Introduction

Simultaneous Evolution of Competitiveness and Defense Switching from Toxin Production to Growth-Based Strategies
by Tessa Jones, Karl Mechtenberg, Charles Jorgenson, Shannon Kulseth, Katie Valkenburg, Michael Zehfus, Paul Brown, and David H. Siemens

Currently, many ecologists believe that there is a tradeoff between defense (resistance and tolerance) and competitiveness (growth) (see McKey 1974, Hamilton et al. 2001). In other words, because plants have limited resources an optimal balance must be found between the two strategies. Until recent experimental programs at BHSU, this idea has gone largely untested.

Because plants are simultaneously being selected for defense and competition, we believe they would evolve a more comprehensive strategy. Two closely related local plants, Aribis Holboellii and Aribis Drumundii, were selected for the experiment. A. drummondii grows in a highly competitive habitat, while A. holboellii grows in less dense areas (see fig. 1).

Methods

The Ecology Lab at BHSU provided a controlled environment for species of plants to grow. In eight, flats 256 plants were grown; half of the flats contained grass to mimic plant-plant competition. Caterpillars were forced to feed on plants form each treatment to induce chemical defense (Glucosinolates) responses. Glucosinolates levels, plant size, and relative growth were measured from treatments.

Results & Discussion

As shown in Fig. 2, growth in A. drummondii was relatively unaffected by the presence of grass. However, A. holboellii showed a significant reduction in growth when competition was present The better competitor, A. drummondii, was found to have higher induced level of glucosinolates, in direct contrast to theoretical predictions.

However, the toxins may have a dual role in defense and competition. Further, the response to the simultaneous challenges of competition and herbivory involved a sophisticated switch between resistance and tolerance strategies. We believe A. drummondii has evolved a comprehensive strategy because of its highly competitive habitat (fig 4). This involves “switching” from a defensive to a competitive strategy (fig. 3).

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