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Fidelidad por un sitio en dos especies de Cyclocephala (Scarabaeidae) a Xanthosoma robumstum (Araceae)
Site fidelity in two species of Cyclocephala (Scarabaeidae) to Xanthosoma robumstum (Araceae)
Mark and recaptured methods were utilized to determine the site fidelity of two species of scarabid pollinators to a patch of Xanthosoma robustum in Monterverde, Costa Rica. Sex ratios remained constant with previous studies, exhibiting a higher proportion of males found per inflorescence than females. One-way ANOVA tests of variance supported evidence that female Cyclocephala are more prevalent in first day inflorescences. One-way ANOVA tests also concluded that scarab pollinators exhibit no fidelity to the study patch, The existence of a more resource rich patch in the area could account for the low fidelity observed. The selection for low fidelity through differential spadix heating in Xanthosoma may be utilized to promote genetic variance within the patch.
Los mtodos para marcar y recapturar fueron utilizados para determinar la fidelidad al lugar en dos especies de escarabajos polinizadores de la familia Scarabidae de un parche de Xanthosoma robustum en Monteverde, Costa Rica. La proporcin de sexos se mantuvo constante con estudios anteriores, que muestran una mayor proporcin de machos que se encuentran por inflorescencia que las hembras. Una prueba ANOVA de una va apoya la evidencia de que las hembras Cyclocephala son ms frecuentes en las inflorescencias de un da. La misma prueba tambin concluye que los escarabajos no exhiben fidelidad por los parches. La existencia de un parche con recursos ms ricos en la zona podra explicar la baja fidelidad observada. La seleccin de baja fidelidad a travs de las inflorescencias de Xanthosoma deben ser utilizados para promover la variacin gentica dentro del parche.
Text in English.
Pollination by insects
Costa Rica--Puntarenas--Monteverde Zone
Polinizado por insectos
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2009
Ecologa Tropical Otoo 2009
t Monteverde Institute : Tropical Ecology
Site fidelity in two species of Cyclocephala (Scarabaeidae) to Xanthosoma robumstum (Araceae) Margaret Hendricks Department of Ecology and Evolutionary Biology, University of Colorado, Boulder ABSTRACT Mark and recaptured methods were utilized to determine the site fidelity of two species of scarabid pollinators to a patch of Xanthosoma robustum in Monterverde, Costa Rica. Sex ratios remained constant with previous studies, exhibiting a higher proportion of ma les found per inflorescence than females. One way ANOVA tests of variance supported evidence that female Cyclocephala are more prevalent in first day inflorescences. One way ANOVA tests also concluded that scarab pollinators exhibit no fidelity to t he study patch, The existence of a more resource rich patch in the area could account for the low fidelity observed. The selection for low fidelity through differential spadix heating in Xanthosom a may be utilized to promote genetic variance within the patch. RESUMEN Mtodos de marcaje y recapture fueron utilizados para det erminar la fidelidad por un sti o en dos e species de escarabajos de la familia Scarabidae polinizadores de un parche de Xanthosoma robustum en Monteverde, Costa Rica. La proporcin de sexos permanece constante con previos estudios, exhibiendo una proporcin mayor de machos que hembras encontrados por inflorescencia. Una prueba ANOVA de una va soporta la evidencia de que las hembra s de Cyclocephala son ms prevalentes en las inflorescencias de un da. La misma prueba tambin concluye que los escarabajos no exhiben fidelidad por los parches. La existencia de ms parches en el rea pueden ser los factores influyentes en la baja fide lidad observada. La seleccin de baja fidelidad a travs de las inflorescencias de Xanthosoma deben ser utilizados para promover variacin gentica dentro del parche. INTRODUCTION Basic models for habitat selection presume that individual settlement deci sio ns are the result of the thorough investigation and evalua tion of all available habitats. T raditionally the habitat selected reflects the site with optimal qua lities (Switzer 