The Pattern of Colonization of Epiphytes on Calabash Trees (Crescentia alata HBK.) in Guanacaste Province, Costa Rica

We examined the structure of the community of epiphytes growing on Crescentia alata trees in Santa Rosa National Park, Costa Rica. Four orchid species were dominant in this community, showing a distinct order of colonization of the calabash trees, with Oncidium cebolleta colonizing first, Encyclia cordigera second, Brassavola nodosa third, and Laelia rubescens fourth. We were unable to find any differentiation of the locations of these species based on the physical characteristics of the sites. It is hypothesized that the colonization pattern observed is due to the quantity of propagules produced by each species. THE NUMBER OF SPECIES found in a community is determined by five major factors-the time for colonization, the distance over which colonizers must travel, the number of potential colonizers, the size of the area to be colonized, and the biotic interactions between colonizers (MacArthur and Wilson 1967). Two approaches have been used to determine the effects of these factors upon the community organization of oceanic islands (e.g., Diamond 1969, Simberloff and Wilson 1969) and a variety of island-like situations (e.g., Vuilleumier 1970, Brown 1971, Culver, Holsinger, and Baroody 1973, Opler 1974). One approach applied to both plant (Johnson, Mason, and Raven 1965) and animal (Hamilton, Barth, and Rubinoff 1964, Wilson and Taylor 1967) communities has been to use correlative techniques to describe the patterns of species number and distribution from island to island on a large geographical scale. The second approach, applied exclusively to animal communities, has been to manipulate the five factors experimentally (e.g., Simberloff and Wilson 1969, Addicott 1974, Schoener 1974). In this paper we describe the community structure of plants occurring on a local scale using correlative techniques and suggest that this system is one in which experimental work on the relative roles of these five factors in determining community structures may be performed. STUDY AREA AND METHODS Individual Crescentia alata HBK, the calabash tree (Bignoniaceae), constitute an island-like situation for epiphytic plants. The calabash tree is an ideal epiphyte host because its soft, deep bark provides easy anchorage for epiphytes and recess for humus accumulations (Pittendrigh 1948). The study site, a 10 ha calabash "orchard" in Santa Rosa National Park, Guanacaste Province, Costa Rica, has been described by Janzen et al. (1980). It is surrounded by dry, deciduous thorn-forest, the dominant vegetation type in this region (Holdridge 1967). No individual calabash tree at this site is located more than 100 m from the edge of the thorn-forest. The spherical canopies of the calabash trees range from 0.5 to 7.5 m radius. Since trees are spaced at least 5 m apart, canopies of individuals rarely intertwine. Each calabash tree was considered an "island" which may be colonized by epiphytic plants: one cactus species, three bromeliads, and five species of orchids. Nomenclature is that of Janzen and Liesner ( 1980). We measured the canopy radius of a randomly chosen calabash tree and recorded the species of epiphytes present (sample sizes are indicated in figure 1). Canopy radius was used because it was both easily measured and accurate with repetition. The existence of a species-area relationship was investigated by means of regression analysis using a logarithmic curve fit after making a scatter plot of the data. To explore the temporal pattern of colonization, we calculated the percent occurrence of each epiphytic species in one-species, two-species, three-species, etc. communities. To study the role of biotic interactions, we contrasted the relative abundance of each epiphytic species on small trees (less than 2 m canopy radius) with that on large trees (greater than 4 m canopy radius). To obtain these numbers, at least two branches were randomly chosen on a minimum of five inBIOTROPICA 14(2): 137-14