The Pattern of Local Web Distribution in a Desert Spider: Mechanisms and Seasonal Variation

Abstract

Organization is basic to all biological systems. It can be considered as the non-random arrangement of matter, and, according to Pittendrigh (1958), differs from mere order in that it is relative and end-directed (i.e. organized with respect to something). Organization occurs at all levels of biology, from molecular and cellular structure in the individual organism to functional relationships in the ecosystem. The organization of a population through spatial arrangement of its individuals in the environment is termed pattern (Pielou 1969). Most definitions of pattern are based on the kind of mechanism determining its character. Hutchinson (1953) categorized pattern into five types on the basis of such determinant mechanisms. These include: (a) vectorial pattern, in which the noted distribution is influenced by external factors such as gradients of temperature and humidity; (b) reproductive pattern as contributed by the incomplete dispersal of offspring; (c) social pattern, delineated by signalling and communication; (d) coactive pattern, where interaction among species determines distribution; and (e) stochastic pattern, where distribution depends on random forces. In practice, it is often difficult to pigeonhole pattern into such groups, since, in a natural situation, one or more of these mechanisms may contribute to the pattern exhibited by an animal population. Delineation of the importance of various mechanisms in determining local pattern character is valuable, though, for the understanding it can provide of the function of pattern and the underlying adaptation it represents. The study of pattern and thus of the adaptation basic to pattern is not merely 'an optional preoccupation with fascinating fragments of natural history: it is the core of biological study' (Pittendrigh 1958). Adaptation suits individuals to their environment in such a way that reproductive efficiency is achieved and the genotype of the most efficient reproducer predominates. Such adaptations can appear in the pattern of individuals occupying a habitat and thus much can be learned from the study of local pattern. Previous study (Riechert, Reeder & Allen 1973) of the local distribution of the funnel web spider (Agelenopsis aperta (Gertsch)) in desert grassland and recent lava bed habitats in south-central New Mexico, distinguished the operation of both intrinsic and extrinsic determinants of pattern. Adult spiders evidenced both social pattern in a mean spacing of individuals and vectorial pattern in the association of their webs with specific habitat features. The results reported here concern the change of local pattern with age of individuals and season. The functioning of the pattern of local distribution for Agelenopsis is discussed in the light of this additional information.