Food webs in phytotelmata: "bottom-up" and "top-down" explanations for community structure.

The field study of food webs and the processes maintaining them is hampered by the sheer complexity and unreplicated nature of natural systems. The animal communities in phytotelmata--plant-held waters--are a convenient exception to this generalization. Tree holes, bamboo internodes, pitcher plants, tank bromeliads, and water-retaining plant axils contain a rich fauna, principally of arthropods, which constitute more or less complex, highly discrete food webs. They are widespread and replicated. The explanations for the community structure observed in these systems may call on "bottom-up" mechanisms such as simple environmental limitations, competition, predation, and facilitation, or they may adduce grander "top-down" theories, which explore biogeographic, energetic, dynamic, or biodiversity-related constraints. The existence of the bottom-up mechanisms is well established in experimental systems, and their consequences may be apparent in naturally occurring food webs. Top-down mechanisms demand a more holistic approach and are more difficult to test either by pattern analysis or experimental manipulation. The synoptic explanation of community composition and structure demands a multidimensional approach best expressed as a heuristic "template." Phytotelmata represent nearly ideal natural instruments for further study of food web dynamics, and exciting opportunities exist for the development and testing of community theories through their manipulation.