Distribution and Host Species Ranges of Umbonia ataliba and Umbonia crassicornis and the Potential for Interspecific Competition Charles A Hernick Department of Ecology, Evolution and Behavior College of Biological Sciences University of Minnesota, Twin Cities ____________________________________________________________________ ABSTRACT Umbonia crassicornis and Umbonia ataliba Homoptera: Membracidae elevational ranges and host tree ranges were studied in Monteverde, Costa Rica. Umbonia crassicornis was known to exist at elevations below 1000 m, while U. ataliba was know n to exist at 1300 1500 m. The possibility of interspecific competition or possible future competition was investigated in ligh t of recent local and global warm ing trends and observed population shifts in birds and herps by Pounds et al. 1999. A census of Umbonia species and host plants was conducted along an elevational gradient, in addition to an experimental test, which examined U. crassicornis survivorship on U. ataliba hosts. Umbonia crassicornis and U. ataliba were not found to coexist at any elevation. However, signature egg and feeding scars were found on Inga mortoniana, a plant commonly used by U. ataliba, in this case found at elevations likely to be used by U. crassicornis. Complete survivorship by U. crassicornis on Acacia angustissima, a known U. crassicornis host, and Inga mortoniana, a known U. atliba host, was observed, proving I mortoniana to be an equally suitable U. crassicornis host, at h igh er elevations. Host species I mortoniana was found to exist at all elevations studied, while other known hosts for U. crassicornis and U. ataliba were limited to lower and higher elevations respectively. If warming trends affect Umbonia species ranges, thereby expanding U. crassicornis populations to higher elevations, interspecific competit ion may occur over I mortoniana. RESUMEN Las elevaciones conocidas de Umbonia crassicornis y Umbonia ataliba Homoptera: Membracidae y sus rboles h uspedes fueron estudiados en Monteverde, Costa Rica. Se sabe que U. crassicornis se encuentra debajo de 1000 m, mientras U. ataliba se encuentra entre 1300 1500 m. La posibilidad de competencia entre especies distintas o de competencia en el futuro fue investigado tomando en cuenta los cambios de poblaciones vistos en pjaros y lagartijas discutido por Pounds et al. 1999. Un censo de Umbonia y sus rboles huspedes fue levantado sobre una gradiente altitudinal, con una prueba experimental que examin si U. crassicornis podra sobrevivir en las planta huspedes de U. ataliba. Se encontr que U. crassicornis y U. ataliba no vi vieron juntos en ningn elevaci n. Sin embargo, marcas dejadas despus de comer y los huevos encontrados en Inga mortoniana, una planta tpicamente usado por U. ataliba, muestra que en estas elevaciones son actualmente usadas por U. crassicornis. Umbonia crassicornis sobrevivi en Acacia angustissima, un rbol hu sped conocido y Inga mortoniana un r bol hu sped de U. ataliba. Este e xperimento significa que I m o r toniana puede ser un arbol huesped de U. crassicornis en elevaciones alt as. Se discuti que I m ortoniana vivi en todas las elevaciones estudiadas. Esto significa que si U. crassicornis es afectada por el aumento e n la temperatura ambiental y la competencia entre especies distintas es posible para I. m ortoniana
INTRODUCTION The treehoppers of the genus Umbonia Homoptera: Membracidae are widely distributed throughout the Americas from Southern Florida to Brazil Wood 1983. Two species, Umbonia crassicornis and Umbonia ataliba, exist within the Monteverde, Costa Rica area. Although closely related, these specie s of Umbonia exhibit different mating behaviors and population densities Masters 1997. Umbonia crassicornis exhibits patterns of outbreeding and maintains a large populat ion along the R o Guacimal below 1000 m. In contrast, U. ataliba characteristically inbreeds and maintains a smaller populat ion on the upper banks of the R o Guacimal at approximately 1300 1500 m in elevation Masters 2000. Both species occupy riparian corridors; however within this area U. crassicornis has a greater a ffinity for disturbed or open areas, while U. ataliba prefers closed canopy forest Masters 1997. Both Umbonia species feed on phloem of young branches of genera within the tribe Ingaeae Mimosaceae, with few exceptions e.g. Acacia. Umbonia crassicorni s is known to feed on the species Acacia centralis, Acacia angustifolia, Calliandra bijuca, and Enterolobium cyclocarpum, whereas U. ataliba is known to use host plants such as Zygia palmanum, Inga punctata and Inga sierrae Masters 2000. The specific ran ges of these host plants were largely unexamined to date, thus, it is unknown whether Umbonia