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Spatial Distribution Patterns of Mistletoe (Antidaphne viscoidea: Eremolepidaceae, and Phoradendron undulatum: Viscaceae) on Psidium guajava (Mrytaceae) in Monteverde, Costa Rica
Digitized by MVI
Spatial distributions of mistletoe are noticeably clustered but unexamined in Monteverde, Costa Rica. To better
understand their spatial structure, I examined the dispersion of two species of epiphytic mistletoe (Antidaphne
viscoidea: Eremolepidaceae and Phoradendron undulatum: Viscaceae) within and between host trees. Six sites
on one hill in Caitas, Costa Rica were surveyed for data relating to dispersion structure (nearest neighbor
distance and frequency within host trees). Additional study was done on the spatial dispersion on the host tree
Psidium guajava, and it was found to have clustered dispersions. Indices of dispersion and tests comparing
observed results to Poisson distributions showed A. viscoidea and P. undulatum were distributed contagiously
within host trees and between host trees. I suggest this may be due to bird disperser foraging behavior.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone
Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology Fall 2003
Ecologa Tropical Otoo 2003
t Monteverde Institute : Tropical Ecology
Spatial Distribution Patterns of Mistletoe (Antidaphne viscoidea: Eremolepidaceae, and Phoradendron undulatum: Viscaceae) on Psidium guajava (Mrytaceae) in Monteverde, Costa Rica Matthew Greenfield Department of Biology, Whitman College A bstract Spatial distributions of mistletoe are noticeably clustered but unexamined in Monteverde, Costa Rica. To better understand their spatial structure, I examined the dispersion of two species of epiphytic mistletoe (Antidaphne viscoidea: Eremolepidaceae and P horadendron undulatum: Viscaceae) within and between host tre es. Six sites on one hill in Ca itas, Costa Rica were surveyed for data relating to dispersion structure (nearest neighbor distance and frequency within host trees). Additional study was done on the spatial dispersion on the host tree Psidium guajava, and it was found to have clustered dispersions. Indices of dispersion and tests comparing observed results to Poisson distributions showed A. viscoidea and P. undulatum were distributed contagiously within host trees and between host trees. I suggest this may be due to bird disperser foraging behavior. Resumen Las distribuciones espaciales de murdago se son arracimadas notablemente pero unexamined en Monteverde, Costa Rica. Entender mejor su estructura espacial, yo examine" la dispersin de dos especie de murdago de epiphytic (Antidaphne viscoidea: Eremolepidaceae y Phoradendron undulatum: Viscaceae) dentro de y entre rboles de anfitrin. Seis sitios en una colina en Ca itas, Costa Rica se inspeccio n para los datos que relacionan a la estructura de la dispersi n (ms cercana distancia de vecino y frecuencia dentro de rboles de anfitrin). El estudio adicional se hizo en la dispersin espacial en el Psidium guajava de rbol de anfitrin, y se encontr para se haber arracimado dispersiones. Los ndices de dispersin y pruebas que comparan los resultados observados a distribuciones de Poisson mostraron A. viscoidea y P. undulatum se distribuy contagio samente dentro de rboles de anfitrin y entre arboles 'de anfitrin. Sugiero que esto puede estar debido a disperser de pjaro que adentra la conducta. Introduction The distribution and dispersion of parasitic mistletoe populations are influenced by the dispersion of a host species population (Donohue 1995). Within the distribution of a population, dispersion characterizes the spacing of individuals with respect to on e another, forming patterns that vary from
tight aggregation in discrete clumps to even, or regular, distribution (Ricklefs 1990). The distribution (and thus dispersion) of the mistletoes Antidaphne viscoidea, and Phoradendron undulatum are limited to bran ches of the host plant Psidium guajava (Sargent 1995, Willoughby 1995). To study the spatial distribution of the mistletoe species, it is necessary to conduct a concurrent study on the spatial distribution of its host tree. The dispersal of parasitic organisms is similar to seed dispersal of non parasitic plants within a diverse forest, but all sites are located on host organisms. Just as non parasitic seeds must try to find an available germination site on the ground, Antidaphne viscoidea and Phoradendron undulatum try to disperse their seeds to suitable sites on host trees to germinate and establish as parasitic organisms. Antidaphne viscoidea and Phoradendron undulatum are host specific, and able to establish on only a few trees, primarily Psidium guajava (Willoughby 1995). Continuing this