Flower Structural Complexity and Microhabitat Separation Among Resident Insects on Old-field Flowers

Abstract

Five species of old-field flowers were studied with respect to their resident insect fauna. Insect assemblages consisted of 2-3 species for a total of eight different species. Although most species of insects were clumped with respect to their distribution among flowers, it was found that intensity of clumping decreased with increased density. Additionally, variability of insect body length increases with increasing density. Both relationships may serve to reduce the frequency of intraspecific encounters. Interspecific separation involved microhabitat partitioning within flowers rather than negative association among flowers. Size differences between species and total insect density were related to the complexity of floral structure. In view of the observed relationships, it appears that flower structural complexity is important in determining the total density of resident floral insects, and in setting morphological limits as to how similar coexisting species may be on a given species of flower. Introduction For quite some time now ecologists have been concerned with elucidating competitive interactions from a wide variety of perspectives. The classic works of Gause (1934) and Park (1954) and the more recent laboratory studies of Istock (1966), Vandermeer (1969) and Culver (1973) involved analyses of numerical changes resulting from interspecific competition. Field studies, both observational and experimental (Connell, 1961; McClure and Price, 1975; Branch, 1976), lend evidence that such competitive interactions are real phenomena of nature as well, and that the identification of limiting factors is feasible. An alternate approach to the study of competition and resource partitioning concerns the theoretical limit to how similar species can be in their use of a shared resource (MacArthur and Levins, 1967; MacArthur, 1969, 1972) and the inference of this limit as drawn from morphological data. Size, weight or other morphological characters representative of resource utilization along one or more important niche dimensions are most often used (Hutchinson, 1959; Schoener, 1965; Diamond, 1973; Brown and Lieberman, 1973; Pulliam, 1975; Davidson, 1977; De Vita, 1978). The uses and inferences from such data are reviewed by Hespenheide (1973). This report considers interspecific and intraspecific separation by microhabitat within old-field insect assemblages residing on flowers and consisting of 2-3 species. With a consideration of observed morphological and numerical data, an attempt is made to identify the factors determining insect density and distribution among five species of flowers. Methods The study area was situated in the southwestern region of Ohio, comprised largely of farmland and scattered forest remnants. However, a small proportion of the farms remained unplanted and thus were in various stages of old-field succession. During the spring and summer of 1976^ a search was made of old-fields which supported relatively large herb populations in bloom. Twenty-four flower species were perused for their resident insect assemblages (transient insects such as flies and bees were not considered), and those flowers with at least two abundant insect species were 1 Present address: Department of Biological Sciences, University of Southern California, Los Angeles, California 90007.