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1 Effect of sunlight intensity and canopy coverage on herbivory in Gunnera insignis (Gunneraceae) Carolyn Rivers Department of Biology, Wofford College ABSTRACT Studies on the influence of sunlight on herbivory h ave generally revealed that increased amounts of le af damage occur in cool, shaded habitats. This study i nvestigates the effects of both sunlight and canopy coverage on herbivore damage in Gunnera insignis by the undescribed beetle Macrohaltica sp (Coleoptera: Chrysomelidae). This beetle is though t to feed on G. insignis because it can obtain secondary metabolites that make it undesirable to predators. In this investigation, all 90 plants studied revea led herbivore damage, with 71% of plants showing eviden ce of herbivory on the majority of leaves. The highest levels of leaf damage occurred on plants in both high light and high shade habitats, while tho se in moderate sunlight conditions revealed reduced herbi vore damage. Resource availability, nutritional qua lity, and plant defenses influenced by sunlight could acc ount for the variation in leaf consumption on G. insignis Because light environment influences a number of factors related to the survival of a plant, the effects of sunlight on herbivory are complex and re quire further investigation. RESUMEN Estudios previos en la influencia de la luz solar e n herbivora generalmente han mostrado un mayor da o en las hojas en sitios fros y con sombra. Este estud io investiga el efecto de ambos, luz solar y cobert ura del dosel en dao por herbivora en Gunnera insignis por el escarabajo Macrohaltica sp. (Coleoptera: Chrysomelidae). Se cree que este escarabajo se ali menta de G. insignis para obtener metabolitos secundarios que lo hacen indeseable para los depred adores. En esta investigacin, todas las 90 planta s estudiadas mostraron dao por herbivora, con un 71 % de las plantas mostrando evidencia de hebivora e n la mayora de las hojas. Los niveles ms altos de dao en las hojas ocurren en plantas con altos nive les de luz solar y sombra, mientras que aquellas en sitios con moderada luz solar presentan menor dao. La disponibilidad de recursos, calidad nutricional y l a influencia de las defensas en las plantas influen ciados por la luz solar podra ser la causa de la variaci n en el consumo de hojas de G. insignis Debido a que la luz solar influencia varios factores relacionados c on la sobrevivencia de las plantas, el efecto de la luz solar en herbivora es complejo y requiere de investigaci ones futuras. INTRODUCTION The evolution of plant-herbivore interactions has l ead to a variety of mechanisms used by plants to reduce the extent of herbivory. Host spec ialization by herbivores appears to be high in the tropics due to the high diversity of in sects and niche partitioning (Coley and Barone 1996). Moth larvae, adult beetles, leaf cutt er ants, and grasshoppers commonly defoliate tropical plants and significantly reduce a plants ability to reproduce (Whitham et al. 1991). In the tropical plant Piper arieianum 10% defoliation significantly reduces plant growth, seed production, and delays flowering (Marquis 1991). Clark and Clark (1991) outline four general options to reduce the i mpact of herbivory: (1) inaccessibility by being too rare or in unattainable locations, (2) predator satiation or flushing of edible
2 tissues at times when herbivores are inactive, (3) mechanical obstacles, such as toughness or spines, and (4) chemical defenses such as tannin s or alkaloids. Gunnera insignis is one plant that employs a number of defensive me chanisms to deter herbivory. It is found in high sunlight envi ronments with low soil nitrogen. The tough, robust, and leathery leaf tissue of G. insignis (Gargiullo et al. 2008) limits the ability of many herbivorous insects to destroy its leaves (Eberhard et al. 1993). It may also produce secondary metabolites (Eberhard et al. 1993). However, one undescribed metallic purple species of Chrysomelidae beetle, Macrohaltica sp. has been able to overcome these defenses (Eberhard et al. 1993). Se condary compounds, such as alkaloids, generally deter herbivores by providing an undesirable taste, slowing growth rate, or being poisonous (Rosenthal and Berenbaum 1 992). However, Macrohaltica sp. appears to be able to incorporate one or more of th ese secondary compounds into its system in order to become unpalatable to its predat ors (Eberhard et al. 1993). The larvae of Macrohaltica sp reared on G. insignis produce a burning sensation when eaten; adults have a similar, but less intense taste and appear t o be