Contagion of mistletoe in a tropical Psidium guayaba pasture


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Contagion of mistletoe in a tropical Psidium guayaba pasture

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Title:
Contagion of mistletoe in a tropical Psidium guayaba pasture
Translated Title:
Contagio de muérdago en un potrero tropical de Psidium guayaba
Creator:
Patel, Amish
Publication Date:
Language:
Text in English

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Subjects / Keywords:
Mistletoes ( lcsh )
Matapalos ( lcsh )
Seeds--Dispersal ( lcsh )
Semillas--Dispersión ( lcsh )
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
CIEE Fall 2003
CIEE Otoño 2003
Genre:
Reports

Notes

Abstract:
Mistletoe is a plant parasite that is unusual in having a specialized dispersal mutualism. The behavior of its avian dispersers assures deposition on tree branches of its preferred hosts. In returning to the same plant, however, such behavior may cause centers of ever-increasing density, akin to contagion in other host-parasite systems. The purpose of this study was to determine if mistletoe species Antidaphne viscoidea or Phoradendron undulatum demonstrate contagious dispersion of their seeds. The study was conducted in a pasture of Psidium guayaba trees on a farm near Monteverde, Costa Rica. In each of 20 trees, I counted the number of fruiting individuals, non-fruiting individuals, seedlings, and defecated seeds of both species of mistletoe. Running simple regressions found positive correlations between the number of seeds and the number of fruiting individuals of P. undulatum (p<0.0001.) Similarly, there was a positive correlation between the numbers of adult individuals and seedlings for A. viscoidea (p=0.0003.) Therefore, there is a contagious dispersion of mistletoe seeds in P. guayaba; trees with numerous mistletoe parasites have more parasite seeds and seedlings than trees with few or none. Contagion in mistletoe creates a general problem in dispersal, in that genetic variability is likely to decrease (Reid et al. 1995), while competition and density-dependent mortality are likely to increase. How mistletoe counters these potential problems requires further study. ( , )
Abstract:
El muérdago es una planta parasitaria que es inusual en tener un mutualismo con dispersión especializada. El comportamiento de sus dispersores aviares asegura la deposición de ramas de los árboles de sus huéspedes preferidos. Al volver a la misma planta, sin embargo, tal comportamiento puede causar centros de densidad cada vez mayor, similar al contagio en otros sistemas huésped-parasito. El propósito de este estudio fue determinar si las especies de muérdago Antidaphne viscoidea o Phoradendron undulatum demuestran dispersión contagiosa de sus semillas. El estudio se realizó en un potrero de árboles de Psidium guayaba en un potrero cerca de Monteverde, Costa Rica. En cada uno de los 20 árboles, conté el número de individuos de fructificación, los individuos no frutales, plantones y semillas defecadas de ambas especies de muérdago. La ejecución de regresiones simples encontró una correlación positiva entre el número de semillas y el número de individuos de fructificación de P. undulatum (p < 0.0001). Del mismo modo, existe una correlación positiva entre el número de individuos y plántulas de A. viscoidea (p = 0,0003) para adultos. Por lo tanto, hay una dispersión contagiosa de semillas de muérdago en P. guayaba; árboles con numerosos parásitos de muérdago tienen más semillas de parásito y semilleros que árboles con pocos o ninguno. Contagio de muérdago crea un problema general en la dispersión, en la que la variabilidad genética es probable que disminuya (Reid et al. 1995), mientras que la competencia y la densidad dependiente de la mortalidad es probable que aumenten. Como el muérdago encuentra estos problemas potenciales requiere mas estudio.
Biographical:
Student affiliation: CIEE Program, Monteverde, Puntarenas, Costa Rica
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Digitized by MVI

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M39-00441 ( USFLDC DOI )
m39.441 ( USFLDC Handle )