1993). Site fideli ty can be defined as the extent of which an individual or species is confined to a certain community or location ( Buse 1988). T he site for which an individual remains faithful could be a territory, breeding ground, ne sting site or foraging area (Lanyon and Thompson 1986). Moreover, s ite fidelity ca n be determined by characteristics such a s reproductive potential (Burger 1982), population pressure (Weatherhead and Boak 1986) and habitat stability or can be the result of individual life history traits, such as age, or previous r eproductive success (Switzer 1993). Typically, sites that demonstrate predictability or stability of resources often experience high rates of fidelity (Switzer 1993). Site fidelity of insect pollinato rs to
habitats is often determined by previous assessments of viable habitats within an area (Lanyon 1986) Site faithful adults seem to recei ve substantial fitness advantages for familiarity to local condition ( Gavin and Bollinger 1988). Insect fide lity is commonly correlated with previous reproductive success. Al though it can be said that plants within a given territory for which insect fidelity exists benefit from a constant strea m of pollinators, in some cases fide lity can be selected against For plants that exist in clumps or patches, a decrease in site fidel ity serves by pollinators could promote optimal genetic out crossing for the plant and ultimately maximize both individual and patch fitness. In the case of insect pollinators who maintain mut u alistic relationships with their specific host plants, fidelity is traditionally promoted by the plant and then maintained by the insect pollinator. Therefore, mutualisms between an insect pollinator an a plant could be useful in studying site fidelity by a polli nator. From the perspective of a pollinator, if the resource the plant produces is beneficial, the pollinator should exhibit site fidelity. From the perspective of the plant, site fidelity may lead to gentic inbreeding, and there a plant may dissuade fidel ity by some physiological machanism. For this strudy, I used the Cyclocephala Xanthosoma system to examine site fidelity from the perspective of the insect pollinator. Xanthosoma is an Areceae that has a mutualism involving scarab beetle pollinators. Many aroids exhibit a unique mutualism involving scarab beetle pollination; s carab beetle pollination is a fundamentally different strategy than traditional rapid pollination by foraging animals (Young 1998). In the case of the aroid Xanthosoma robustum and i ts beetle pollinators, it seems probable that fidelity of Cyclocephala pollinators should be correlated to inflorescence abundance. Previous studies conducted on the Xanthosoma : Cyclocephala pollination syndrome have shown little movement of Cyclocephala be tween near inflorescences; forgoing eligible neighbors for distant inflorescences. (Goldwasser 1987) Although the pollination syndrome has been intensely studied, l ittle is known about the fidelity of Cyclocephala to Xanthosoma robustum The study co nducted serves to determine the site faithfulness of two Cyclocephala species to the Pacific Slope patch of Xanthosoma robustum Sin c e no other known patches of Xanthosoma exist in Monteverde it can predicted that there will be an increase in beetle popu lations on the Pacific Slope due to immigrations by beetles that previously visited the Atl antic Sl ope patch. As a result, predictions can also be made that the estimated population sizes for individual species should be double that of those estimated in t he spring of 2009. Increases in population size due to an influx of displaced individuals should result in high fidelity of scarab pollinators to the Pacific Slope patch of Xathosoma robustum MATERIALS AND METHODS Study Site This study was conducted over the period of October 27 th to November 16 th 2009 in Monteverde, Costa Rica. Data was collected from a patch of Xanthosoma located on Cloud Forest Reserve. Historically two main patches of Xanthosoma have been known in the Monteverde area, one patch on the Atlantic Slope and one patch on the Pacific Slope. In June 2009, the Atlantic Slope patch of Xanthosoma robustum was elimated.