distasteful to three species of lizard (Eberhard et al. 1993). Adult Macrohaltica spp. beetles can be found in large aggregations on Gunnera insignis, but little research has been done on the feeding pa tterns of such aggregations (Eberhard et al. 1993). Adults often chew small rou nd holes in the foliage (White 1983) but in some cases will completely strip leaves of p hotosynthetic tissue, reducing the leaf to its main veins (Eberhard et al. 1993). One common area of plant-herbivore study investigat es the effects of sunlight on herbivory rates. Studies examining the influence o f light availability on patterns of herbivory have generally shown that shade tolerant plants tend to experience increased rates of herbivory than those in high light environ ments (Baraza et al. 2004, Lowman 1985). Generally, leaves found in sunnier environme nts, such as a forest canopy, have tougher leaves that slow the rate of herbivore cons umption (Coley and Barone 1996, Lucas 2000). However, the beetle, Macrohaltica jamaicensis appears to show a different pattern. In the morning, these beetles remain tigh tly grouped in shaded sites until direct sunlight falls on them; they then move about, formi ng new clumps throughout the day (Eberhard et al. 1993). Laboratory studies on the relationship between temperature and feeding rates have shown that insects in warmer hab itats consume more leaf tissue (Niesenbaum and Kluger 2006). If true for Macrohaltica spp ., then herbivory should be greater on plants more directly exposed to sunlight as appears to occur in M. jamaicensis A similar prediction follows for Macrohaltica sp. on Gunnera insignis (Eberhard et al.1993). The following experiment investigates patterns of h erbivory on Gunnera insignis by Macrohaltica sp. to determine if increased herbivory can occur in s unlight habitats MATERIALS AND METHODS Data were obtained from 90 Gunnera insignis plants located in regenerating pasture along the Mirador Road near the Santa Elena Reserve in Monteverde, Costa Rica between April and May of 2008. Plants of a range of sizes were chosen for analysis, but no
3 extremely young individuals were included. All meas ures of herbivore damage were made on the third leaf from the top of each plant i n order to take a representative sample of the entire plant. I assessed damage by cutting four 226 cm2 circles at the locations shown in Fig. 1. A plastic grid (4 squares/cm2) was used to measure the leaf area consumed. I then totaled the missing leaf area and calculated the percent herbivory damage of the four pieces. The number of leaves co ntaining herbivory was also recorded. Sunlight data were obtained using a light meter and taken on a single sunny day between 11001300 hrs. and measured in klux. A densiometer measured canopy coverage. The pattern of Macrohaltica sp damage was analyzed using a polynomial regression analysis for both sunlight and canopy coverage. A frequency distribution was constructed to depict the incidenc e of damaged leaves per plant. RESULTS All 90 plants experienced herbivory, with the major ity of leaves showing evidence of damage (Fig. 2). Seventy-one percent of plants exa mined contained over 84% of leaves with herbivory per plant. The majority of plants ( 67%) showed 100% of leaves with leaf damage. Few beetles were observed on Gunnera insignis, and herbivory on most leaves did not appear to be fresh. Leaves in high levels of either sunlight or shade s uffered more damage than leaves in moderately sunlit conditions (Fig 3; adju sted R2 = 0.129, f = 7.603, P = 0.0009, n = 90). The highest amount of herbivory (96.57%) occurred in sunlight levels of 101.5 klux. The second highest level of herbivory (95.6) %) was in a shaded area, registering 4.3 klux. At middle ranges of sunlight, leaves had 34.32% herbivory or less. The same pattern was true for canopy coverage (Fig 4; adjusted R2 = 0.114, f = 6.711, P = 0.002, n = 90), with the greatest amount of herb ivore damage occurring with 70.5% canopy coverage and the second greatest with 2.9% coverage. Moderate canopy coverage revealed only 34.32% herbivory or less. Figure 1. Location of leaf samples taken from G. insignis (Original picture:Fuller and Hickey 2005)
4 nnn Percent of leaves with herbivory per plant n nLight intensity (klux) Figure 2. Percent of leaves per plant showing herbi vorous damage. All plants contained leaves with evidence of herbivory (n = 90, mean = 9 4.01% 8.99%,). ___________________________________________________ ______________________ Figure 3.Influence of sunlight intensity on herbivo ry of Gunnera insignis (y = 0.0093x2 1.0081x + 27.845; n = 90).
