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Patrones de abundancia del pecolo y diversidad de los escarabajos de corteza (Scolytinae) de los rboles de Cecropia en Monteverde, Costa Rica
Patterns of petiole abundance and diversity in bark beetles (Scolytinae) of Cecropia trees in Monteverde, Costa Rica
Cecropia trees regularly shed their woody, large petioles year round. This provides bark beetles from the
family Scolytinae with a dependable and stable microhabitat. Bark beetles bore into the petioles, lay their
eggs, and feed on the pith (Wood 1983). This study was conducted in Monteverde, Costa Rica. It examined
the abundance and diversity of bark beetles between Cecropia polyphlebia in the Monteverde Cloud Forest
(1550 meters) and an intermediary phenotype at Bajo del Tigre (1400 meters). It also investigated the
percent of petioles colonized, species richness, evenness, and the influence of petiole moisture and length
on the abundance and diversity of beetles. I analyzed 90 dead, woody petioles from the ground and from
hanging vegetation from each site. Petioles were dissected and beetles were removed and identified by one
of six Scolytinae morphospecies. Differences in the abundance and diversity of beetles between the two
species were then compared. This study suggests that there was no difference in species diversity between
the two tree species (p > 0.05). However, a difference in abundance between the hybrid phenotype and C.
polyphlebia was observed. The hybrid phenotype possessed a higher abundance of beetles. Also moisture
content (ANCOVA, F = 4.93, df = 2, p = 0.0083), the species of Cecropia (ANCOVA, F = 11.2, df = 1, p
=0.001), and petiole length with Cecropia species (ANCOVA, F = 5.16, df = 1, p = 0.0244) had an
influence on the abundance of beetles. The higher abundance of beetles in the hybrid phenotype could
indicate differences in interspecific competition due to phenotypic traits of the Cecropia species.
Differences in moisture content and petiole length between the two Cecropia species may reflect petiole
preference and favorability of tree hosts. Furthermore, the higher abundance of beetles in the lower
elevation hybrid phenotype could indicate that the majority of bark beetles may not be affected by climate
change unlike many other species of animals and plants that are being severely impacted by the increasing
Este estudio se realiz en Monteverde, Costa Rica. Examin la abundancia y la diversidad de los escarabajos de corteza entre Cecropia polyphlebia (1550 metros) en el Bosque Nuboso de Monteverde y un fenotipo intermedio en el Bajo del Tigre (1400 metros). Tambin investig el porcentaje de pecolos colonizados, riqueza de especies, igualdad y la influencia de la humedad del pecolo y la longitud de la abundancia y diversidad de escarabajos.
Text in English.
Bark beetles--Behavior--Costa Rica--Monteverde Cloud Forest Reserve
Escarabajos de corteza--Comportamiento--Costa Rica--Reserva Bosque Nuboso Monteverde
Tropical Ecology 2008
Bark beetle feeding preferences
Ecologa Tropical 2008
Preferencias alimenticias del escarabajo de corteza
t Monteverde Institute : Tropical Ecology
Patterns of Petiole Abundance and Diversity in Bark Beetles Scolytinae of Cecropia trees in Monteverde, Costa Rica Sonali Gandhi Department of Biology, University of Southern Indiana ABSTRACT Cecropia trees regularly shed their woody, large petioles year round. This provides bark beetles from the family Scolytinae with a dependable and stable microhabitat. Bark beetles bore into the petioles, lay their eggs, and feed on the pith Wood 1983. This study was conducted in Monteverde, Costa Rica. It exam ined the abundance and diversity of bark beetles between Cecropia polyphlebia in the Monteverde Cloud Forest 1550 meters and an intermediary phenotype at Bajo del Tigre 1400 meters. It also investigated the percent of petioles colonized, species richne ss, evenness, and the influence of petiole moisture and length on the abundance and diversity of beetles. I analyzed 90 dead, woody petioles from the ground and from hanging vegetation from each site. Petioles were dissected and beetles were removed and id entified by one of six