Natural History of the Cyclocephala Xanthosoma System The pollination of the aroid Xanthosoma robustum is the result of the mutualistic partnership with scarbid beetles of the genus Cyclocephala (Goldwasser 1987). Two species of Cyclocephala have be en found to pollinate local pop u l ations of Xanthosoma robustum in Mon e tverde Cyclocephala nigerrima and Cyclocephala sexpunctata (Goldwasser 1987). Both species of beetles can be portray ed as sexually dimorphic ; males can be distinguished by enlarged tarsi on their forelegs, with females characterized by grooves on elytral edges. Cyclocephala sexpunctata are categorized by yellow brown elytra with brown spots and ar e typically range in size from 1.0 to 2.5 cm. Cyclocephala nigerrima can be differentiated by black elytra and are marginally larger ranging in size from 1.0 to 3.0 cm ( Goldwasser 1987). Both Cyclocephala species utilize Xanthosoma as a resource for mating as well as for a food source. Xanthosoma robustum is a tall herbaceous plant with characteristic sizeable Flowering of Xanthosoma robustum predominantly occ urs during the rainy season; each plant produces inflorescences every nine days with up to 5 15 inflorescences per season (Goldwasser 2000). As a result, the flowering phenology of Xanthosoma is such that beetles cannot move between inflorescences on the s ame plant (Young 1998). Inflorescences are protogynous (Young 1986) and agamous; male flowers are located on the top of the spadix and are separated from the basally located female flowers by a ring of sterile flowers. (Venkatesan 2001). Xanthosoma employ s the arecous pollination strategy of spadix heating (Gibernau 1999). Thermogenetic respiration, or the release of heat through the conversion of metabolized carbohydrates, serves as the main attractant for beetle pollinators (Craig 2001). On the first ni ght of opening the spadix heats rapidly to 40 42 C and emits a pleasant smelling odor to attract Cyclocephala pollinators. The beetles crawl into the chamber containing the female flowers where they mate and feed on the sterile flowers for a period of 24 h ours (Ghose 2009). On the second night the spadix heats again to release the male pollen; scarab visitors then crawl up the spadix, inadvertently collecting pollen, which is then carried to newly, opened inflorescences (Goldwasser 2000). Therefore, the pol lination syndrome of Xanthosoma operates on a 48 cycle. Data Coll ection Data collection was conducted every morning, for eleven days. Each day new inflorescences w ere located and noted within t he patch. Beetles found within each inflorescence were counte d, sexed and species were determined. Mar k and recapture methods involved the elytral ma rking of individuals by paint pens. I nflorescences were then marked with corresp onding paint pen colors on flagging tape. As well as tagging newly emergent inflorescences, second day inflorescences previously flagged wer e checked for recaptures or the presence of newly immigrated individuals. When recaptures were disco vered, a measuring tape was utilized to de termine average flight distance from the original inflorescence.
Statistical Analysis Mark and recapture techniques allowed for the calculat ion of population estimates through the Schnabel Method ; population size was determined for both Cyclocephala sexpu nctata and Cyclocephala nigerrima. The difference in occurrence of females on day 1 versus day 2 was determined through the use of a one way ANOVA test for variance. One way ANOVA tests were also utilized to determine fidelity of each species to second da y inflorescences. RESULTS Results for sex ratios for total beetles recorded within an inflorescence showed 138:19 males to females; sex r atios for recaptures showed 4:1 males to females (Figure 1) The percentage of marked males was 87.9 % (n= 138); the percentage of females marked was 12.1% (n=19). The total number of recaptures was 17 individuals; males accounted for 82.4% (n=14) and females accounted for 17.6% (n=3). There were a greater number of females in first day inflorescences than in second day inflorescences ( F 1 36 = 5.1892, p=0.0288 ; Figure 2 ). A second one way ANOVA was utilized to determine the fidelity of separate sexes in Cyclocephala nigerrima and Cyclopcephala sexpunctata to the patch of Xanthosoma robustum Neither species n or sexes showed fidelity to the Xanthosoma patch (F 1 40 = 1.848, P= 0.3623; Figure 3 ) The Schnabel method was utilized to calculate population estimates for both species of Cyclocephala 676 individuals were calculated for the population of C. nigerrima an d 192 individuals for C. sexpunctata (Figure 4). FIGURE 1. Recapture rates show a higher proportion of males to females. Male recapture rates account for 82.4 % (n= 14) and female recapture rates account for 17.6 % (n=3) 0 2 4 6 8 10 12 14 16 Males Females Number of Individuals Sex Male vs Female Recaptures Males