comparison, mistletoe use birds of the genus Euphonia as a specialized disperser for their seeds (Sargent 2000), similar to plants using specialized avian and mammalian seed disperse rs. Distributions can be described by comparing frequencies of a population to a Poisson distribution. The Poisson distribution is used as a null model in biological studies, because it is considered to be a perfectly random distribution. A Poisson distrib ution has an equal mean and variance. If the variance is found to be less than the mean, the distribution is more even than a Poisson and best described as a regular distribution. In ecological studies it is more common to find a variance larger than the m ean, these distributions are considered contagious, and have the tendency of being more clustered than the Poisson distribution (Southwood 1966). The purpose of this study is to examine and describe mistletoe spatial dispersion patterns within and across h ost trees, and to find explanations for those patterns. I hypothesize that the dispersion of mistletoes within a tree and across hosts will both be clustered, and that clusters of Psidium guajava will as a group either have mistletoe or not T hese hypothes are based on my perception of the spatial patterning of P. guajava and on the foraging behavior of mistletoe dispersers. Materials and Methods This study took place near Santa Elena, Puntarenas, Costa Rica, between October 24 and November 14,2003 Measurements were taken from six sites and additional observations of mistletoe were taken from off site Psidium guajava trees. Each site consisted of a localized group of P. guajava containing at least six trees occupied by Phoradendron undulatum. The largest site encompassed an area of 4000 m 2 and the smallest occupied an area of 200 m 2 Psidium guajava is a small tree (less than ten meters tall) with reddish peeling bark. I noticed it thrives in disturbed habitats and may quickly expand from a single tree into a dense thicket of trees. Fruit eating birds have been observed eating the fruits, and are possible dispersion agents of mistletoe into P. guajava branches (personal observation). Antidaphne viscoidea (Eremolepidaceae) and Phoradendron undulatum (Viscaceae) are both mistletoe capable of photosynthesis. They graft themselves to the branches of their host
and share the water and minerals of the host xylem. Phoradendron undulatum has light green opposite leaves pointed at the tips that seem to be su cculent. Antidaphne viscoidea has darker and more rounded leaves than P. undulatum and tends to be woody. Both are dispersed by birds and establish themselves on the tips of branches (Sargent 1995). For each Psidium guajava a measurement of circumference (cm) at breast height was taken with a tape measure and then converted (divided by 3.14) to a diameter at breast height (DBH). For individuals with forks in the trunk below breast height, each section was measured and combine d into one circumference before converting to diameter. The distance from the trunk of a P. guajava to the trunk of its nearest neighbor (NND) was measured by either pacing distance by foot (14 strides = 10 meters), with a tape measure, or with a measuring pole (3 meter long, 3 centimeter diameter PVC pipe, with colored tape marking 20 cm and 50 cm intervals). The distance to the nearest P. guajava occupied by P. undulatum, and the distance to the nearest P. guajava unoccupied by P. undulatum were also coun ted and recorded for each tree (this is because P. undulatum was common throughout all sites, while A. viscoidea was common only in one site). The number of A. viscoidea and P. undulatum was recorded for each tree. To avoid counting un established seedlings, only mistletoe with at least two mature leaves were included in this study. The distance to the nearest neighbor of each mistletoe was measured with a measuring pole and recorded. Data were analyzed by comparing mistletoe and Psidium guajava nearest neighbor distance frequencies and the number of mistletoe per tree to Poisson distributions. Indices of dispersion were calculated using the mean and variance for each data set. Chi squa red tests were used to compare observed frequency distributions to theoretical Poisson distribution frequencies or other expected values. Results I sampled 278 Psidium guajava trees and found 163 Antidaphne viscoidea and 1073 Phoradendron undulatum. Chi sq uared tests against theoretical frequencies and the indices of dispersion (I D ) were used to test for goodness of fit to the Poisson distribution and to examine the possibility of a contagious distribution of mistletoe within hosts and/or between hosts. Dispersion of Host trees (Table 1) Nearest neighbor distances for the 278 Psidium