The Effect of Simulated Herbivory on the Growth and Reproduction of A. elliptica, an Invasive Exotic Species in South Florida 2000 Young Scholars Program University of Miami Department of Biology August 2, 2000 The Millennium Exotics Jena Greenman Zakiya Grubbs Grace Kaissal Dorothy Legros Bobbie McGruder Haris Sayeed Teacher: Otto Rodriguez Undergraduate Facilitator: Ana Lugo Graduate Facilitator: Tony Koop Faculty Mentor: Dr. Ron Hofstetter
1 ABSTRACT Invasive exotic species have negative effects on native communities, reducing biological diversity, changing habitat structure, and modifying environmental processes. One possible reason why they are so successful is that they have escaped from herbivore pressure in their native ranges. This study examined the effect of simulated herbivory on the growth and reproduction of an invasive exotic species: Ardisia elliptica. We simulated herbivory by punching holes in adult Ardisa elliptica leaves. All adult leaves were treated (punched) once every two months with one of three treatments (1,2,4 holes per leaf; an additional treatment served as a control). Herbivory effects on plants were assessed by measuring diameter, height, number of leaves, number of clusters and number of fruits/flowers for each of the potted plants. Overall, we found that the growth was uniform over time (1 year). The variables whose differences were significant were (for growth) diameter and the number of leaves. There was no significant effect in the reproduction variables. We concluded from this study that simulated herbivory had little to no effect on the growth and reproduction of Ardisia elliptica. In South Florida, A. elliptica is probably not experiencing any negative effects due to natural herbivory.
2 INTRODUCTION A native species is one living and growing where it originally started. An exotic species is a species brought by humans into a certain place where it does not occur naturally. An invasive exotic species is an exotic species that is invading and taking over the natural communities. Invasive exotics have several negative effects on native communities including, changing habitat structure, modifying environmental processes, and reducing biological diversity (Vitousek et al. 1996). This can become a very serious problem because invasive exotic species grow and reproduce much more successfully in a foreign land as opposed to its native land. Invasive exotics take over the land by reproducing massively. In turn, they out-compete and displace the native species. An example of such an invasive exotic is the Eurasian zebra mussel. The Eurasian zebra mussel entered North America in the ballast water of ships. This species spread rapidly as soon as it arrived. It covered the bottoms of rivers, lakes, and then into the waterworks of municipalities and industries. The cost for clearing blocked pipes will reach $3.1 billion over a 10-year period. Zebra mussels reduce natural algae populations, biological productivity, and increase the concentrations of nutrients in entire ecosystems. They are still continuing to spread into lakes, rivers, and canals throughout North America, posing extensive environmental problems (Vitousek, et. al., 1996) One possible reason why invasive exotic species are so successful in their new ranges is that they have escaped from herbivore pressure in their native ranges. Herbivory is the predation on a leaf or shoot by herbivores. Generally herbivory is when an animal (herbivore) eats away that plant s leaves. Enemy herbivores eat away the plants leaves, suck up the sap, and cause leaf death (Wineriter and Buckingham 1988). Herbivory harms a plant s growth and reproduction reducing the leaf area by which a plant acquires it solar resources. If a plant is released from herbivore pressure, it will grow more rapidly as opposed to a plant suffering from herbivory. Some scientists have used this fact to help control invasive exotics using biological control
3 (Center et al. 1998). Biological control is a process where natural herbivores of the invasive exotic plant are used to reduce growth and reproduction. The main goal of this study was to examine the role that herbivory may be playing in the invasion of southeastern Florida by Ardisia elliptica. Our specific goal was to measure the effect of simulated herbivory on the growth and reproduction of A. elliptica. BACKGROUND Ardisia elliptica is a tropical understory shrub from Southeast Asia. It grows up to 17 feet tall. The plant has one shoot and with leaves that grow perpendicularly. The stem is smooth and new leaves are often reddish. Leaves are oval in shape, fleshy with smooth margins. The inflorescences and fruit are axillary. As the fruits mature they are green, then a red color and then turn black when ripe (Langeland et. al., 1998) Humans brought A. elliptica to Florida in 1900. It was initially brought in as an ornamental plant, but little did humans know of its future effects. In Florida A. elliptica is now abundant around old fields, disturbed wetlands and hammocks, forming dense mini forests made up of A. elliptica only. A. elliptica also invades cypress and mangrove areas. In these habitats it grows so densely that it excludes native plants and animals (Langeland et. al., 1998) The Exotic Group of the 1999 Young Scholars Program examined herbivore damage on the invasive exotic A. elliptica and the native A. escallonioides. The purpose of their project was to compare the amount of natural herbivory between both species. In S. Florida, the 1999 Exotic Group sampled 6 leaves from 39 plants of each species at three different sites. In the lab, they measured the amount of leaf area eaten by herbivores for every plant. They found that the exotic Ardisia does have some herbivore damage in S. Florida. In all three sites, A. elliptica