FIGURE 2. A one way ANOVA shows statistically significant evidence for a higher occurrence of females in Day 1 inf lorescences compared to Day 2 FIGURE 3 The one way ANOVA for % Fidelity of male and female Cyclocephala sexpuntcta and Cyclocephala n igerrima to Xanthosoma robustum is n ot statistically significant. No site fidelity is experienced for each species or sex. 0 5 10 15 20 25 30 Day 1 Day 2 Females 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Males Females C. sexpunctata C. nigerrima % Fidelity
FIGURE 4. The Schanbel method was utilized to determined population sizes. 676 individuals for C. nigerrima and 192 individuals for C. sexpunctata DISCUS S ION Sex ratios obtained correspond with previous studies that observed a strong bias for males in the population; traditionally a 1:3 female to male ratio was detected. Usually i t has been observed that female scarab beetles spend a reduced amount of time within individual inflorescences than that of their male counterparts; typically females visit Xanthosoma inflorescences for 40% as long as males (Goldwasser 1987). Although it is known that females visit inflorescences for a shorter period of time, it still remains unknown how females allocate their time outside of inflorescences. A higher ratio of males in the pollinator population serves to account for the difference s between relative numbers in the population; theref ore it can be determined that no biased sex ratio is implemented at birth (Goldwaser 1987). There was low site fidelity by both beetle species to X. robustum It is possible low fidelity was found for two differe nt reasons: (1) t he existence of another patch in the area or (2) the selection for low site fidelity of beetles by Xanthosoma itself Traditionally, individuals that experience high reproductive success like beetles, show elevated probabilities for site faithfulness in the next breeding season (Hoover 2003). Due to the fact that t he Pacific Slope patch of X. robustum e xists as a predictable and stab le resource, it would seem logical for scarab beetle pollin ators to e xhibit high fidelity; h owever, low levels of site fidel ity are found with Cyclocephala. The existence of 0 100 200 300 400 500 600 700 800 C. nigerrima C. sexpunctata Population Estimates
another patch of X. robustum that contains more inflorescences and therefore more mating opportunities, may account for the lack of beetle fidelity I t has b een found that site fidelity is positively correlated with the cost of changing territories; in other words, organisms will stay in one place if the cost of finding a new site is high The higher turnover rates experienced by short lived sp ecies ser ves to diminish movement costs that could potential ly discourage the movement of individua ls to new habitat ; therefore short lived species are more prone to migrating to better quality habitats For habitats that exhibit predictability of resources, the movement of current residents out of the existing territory should only occur in the event that a higher quality option becomes available (Switzer 1993). Taking into account the influence of move ment costs, turnover rates, and the predictability of resources inflicted on the individual when changing habitats, the alternative patch must be more resource rich than the existing patch in Monteverde. The benefits derived from the other resource rich p atch much outweigh the movement costs inflicted on Cyclocephala. Seasonality may also account for low fidelity. One mechanism that may explain the difference in resources among patches is the seasonality in phenology of Xanthosoma Xanthosoma robustum ha s been recored as having a peak in flowering during the wet season on the Pacific slope in Monterverde, Costa Rica. It is possible that, in another location, there where more flowers that the patch I studied. Since the Atlantic slope of Costa Rica has cons tant abiotic inputs in the form of rain and mist and, therefore, shows less seasonality than the Pacific slope, it is possible that there is a higher abundance of flowers on the Atlantic slope below Monteverde, and Cyclocephala beetles show high fidelity o f patches on the Atlantic slope. Furthur work on this system should focus on comparisons of Atlantic and Pacific slope phenology and fidelity. Due to the low fidelity observed by beetle pollinators it can be concluded that