ranges are limited by host plants or other factors. Interspecific competition occurs when individuals of one species suffer a reduction in fecundity, survivorship or growth as a result of resource exploitation or interference by individuals of another species Began et al. 1990. For competition to occur, t hese species must occur at the same place and at the same time. Thus far, studies on U. crassicornis and U. ataliba have only looked at host species use at the core of Umbonia population areas, not in areas where U. crassicornis and U. ataliba may co exist The potential for contemporary and future range overlap is unknown for U. crassicornis and U. ataliba, since there exists no systematic study of host plant use and ranges. Climate changes more rapidly with elevation about 1 C per 160 m than it does wi th latitude about 1 C per 150 km, so, rapid changes in montane communities are expected as climate changes Grabherr 1994 in McCarthy 2001. Climatic stress is known to have caused extinctions in amphibians and upslope shifts in the range of bird specie s, potentially creating competition for resources where there once was none McCarthy 2001, Pounds et al. 1999. Although many other factors are known to influence the geographic range of any given species, recent global climate changes have warranted a cl oser look at the dynamics of insect populations, such as Umbonia species in Monteverde. This study investigated the possibility of U. crassicornis and U. ataliba range overlap and the potential for interspecific competition in U. ataliba hosts. It was thou ght that U. ataliba populations would be small and located at the highest elevations of the gradient, while U. crassicornis would be more widespread, pushing into areas and using host species used by U. ataliba.
METHODS Part I Distribution of Umbonia spp. and Their Hosts at Different Elevations A census of Umbonia species was conducted along and elevational gradient consisting of four plots along the R o Guacimal and its tributaries, from 870 1580 m on the Pacific s lope of the Cordillera de Tilar n in November of 2001. A search for Umbonia crassicornis and U. ataliba was conducted at all sites to define their ranges and identify any range overlap. Known host tree species were searched in addition to other Mimosoids which cou ld also be used as hosts. The R o Guacimal was chosen for its bordering riparian forest, continuous over a broad elevational gradient where both species were thought to exist Masters 1997. The four plots were each 900 m 2 and of differing shapes because of topographical limitations. Plots were located at the Mata Farm from 870 880 m, Bajo del Tigre from 1100 1180 m, La Lecheria fr om 1360 1365 m, and El Estaci n from 1560 1570 m. Each plot was searched for 3.5 person hours or until all potential host plants were sear ched. Within each plot, the following seven host plants were searched for; Acacia centralis, Acacia angustifolia, Calliandra bijuca, Enterolobium cyclocarpum, Zygia palmanum, Inga punctata and Inga brenesii. The presence of these, in addition to other pote ntial Mimosoid hosts were recorded to identify the ranges of each host species. Part II Experimental Test of Host Plant Use The capacity of U. crassicornis to use the known host plants of U. ataliba is important to evaluate the likelihood of interspecifi c competition, particularly if U. crassicornis can colonize higher elevations. Ninety five U. crassicornis adults were removed from Acacia angustissima trees at 815 m and 840 m and moved to experimental host plants at 1520 m. Twenty U. crassicornis adults were relocated to non host mimosoids, known U. ataliba hosts: Zygia palmanum, Inga mortoniana and Cojoba costaricensis. Fifteen additional U. crassicornis adults were moved to Acacia angustissima in addition to Psidium guajava, a non host, non mimos oid species, acting as a positive and negative control respectively. The positive control was the A. angustissima, a known host plant species at lower elevations, where survival was expected; the negative control being P. guajava, a non host plant, non mim osoid where survival was not expected. The experiment lasted 21 days. Groups of five U. crassicornis were placed in fine mesh bags around a branch tip. One bag per branch tip, with four bags on each tree, the exception being A. angustissima, which only had three bags. Groups were distributed over multiple branches to minimize variation in branch quality, thus countering the effects of a bad branch or species loss do to any other factor besides feeding capacity. Each group was observed every one to two days for evidence of mortality as well indications of feeding. The number of days each individual survived was quantified.