guajava were organized into relationships based on the presence of Phoradendron undulatum. The frequencies constructed from these relationships were used to create indices of dispersion and tested against theoretical Poisson frequencies with chi squared tests. Any given P. guajava was in two of the three categories; every tree was included in the "any" tree category, trees with P. undulatum were included in the "occupied" cate gory; trees without P. undulatum were included in the "unoccupied" category. Each of the three categories had three frequency distributions created: one for the distance to the nearest "any" neighbor, one for the distance to the nearest "occupied" neighbor and one for the distance to the nearest "unoccupied" neighbor. Whether categories of trees generally had occupied or unoccupied trees as nearest
neighbors were also tested. A chi squared test showed occupied hosts tend to have other occupied hosts closer than unoccupied hosts ( x 2 = 40.96, df = 1, P < .001). Unoccupied hosts generally had an unoccupied tree as the nearest neighbor (x 2 = 38.44, df = 1, P < .001). A test from any Psidium guajava showed that although more trees have occupied hosts as nearest ne ighbor, it is not significant (x 2 = .36, df = 1, P = .555). From any given Psidium guajava tree the average distance was, 1.78 m 3.07 m to the nearest conspecific, 2.47 m 3.65 m to the nearest occupied host, and 5.6 m 7.5 m to the nearest unoccupied n eighbor. The distribution of distances to nearest conspecific tree had a variance of 9.4 and was found to be dispersed non randomly (X 2 = 10.641, df = 5, P = .059) (I D = 598.84, df = 113, P < .001). The distribution of distances to nearest occupied tree had a variance of 13.3 and was found to be dispersed non randomly (x 2 = 11.717, df = 6, P = .069) (I D = 511.77, df = 95, P< .001). The distribution of distances to nearest unoccupied tree had a variance of 56.28 and was found to be dispersed non randomly (x 2 = 117.4, df = 11, P < .001) ( I D =1116.35, df=111 P<.001). From any given occupied Psidium guajava tree the average distance was, 2.28 m 3.93 m to the nearest conspecific, 2.81 m 4.4 m to the near est occupied neighbor, and 8.97m 8.47 m to the nearest unoccupied host. The distribution of distances to nearest conspecific tree had a variance of 15.44 and was found to be dispersed non randomly (x 2 = 23.933, df = 6, P = .0005) (I D = 413.92, df = 61, P < .001). The distribution of distances to nearest occupie d tree had a variance of 19.39 and was found to be dispersed non randomly (x 2 = 28.76, df = 7, P = .0002) (I D = 427.59, df = 62, P < .001). The distribution of distances to nearest unoccupied tree had a variance of 71.77 and was found to be dispersed non ra ndomly (x 2 = 128.36, df = 16, P < .0001) (I D = 504.1, df = 63, P<. 001). From any given unoccupied Psidium guajava tre e the average distance was, .94 m .52 m t o the nearest conspecific, 1.82m 1.12 m to the nearest occupied neighbor, and 1.1 m .73 m to the nearest unoccupied host. The distribution of distances to nearest conspecific tree had a variance of .27 and seemed to be dispersed randomly (x 2 = 5.30, df = 2, P = .0704) (I D = 14.19, df = 49, P = 1). The distribution of distances to nearest o ccup ied tree had a variance of 1 .24 and did not fit the Poisson distribution but had a low index of dispersion compared to other distributions (x 2 = 17.486, df = 5, P = .0037) (I D = 21.92, df = 32, P = .975). The distribution of distances to nearest unoccupied tree had a variance of .537 and was found to be dispersed randomly (x 2 =2.856, df = 3, P = .4143) ( I D = 23.04.1, df = 47, P = 1). Dispersion of Antidaphne viscoidea (Table 2, Table 3, Figure 1 ) Antidaphne viscoidea had an average nearest neighbor distance of 87 cm (N = 63) and an average of 0.64 individuals per tree (N = 278). Nearest neighbor data suggest a non random and specifically contagious distribution for A. v iscoidea (x 2 = 1.25*10 34 df = 103, P < .001) ( I D = 51496, df = 162, P < .001). The data on number of A, viscoidea per tree also support a hypothesis of a non random and contagious distribution (X 2 = 64.407, df = 3, P < .001) ( I D = 5966, df = 277, P < .001). Dispersion of Phoradendron undulatum (Table 2, Table 3, Figure 1) Phoradendron undulatum had an average nearest neighbor distance of 15 cm (N = 1073) and an average of 4 individuals per tree (N = 278). Nearest neighbor data suggest a non random and
