Morphometrics and ecological isolation in sympatric Spiranthes (Orchidaceae) in southwestern Ontario

Abstract

To determine the extent to which three Spiranthes taxa are distinct and the extent to which they are isolated in a region of close sympatry, morphological and ecological data were collected for each of 251 flowering plants selected from 22 stations in southwestern Ontario. The morphological data, consisting of 36 quantitative characters per plant, were subjected to statistical analysis to allow an objective classification into groups and the ecological data were subsequently evaluated in terms of these groups. Three morphological groups obtained by principal-component analysis corresponded to three subjectively determined flower-color categories and are referable to S. cernua, S. magnicamporum, and S. ochroleuca. Clustering of distance coefficients followed by discriminant analysis of major divisions also produced the same three morphological groups, but the separation was more obvious. Possible hybrids involving S. cernua, with either of the other taxa, were infrequent (11 of 251). Further discriminant analysis using population samples from the region of sympatry along with topotype population samples of two of the taxa revealed no character convergence, but some character displacement was suggested. Interpopulation variation was least in plants referable to the agamospermic S. cernua and maximal in the group referable to the sexual S. ochroleuca. The best characters for separating the three groups in the region of sympatry are the same characters employed over a broad geographical area. Spiranthes ochroleuca and S. magnicamporum were shown to be isolated by soil pH. Although S. cernua has a broad pH tolerance, it was found to be isolated from S. ochroleuca and S. magnicamporum by its earlier flowering time and preference for moist sites. Ecological discreteness of the three Spiranthes was further demonstrated by coordinate analysis of distance and Jaccard's coefficients for various plant associates. Other potential isolating mechanisms which may also contribute to the three distinctive morphologies are discussed. Taxonomic recognition at the species level is supported.