ECOLOGICAL DIFFERENTIATION IN SOME CONGENERIC SPECIES OF COSTA RICAN FLOWERING PLANTS' WILLIAM C. BURGER2 Initial work for a new flora of Costa Rica disclosed a number of difficulttaxonomic problems in the Piperaceae, Chloranthaceae, and Moraceae families.Herbarium studies indicated that there were very closely related taxa in a numberof genera. These closely related taxa could be interpreted either as species com-plexes or as single variable species. Field work in Costa Rica showed that manyof these closely related taxa do not grow together. They may grow in the samegeneral geographic area, but these closely related populations are usually foundin different habitats or at different altitudinal levels. These observations havebeen very important in making taxonomic decisions, but they may also be im-portant in recognizing processes of population differentiation and speciation inthe wet tropics. Delimitation of taxa in the neotropics is often based on rather few herbariumcollections with little biological or ecological data. Such is the case in thisstudy where estimates of affinity are based primarily on similarity or dissimilarityin vegetative and floral characters. Palynological, cytological, or biochemicaldata are not presently available for these species. The data are based on plants collected in Costa Rica and the adjacentprovinces of western Panama. While this may seem to be a very small areafrom which to make general speculations, it is rather well sampled when com-pared to other wet tropical areas. Not only does the area of Costa Rica andwesternmost Panama have the benefits of decades of botanical exploration, butit also represents an area of isolated highlands with considerable endemism.This area is a minor but natural phytogeographic region, though its lowlandspecies are often widespread. Despite the small area there is a great altitudinal range (0-3800 m) andthe patterns of rainfall are very different in different parts of the region. Thedeciduous forest formations of the northem Pacific lowlands can have less than20 mm of rainfall during the dry season (December through April), while onthe Caribbean side of the mountains, as little as 30 km away, the rainfallaverages over 50 mm in the dryest month of the year. However, the rainfalldata alone can be misleading, especially at higher elevations. Turrialba on theCaribbean slope has an average annual rainfall of around 2400 mm, not muchgreater than some areas on the Pacific slope, such as Puriscal. The dry seasonon the Pacific slope, however, is much more severe and lacks the frequentcloudiness and misting of Turrialba and the Caribbean slope. These seasonaldifferences in cloud-cover and in rainfall affect the vegetation, and one can 1 This work was done in conjunction with preparation of a new flora for Costa Rica whichhas been supported in part by National Science Foundation Grants GB-3106, GB-7300, andGB-28446. 2 Department of Botany, Field Museum of Natural History, Roosevelt Road at Lake ShoreDrive, Chicago, Illinois 60605.ANN. MissoURI BOT. GARD. 61: 297-306. 1974.