specifically contagious distribution for P. undulatum (x 2 = 1981891, df = 24, P < .001) ( I D = 106210, df =1072, P < .001). The data for number of P. undulatum per tree also suggest a non random and contagious distribution (x 2 = 3065, df = 9, P < .001) (I D = 10387, df = 277, P < .001). Discussion The dispersion of the host is key crucial to understanding the dispersion of a parasitic organism. What seems to be clustering in the population of a parasite could actually be a random distribution within a clustered distribution of hosts. I found Psidium guajava to be contagiously distributed. Comparing indices of dispersion for the distribution of all host trees (I D = 598.84, df = 113, P < .001) with just the distribution of occupied trees (I D = 427.59, df = 62, P < .001) shows that while both are clustered, the occupied trees are not more clustered than the population as a whole. But when the type of nearest neighbor is examined occupied trees tended to have other occupied trees as nearest neighbors (x 2 = 40.96, df = 1, P<.001). These data suggest mistletoe are clustered within a grove of P. guajava and do not extend throughout the entire grove, as then some groves would be occupied and the occupied trees would have a higher level of clustering. I hypothe sized groves of P. guajava would either be occupied or unoccupied. There seems to be some limits on the dispersal of seeds preventing mistletoe from occupying complete groves. I observed mistletoe clustered in trees and found that occupied trees tended to be clustered in groups. By comparing the indices of dispersion of mistletoe (Table 2, Table 3) to the indices of dispersion for Psidium guajava (Table 1) it is obvious the mistletoe are more clustered, the indices for mistletoe are larger by a factor of te n. Further comparison of the indices of dispersion shows that nearest neighbor distances are greater than the number of mistletoe per tree by almost a factor of ten for both Antidaphne viscoidea (106210 and 10387) and Phoradendron undulatum (51496 and 5966 ). This shows that the mistletoe are clustered on a very local scale within a tree, and only in a few trees. Within Psidium guajava trees it was common to find clusters of three, four, five or even eight mistletoe growing only centimeters apart. It seems doubtful that seeds are repeatedly dispersed to the same place. More likely all mistletoe in a single cluster were dispersed at the same time. Contagious distributions must be due to a multit ude of factors, but one I find particularly important is avian disperser behavior. Mistletoes have adapted their seeds to stick on a substrate before germination and establishment. Surrounding the mistletoe seed is a viscous endocarp surrounding the seed c alled viscin (Reid 1995). Literature suggests the viscin prevents birds from easily defecating or otherwise voiding the seed. The bird must use a wipe and sidestep motion to remove the seed, thus leaving the seed stuck on a branch (Sargent 2000). I observe d birds feeding on mistletoe, but the seeds were defecated quickly and easily without a wipe or sidestepping motion. I also observed seeds stuck under and on the sides of branches. These small clusters of seeds could be the beginnings of a future cluster o f mistletoe. Seeds fall with the bird dropping but instead of falling off the branch the viscin fixes them to a possible place for establishment. The behavior of
foraging birds and the placement of their droppings, leads me to believe mistletoe have very small seed shadows, with most seeds remaining within the tree. Part of my hypothesis was supported by my data. I found mistletoe tended to be clustered in groups and the groups were found in just a few trees in close proximity to each other. Trees in a cl uster were not occupied or unoccupied as a group; there were smaller clusters of mistletoe within clusters of Psidium guajava. Perhaps mistletoe colonize a cluster by radiating from a point of origin. Birds could be responsible for the clustering of mistle toe by eating many seeds and defecating them all at once. Bird dispersers may also move between neighboring trees while feeding and establish mistletoe on each neighboring host. Further research needs to be performed on the behavioral habits of bird disper sers of mistletoe, as it is unknown how clusters of mistletoe form. Another possibility is research to find factors that facilitate mistletoe establishment. Studies could be expanded to larger scales to discover if there are further patterns on a larger sc ale due to patterns of Psidium guajava dispersion and bird behavior. Acknowledgements First, I must thank my parents, because without their support I wouldn't be here. I would like to thank Jose Luis Argue das who was my source of sites. Also, Heidi Dobson and Bruce Hewlett deserve my thanks for providing a base of ecological knowledge, and encouraging me to study here in Costa Rica. Finally I want to thank Alan, Karen, Carmen, Matt and Andrew for helping me u nderstand the Poisson distribution, and for numerous other reasons.