4 had significantly less herbivore damage than the native Ardisia; however, this was due mostly to a significant difference at the northernmost site (Dominique et. al., 1999). Across S. Fl., A. elliptica had an average of 1.7% of its leaf area eaten, while the native A. escallonioides had about 3.5%. MATERIALS and METHODS To determine how the loss of leaf area may affect the growth and reproduction of A. elliptica, we conducted a simulated herbivory experiment. Herbivory was simulated by punching holes in leaves with a hole puncher (hole area = 0.335 cm 2 ). There were three different treatment groups and a control group. All adult leaves on the 116 potted plants were given 1 of 3 herbivory treatments (1, 2, or 4 holes per leaf) or, left untouched (control group) once every two months. Growth and reproduction were monitored on Ardisia elliptica plants every two months. Growth was measured as changes in height, basal diameter, number of branches, and number of leaves. Reproduction was measured as the number of inflorescences and the number of flowers/fruits per plant. The data were recorded on an Excel spreadsheet in the computer and were summarized by calculating group average and standard deviation for each variable. The data were analyzed by doing an analysis of variance (ANOVA) on each of the variables. ANOVA is a statistical procedure that is used to test the effect of the treatment. RESULTS The diameter under the control group increased more rapidly than in the other treatments. Under the one-hole treatment the plants were smallest in average diameter in July 1999. In September 1999 the control plants had the third largest diameter. In November of 1999 they increased to become the second largest in all measurements of growth and reproduction, as
5 expected (Fig 1a & 1b). What we expected was that the control group would have the largest growth rate. This was evident in everything that was measured except in the measurement for the number of branches. Across all two-month periods, the plants with the two-hole treatment consistently had a greater average number of branches (Fig. 4a & 4b). The graphs (Fig. 4a & 4b) show a growth spurt from March to May of 2000 for both the number of branches and the number of leaves. The effect of simulated herbivory was tested on the final measurements of reproduction. In May, 2000 the plants under the two-hole treatment had the greatest number of fruiting clusters followed by those under the one-hole treatment and control group respectively. For July, 2000 the control plants had the greatest number of flower/fruit clusters. They were followed by the one-hole treatment plants. It is evident from the graphs that there was a very large increase in number of clusters for the plants under the control group from May to July, 2000 (Fig 5a & 5b). The final numbers of fruits/flowers show the plants in the two-hole treatment had a slightly greater average than control plants (Fig. 6). In short, the only significant change was in the control group and four-hole treatments in the diameter and the number of leaves. DISCUSSION All of the patterns of reproduction on Ardisia elliptica were not what we expected. When our group began the experiment, we predicted that simulated herbivory would reduce the growth and reproduction of the exotic A. elliptica. Overall, the experiment showed that there was no significant effect due to the simulated herbivory. The only significant effect occurred in the values of diameter and the number of leaves, but that was only between the control and the 4- hole treatment. We can thus conclude that herbivory doesn t play a significant role in the
6 regulation of A. elliptica. Even though in some cases growth may be reduced slightly, reproduction is not affected. Although this study tested how different amounts of leaf-area loss (simulated herbivory) affects the growth and reproduction of A. elliptica, it doesn't say how A. elliptica in natural populations are responding to natural herbivory. To know if A. elliptica in natural populations respond to natural herbivory, we extrapolated from our simulated herbivory data. In order to extrapolate from our data, we had to express our treatments in terms of percent leaf area lost. To simplify calculations, we assumed that leaves were treated only once (a highly conservative assumption). We estimated that the 0, 1, 2, and 4 hole treatments represent a loss rate of 0%, 1.3%, 2.7%, and 5.2%, respectively. Last years Young Scholars found that in the field, 1.7% was the average percentage of leaf area lost for populations of A. selliptica. 1.7% falls in between the 1 hole (1.3%) and the 2 hole (2.3%) treatments. Because both the 1and 2 hole treatments did not have a significant effect on the growth and reproduction of A. elliptica, we concluded that A. elliptica growing in wild populations is not being affected by current levels of herbivory. Future studies of A. elliptica should find out what amounts of herbivory do affect growth and reproduction. ACKNOWLEDGEMENTS We would like to thank our parents for supporting us, Dr. Gaines for directing this program, and Dr. Krempels for her wonderful lectures. We would also like to thank Tony Koop for the weird, but fun adventures and for taking care of the plants in our absence, also Ana Lugo, Otto Rodriguez, and Dr. Hofstetter for the help on our project. We would also like to thank all of our teachers and guest speakers for teaching us things we used in this research project and will use further in life. Lastly, our gratitude to the National Science Foundation for funding this project.