RESULTS Part I Distribution of Umbonia spp. and Their Hosts at Different Elevations Of the four sites censused, Umbonia species were only found at two sites, with U. crassicornis and U. ataliba being only found at one site each with no range overlap. The frequency distribution of U. crassicornis over life stages appeared even Figure 1. At the lowest elevation site 870 880 m two trees were found with U. crassicornis. On one tree, a single female with eggs; while the other tree had three single females with eggs, three adult groups and three nymph groups. Umbonia ataliba was observed at the 1360 1365 m site. The frequen cy of observed females with eggs was two, as were adult groups and nymphs groups. Each of the U. ataliba groups were found on separate trees as opposed to U. crassicornis, where nine groups were found on a single tree. Of the two trees found U. crassicornis, both tree species were A. angustissima. Of the six trees found with U. ataliba, three were Z. palmanum, while the remaining three were; I mortoniana, I. punctata, and I Sierrae. No tree host species was used by both Umbonia species Figu re 2. Inga mortoniana was the only tree host found at every elevation surveyed, with an estimated range of 850 1580 m. Most species had smaller ranges: Albizia adinocephala, Lysiloma divericanum, Acacia angustissima, Inga marginata, Calliandra bijuca, Z ygia palmana, Inga micheliana only being found at one elevation each Figure 3. Part II Experimental Test of Host Plant Use Only A. angustissima and Inga mortoniana had complete U. crassicornis survivorship, meaning that 100% of transferred insects survived. Zygia palmana, C. costaricensis, and P. guajava all experienced lower mean survivorship Figure 4. A significant difference was found between survivorship on host plants ANOVA, F = 8.096, p < 0.0001, DF = 4. Using a Post hoc Fisher's PLSD, significant differences were found between survivorship on P. guajava and all other hosts tested all p < 0.008. Significant differences were also found between C. costaricensis and both A. angustissima and I mortoniana p = 0.0393 and p = 0.0262 respectively. Overall, survival was increasing from the lowest P. guajava, to C. costaricensis, to Z. palmanum, with I. mortoniana and A. angustissima having the highest survivorship. DISCUSSION Umbonia crassicornis and Umbonia ataliba were not found to co exist at any elevations studied. At the lowest elevation site 870 880 m, U. crassicornis was found with groups of individuals at various life and reproductive stages. Reproducing groups were also found for U. ataliba at the 1360 1365 m site. Other groups of both U. crassicornis and U. ataliba were also observed in non study sites, at elevations of 815 m and 1520 m respectively. Neither species was found between 880 1360 m, the area of pot ential overlap. Therefore direct evidence of current range overlap and competition remains nonexistent. The 1100 1180 m site was a prime candidate for overlap, containing both forest types and plants species suitable for either Umbonia species. In genera l, insect populations are known to fluctuate annually making the likelihood of finding
insects such as Umbonia variable. This in addition to the plot shape of this site, a 2 x 450 m plot, essentially transecting a ridge top, may have decreased viewable hosts. A previous study had estimated densities at 4 14 and 27 207 females per hectare for U. ataliba and U. crassicornis respectively Masters 1997. Such variable densit ies posed a challenge for creating plots that would accurately depict populations and ranges. The fact that direct evidence of overlap was not found in this study is inconclusive, since scarring was found on hosts of both species at two elevations, 870 88 0 m and 1100 1180 m, possibly indicating range overlap. Although the scarring was distinctively Umbonia created, is the same for both species. This finding, in addition to a better understanding of host plant ranges and feeding potentials, makes the futu re population dynamics of Umbonia species very interesting as global climate continues to change. The data suggest that U. crassicornis survivorship at higher elevations is not limited by climate, but it may be limited by hosts. Known U. crassicornis hosts were only found at elevations from 850 1145 m. In contrast, known hosts for U. ataliba were fou nd at every elevation, mainly, I mortoniana, whose range extended down to 850 m. Umbonia ataliba however, was not found to have a corresponding range. At