Scolytinae morphospecies. Differences in the abundance and diversity of beetles between the two species were then compared. This study suggests that there was no difference in species diversity between the two tree species p > 0.05 . However, a difference in abundance between the hybrid phenotype and C. polyphlebia was observed. The hybrid phenotype possessed a higher abundance of beetles. Also moisture content ANCOVA, F = 4.93, df = 2, p = 0.0083, the species of Cecropia ANCOVA, F = 11.2, df = 1, p =0.001, and petiole length with Cecropia species ANCOVA, F = 5.16, df = 1, p = 0.0244 had an influence on the abundance of beetles. The higher abundance of beetles in the hybrid phenotype could indicate differences in interspecific c ompetition due to phenotypic traits of the Cecropia species. Differences in moisture content and petiole length between the two Cecropia species may reflect petiole preference and favorability of tree hosts. Furthermore, the higher abundance of beetles in the lower elevation hybrid phenotype could indicate that the majority of bark beetles may not be affected by climate change unlike many other species of animals and plants that are being severely impacted by the increasing temperatures. RESUMEN Los Ã¡rbo les de Cecropia sueltan regularmente sus largos y leÃ±osos durante todo el aÃ±o. Esto proporciona corteza escarabajos de la familia Scolytinae de un microhÃ¡bitat fiable y fijo. Los escarabajos de la corteza habitan en los pe cÃolos, colocan sus huevos, y c omen la mÃ©dula Wood 1983. Este estudio fue realizado en Monteverde, Costa Rica. ExaminÃ³ la abundancia y la diversidad de escarabajos de corteza entre Cecropia polyphlebia 1550 metros y un fenotipo intermedio en el bosque nuboso de Monteverde y un feno tipo intermedio en del de Bajo Tigre 1400 metros. TambiÃ©n investigÃ³ el porcentaje de pecÃolos colonizados, riqueza de especie, la equidad, y la influencia de la humedad de pecÃolo y longitud en la abundancia y la diversidad de escarabajos. AnalicÃ© a 90 peciolos leÃ±osos del suelo y de la vegetaciÃ³n colgante de cada sitio. Los pecÃolos fueron disecados y los escarabajos fueron sacados y fueron identificadas como seis diferentes especies de Scolytinae. Las diferencias en la abundancia y la diversidad de es carabajos entre la dos especie entonces fueron comparadas. Este estudio sugiere que no habÃa diferencia en la diversidad de la especie entre la dos especie de Ã¡rbol p > 0.05. Sin embargo, se encontrÃ³ una diferencia entre C. polyphlebia y el hÃbrido. El f enotipo hÃbrido poseyÃ³ una abundancia mÃ¡s alta de escarabajos. TambiÃ©n humedad contenida ANCOVA, F = 4.93, df = 2, p = 0.0083, la especie de Cecropia ANCOVA, F= 11.2, df = 1, p = 0.001, y la longitud de pecÃolo con la especie df de Cecropia ANCOVA, F = 5.16, df = 1, p = 0.0244 tuvo una influencia en la abundancia de escarabajos. La abundancia mÃ¡s alta de escarabajos en el
fenotipo hÃbrido podrÃa indicar las diferencias en la competencia intraespecÃfica debido a rasgos fenotÃpicos de la especie de Ce cropia. Las diferencias en el contenido de la humedad y la longitud de peciolo entre la dos especie de Cecropia pueden reflejar la preferencia de peciolo y favoritismo por Ã¡rboles hospederos. AdemÃ¡s, la abundancia mÃ¡s alta de escarabajos en la elevaciÃ³n mÃ¡ s baja podrÃa indicar que la mayorÃa de escarabajos de corteza no puede ser afectada por el cambio del clima a diferencia de muchas otra especie de animales y plantas que son impresionados severamente por las temperaturas crecientes. INTRODUCTION Woody petioles are rarely considered a habitable location for any subcortical feeding insects, but beetles from the family Scolytinae use several substrates such as xylem, petioles, and the pith of dead twigs as breeding grounds Jordal and Kirkendall 1998. Int erestingly enough, several species from the family Scolytinae have a unique ecological relationship with Cecropia trees. These beetles, known colloquially as bark beetles, breed within the fallen, dead, woody petioles that Cecropia trees offer year round Jordal and Kirkendall 1998. Therefore, these petioles can provide information about the diversity and abundance of bark beetles found in