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Mistletoe is a plant parasite that is unusual in having a specialized dispersal mutualism. The behavior of its avian
dispersers assures deposition on tree branches of its preferred hosts. In returning to the same plant, however, such
behavior may cause centers of ever-increasing density, akin to contagion in other host-parasite systems. The
purpose of this study was to determine if mistletoe species Antidaphne viscoidea or Phoradendron undulatum
demonstrate contagious dispersion of their seeds. The study was conducted in a pasture of Psidium guayaba trees on a
farm near Monteverde, Costa Rica. In each of 20 trees, I counted the number of fruiting individuals, non-fruiting
individuals, seedlings, and defecated seeds of both species of mistletoe. Running simple regressions found positive
correlations between the number of seeds and the number of fruiting individuals of P. undulatum (p<0.0001.)
Similarly, there was a positive correlation between the numbers of adult individuals and seedlings for A. viscoidea
(p=0.0003.) Therefore, there is a contagious dispersion of mistletoe seeds in P. guayaba; trees with numerous
mistletoe parasites have more parasite seeds and seedlings than trees with few or none. Contagion in mistletoe
creates a general problem in dispersal, in that genetic variability is likely to decrease (Reid et al. 1995), while
competition and density-dependent mortality are likely to increase. How mistletoe counters these potential problems
requires further study.
El murdago es una planta parasitaria que es inusual en tener un mutualismo con dispersin especializada. El comportamiento de sus dispersores aviares asegura la deposicin de ramas de los rboles de sus huspedes preferidos. Al volver a la misma planta, sin embargo, tal comportamiento puede causar centros de densidad cada vez mayor, similar al contagio en otros sistemas husped-parasito. El propsito de este estudio fue determinar si las especies de murdago Antidaphne viscoidea o Phoradendron undulatum demuestran dispersin contagiosa de sus semillas. El estudio se realiz en un potrero de rboles de Psidium guayaba en un potrero cerca de Monteverde, Costa Rica. En cada uno de los 20 rboles, cont el nmero de individuos de fructificacin, los individuos no frutales, plantones y semillas defecadas de ambas especies de murdago. La ejecucin de regresiones simples encontr una correlacin positiva entre el nmero de semillas y el nmero de individuos de fructificacin de P. undulatum (p < 0.0001). Del mismo modo, existe una correlacin positiva entre el nmero de individuos y plntulas de A. viscoidea (p = 0,0003) para adultos. Por lo tanto, hay una dispersin contagiosa de semillas de murdago en P. guayaba; rboles con numerosos parsitos de murdago tienen ms semillas de parsito y semilleros que rboles con pocos o ninguno. Contagio de murdago crea un problema general en la dispersin, en la que la variabilidad gentica es probable que disminuya (Reid et al. 1995), mientras que la competencia y la densidad dependiente de la mortalidad es probable que aumenten. Como el murdago encuentra estos problemas potenciales requiere mas estudio.
546
Text in English.
650
Mistletoes
Seeds--Dispersal
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
4
Matapalos
Semillas--Dispersin
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
653
Tropical Ecology Fall 2003
Ecologa Tropical Otoo 2003
655
Reports
720
MVI
773
t Monteverde Institute : Tropical Ecology
856
u http://digital.lib.usf.edu/?m39.441



PAGE 1

Contagion of Mistletoe in a Tropical Psidium guayaba Pasture Amish Patel Fall 2003, CIEE Program, Monteverde, Puntarenas, Costa Rica _____________________________________________________________________ ABSTRACT Mistletoe is a plant parasite that is unusual in having a specialized dispersal mutualism. The behavior of its avian dispersers assures deposition on tree branches of its preferred hosts. In returning to the same plant, however, such behavior may cause cen ters of ever increasing density, akin to contagion in other host parasite systems. The purpose of this study was to determine if mistletoe species Antidaphne viscoidea or Phoradendron undulatum demonstrate contagious dispersion of their seeds. The study wa s conducted in a pasture of Psidium guayaba trees on a farm near Monteverde, Costa Rica. In each of 20 trees, I counted the number of fruiting individuals, non fruiting individuals, seedlings, and defecated seeds of both species of mistletoe. Running simpl e regressions found positive correlations between the number of seeds and the number of fruiting individuals of P. undulatum (p<0.0001.) Similarly, there was a positive correlation between the numbers of adult individuals and seedlings for A. viscoidea (p= 0.0003.) Therefore, there is a contagious dispersion of mistletoe seeds in P. guayaba; trees with numerous mistletoe parasites have more parasite seeds and seedlings than trees with few or none. Contagion in mistletoe creates a general problem in dispersal , in that genetic variability is likely to decrease (Reid et al. 1995), while competition and density dependent mortality are likely to increase. How mistletoe counters these potential problems requires further study. __________________________________________________________________________________________________ RESUMEN Las especies del matapalo en los trópicos han coevolucionado este chamente con la e specie de ave que lo dispersa. El sistema de dispersión se bare co mportamiento del ave de comer el fruto del mat apalo y defeca en ramas cercanas . El propó sito de este estudio debía determinar si viscoidea de especie de muérdago Antidaphne o undulatum de Phoradendron demuestran dispersión contagiosa de sus semillas. El es t udio se condujo en un pasto de á rboles de guayaba de Psidium en una granja Monteverde cercano, Costa Rica. En cada uno de 20 ár boles, yo conté el nú mero de individuos de fruiting, no individuos de fruiting, los semilleros, y las semillas defuestas de ambas especie de muérdago. Los retrocesos sencillos corrientes encontraron las cor relaciones positivas entre el nú mero de semillas y el nú mero de individuos de fruiting de P. undulatum (p<0.0001.) Semejantemente, había una correlac ión positiva e ntre lo s números de individuos y semilleros adultos para A. viscoidea (p=0.0003.) Por lo tanto, hay una dispersión contagiosa de semillas de muérdago en P. guayaba; árboles con numerosos parásito s de muérdago tienen más semillas de parásito y semilleros que á rbol es con pocos o ninguno. El método de la dispersió n asegura una planta conveniente de anfitrión para la germinación de la semilla, sin embargo, el efecto de fundador puede causar bottlenecking genético (Reid et al. 1995.) Con el tiempo, la perdida de la variabilidad genética reducirá la habilidad de la especi e para evolucionar a cambios bióticos y no bió ticos en su ambiente (Pescador 1930.) Este método de la dispersió n contradice aparentemente las teorías de la dispersión de la semilla presentadas por H. F. Howe y J. Smallwood (Howe et al. 1982.) Key words: mistletoe; parasite; contagion; seed dispersal; Psidium guayaba; Antidaphne viscoidea; Phoradendron undulatum