Literature Cited Donohue, K. 1995. The Spatial Demography of Mistletoe Parasitism on a Yemeni Acacia. International Journal of Plant Science. 156(6):816 823. Reid, N., M. S. Smith, and Z. Yan. 1995. Ecology and Population Biology of Mistletoes. In: Lowman, M. D., and Nadkami, N. M.. Forest Canopies. Academic Press, San Diego. Pp. 285 306. Ricklefs.R. E 1990. Ecology. W. H. Freeman and Company, New York, p.282. Sargent, S. 1995. S eed Fate in a Tropical Mistletoe: The Importance of Host Twig Size. Functional Ecology. 9,197 204. -----. 2000. Mistletoes and Where They Grow. In: Monteverde Ecology and Conservation of a Tropical Cloud Forest. Lowman, M. D., and Nadkami, N. M.. Editors pp. 288 289. -----. 2000. Specialized Seed Dispersal: Mistletoes and Fruit eating Birds. In: Monteverde Ecology and Conservation of a Tropical Cloud Forest. Lowman, M. D., and Nadkami, N. M.. editors, pp. 81 82. Southwood, T. R. E., 1966. Ecological Methods. Uni versity Printing House, Cambridge. P. 27. Willoughby, S. 1995. Preferential Parasitism by Mistletoe Species on Different Host Trees. In: UCEAP Spring 1995 Tropical Biology, pp. 126 131.
______________________________________________________________________________________________ ____________ TABLE 1:One variable statistics and indices of dispersion for nearest neighbor distances, and chi squared values of nearest neighbor distance frequen cies compared to theoretical Poisson frequencies of Psidium guajava trees with special attention to if a tree is occupied or unoccupied in Santa Elena, Costa Rica. ____________________________________________________________________________________________ _______ _______ Relation Mean (m) Standard Deviation Variance Index of Dispersion DF P Value Chi Square Value DF P Value Any to Occupied 2.469 3.647 13.3 511.77 95 <.001 11.717 6 0.068 Any to Unoccupied 5.596 7.502 56.28 1116.35 111 <.001 117.397 11 < .000 Any to Any 1.775 3.067 9.41 598.84 113 <.001 10.641 5 0.059 Occupied to Occupied 2.811 4.403 19.39 427.59 62 <.001 28.755 7 0 .000 Occupied to Unoccupied 8.97 8.472 71.78 504.11 63 <.001 128.358 16 <.000 Occupied to Any 2.275 3.929 15.44 413.92 61 <.001 23.933 6 0 .000 Unoccupied to Occupied 1.815 1.115 1.24 21.92 32 0.975 17.486 5 0.003 Unoccupied to Unoccupied 1.096 0.733 0.54 23.04 47 1 2.856 3 0.414 Unoccupied to Any 0.941 0.522 0.27 14.19 49 1 5.306 2 0.07 0 __________________________________________________________________________________________________________ TABLE 2: One variable statistics and indices of dispersion for nearest neighbor distances, and chi squared values of nearest neighbor distance freque ncies compared to theoretical Poisson frequencies for Antidaphne Viscoidea and Phoradendron undulatum in Santa Elena, Costa Rica. __________________________________________________________________________________________________________ Standard Index of Chi squared Species Mean (cm) Deviation Variance Dispersion DF P Value Value DF P Value Antidaphne viscoidea 86.798 166.107 2.76*10^4 5.15*10^4 162 <.001 1.25*10^34 103 <.001 Phoredendron undulatum 14.586 38.015 1.45*10^3 1.06*10^5 1072 <.001 1.98*10^6 24 <.001 __________________________________________________________________________________________________________ TABLE 3: One variable statistics and indices of dispersion for number of mistletoe per host tree, and chi squared values of number of mistletoe frequencies compared to theoretical Poisson frequencies for Antidaphne Viscoidea and Phoradendron undulatum in Santa Elena, Costa Rica. __________________________________________________________________________________________________________ Standard Index of Chi squared Species Mean (cm) Deviation Variance Dispersion DF P Value Value DF P Value Antidaphne viscoidea 0.064 3.7 13.79 5.97*10^3 277 <.001 64.41 3 <.001 Phoredendron undulatum 4.011 12.3 150.41 1.03*10^4 277 <.001 3065.58 9 <.001 _______________________________________________________________________________________________________