types of Cecropia . Each Cecropia tree can be considered a mini ecosystem that is colonized by several wood boring bee tles. With high competition, the various genera of bark beetles demonstrate niche stratification in order to coexist. Competition can be greater when sharing a petiole Larimer et al. 2006. So, it is not uncommon to find two or three species of bark beet les in each petiole due to niche partitioning. For example, Scolytodes aratus panamensis is notorious for having one to four pairs of adults per petiole Wood 1983. This is a relatively high number of beetles per petiole. Bark beetles are extremely uniqu e in their breeding and habitat locations. They colonize the leaves around three to fifteen days after they have fallen Wood 1983. Also, their breeding systems can have eight different inbreeding origins Larimer et al. 2006. Bark beetles breed by havi ng the adult male Scolytine beetle bore an entrance hole through the hard cortex into the soft pith of the petiole. Then a female bark beetle assumingly joins the male within the petiole by following a pheromone trail. After meeting, the female begins to b urrow in an irregular pattern and lays one to twenty eggs through the tunnels. The pupae eat away at the pith and escape to find another petiole to inhabit Wood 1983. The goal of this study is to determine if some species of Scolytine beetles are exclus ive to one species of Cecropia . It has been hypothesized that bark beetles will not be able to easily colonize another species of Cecropia because they are highly specialized and have unique breeding systems. Abundance was predicted to be greater in the hy brid species otherwise known as the intermediary phenotype due to physiological traits that may favor bark beetles over other species. For instance, the Mullerian bodies are less distinct in the hybrid phenotype. This results in a drop of Azteca colonies. However, higher diversity will be found in C. polyphlebia . C. polyphlebia is the only species of Cecropia found at high altitudes Longino 2000. This would lead to a concentration of bark beetles preying upon these Cecropia woody petioles, which can lead to higher diversity due to high competition for similar resources. However in the hybrid range, more than one species of Cecropia can coexist, which may lead to lower competition of resources and thus less diversity Longino 2000.
METHODS Two study sit es were used in Monteverde, Costa Rica to obtain Cecropia petioles for the research of the Scolytine beetles from July 17, 2008 to August 1, 2008. The high elevation species C. polyphlebia was collected from the Monteverde Cloud Forest 1550m, and the low er elevation hybrid species was collected from the Bajo del Tigre of the Children s Eternal Rainforest 1400 m. At each site, 30 petioles per tree were collected, thus 90 petioles at each site. Petioles were arbitrarily collected from a variety of locatio ns from the ground to hanging vegetation up to 4 meters high, and a variety of petioles were selected. Petioles were placed in plastic bags for transport. Petiole moisture and petiole length were determined for each petiole. Petiole moisture was determine d using a procedure outlined by Jordal and Kirkendall 1998. They ranked each petiole on a scale of zero to two. Zero was desiccated, one was wet, and two was soft and moist. Petioles were dissected, and all the beetles were removed and placed in containe rs. Beetles were initially placed in containers filled with ethanol, but classification of color may have been compromised with prolonged time in the containers. Containers without ethanol were used afterwards. Scolytine bark beetles were identified by mo rphospecies. A dissecting microscope was used to distinguish physical characteristics of color, presence of hair on abdomen, presence of hair on elytra, size, and color. A modified t test was used to compare diversity. I then calculated abundance, evennes s, and species richness for both sites. A two way ANCOVA analysis was performed to see if the type of species, moisture content, or length of petiole influenced the abundance of beetles. RESULTS A total of 180 petioles were dissected from two different s pecies of Cecropia. 