David Mortensen

David Mortensen

I studied Ecology at Duke University at a time (the late 70’s and early 80’s) when asking applied questions was considered a lower form of science. I remember squatting in my field site on the north slope of the Brooks Range contemplating the political implications of a positive feedback in an environment with elevated CO2. Political implications were out of bounds in those days. Our work was to assess the impact of elevated carbon dioxide concentrations on the carbon source/sink dynamics in the tundra and taiga of Alaska. Of course the political implications were profound, increasing carbon dioxide concentrations resulted in warmer artic temperatures which in turn increased the depth of thaw of the permafrost which exposed an extensive carbon-rich peat reserve to decomposition…in short a positive feedback. I am delighted to say that our discipline has come a long way since those days. So long in fact that our most widely cited journal is the Journal of Applied Ecology. In fact, the application of ecological theory to solve largely human induced problems is where much of the action lies.

Today my research addresses applied plant ecology questions using the latest in empirical methods and mathematical models. My title, weed ecologist, indicates I do so working with weedy plants as my study system species. I have a strong commitment to conducting fundamental research then extending those insights to enhance ecologically enlightened management. Most of my work couches questions in a population dynamics framework, often a spatially explicit population dynamics framework. By the way, I am a strong believer in group chemistry, from here on “my” research will appropriately be written as “our” research. The “our” changes over time but right now consists of five graduate students (four doctoral students and one Masters student) and one post-doctoral scientist. I expect an additional post-doc to join our lab in the coming fall. Research underway in our lab is conducted in one of the following three study systems: ecological compensation of agricultural landscapes, invasive annuals in Appalachian Ridge/Valley forests, and invasive genotypes (herbicide resistant Conyza canadensis).

Ecological Compensation: the role of within field management and edges

Ecological compensation refers to landscape that is managed to provide extra ecosystems services to compensate for those lost due to intensive landscape management. In an agricultural landscape for example, management within fields or along edges strongly influences the resulting ecosystems services. We are currently engaged in preliminary research describing the flora and fauna of field edges and soon will be conducting experiments to quantify specific ecosystem services (ie. within field weed seed herbivory). A model assessing the relative importance of field perimeter to area relationships is being developed to provide a conceptual context for our empirical field studies.

Reducing equilibrium weed population densities in organic production systems. For much of my career I have conducted research to advance ecologically-based weed management. Our current research involves assessing mechanisms by which organic crops tolerate higher infestations of weeds than conventionally managed crops. This buffering seems to result from several factors currently under study in our group and involve differential time of emergence and enhanced moisture and nutrient supply rates of organic soils. This work is aided by research support from the national USDA small farming systems laboratory and is conducted through a collaborative effort between our lab, USDA-ARS and the Rodale Institute. This research couples empirical studies with leslie matrix modeling to extend the results of the field studies.

Invasive annuals in Appalachian Ridge/Valley forests

This work involves a survey and monitoring component and work to develop a predictive capability for where invasives could invade. In collaboration with The Nature Conservancy a large (1400 quadrat) survey was conducted along the Potomac River to determine the extent of invasion in a deciduous forest managed by The Nature Conservancy. In addition, the survey provided data to for testing plant by site associations. The predictive capacity, where do invasives invade, will be developed based on this and other surveys conducted in the lab. In addition to this descriptive work, a long-term experiment is underway in which the influence of habitat heterogeneity on the invasion success of Microstegium vimineum is being quantified. As that four-year study draws to a close we are working with the data to assess what environmental factors influenced invasion success and applying spatially explicit quantitative methods to assess the temporal and spatial dynamics of this troublesome invasive.

Invasive genotypes (herbicide resistant Conyza canadensis)

I have been an outspoken critic of widespread adoption of herbicide tolerant crops. In the long run I believe they will threaten the stability of our food supply. While I have raised flags about adoption of such crops (company’s insert herbicide resistant genes in a crop that was otherwise susceptible to a herbicide), their adoption has hurtled along at a phenomenal rate. For example, today approximately 85 percent of the soybean planted are herbicide tolerant, almost all to the active ingredient glyphosate. Resistant weeds have begun to appear, the most widespread and difficult to manage being horseweed or Conyza canadensis. Our research looks at the dispersal dynamics by empirically quantifying the dispersal kernel. Detailed life-history experiments are also being conducted to estimate the recruitment kernel. We are coupling the empirical work with a spatially explicit population dynamics model that enables us to ask policy questions regarding the marketing of these crops and extending the herbicide tolerance trait to other crop types, effectively increasing the proportion of the landscape in which the selection pressure is applied. There are number of aspects of this study system that welcome further study by a graduate student. For example, how does dispersal ability influence the fitness of seed? Do long distance dispersers carry a fitness disadvantage? This is a very important question as we are finding seed moving greater than 1000 meters in the field and our aerial sampling coupled with atmospheric modeling suggest seed could move 10’s of km a day.

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