I am interested in why some plants are well defended against insects and others are not. Plant defense theory has led to development of numerous hypotheses. Four of these hypotheses have been especially important as frameworks for research: a) optimal defense hypothesis, b) growth rate hypothesis, c) carbon: nutrient balance hypothesis, and d) growth-differentiation balance (GDB) hypothesis. These hypotheses are not mutually exclusive. The GDB hypothesis subsumes the others, but it is also the most difficult to test. Furthermore, the hypotheses cannot be tested directly; rather sub-hypotheses can be tested, which requires clearly articulating the assumptions and domain of the test. I have written several journal articles to clarify the issues.
- An increase in prey quantity does not simply yield an increase in predators. For instance, well-fed wasp colonies just slightly cooler on average than warmer colonies produced only a third as many offspring as warm, well-fed colonies and no more than cool, poorly-fed colonies. The difference in temperature between warm and cool colonies averaged just 1.3 degrees C per day, which is within the range predicted (0.6 to 2.5 C) for global increase over the next 50 or so years, due to global warming. (And this ties to what others have found: an increase in environmental temperature does not necessarily mean more favorable conditions for prey and, then correspondingly, higher prey populations.) These results illustrate the non-linearity of relationships among factors, and so the difficulty in making predictions without a much better understanding of these relationships.
- When prey are scarce, plant defensive chemicals in the diet of prey (insect herbivores) had a greater impact on predator growth rate than when prey were plentiful. That is, again, we see interactive effects. This is important because often predators are faced with prey scarcity, and so may be forced to subsist on prey containing plant defensive chemicals.
- Prey scarcity changes foraging behavior of predators and affects prey choice. For example, wasps from low-fed colonies attacked unpalatable prey sooner than wasps from high-fed colonies did, but not without considerable effort to avoid use of the patches with unpalatable prey. Furthermore, only wasps from low-fed colonies foraged on cool days, which is risky since temperature may drop to a level preventing the wasps returning that day. The message is that foraging behavior of predators feeding on insect herbivores is influenced by both quantity and quality of prey, with quality in large part a function of plant defensive chemicals.
- Tracy Curtis (PhD 2005) and I worked in collaboration with researchers elsewhere on a synthesis of the genetics, behavior and ecology of Polistes dominulus, an invasive paper wasp. This European-Asian species has been accidentally introduced into North America and other continents. To ascertain why it has been so successful, we have compared it to a native North American species.
With the increase in carbon dioxide and other greenhouse gases in the atmosphere, temperature has been increasing. Most of that increase is due to warmer nights (due to more cloud cover). Since temperature constrains plant growth more than photosynthesis, the prediction is that with warmer nights, plants will allocate more of their photosynthate to growth (most of which occurs at night) and, therefore, put less into defense. If that occurs, it could have serious consequences for plant defense against insects. Most plants have an array of chemical defenses but amounts vary because plants also have to allocate photosynthate to growth. We found that night-time temperatures does indeed affect the level of plant defenses, but different toxins are affected in different ways. We are not able to make reliable predictions about effects on particular toxins or groups of defenses because that would require a more precise accounting (for example, as called for in the GDB hypothesis) than currently can be done.
Prior work in my laboratory indicated that the effect of plant defensive chemistry on insect herbivores, and on insect predators eating the herbivores, is a function of temperature, as well as the combination of plant defensive chemicals. In particular, we found that the potential effect of warmer nights on growth of insect herbivores is not simply a function of an average temperature, but rather there can be interactive effects between night-time temperature and dietary plant defensive chemicals. For example, the negative effect of rutin on molting duration of caterpillars decreased with increased night-time temperature. So night-time temperature determined how much of the larval stadium (or instar) was susceptible to rutin effects and, thus, how much of the defenseless molting period was prolonged. (Caterpillars cannot defend themselves against insect predators and parasitoids while molting.)
In summary, it became clear to us that there is no simple pattern of interactive effects between plant defensive chemicals and temperature in plants or on insects. Therefore, global warming, especially with disproportionately warmer nights, could profoundly influence insect growth and, thus, populations in ways that are difficult to predict.
- HHMI-Interdisciplinary Research for Undergraduates
- Freshman Research Immersion program
- Overcoming Misconceptions in Ecology
Recent advances in science education show that teaching science by lecturing and having students read thick textbooks is not very effective. I have been involved in several science education grants aimed at providing university faculty, graduate students and elementary school teachers with alternatives.
One grant addressed the misconceptions in ecology and evolution that undergraduates have. The focus here was to identify and challenge the misconceptions using the 5E learning cycle method, which is based on constructivism learning theory. I collaborated with faculty and doctoral students on development and implementation of modules for large enrollment ecology courses that combine "the power of story" and the 5E teaching cycle. We also refined a concept inventory. Related to this project, I have reviewed ecology textbooks to determine whether the big ideas in the area of plant-herbivore interactions are present and identified some of the major misconceptions that people have about plant-herbivore interactions.
Another grant developed science units (5E teaching cycles) for elementary school teachers via a partnership of the teachers, university faculty, and graduate students. We developed modules that addressed children's misconceptions in biology, chemistry, geology and physics, and ran workshops to help the teachers and doctoral students plan how to co-teach those units.
Stamp N, O'Brien T (2004) GK-12 partnership: a model to advance change in science education. BioScience 55:70-77
A third grant focused on development of a series of workshops on university science education to help faculty and graduate students improve their teaching. Angela Pagano (PhD 2006) worked with us on this project.
Today tomato is the number one vegetable fruit produced in the world. How and when wild tomato arrived in Mesoamerica from the western slopes of the Andes in South America no one knows. Exactly how and when Mesoamerican farmers began nurturing wild tomato is not known either. With domestication of tomato, the key mutation, from a two-chambered fruit to a fruit with many seed chambers, created a much bigger fruit. By the time the Spanish conquistadors arrived in Mexica in1519, the domesticated tomato was much larger than its wild ancestors, a change of a marble-size to as much as baseball-size. It was the Spanish conquistadors who introduced tomato to Europe, which in turn led to tomato seeds carried throughout the Americas and Asia. The Aztecs also provided the winning recipe, salsa, a tomato-based condiment for vegetables and meat, loaded with flavors that stimulate our taste buds, an unusual combination of sweet, sour, salty, bitter and especially savory (aka umami). Even so, given that a mere fraction of the world's plants can be domesticated, why has tomato been so successful? A combination of factors provide the clues:
- Tomato's weediness gave it a foothold in house-hold gardens
- Mutations changing fruit from marble-size to baseball-size were favored by early Mesoamerican farmers
- Tomato's unusual balance of tastants—sweet, sour, salty, bitter and savory (umami)--set it apart as a condiment
- Tomato's flowering and seed set attributes favored cultivation
- Buzz pollinators were available in the Northern Hemisphere
- Tomato's genetics favored development of modern cultivars
- Tomato's genetics positioned it as a model fruit for research about fruiting in general