90 petioles were from C. polyphlebia and 90 petioles were of the intermediary phenotype. 44% of the petioles were inhabited in the hybrid but only 20% of the petioles were colonized in C. polyphlebia . 32% of all the petioles were coloni zed. A statistically significant difference in abundance was found between the two Cecropia species Table 2. The hybrid species had a total number of 304 beetles, whereas C. polyphlebia had 85 beetles Table 2. In both species of Cecropia , the most abu ndant beetle species was Morpho 3 and second most abundant was Morpho 1 Table 2. The least abundant beetle species found in C . polyphlebia was either Morpho 5 or 6 Table 2. The least abundant beetle species found in the hybrid Cecropia was Morpho 2 Tab le 2. No significant difference in Shannon Weiner diversity index was found between C. polyphlebia H = 1.26 and the hybrid H = 1.23 Modified t test, df = 127.9, p > 0.05; Table 2. Six species of beetles were found in C. pol y phlebia and five species in the hybrid Table 1, Table 2. No significant difference in evenness was found. C. polyphlebia had an evenness of 0.76, and the hybrid Cecropia had an evenness of 0.77 Table 2. S marg for C. polyphlebia was 1.13, and the hybrid Cecropia S marg was 19.2 1 Table 2. Moisture content was found to increase as the abundance of beetles increased ANCOVA, F = 4.93, df = 2, p = 0.008; Figure 3. Also, the hybrid species had
a greater abundance of beetles compared to C. polyphlebia ANCOVA, F = 11.2, df =1, p = 0.001; Figure 2. Furthermore, as the petiole length increased within each species of Cecropia , the abundance of beetles increased ANCOVA, F = 5.16, df = 1, p = 0.024; Figure 1. Several larvae were observed in many of the occupied petioles. Furthermore, there were many insects and non beetle species found in the Monteverde Cloud Forest. DISCUSSION Based on results from this study, it was concluded that bark beetles were able to successfully colonize and reproduce in the hybrid species of Cecropia bette r than in C. polyphlebia . This is supported by the fact that there was also a higher abundance of beetles found within the hybrid Cecropia . This indicates that there was not a significant difference in diversity, but there was a significant difference in ab undance of beetles colonized. Also, 44% of the hybrid petioles were colonized and 20% of the C. polyphlebia petioles were colonized. One possible explanation for a higher colonization percentage in the hybrid Cecropia is that the hybrid species offers a be tter habitat for bark beetles than C. polyphlebia. Through observation, there were significantly less insects other than beetles found in the hybrid petioles than in the C. polyphlebia petioles. This may reduce inter specific competition between species and allow for bark beetles to thrive in the hybrid Cecropia . Also, the intermediary phenotype may inhibit other species that generally colonize Cecropia petioles. For example, hybrid MÃ¼llerian bodies, similar to extrafloral nectaries, are less distinct and ar e covered in long hairs. Furthermore, colonies of Azteca that s hares a mutualistic relationship with Cecropia drop out around the elevation where the hybrid species is found Longino 2000. These factors may explain a higher abundance of beetles in the hybr id Cecropia . Bark beetles regularly inbreed and generally mature and mate with their sisters before even leaving the petiole Beaver et al 2001. In bark beetles, usually one to four pairs of adults was found per petiole Wood 1983. However the hybrid Cecropia , had up to 26 beetles per petiole in this study. Bark beetle reproductive fitness may be greatly increased in the hybrid phenotypes due to reduced inter specific competition Jordal and Kirkendall 1998. Additionally, bark beetles are colonizing o rganisms and regularly inbreed. Therefore, they are not affected by inbreeding depression that may affect their reproductive fitness Beaver et al. 2001. Even though there was a difference in abundance between the hybrid and C. polyphlebia , there was no t a significant difference in diversity. Most beetle species may be able to colonize both of the Cecropia species, but beetles