Dr. Karowe's Research

     Long-term studies in Dr. Karowe's laboratory seek to understand 1) the effects of elevated atmospheric carbon dioxide on the nutritional quality and defensive chemistry of host plants, and 2) the consequences of these carbon dioxide-induced changes for the growth, survivorship, and behavior of insect herbivores, parasitoids, and hyperparasitoids.



 Pieris on lantana

    The carbon dioxide content of our atmosphere is expected to double by the end of this century, and my students and I are currently investigating several potential ecological consequences of this global atmospheric change.  Growth under elevated carbon dioxide typically results in increased plant growth, decreased leaf nitrogen and water contents, and increased leaf carbon:nitrogen ratios.  Presumably to compensate for decreased leaf nitrogen content, insect herbivores increase consumption rates but nevertheless usually display reduced survivorship and/or growth.  Thus, it is becoming clear that elevated carbon dioxide is likely to exert substantial direct effects on plants, and may therefore profoundly influence natural ecosystems.

  Cotesia glomerata laying a clutch of eggs in a Pieris caterpillar. Courtesy of Wageningen Agricultural University.

    Our current research compares these direct effects of elevated carbon dioxide between two contrasting multiple trophic level systems:  one based on crucifer host plants and the other based on legume host plants.  Since a major direct effect of elevated carbon dioxide is dilution of plant nitrogen, the hypothesis underlying this study is that association of host plants with nitrogen-fixing symbionts (legumes) will buffer all trophic levels against the direct effects of elevated carbon dioxide.  

 

    We grow plants at ambient and elevated carbon dioxide at the University of Michigan Biological Station.  We then analyze leaf nutritional quality (nitrogen and water contents and C:N ratio) and secondary chemistry (glucosinolate and alkaloid content) at each carbon dioxide level.  Consequences of elevated carbon dioxide for insect herbivores are identified by measuring larval survivorship, growth, efficiency of food utilization, and oviposition preference among host plants grown under each carbon dioxide level.  We also determine effects on parasitoid and hyperparasitoid survivorship and growth by rearing wasps in caterpillars fed host plants grown under each carbon dioxide level.
         The ultimate goals of this research are to expand our understanding of the direct ecological consequences of elevated carbon dioxide to include the third and fourth trophic levels, and to begin to assess mechanisms responsible for variability among different multiple trophic level systems in their susceptibility to the direct effects of elevated carbon dioxide.

         This work has been supported by grants from the National Science Foundation, the United States Department of Agriculture, and Western Michigan University.
 
 

Over the last several years, I have also worked with a number of undergraduate and graduate students on a very different set of questions about the ecology of nitrogen acquisition by insectivorous plants. Insectivory, the eating of insects, is one of the most dramatic adaptations of plants to low-nutrient environments. In Northern Michigan, nitrogen-poor bogs and swales contain two types of plants that have evolved insect-eating leaves: pitcher plants and sundews. In essence, these insectivorous plants have converted at carbon-capture tissue (the leaf) into a nitrogen-capture tissue. While it is a common belief that such plants obtain a substantial portion of their nitrogen from insects, few studies have actually demonstrated this (pitcher plants and sundews also have roots that can absorb nutrients from the slowly decaying plant matter). It is also not entirely clear which plant traits have the largest impact on prey capture rates. Finally, it is also not entirely clear what the ultimate effect is of the species (called inquilines) that live inside the pitcher on nitrogen availability to the plant.

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Insectivorous plants at UMBS, including pitchers with little venation (left) or heavy venation (middle), and a sundew plant(right).

Most nitrogen atoms have 7 protons and 7 neutrons, so are referred to as N14. A small percent of nitrogen atoms have an extra neutron, and comprise the stable isotope N15 (which is a "stable" isotope because it is not radioactive). The ratio of N15 to N14 increases as nitrogen is passed up through the food chain, so the nitrogen in insect prey (which is absorbed through the leaves) has a different ratio than nitrogen from decomposing plant material (which is absorbed through the roots). The N15 to N14 ratio of the pitcher itself therefore indicates how much nitrogen came from insect prey vs. from decomposing plant material. Using the UMBS stable isotope mass spectrometer, my students and I have addressed several questions, including:

1) What percent of their nitrogen do insectivorous plants at UMBS obtain from insects they capture?

2) Which traits (coloration, hair density, microsite pH) make some plants or pitchers better at capturing insects?

3) How do the species that live inside pitchers (the inquiline mosquitoes, midges, mites, and rotifers) affect nitrogen availability by the plant? Is the amount of nitrogen they make available to the plant through enhanced prey decomposition more than the amount they steal for their own growth?

 

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