In many undergraduate ecology courses, students are introduced to the concepts of life history, resource allocation to fitness-associated traits in animals and plants, and the potential ecological and evolutionary implications of different patterns of energy allocation to offspring. The laboratory exercise described here allows students to examine the concept of plant allocation of resources to traits of wind-dispersed seeds including seed mass, wing area, and plant height that could potentially influence dispersal distance. Students also evaluate the role of external environmental variables that may influence seed dispersal, particularly wind speed. Using locally available plant species with winged or plumed seeds, instructors can guide students to test concepts related to the consequences of seed physical traits on seed dispersal and the dispersion of seeds in the environment. With forceps, stopwatches, and easy-to-make cylinders through which seeds are dropped students can evaluate the importance of seed traits for dispersal under highly controlled conditions. A terminal velocity can be generated for each seed, and from the terminal velocity, potential seed dispersal distance can be calculated. An outdoor portion of the lab exercise allows students to re-examine their findings regarding seed traits and dispersal potential, in light of prevailing field conditions.
Results/Conclusions
The student learning goals for this exercise include: 1) application of ecological concepts related to parental allocation to offspring and consequences for those offspring, 2) graphing the relationship between seed traits, e.g., seed mass, wing area, and terminal velocity of seeds, 3) calculation of the regression equations between seed traits and terminal velocity, 4) use of terminal velocity values from laboratory studies to calculate potential seed dispersal distance, and 5) testing ideas related to seed traits and dispersal distance in the field to determine if results from the controlled laboratory conditions are supported by patterns of seed dispersal in the field. Students prepare a short laboratory report to evaluate their findings. This exercise has been conducted using seeds of at least two species of milkweed (Asclepias), two species of maple (Acer), and goatsbeard (Tragopogon). Students report that they enjoy the laboratory portion of this exercise, since it generates simple-to-observe outcomes based on hands-on tests of ideas in biology. Pitfalls associated with this exercise, e.g., weak correlations between seed traits and terminal velocity for seeds of certain species will also be examined.