may be able more successful in the hybrid species of Cecropia . This would lead to higher abundance of beetles in the hybrid Cecro pia . Also, altitude may play a role in affecting abundance of bark beetles but may not prohibit species from traveling to different altitudes. Studies done by Wilkinson 2002 and Strauss 2007 both found that all species were found at both of the sites s eparated by altitude. Further studies should be completed to specifically observe the altitudinal effects on bark beetles. Interestingly enough, Morphospecies 6 was found only in the Monteverde Cloud Forest. Although this is contradictory to previous stu dies, this may be due to a variation in sample sizes and the time in which the study took place. Furthermore, a high number
of petioles studied were filled with larvae, and there were many more beetles per petiole. Collection of petioles may have occurred during a community turnover time, where many bark beetles were emerging for their migratory phase to locate new hosts for breeding material Berryman 1983. In addition, differences in the species of Cecropia with varying petiole lengths affected the abu ndance of bark beetles. Therefore, bark beetles may prefer specific habitats and petiole lengths. Overall, as petiole length increased in each species of Cecropia , the abundance of beetles increased as well. Larger petioles would have less intense competi tion for similar resources. Also spatial partitioning would be more successful in longer petioles. Jordal and Kirkendall 1998 found that many bark beetles were found in a specific tissue of the Cecropia petioles. For instance, S.maurus colonized the base of the petiole, but S. acares and S. cecropiavorus colonized the fibrous tissue. With longer petioles, there are more resources to share thus allowing a greater abundance of beetles to be supported. As petiole moisture increases, abundance of beetles in creases. Bark beetles may prefer petioles that are in the shade and moist. Therefore, bark beetles favor certain tree sites. This coincides with the study done by Jordal and Kirkendall 1998. They found moist petioles in shaded forest sites had ten or mor e egg tunnels per petiole in comparison to dry, sun exposed sites, which were characterized by Hypothenemus species. In this study, there were several petioles that were not colonized at all. Only 32% of the petioles studied were colonized. In the stud y done by Jordal and Kirkendall 1998 83% of their petioles were colonized. The difference in colonization may be counterbalanced by the high amount of bark beetles found per petiole. Bark beetles complete their whole life cycle protected within the petio le Rudinsky 1962. However, they may leave to search for a new host or reproduce. My results may indicate that collection of data may have been during an inbreeding season, where many are drawn to petioles to reproduce. This can explain why there were man y beetles per petiole and why few were colonized. With the high abundance of bark beetles found in lower elevations, climate change may not be a factor that affects bark beetles. However, Scolytinae beetles prefer moist petioles, which means they may hav e a harder time finding petioles that are not desiccated in the future. However, there was one species found in the higher elevations that was not found in the lower elevations Morpho 6. This specific species of bark beetle may suffer from climate chang e if this species is only found in higher elevations. Further investigation may want to study the effects of climate change on bark beetles. For example, the ever increasing dry days in Monteverde may have significant effects on bark beetle habitats. As th e Earth warms and continues to alter the face of our landscape, it is essential to see the impacts climate change will have on biodiversity. Biodiversity is threatened as global warming changes the very basics of our ecosystems. The carbon enriched atmosph ere has wounded species and ecosystems for millions of years. As biodiversity is rapidly declining due to climate change and anthropogenic factors, this study shows that bark beetles may not be affected by climate change and may even benefit.
