Introduction
Fetid Adder’s Tongue (Scoliopus bigelovii) is a plant species in the Liliaceae family, native to California, that occurs on the UCSC campus. Though beautiful and unique, very little is known about this species. Fetid Adder’s Tongue blooms throughout the winter into early spring. Its flowers consist of three petals of off-white color and deep purple veins, have an ill scent, and are fly pollinated. One of the plant’s more notable features, however, is its leaves - Fetid Adder’s Tongue occurs on the UCSC campus; it has two leaves per plant which are often spotted with deep purple mottling. Generally, leaf spotting is associated with herbivory defense mechanisms, but this has not been studied in this species. My research aimed to fill this knowledge gap in the evolution and morphology of Fetid Adder’s Tongue.
I conducted research at Nisene Marks State Park (Site A) and just north of Hahn Student Services (Site B). I marked fifty leaves at each site at the beginning of their growth period and tracked herbivory levels on each leaf over the course of six weeks. I used ImageJ to determine the surface area of leaves covered by spots and the surface area of leaves affected by herbivory. I then used linear regression models to examine the relationships between the area of leaf with spotting and the change in leaf area removed by herbivory.
Results
Figure 1. Percent Spotted vs Percent Herbivorized at Nisene Marks State Park.
Figure 2. Percent Spotted vs Percent Herbivorized at Nisene Marks State Park.
Figure 3. The increase in percent of leaf herbivory at week 1 depending on percent of leaf area with spots at Site A.
No strong correlation was found between the amount of leaf spotting and level of herbivory at either site A or B (Figures 1, 2). However, at Site A during week 1, the amount of herbivory on leaves with <20% spotted area was greater than leaves with between 20-40% spotted area, and the amount of herbivory on leaves with >40% spotting was the lowest (Figure 5). Over the
course of the study period, leaves with >40% spots had the highest rate of increase in herbivory. However, a chi-squared test (0.29) and p-value (0.86) revealed that the null cannot be rejected The rate of increase of herbivory was progressively lower in leaves with less spotted surface area.
Discussion
Overall, the data collected did not support the prediction that leaf spotting deterred herbivores, and the results of this study were inconclusive. This does, however, provide a framework for further questions surrounding the evolutionary significance of leaf spotting. The lack of conclusive evidence may be due to several factors.
While the overall sample size of the study was large (100 leaves total), the sample size of leaves that experienced herbivory by the end of the study was significantly smaller. A total of 21 leaves and site A and 19 leaves at site B showed signs of herbivory. In addition, the actual amount of
leaf consumed was consistently below 10% aside from a few individual leaves, thus this plant species may not experience especially high levels of herbivory in general. A sample size this small may not be representative of the total population and therefore may give inaccurate results.
During week 1, high amounts of leaf spotting and low frequency of herbivory were related. By week 6, however, these results were reversed with high amounts of leaf spotting and high frequency of herbivory being related. This flip in correlation may be due to herbivore preference. If an herbivore has multiple options of leaves, it may at first choose leaves with less spotting. Once those leaves experience herbivory, however, it may show a preference for leaves with no signs of previous herbivory over leaves with low spotting with herbivory. An alternate explanation for the variance among correlation over time may be due to the evolutionary significance of leaf spotting throughout the lifecycle of the plants. During week 1, leaf spotting seemed to decrease herbivory. It may be that during the early stages of development it is more important to deter herbivores than it is at later stages of development. Decreasing leaf surface area through herbivory decreases the plants photosynthetic properties, causing decreased overall plant growth. During later stages of a plant's development, however, it has already flowered and potentially reproduced, leaving leaves with less importance to the plant’s survival. These hypotheses, if correct, would explain why at week 6 leaves with high levels of spotting have a higher frequency of herbivory. Further studies on both leaf spotting and herbivore preferences would have to be conducted to make any definitive conclusions.
Acknowledgements
I would like to thank Stacy Philpott, my faculty advisor, for helping me throughout my project. I would also like to thank Alex Jones, manager of the UC Santa Cruz Campus Natural Reserve, for showing me Fetid Adder’s Tongue sites and getting my research permits approved. A final thank you to the Norris Center for funding my research.