X. robustum pheneology successfu lly operates to deter beetle fidelity. Mark and recapture studies showed exceptionally low recapture rates; confirming long distance pollen flow (Goldwasser 1987). Xanthosoma may effectively control Cyclocephala fidelity through thermogenetic r espiration; due to the fact X. robustum plants utilize differential heating times between day one and day two inflorescences and between plants they successfully control pollen flow via beetle pollinators as pollinators cannot predict at what time an inflorescence will be receptive (Goldwosser 1987). Due to the fact that Xanthosoma exists in a clumped formation, there is an increased likelihood that the patch will experience genetic bottlenecking through restricted gene flow. However the variability in inflorescence heating functions operate to promote long distance flights of pollinators, insuring the optimal outcrossing of genes. During the rainy season the scarab population experiences high rates of turnover, mark and recapture trials show direct evidence for con tinuous recruitme nt in local beetle populations. Continuous recruitment of Cyclocephala may be the result of increased immigrations from beetles previously inhabiting the Atlantic Slope patch. Although there is a constant influx of beetles into the populat ion, no exponential increase in population size was noted. Therefore, there must be unremitting departure of existing scarab beetles from the population, ultimately leading to low fidelity (Goldwasser 1987). The departures
from the patch contribute to the notion that a more resource rich habitat exists in the Monteverde region. The assumptions of the Schnabel Method of population estimation presume that there is random mixing within the population (Krebs 1989). I t seems probable that scarab pollinators move in a nonrandom pattern through the Monteverde patch of Xanthosoma; as a result the population numbers obtain by the Schnabel Method could be overestimated. Based on recapture rates, it seems probable that both species of Cyclocephala move in a unidirectio nal pattern through the patch. However, my estimate of 614 individuals in the total population supports my prediction that population numbers would more than double (from 200 in Spring 2009) due to the absence of the Atlantic Slope patch of Xanthosoma. Li ttle remains known about the influence of differential spadix heating and its selection for low fidelity in beetle pollinators, further studies should b e conducted to determine if thermogenetic heating by Xanthosoma robustum results in increased genetic variance. LITERATURE CITED B URGER J. 1982. The role of reproductive succe ss in colony site selection and abandonment in black skimmers ( Rynchops niger ). Auk 99 : 109 115. BUSE A. 1988. Habitat selection and group of b eetles (Coleoptera). Holarctic Ecology 11 : 241 247. CRAIG J. 2001. Determinants of morphological characteristics: thermogenetic heating and Scarabaeidae pollination in Xanthosoma robustum (Areceae). Tropical Ecology and Conservation Spring 2001 : 30 40 GHOSE S. 2009. The effects of Xanthosoma spp (Areceae) temperature ,scent, and flowers on the mating frequency of Cyclocephala sexpunctata (Scarabaeidae). Tropical Ecology and Conservation Spring 2009 : 9 18. GIBERNAU et al. 1999. Beetle pollination of phil odendron solimoesense (Areceae) in French Guiana. International Journal of Plant Sciences 160 : 1135 1143. GOLDWASSER L. 1987. Mutualism and its ecological and evolutionary consequences PhD Dissertation. University of California, Berkley, CA. Pp. 243 324. GOLDWASSER L. 2000. Scarab beetles, elephant ear ( Xanthosoma robustum ) and their associates. Monteverde: Ecology and C onservation of a Tropical Cloud Forest : 268 271. HOOVER J. 2003. Decision rules for site fi delity in a migratory bird, the prothronotary warbler. Ecology 84 : 416 430.
KREBS C. 1989. Schnabel Method. In: Ecological Methodology Harper and Row, Publishers, New York, NY. Pp. 30 36. LANYON S.M. and Thompson, C. F. 1986. Site fidelity and habitat quality as determinants of settlement pattern in male painted buntings. Condor 88 : 206 210. SWITZER Paul. 1993. Site fidelity in predicta ble and unpredictable habitats. Evolutionary Ecology 7: 533 555. VENKATASEN M. 2001. The movement, composition and behavior of scarab beetl es and other insects visiting inflorescence of Xanthosoma robustum (Areceae). Tropical Ecology and Conservation :1 11. WEATHERHEAB P. J. and Boak, K.A. 1986. Site infi delity in song sparrows. Animal Behavior 34 : 1299 1310. YOUNG H. 1986. Beetle pollina tion of Diffenbachia longispatha (Areceae). American Journal of Botany 73: 931 944.