ACKNOWLEDGEM ENTS I wou ld like to thank Tania Chavarria Pizarro for advising me throughout this entire research project. I would also like to thank Karen Masters for helping and supporting me throughout the planning process as well as with any follow up questions I ev er had. I would like to thank Pablo and Moncho for helping me with any questions or supplies I needed. I really want to thank Nathan Spaht and Caitlin Loooby for helping me collect data and keeping my sanity. Finally, I want to thank every person on the S ummer 2008 program, my trip and life will not be the same without you! LITERATURE CITED Beaver, R.A., B.H. Jordal and L.R. Kirkendall. 2001. Breaking taboos in the tropics: incest promotes colonization by wood boring beetles. Global Ecology and Biogeog raphy 104: 345 357. Berryman, A.A. and K.F. Raffa. 1983. The Role of Plant Resistance in the Colonization Behavior and Ecology of Bark Beetles Coleoptera: Scolytidae. 53:27 49. Jordal, B.H. and L.R. Kirkendall. 1998. Ecological relationships of a gu ild of tropical beetles in Cecropia petioles in Costa Rica. Journal of Tropical Ecology 142: 153 176 Larimer, A., A. Moe, M. Moyano and R. Oono. 2006. Distribution and dispersal of two bark beetle species in Cecropia petioles. OrganizaciÃ³n para Estudi os Tropicales. Course Book. Longino, J.T. 2000. Myrmecophytes. pp. 293. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest . Nadkarni, N.M. and Wheelwright, N.T. Ed.. Oxford University Press: Oxford, New York. Rudinsky, J.A. 1962. Ecolo gy of Scolytidae. Annual Review of Entymology. 27:327 348. Strauss, A. 2007. Differences in bark beetle Coleoptera: Curculionidae community composition within Cecropia obtusifolia Cecropiaceae petioles in two habitats in Monteverde, Costa Rica. CIEE T ropical Ecology and Conservation, Fall 2007: 77 86. Vitug, S. 2005. Scolytinae within Cecropia petioles. UCEAP IMV Tropical Biology and Conservation , Fall 2005:1 9. Wilkinson, K. 2002. Distribution and Niche Partitioning of Beetles from Two Cecropia spp. Cecropiaceae in Monteverde, Costa Rica. CIEE Tropical Ecology and Conservation, Fall 2002: 60 70. Wood, S.L. 1983. Scolytodes atratus panamensis Escarabajit de Guaramo, Cecropia petiole Borer. pp. 768 769. In: Costa Rican Natural History , D.H. Janze n Ed.. University of Chicago Press: Chicago, Illinois.
Table 1. The abundance of each morphospecies found in C. polyphlebia and the hybrid Species C . polyphlebia Hybrid Morpho 1 25 106 Morpho 2 6 13 Morpho 3 40 142 Morpho 4 8 19 Morpho 5 3 24 Morpho 6 3 0 Table 2. The abundance N, evenness E, Shannon Weiner Index H , Species Richness S, and S marg S marg between C. polyphlebia and the hybrid Modified t test, df=127.85 p=0.1 0.2. C. polyphlebia Hybrid E 0.76 0.77 N 85.00 304. 00 S 6.00 5.00 H' 1.36 1.23 S'marg 1.13 19.21 Table 3. Degrees of Freedom, F Ratio, and Probability > F for Species of Cecropia , Moisture content, and Species of Cecropia and Length using 2 way ANCOVA test. Source DF F Ratio Prob > F Species 1 11.24 19 0.001 Moisture 2 4.9336 0.0083 Species and Length 1 5.1569 0.0244
Figure 1. Petiole length mm in C. polyphlebia blue and hybrid red in relation to the abundance of beetles ANCOVA, F = 5.16, df = 1, p = 0.0244 0 1 2 3 4 5 6 7 8 9 10 polyphlebia Hybrid Species Abundance of Beetles Figure 2. The abundance of beetles in C. polyphlebia and the hybrid Cecropia ANCOVA, F = 11.2, df = 1, p = 0.001 0 3 6 9 12 15 18 21 24 27 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 petiole length # beetles
0 1 2 3 4 5 6 7 8 9 10 0 1 2 Petiole Moisture Abundance of Beetles Figure 3. The abundance of beetles versus moisture content in Cecropia ANCOVA, F = 4.93, df = 2, p = 0.0083