5 ___________________________________________________ ____________________ Figure 4. Influence of canopy coverage on herbivory of Gunnera insignis (y = 0.0101x21.1362x+38.727; n = 90). DISCUSSION Results indicate that Gunnera insignis is highly subject to herbivorous activity by Macrohaltica sp Figures 3 and 4 suggest that higher levels of he rbivory on G. insignis occur at the extreme ranges of sunlight exposure an d canopy coverage. It could be that another organism feeds on G. insignis but Eberhard et al. (1993) made no note of this i n their study on G. insignis and Macrohaltica spp Furthermore, the toughness of G. insignis is difficult to overcome for most herbivores. It is more likely that resource availability, nutritional quality, and plant defens es, as influenced by sunlight, account for the variation in leaf consumption patterns on G. insignis The availability of water and light can cause varia tions in the concentrations of plant compounds, such as nitrogen, and some seconda ry metabolites (Baraza 2004). The leaves of Gunnera insignis are long-lived (Fuller and Hickey 2005) and thus h ave to minimize herbivore damage over a long period of tim e. G. insignis located in shaded areas do not have the ability to turn over tissues as quickly as those in sunlight, and thus cannot afford to loose large quantities of tissue t o herbivory. Furthermore, shaded leaves have higher water content (Young 1980), making them more attractive to herbivores. Shaded plants must therefore produce increased amou nts of secondary metabolites to deter the majority of herbivores. Since secondary compounds in G. insignis provide Macrohaltica sp with a defense against predators (Eberhard et al. 1993), Macrohaltica n n% Canopy Coverage
6 sp may be attracted to shade plants with high levels of these compounds. Therefore, herbivory increases from Macrohaltica sp. but is reduced overall from other herbivores. For plants located in sunny habitats, increased lea f temperature (as a result of sunlight) can increase herbivore activity (Neisenba um and Kluger 2006). Studies of Lepidoptera larvae have shown high consumption and growth rates at elevated temperatures (Sherman and Watt 1973). This pattern increases general herbivore damage to Gunnera insignis, therefore G. insignis must provide a means of defense. G. insignis located in high sunlight conditions may also be abl e to produce more secondary compounds because of increased photosynthate produc tion (Patterson 1979). This may increase herbivory from Macrohatlica sp. but it decreases overall herbivore damage once again. In moderately shaded areas, Gunnera insignis does not have excess water or sunlight to produce large amounts of secondary meta bolites. However, the reduction in these resources may still contribute to decreased h erbivory. Plants in moderate sunlight have less water content than shade plants (Young 19 80), making them less appealing to herbivores. Furthermore, decreased sunlight result s in decreased leaf temperature (Gates 1968), which reduces beetle metabolism and in turn reduces herbivory. Toughness may be the only defense needed to deter herbivores, and in such environments, investment in secondary metabolites is not necessary. Insects are active at different times of the year, therefore rates of herbivory damage generally require repeated analysis (Coley a nd Barone 1996). It is thought that Macrohaltica sp. enters reproductive diapause (Eberhard et al. 1993 ), which may explain why so few beetles were seen feeding on Gunnera insignis and leaf damage appeared to be old Additional research could determine if this same pa ttern occurs on G. insignis in the wet season. Furthermore, more research is need ed to determine if it is secondary metabolites that are attracting Macrohaltica sp. while simultaneously deterring other herbivores and if the quantities of these secondary metabolites differ under varying degrees of sunlight. This study demonstrates that the influences of sunl ight on herbivory are complex with numerous factors contributing to plant-herbivo re dynamics. Sunlight may be the ultimate factor that determines the mechanism and e xtent of herbivore defense, but other factors are surely involved. Since light influence s temperature, water availability, nutritional quality, and possibly leaf defense, add itional investigation of this complex dynamic is needed both in the field and in the labo ratory. ACKNOWLEDGEMENTS Special thanks to Dr. Karen Masters for assistance and advice and to Tania Chavarria for inspiration. Thank also to the Santa Elena Reserve for transport ation to and from El Mirador Road and the Estacin Biolgica Monteverde for use of its facilities. Th ank you also to Taegan McMahon, Pablo Allen, Kather ine Heal, and Megan Copley for opinions and advice alon g the way. LITERATURE CITED BARAZA, E., J. GOMEZ, J. HODAR, AND R. ZAMORA. 2004. Herbivory has a greater impact in shade than in sun: response of Quercus pyrenaica seddlings to multifoactorial environmental variation. Canadian Journal of Botany 82: 357-364.