PAGE 2

MISTLETOES ARE SHRUB BY ANGIOSPERMS that are parasites of plants. Nearly 1400 species of mistletoe occur in a variety of habitats on every continent except Antarctica, with most in the tropics (Reid et al. 1995.) The ecology of mistletoe is unique in fruit and seed anatomy, seed dispersal, and seed germination and development. The fruit contains a seed, surrounded by a sticky mesocarp and a tough epicarp. Specialist avian dispersers are attracted to the pulp and deposit the still sticky seed on the branch once it passes through the gut or is regurgitated. Often, only one species of bird will consume and disperse a particular mistletoe species' fruit. The sticky mix of mesocarp and seeds e nsures that the seeds will remain on a host stem (Reid et al. 1995.) With such co evolution between mistletoe species and their dispersers, the question arises as to how well the respective bird actually disperses the mistletoe's seeds. In the Mediterranea n ecosystem of Chile, it was shown that previously parasitized host plants of the mistletoe Tristerix aphyllum receive seeds much more frequently than expected by their relative abunda nce (Martinez del Rio et al. 1996 ). This is functionally equivalent to t he contagion that occurs when the abundance of a pathogen increases rapidly within a host. Though this mistletoe is further specialized in living within cacti (hence it is an internal parasite), I suggest that its contagious dispersal also occurs within ex ternally parasitic mistletoe species. The purpose of this study was to determine if Antidaphne viscoidea (Santalaceae) or Phoradendron undulatum (Viscaceae) demonstrated contagious dispersion in the preferred host tree Psidium guayaba (Myrtaceae.) METHODS I conducted my study i n a pasture on the farm of José Luis Argue das in Cañitas de Santa Elena, Costa Rica. The site was selected due to its relative abundance of P. guayaba and the presence of two mistletoe species: A. viscoidea and P. undulatum. I randomly selected seventeen P. guayaba trees in the isolated pasture and counted the number of individuals of both species of mistletoe in the following categories: fruiting individuals, non fruiting individuals (individuals >10 cm) and seedlings (indivi duals <10 cm). I also counted the total number of seeds for P. undulatum, as A. viscoidea had not yet flowered. I concluded my data collection with measuring nearest neighbor distances (trunk to trunk) and crown diameters along the cardinal directions for the P. guayaba trees. STUDY SITE The study was conducted on the farm of Jose Luis Argue das in Cañitas, approximately 10km northeast of the Monteverde Biological Station. The site is at an elevation of 1325 m and was isolated from surrounding farms by large windbreaks and secondary growth forest to the north and west. I observed that P. undulatum fruited from October 22 nd to November 10 th and A. viscoidea was observed to begin fruiting on November 14 th . This study was conducted from October 28 th to November 13 th 2003. RESULTS At the study site, there was a small grove of trees with no mistletoe. The collected data were interpreted using simple regressions. The numbers of P. undulatum seeds, seedlings, and non fruiting individuals in a P. guayaba tree are highly correlated with the number of fruiting

