Disturbance and Variance: Detecting change in terrestrial and aquatic ecosystems

James Rusak

Our workshop explored the relationship between disturbance and variance across a variety of temporal, spatial and ecosystem scales. Some of the overarching questions we posed at the outset related to how often perturbations affect system variability, whether altered variance a useful metric for detecting either direct effects or legacies of disturbance, and what are the best approaches to detect changes in variability at different spatial and temporal scales or levels of organization. Jennifer Fraterrigo (Coweeta) opened the talks with a comprehensive examination of changes in spatial variability of terrestrial forest ecosystems in the Appalachians as a result of historical land use change. She showed that despite little or no changes in mean values for soil chemistry and structure, historical land use practices had dramatically altered the variance (both increases and decreases) associated with these parameters when compared with relatively undisturbed sites. James Rusak (North Temperate Lakes) provided an overview of disturbance-variance relationships over time in a variety of aquatic ecosystems. He demonstrated large increases in population, community and ecosystem variability (again often with no detectable difference in mean response) in multiple whole-lake experiments that mimicked current anthropogenic stresses on aquatic ecosystems (eutrophication, acidification, species introductions). Pierre Legendre (invited speaker, U. of Montreal) presented an innovative multivariate technique capable of partitioning variation at essentially all spatial and/or temporal scales present in a dataset (biological or physical) as they relate to measured environmental (or disturbance) variables. Using a variety of terrestrial and aquatic datasets, sampled over both space and time, he showed the usefulness of this new technique at extracting patterns that would otherwise have gone unnoticed.

In the half hour discussion that followed, the audience generated a number of further questions and issues. Were there certain types of disturbance that always elicited a similar response in variance? Were there thresholds that had to be crossed before increased variance was observed? How could the effects of multiple disturbances on ecosystem variance be reconciled? How did altered variance affect predictability of ecosystem function? The workshop was very well attended and researchers from a number of LTER sites expressed at interest in collaboration given the prevalence of disturbance as important variable in generating a wide variety of ecosystem behaviors across a wide range of ecosystem types.