally predict compensatory dynamics (negative temporal covariation) among competing populations and long-term datasets that demonstrate synchronous dynamics (positive temporal covariation) among competing populations.  My work addresses this disparity from two standpoints.  By developing and applying a novel statistical approach to long-term data, I was first to demonstrate that synchronous and compensatory dynamics need not be mutually-exclusive, but can exist together when confined to different temporal scales (Vasseur et al. 2005; Vasseur and Gaedke 2007; Vasseur et al. in prep).  Second, my work has demonstrated that the interaction between environmental and biotic factors (species interactions) fundamentally alters the expected patterns of covariation among pairs and groups of interacting species (Vasseur and McCann 2005; Vasseur and Fox 2007; Vasseur and Fox 2009; Vasseur et al. 2011; DeLong and Vasseur 2012).   Currently, our ability to utilize the vast amount of long-term population time-series data being collected by projects like NEON (National Ecological Observatory Network) is impeded by our inability to infer the processes generating temporal dynamics.  My research is integral to inferring mechanistic processes from long-term data.  Within the context of this meta-theme, my research comprises three overlapping threads:

i) The impact and maintenance of biodiversity in aquatic communities          

Like a diverse economic portfolio of securities is thought to minimize risk and yield more stable returns, ecological theory suggests that a diverse community should exhibit a more stable ‘function’ (such total biomass, carbon sequestration, etc.). This idea requires that competing populations exhibit compensatory dynamics, yet prior to my work, evidence of compensatory dynamics in nature were rare and the common belief was the climate-forcing of populations was driving synchronization.  By developing and applying a new statistical methodology (based on spectral analysis), I demonstrated that biotic interactions and climate forcing operate atdifferent scales in aquatic plankton communities, emphasizing that both process co-occur and that both are required to understand the importance of biodiversity (Vasseur et al. 2005; Vasseur and Gaedke 2007).  I am currently using a refinement of this method to search for consistency in the patterns of compensatory and synchronous dynamics across a broad sample of 54 north-temperate lakes (Vasseur et al. in prep CIEE working group paper).  This work will provide more general insight into the drivers of temporal patterns and the importance of biodiversity in aquatic communities.

Although Yale professor G.E. Hutchinson first stated the problem long ago, ecologists still question why, and how, so many species of plankton coexist in aquatic systems.  This is largely because the homogenization of aquatic habitat (particularly in the open-water pelagic zone) yields little opportunity for species to find their ‘niche – i.e. the place where they are more successful than their competitors.   My research addresses this question by considering the environment a temporally variable arena in which biotic interactions (competition, predation) play-out.  I have developed mechanistic theory describing how temperature variation changes the interaction between predators and their prey (Vasseur and McCann 2005; Vasseur 2007a; Vasseur 2007b) and have recently expanded this theory and conducted experiments to understand how competitive interactions are impacted by temperature variation (Scranton and Vasseur, in prep; Cheung et al. in prep).  Considered together, work in this thread is changing our expectations of covariation patterns among populations and preparing the field to use time-series data to infer the mechanistic underpinnings of population dynamics. 

ii) The causes and consequences of spatial population synchrony

Many biological populations are confined to live in local pockets or ‘patches’ of suitable habitat within a larger inhabitable landscape and much work has been devoted to understanding how such ‘metapopulations’ function.  Recently, I experimentally demonstrated that metapopulations of a single-celled protist (Tetrahymena) can be understood as a physical system of coupled oscillators (e.g. lasers, Huygen’s clocks and Josephson-coupled superconductors); provided that there is the scope for oscillations within patches (e.g. a predator-prey cycle), passive dispersal of a very small number of individuals generates ‘phase-locked’ synchronization (Vasseur and Fox 2009; Fox et al. 2011) – a phenomenon which had been widely seen but poorly understood in nature.  The generality of this behavior provides an important and general simplification for the study of spatial metapopulations.  Unfortunately, we have further shown that spatial synchronization consistently and predictably increases extinction risk witout regard for the differences in rates or the type of local population dynamics (Vasseur 2007b; Yaari et al. 2012).  This work has opened a new area of inquisition that seeks to understand the mechanisms facilitating persistence in spatially synchronized metapopulations. 

iii) Eco-evolutionary dynamics of competitive communities

Mounting evidence that ecological and evolutionary changes can occur on concurrent timescales has provided a fresh set of ideas and tools for addressing classical problems in ecology.  Recognizing the lack of a formal theory for the coexistence of toxin-producing plants and their competitors, I used an eco-evolutionary approach to show that evolution can maintain coexistence in the absence of any ecological mechanism (Vasseur et al. 2011).  Moreover, I have used eco-evolutionary theory to demonstrate that the traits (those characteristics subject to evolutionary change) exhibited by competitors tend to be more similar under the type of resource competition common to aquatic phytoplankton (where resources like phosphorous and nitrogen are non- or only partially-substitutable; Vasseur and Fox 2011; Fox and Vasseur 2008).   Since the traits belonging to a population impact their dynamics (e.g. similar traits yielding similar dynamics – which I showed in Rocha et al. 2011; 2012), this work is continuing to reveal the mechanistic underpinnings of synchronous and compensatory dynamics in competitive communities.  Recently, Paul Turner, David Post and I established a Program in Eco-Evolutionary Interactions through the Yale Institute for Biospheric Studies.  This program will provide an experimental test-bed for validating theory-driven predictions.