Cascading Evolutionary Change in Lakes


There has been much interest in the potential for evolutionary diversification to impact ecological properties and promote reciprocal interactions between ecological and evolutionary forces, or eco-evolutionary dynamics.  It has only recently been shown that intraspecific diversification can impact the properties of populations, communities, and ecosystems.  However, this body of research has focused on feedbacks between variation in one organism and the rest of the environment.  This is important because natural systems are inherently complex and evolutionary changes in one organism, and associated ecological impacts of these changes, may alter the selective landscape and promote a series of evolutionary changes that propagate throughout the food web.  In collaboration with David Post (Yale University), we are exploring the importance of cascading evolution in lakes in Connecticut.  In this study system, lakes contain populations of a dominant fish predator, the alewife (Alosa pseudoharengus), that either does (anadromous) or does not (landlocked) migrate between the marine and freshwater environments for the purposes of spawning.  Our research has shown that intraspecific variation in alewives has driven evolutionary divergence in their zooplankton prey (Daphnia).  This includes significant changes in life history traits as well as divergence in phenotypic plasticity (Daphnia responses to predators).  We have also shown that evolution in Daphnia, in turn, has reciprocal impacts on community properties and ecosystem function.  Current and future topics of research include: cascading selection on multi-generation plasticity in Daphnia, life history trade-offs in Daphnia, the impact of alewife variation on Daphnia behavior.  

Evolutionary dynamics in an invasive species


Biological invasions are increasingly recognized as significant agents of global change that threaten native biodiversity.  In collaboration with Jennifer Howeth (Univ of Alabama), we are exploring the mechanisms of invasion and proliferation in an invasive species of zooplankton in the United States.  Daphnia lumholtzi, is native to Africa and Australia and was first detected in a lake in Texas in 1990.  D. lumholtzi has subsequently spread as far north as Minnesota, as far west as California, and it is found in >152 bodies of water.  D. lumholtzi invasions are potentially favored by an enhanced ability to produce morphological defenses (head spines) that reduce susceptibility to predation and/or a superior competitive ability compared to native species.  In this research we are evaluating patterns of trait variation and the evidence for evolution across an extensive geographic range.   In addition, many species of zooplankton, such as D. lumholtzi, produce resting eggs that sink to the sediment and remain viable for decades.  By collecting sediment cores from lakes and determining the age of each layer of sediment, eggs that are nearly 100 years-old can be successfully hatched.  Thus, we are also quantifying the dynamics of trait variation in D. lumholtzi (and native species) using resurrected populations from sediment cores.

Research in the Walsh Lab

Our research has two overarching goals: (1) to further understanding of the ecological  mechanisms (i.e., predation, competition, human-induced environmental change) that drive microevolutionary changes in natural populations, and (2) to quantify the importance of evolution as agent of ecological change.


Indirect Effects and Evolutionary Change  


Much work has shown that predators cause evolution via the consumption of prey.  However, the traditional evolutionary perspective is that interactions between predator and prey occur in a vacuum; no consideration is given to feedbacks associated with community- or ecosystem-level processes.  This is important because, by reducing prey abundances and increasing food to survivors, predators almost always have indirect effects.  Indirect effects are ubiquitous in nature and represent a major focus of ecological research.  Moreover, the ecological consequences of indirect effects also provide a link to evolution because it is generally assumed that resource availability influences patterns of evolutionary change.  Research in my lab continues to explore links between the indirect effects of predators and life history evolution in a killifish, Rivulus hartii on the island of Trinidad (in collaboration with David Reznick, UC Riverside).  This research has focused on a killifish, Rivulus hartii, because they exhibit an enhanced dispersal capability, which allows them to colonize a diversity of aquatic habitats.  As a result, Rivulus are located across a series of communities that differ in both the direct and indirect impacts of predators on the ecology of Rivulus.  In this system, we utilize a combination of ecological (mark-recapture studies, field surveys) and evolutionary approaches (common garden experiments, introduction experiments) to quantify selection due to indirect effects.  


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The Walsh Lab

Department of Biology

University of Texas Arlington