7 CLARK, D. AND CLARK D. 1991. Herbivores, herbivory, and plant phenology : Patterns and consequences in a tropical rain-forest cycad. In PRICE, P., M. LEWINSOHN, G. FERNANDES, AND W. BENSON. 1991. Plant-animal interactions: Evolutionary ecology in tropical and temperate regions. John Wil ey & Sons, New York, New York. COLEY, P. AND J. BARONE. 1996. Herbivory and plant defenses in tropical fore sts. Annual Review of Ecology and Systematics. 27: 305-335. EBERHARD ET AL., W., R. ACHOY, M. MARIN, AND J. ULGALDE. 1993. Natural history and behavior of two species of Macrohaltica (Coleoptera: Chrysomelidae). Psyche 100: 90-119. FULLER, D. AND L. HICKEY. 2005. Systematics and leaf architecture of the Gu nneraceae. The Botanical Review. 71: 295-353. GARGIULLO, M., B. MAGNUSON, AND L. KIMBALL. 2008. A Field Guide to Plants of Costa Rica. Oxford University Press, Oxford. GATES, D. 1968. Transpiration and leaf temperature. Annual Re view of Plant Physiology. 19:211-238. LOWMAN, M. 1985. Temporal and spatial variability of insect grazing of the canopies of five Australian rainforest species. Austral Ecolog y. 10(1):7-24. MARQUIS, R. 1991. Herbivore fauna of Piper (Piperaceae) in a Costa Rican wet forest: Diversity, specificity, and impact. In PRICE, P., M. LEWINSOHN, G. FERNANDES, AND W. BENSON. 1991. Plant-animal interactions: Evolutionary eco logy in tropical and temperate regions. John Wiley & Sons. New York, New York. NIESENBAUM, R. AND E. KLUGER. 2006. When studying the effects of light on herbi vory, should one consider temperature? The case of Epimecies hortaria F. (Lepidoptera: Geometridae) feeding on Lindera benzoin L. (Lauraceae). Environmental Entomology. 35(3): 600-606. PATTERSON, D. 1979. The effects of shading on the growth and p hotosynthetic capacity of itchgrass ( Rottboellia exaltata ). Weed Science. 27(5): 549-553. PRICE, P., T. LEWINSOHN, G. FERNANDES, AND W. BENSON. 1991. Plant-animal interactions: Evolutionary ecology in tropical and temperate regions. John Wiley & Sons, New York, New York. ROSENTHAL, G. AND M. BERENBAUM, eds. 1992. Herbivores: Their interactions with secondary plant metabolites. Academic Press, San Di ego, California. SHERMAN,P., AND W. WATT. 1973. The thermal ecology of some Colias butterfly larvae. In Niesenbaum, R. and E. Kluger. 2006. When studying the effects of light on herbivory, should one consider temperature? The cas e of Epimecies hortaria F. (Lepidoptera: Geometridae) feeding on Lindera benzoin L.(Lauraceae). Environmental Entomology. 35: 600-606. WHITE, R. 1983. A field guide to the beetles of North Amer ica. Houghton Mifflin Company, Boston, Massachusetts. WHITHAM, T. J. MASCHINSKI, K. LARSON, AND K. PAIGE. 1991. Plant responses to herbivory: The continum from negative to positive a nd underlying physiological mechanisms. In PRICE, P., M. LEWINSOHN, G. FERNANDES, AND W. BENSON. 1991. Plant-animal interactions: Evolutionary ecology in tropical and temperate regions. John Wiley & Sons, New York, New York. YOUNG, D. AND W. SMITH. 1980. Influence of sunlight on photosynthesis, wa ter
8 relations, and leaf structure in the understory spe cies Arnica cordifolia Ecology. 61: 1380-1390.
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Efecto de la intensidad de la luz solar y la cobertura del dosel en la herbivora en Gunnera insignis (Gunneraceae)
Effect of sunlight intensity and canopy coverage on herbivory in Gunnera insignis (Gunneraceae)
Studies on the influence of sunlight on herbivory have generally revealed that increased amounts of leaf damage occur in cool, shaded habitats. This study investigates the effects of both sunlight and canopy coverage on herbivore damage in Gunnera insignis by the undescribed beetle Macrohaltica sp. (Coleoptera: Chrysomelidae). This beetle is thought to feed on G. insignis because it can obtain secondary metabolites that make it undesirable to predators. In this investigation, all 90 plants studied revealed herbivore damage, with 71% of plants showing evidence of herbivory on the majority of leaves. The highest levels of leaf damage occurred on plants in both high light and high shade habitats, while those in moderate sunlight conditions revealed reduced herbivore damage. Resource availability, nutritional quality, and plant defenses influenced by sunlight could account for the variation in leaf consumption on G. insignis. Because light environment influences a number of factors related to the survival of a plant, the effects of sunlight on herbivory are complex and require further investigation.
Estudios previos en la influencia de la luz solar en la herbivora generalmente han mostrado un mayor dao en las hojas ocurriendo en hbitats fros y con sombra. Este estudio investiga el efecto de ambos, la luz solar y la cobertura del dosel en el dao de herbvoros en Gunnera insignis por el escarabajo Macrohaltica sp. (Coleoptera: Chrysomelidae).
Text in English.
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