1 Azteca Ants Inhabiting Cecropia Trees Are Moving up in Elevation In
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1 Azteca ants inhabiting Cecropia trees are moving up in elevation in Monteverde Meghan R. Garfink Department of Environmental Science and Policy University of California, Davis UCEAP Tropical Biodiversity and Conservation Spring 2019 7 June 2019 ABSTRACT Global climate change impacts ecological relationships of all taxa and kingdoms, yet little has been documented of the impact on insects. This study documents the changing elevational range of four Azteca ant species in Cecropia trees in the Monteverde cloud forest. In Monteverde, Costa Rica, many species of Cecropia form a mutualism with Azteca in which ants receive nutrients in exchange for protection of the tree. Data was collected in May 2019 and compared to similar studies done in 1989 (Longino), 2003 (Mazzei), 2008 (Loope), and 2016 (Jensen). Since 1989, maximum elevational range of Azteca increased by 132 meters. Average elevation of all species of Azteca and Cecropia have increased significantly since 1989 as well. One species of Cecropia, C. polyphlebia, contained Azteca at 1500 m, despite historically not harboring Azteca. This data indicates that Azteca are moving upslope and inhabiting different species of Cecropia in Monteverde, potentially due to climate change. Hormigas Azteca que habitan en árboles de Cecropia en Monteverde se están desplazando hacia arriba en elevación RESUMEN El cambio climático global afecta las relaciones ecológicas de todos los taxones y reinos, sin embargo, el impacto en los insectos ha sido poco documentado. Este estudio documenta el cambio del rango de elevación de cuatro especies de hormigas Azteca que viven en árboles de Cecropia en el bosque nuboso de Monteverde. En Monteverde, muchas especies de Cecropia tienen una relación mutualista con Azteca, en el que las hormigas reciben nutrientes y casa a cambio de proteger el árbol hospedero. Recopilé datos en mayo de 2019 y los comparé con estudios similares realizados en 1989 (Longino), 2003 (Mazzei), 2008 (Loope) y 2016 (Jensen). Desde 1989, el rango de elevación máxima de Azteca ha aumentado 132 metros. La elevación promedio de todas las especies de Azteca y Cecropiatambién ha aumentado significativamente desde 1989. Encontré hormigas Azteca en C. polyphlebia a 1500 m, a pesar de que históricamente no albergaba a Azteca. Estos datos indican que las hormigas están utilizando árboles en elevaciones más altas y habitando diferentes especies de Cecropia en Monteverde, potencialmente debido a cambios en el clima. Elevational range of Azteca and Cecropia Garfink 2 Climate change is affecting species distributions and ecological interactions globally (Welch 2017). Often, these interactions are mutualisms, where both species benefit from the relationship. Mutualisms can be obligate, when one organism cannot survive without the other, or facultative, where both species are not dependent on each other to survive (Jorgensen et al. 2008). Many observed changes in ecological relationships have been correlated to climate change, including the mutualism between Azteca ants and Cecropia trees in Monteverde, Costa Rica (Mazzei 2003, Loope 2008, Jensen 2016). Cecropia are native to lowland, and mid- elevation cloud forests in the American tropics. They grow alongside forests, forest edges, pastures, and residential areas (Longino 1991). Eighty percent of Cecropia are myrmecophytic: plants which lives in a mutualistic relationship with ants (Berg et al. 2005). In this mutualism, ants protect against herbivores and kill vine ends that begin to attach to the Cecropia trunk. Cecropia provide shelter and food from glycogen-rich Mullerian bodies found on the stems of Cecropia leaves (Janzen 1969). Azteca in this mutualism are obligate and therefore cannot survive without nesting in Cecropia wood (Berg et al. 2005). The four species of Azteca found in Monteverde are A. constructor, A. xanthrochroa, A. alfari, and A. coeruleipennis (Longino 1989). There are four species of Cecropia found in the Monteverde area: C. peltata, C. obtusifolia, C. insignis, and C. polyphlebia (also known as C. angustifolia) (Longino 1989). C. peltata is found on the Pacific slope in lower elevations. C. insignis can be found in lowland regions of the Caribbean slope and C. obtusifolia can be found in mid-elevations on both slopes. Typically, non-myrmecophytic Cecropia (C. polyphlebia) is found at higher elevations where it can survive without the ant mutualism (Zuchowski 2007). Azteca are limited by cold, wet environments (Longino 1991). For this reason, C. polyphlebia historically do not partner with Azteca. Instead, C. polyphlebia relies on secondary compounds to defend from predators (Longino 1989, 1991, Jensen 2016). For over 30 years, non-continuous data has been collected in Monteverde comparing Cecropia and Azteca ranges at varying elevations: a primary study from 1989 (Longino on Azteca and Cecropia), and replicate studies in 2003 (Mazzei on Azteca), 2008 (Loope on Azteca and Cecropia), and 2016 (Jensen on Azteca and Cecropia). Comparing these studies, trends show that Azteca are moving up in elevation in Monteverde over the past 30 years. While there is no known cause for these changes, many studies on ecological interactions have been attributed to climate change. To track elevational shifts in Monteverde, I asked the questions (1) what is the current distribution of Azteca and Cecropia, and (2) how does this compare to previous studies? To answer this, I surveyed Azteca and Cecropia at varying altitudes in Monteverde and compared the results to previous work. MATERIALS AND METHODS I collected samples of Cecropia and Azteca from 1000-1800 m on the Caribbean and Pacific slopes in Monteverde, Costa Rica between 6 May 2019 and 19 May 2019. I sampled along trails near the San Luis Valley (1000-1200 m.a.s.l.), San Gerardo (Caribbean slope, 1100- 1300 m), La Calandría (1200-1300 m), Cerro Plano (1400-1500 m), Bajo del Tigre Reserve (1400 m), the Estación Biológica forest (around 1500-1600 m), and Cerro Amigos (1600-1800 Elevational range of Azteca and Cecropia Garfink 3 m). I identified Cecropia in the field with the help of plant expert and Monteverde local, Eladio Cruz, and with additional assistance from the Tropical Plants of Costa Rica (Zuchowski 2007). In the appendix of this paper there are images and notes of Cecropia that I identified for future reference. Also included are tables of my collected data, satellite imagery of the locations surveyed, and the averages, maximums, minimums, and sample sizes from past data. I measured tree height with a Nikon Forestry Pro laser range finder. I then calculated tree height accuracy as +/- 0.37 m by taking the same measurement from one tree 10 times and calculating the standard deviation. I recorded the diameter at breast height with a tape measurer for each tree. Altitude was recorded at each sample with an Oregon Scientific altimeter. I knocked on Cecropia trunks and used an extendable tree trimmer to observe ant activity. If present, 3-4 ants were collected using an aspirator and placed in ethanol for preservation. Ant species were identified in lab with the help of University of Utah professor and ant expert, John Longino. I classified ants with setae on the hind tibia as A. alfari. Ants with a flat, shelf-like metanotal groove, were identified as A. coeruleipennis. Other ants were classified as either A. xanthrocroa or A. constructor. Physical and ecological differences (such as nesting in Cecropia) between the two species are insignificant (Longino pers. comm. 2019), so A. xanthrocroa and A. constructor were combined into the group A. xanthrocroa/constructor for statistical analysis. I produced box plots of the elevational ranges of Azteca and Cecropia in Microsoft Excel. I also made scatter plots with regression lines of the average elevations of all Azteca and Cecropia species per year. A scatter plot and linear regression of maximum elevational height for all Azteca by year was also produced. RESULTS Sixty one Cecropia were sampled between 1000 m and 1800 m. Of the 61 trees, 37 were C. obtusifolia, four were C. peltata, 19 were C. polyphlebia, and one was C. insignis. Forty-two trees of Cecropia contained Azteca. Of the 42 samples, 36 were identified as A. xanthrocroa/constructor, 5 were A. alfari, and 1 was A. coeruleipennis. No trees contained more than one species of ant, except for six individual C. obtusifolia trees which were also being harvested by leaf cutter ants. Azteca spp. were found throughout the sampling area below 1532 m. A. xanthrocroa/constructor had the largest range: 1035-1532 m. A. alfari was found between 1037 m and 1240 m. One A. coeruleipennis was identified and found at 1087 m (Fig. 1). C. obtusifolia was most common throughout the survey. C. obtusifolia was found above 1035 m and below 1532 m. C. peltata was found between 1085-1239 m. One C. insignis was sampled along the distribution and was found at 1238 m on the Caribbean slope. C. polyphlebia was found at higher elevations, yet had the largest range of 564 meters (from 1238-1802 m) (Fig. 2). Five C. polyphlebia samples contained A. xanthrocroa/constructor. Elevational range of Azteca and Cecropia Garfink 4 Fig. 1: Elevational ranges of Azteca ants by species in 2019. Average altitude per species is represented by the horizontal black lines. Minimum and maximum altitude per species is represented through the vertical black lines. (A. xanthrocroa N=35, A. alfari N=5, A. coeruleipennis N=1). Fig. 2: Elevational ranges of Cecropia trees by species in 2019. Average altitude per species is represented through the horizontal black lines. Maximum and minimum altitude per species is represented through vertical black lines. (C. obtusifolia N=35, C. peltata N=4, C. polyphlebia N=19, C. insignis N=1) Elevational range of Azteca and Cecropia Garfink 5 In comparison with previous studies, the average elevation of A. xanthrocroa/constructor has increased by 172.5 m since the original 1989 study by Longino (Fig.