7 LITERATURE CITED Austin, D. 1998. Displacement of Native Ecosystems by Invasive Alien Plants- The Florida Experience. In Jones, D. and B. Gambel, ed., Florida's Garden of Good and Evil. West Palm Beach: Florida Exotic Pest Plant Council. Center, T., Van, T., Rayachhetry, M., Buckingham, G., Wineriter, S., and M. Purcell. 1998. Release and Establishment of Oxyops vitiosa Pascoe for the Biological Control of Melaleuca in South Florida, In. Jones, D. and B. Gambel, eds., Florida's Garden of Good and Evil. West Palm Beach: Florida Exotic Pest Plant Council. Dominique, J., Scott, A., Scott, T., and G. Valdes. 1999. Herbivore damage on the invasive exotic Ardisia elliptica & the native A. escallonioides in southeastern Florida. In Gaines, M., ed., NSF Research Program Ecology 1999. Miami: University of Miami, Department of Biology. Langeland, K. and K. Burks, ed. 1998. Identification and Biology of Non-Native Plants in Florida's Natural Areas. Gainesville: University of Florida Press. Vitousek, P., D' Antonio C., Loope, L., and R. Westbrooks. 1996. Biological Invasions as Global Environmental Change. American Scientist 84: 468-478. Wineriter S. and G. Buckingham. 1998. Biological Control of Melaleuca- Insect Quarantine Research, In Florida's Garden of Good and Evil, ed. Jones, D. and B. Gambel, West Palm Beach: Florida Exotic Pest Plant Council.
8 Table 1. Summery of final (July 2000) growth and reproductive measurements of A. elliptica given four different levels of simulated herbivory over a 1-year period (July 1999 through July 2000). Treatments 1 0 1 2 4 F-value P-value Basal Diameter (cm) 1.00 0.97 0.97 0.92 4.65 042 Shoot Height (cm) 57.5 55.2 57.1 55.2 1.32 0.2646 Number of Leaves 120.5 108.5 106.8 96.0 5.10 024 Number of Branches 7.7 7.5 7.9 7.1 1.09 0.3578 Number of Clusters 13.2 10.7 13.1 9.7 0.96 0.4136 Number of Flowers or Fruits 59.6 48.2 6 4 1.18 0.4136 1 Simulated herbivory treatments consisted of punching 1, 2, or 4 holes per every adult leaf, every 2 months. Note the 0 treatment represented a control treatment.
Basal Diameter (cm) 1.2 1.0 0.8 0.6 0.4 0.2 0 1 2 4 9 1.2 Basal Diameter (cm) 1.0 0.8 0.6 0.4 0.2 Figures 1a & 1b- Mean basal diameter of A. elliptica subjected to 4 different levels of simulated herbivory from July 1999 to July 2000. The lines on top of the bars in the bar graph are the standard error.
10 7 6 Height (cm) 5 4 3 2 1 0 1 2 4 7 Height (cm) 6 5 4 3 2 1 Figures 2a & 2b- Mean height of A. elliptica subjected to 4 different levels of simulated herbivory from July 1999 to July 2000.
11 140 Number of Leaves 120 100 80 60 40 20 0 0 1 2 4 140 Number Of Leaves 120 100 80 60 40 20 0 Figures 3a & 3b- Mean number of leaves of A. elliptica subjected to 4 different levels of simulated herbivory from July 1999 to July 2000.
12 Number of Branches 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 1 2 4 9.0 Number of Branches 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 Figures 4a & 4b- Mean number of branches of A. elliptica subjected to 4 different levels of simulated herbivory from July 1999 to July 2000.
13 14.0 Number of Clusters 12.0 1 8.0 6.0 4.0 2.0 May 00 July 00 0 1 2 4 16.0 Number of Clusters 14.0 12.0 1 8.0 6.0 4.0 2.0 May 00 July 00 Figures 5- Mean number of flower \ fruit clusters of A. elliptica subjected to 4 different levels of simulated herbivory from July 2000.
14 Number of Fruits/Flowers 800 600 400 200 00 0 1 2 4 Treatment Figure 6- Mean number of fruits \ flowers of A. elliptica subjected to 4 different levels of simulated herbivory on July 2000.