Species interactions within and between the inquiline larval fauna of Sarracenia purpurea L.: competition or commensalism?

Abstract

Species interactions such as commensalism and competition greatly affect community structure and composition. The small and diverse communities present in the northern pitcher-plant Sarracenia purpurea L. (Sarraceniaceae) are ideal for the study of species interactions. This study investigated the relationships within and between the inquiline populations of the mosquito Wyeomyia smithii and the midge Metriocnemus knabi contained within the leaves of S. purpurea. Samples of pitcher-plant leaf contents were taken from Waldron Fen, Emmet County, Michigan. The larvae present in each sample were identified, counted and measured. Average densities and larval lengths per unit volume were calculated by species type and dipteran association (i.e., cohabitation with other fly species). The data indicated that both interspecific competition and intraspecific competition may be present in the larval populations of Metriocnemus knabi and Wyeomyia smithii. The co-habitation of M. knabi and W. smithii resulted in reduced M. knabi mean larval length per unit volume when compared with the mean larval length per unit volume of M. knabi found alone, indicating the presence of interspecific competition (1.35 +- 0.15 vs. 2.64 +- 0.40, U=747.5, p<0.05, n=59). The singly-occurring average length of W. smithii decreased with increasing density, indicating the presence of intraspecific competition within the population of W. smithii (Spearman's = -0.302, p<0.05).