PAGE 3

individuals in the same tree (p <0.0001 for each of the three tests, Figures 1,2,3). Similarly, the number of A. viscoidea seedlings in a P. guayaba tree has a high positive correlation with the number of adults in the same tree (p=0.0003, Figure 4.) This shows that trees with a greater number of fruiting P. undulatum also have a greater number of non fruiting individuals, seedling, and seeds. Furtherm ore, the number of seeds, seedlings, non fruiting or fruiting individuals can be used predict the expected numbers for all other life stages. On average, 12% of P. undulatum individuals per tree were fruiting while 30% were non fruiting and 57% were seedli ngs. I found that there is no correlation between the crown diameters of P. guayaba and the number of fruiting P. undulatum within it (p=0.4926, Figure 5.) Nor is there a correlation between nearest neighbor distance and the total number of mistletoe within the tree (p=0.1886); there were isolated trees with lots of mistletoe or none. The trees with mistletoe had a total number of individuals (for both species) ranging from 2 to 165. B ecause A. viscoidea was not fruiting at the time of this study, I could not differentiate the difference between fruiting and non fruiting individuals. Instead, because there was a large relative difference in size, I could differentiate and count the numb er of adults and seedlings. Interestingly, also highly correlated are the numbers of fruiting P. undulatum and adult A. viscoidea individuals (p<0.0001, Figure 6.) While parasitized trees usually had large numbers of P. undulatum, the numbers of A. viscoidea were proportionately less; the numbers of one species could predict the numbers of the other. DISCUSSION The lack of correlation between the tree crown diameter and the number of mistletoe individuals within rules out the possibility that the number of mistletoe was age dependent. Using crown diameter as a judge of age, it shows that older trees do not necessarily have greater number of mistletoe. Therefore, the trends found between the numbers of seeds, seedlings, non fruiting and fruiting adu lts are likely to be dependent on the feeding behavior of the disperser; the avian seed dispersers act as disease vectors for contagion of mistletoe. Mistletoe has a unique seed dispersal mechanism that has co evolved with its disperser. The dispersion of mistletoe is dependent on the abundance of a host plant. This is further complicated because the avian dispersers must spread the seeds to those host plants for the seeds to properly germinate and grow. This study demonstrates that there is contagious dispersion of P. undulatum and A. viscoidea seeds within P. guayaba. A parent plant therefore disperses the majority of its seeds within its own host plant. By simplifying the dispersion to mostly spreading within the same tree, the mistletoe is assured a suitable host plant. Though this dispersal method ensures a suitable host plant for seed germination, it can cause genetic bottlenecking by means of the founder effect (Reid et al. 1995.) When a parent plant colonizes an area with its own off spring, pollinator behavior tends to cause inbreeding amongst the individuals. Inbreeding depression causes loss of heterozygous alleles and, over time, the loss of genetic variability probably reduces the species ability to evolve to biotic and abiotic ch anges in their environment (Fisher 1930.) In addition to inbreeding, contagion causes mistletoe populations on a tree to rise rapidly. This rise in population can either cause increased competition amongst the mistletoe individuals or provide a mutualistic benefit by attracting avian dispersers with more seeds. Future studies can address this question. Eventually, however, the population can get so high that it kills the

PAGE 4

host plant along with all the parasites on it. I observed several tr ees in the pasture that had died, leaving only dry branches and much dead mistletoe. So why did these mistletoe species evolve to have such a dispersal system? Perhaps because of the quick colonization of P. guayaba in open pastures there are always new tr ees to infect. Contrarily, perhaps the scarcity of P. guayaba, large enough to support a parasite, causes pressures to disperse seeds within the same host plant. There is even the possibility that even though the majority of seeds remain within the same tr ee, occasionally a nearby tree will be colonized and contagion begun again. Further studies need to be conducted to learn the benefits of contagion for mistletoe and how it counters the genetic variability loss, competition increase and density dependent m ortality it causes. ACKNOWLEDGEMENTS I thank Amit and Sumati Patel for their generous funding of my research through CIEE, and Alan and Karen Masters for whom without this project would not have been po ssible. I thank Jose Luis Argue das for use of his farm and his family for their generosity while staying in their home. I thank Joel Patrick Napier Stonedale for helping measure nearest neighbor distances in the field, Carmen Rohas for translating the Abstract, and Matt Gasner for reading this paper. _______________________________________________________________________________________________ LITERATURE CITED FI SHER , R. A. (1930). The genetical theory of natu ral selection. Clarendon Press, Oxford, UK. HOWE, H.F., SMALLWOOD, J. 1982. Ecology of seed dispersal. Ann Rev. Ecol. Syst. pp. 201 228. Martínez DEL Rio, C., Silva, A., Medel, R., Hourdequin, M. 1996. Seed dispensers as disease vectors: Bird transmission of mistletoe seeds to plant hosts. Ecology 77. pp. 912. REID, N. 1991. Co evolution of mistletoes and frugivorous birds? Aust. J. Ecol. pp. 457 469. REI D, N., SMITH, M. S., AND YEN, Z. 1995. Ecology and Population Biology of Mistletoes. In: Forest Canopies, M. D. Lowman and N. M. Nadkarni, ed. Academic Press, San Diego, CA, pp. 285 310.


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