the two lowest elevational sites I. mortoniana was found with egg and feeding scars. The scars may have been created by U. crassicornis. Umbonia crassicornis survived as well on I. mortoniana as it d id on A. angustissima a known host, in spite of the fact that it was previously not known to use I mortoniana. Interestingly, both I. mortoniana trees with scars were found under a closed canopy, habitat favored not by U. crassicornis but by U. ataliba. It is possible that U. crassicornis occasionally uses I. mortoniana in contemporary situations. Possible explanations for these observations are many. One is that host plants in shaded areas may occasionally be used by U. crassicornis simply be cause of the constant experimentation for new, better ways to live, a shadow of the fundamental mechanism that drives evolution. Another explanation for these observations may be a new drive for U. crassicornis to explore new habitats as a result of climat e change. These findings are very significant for the future ecology of these two organisms because if climate changes result in population range changes for insects, in the same way as has been suggested for birds, lizards and anurans by Pounds et al. 1999, U. ataliba populations may be driven to interspecific competition with U. crassicornis for host plants. The potential for interspecific competition must be weighed by the limitations of this study; the ho st plant experiment only lasted twenty one days. And although this time frame was a good indicator of survivorship potentials, it was not representative of a full year. Further studies ought to investigate the potential for full life and reproductive cycle s at high elevations. Studies of this sort in addition to a more detailed investigation of the 880 1360 m region are necessary to create a more complete picture of Umbonia interactions. Umbonia spp. may be vulnerable to ongoing climate change in Montever de. Global warming and possibly lowland deforestation have apparently contributed to warmer, drier conditions in Monteverde, and consequently, the upslope colonization of species Pounds et al. 1999; Lawton et al. 2001. Changing climate may provide favora ble conditions for U. crassicornis to establish new breeding populations at higher elevations. This is especially likely considering that the results here show that suitable host trees exist. The use of I mortoniana by U. crassicornis should be the focus of future studies.
AKNOWLEDGMENTS I would like to thank Karen Masters for the endless amount of help, encouragement and harassment that made this project work. I also thank the Mata Family, the Masters Family, The Monteverde Conservati o n League, and La Estacin Biol gica for allowing me to use their lands and facilities for this project. ______________________________________________________________________________________________ LITERATURE CITED Began, M., J.L. Harper, and C.R. Townsen d. 1990. Ecology: Individuals. Populations and Communities. Blackwell. Cambridge. Massachusetts. Grabherr, G., M. Gottfried, and H. Pauli. 1994. Climate effects on mountain plants. Nature. 369:448 Lawton, R.O., U.S. Nair, R.A. Pielke Sr., R.M. Welch. 2001. Climatic Impact of Tropical Lowland Deforestation on Nearby Mountain Cloud Forest. Science 294: 584 587. McCarthy, J.P. 2001. Ecological Consequences of Recent Climactic Change. Conservation Biology. 15:320 331. Masters, K.L. 1997. Behavioral and Ecological Aspects of Inbreeding in Natural Animal Populations: Inferences from Umbonia Treehoppers Homoptera: Membracidae. Princeton University, Princeton, New Jersey. Masters, K.L. 2000. Sex and Social Life of Umbonia Treehoppers. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest. Nadkami, N.M. and N.T. Wheelwright eds. Oxford University Press, New York, New York, pp. 103 104 McCarthy, J.P. 2001. Ecological Consequences of Recent Climatic Change. Conservation Biology 15:320 331. Pounds, J.A., M.P.L. Fogden, and J.H. Campbell 1999. Biological response to climate change on a tropical mountain. Nature. 382:611 614 Wood, T.K. 1983. Umbonia crassicornis Bicho Espino, Thorn Bug, Treehopper. In: Costa Rican Natural History, D.H. Janzen, ed. The University of Chicago Press. Chicago, Illinois, pp. 773 775.
_____________________________________________________________ _________________ Figure 2: Frequency of observed occupied hosts for U. ataliba and U. crassicornis _____________________________________________________________________________________________