A critical evaluation and experimental test of the conservative assembly hypothesis

There are two basic approaches to community ecology. The first examines the role that processes occurring in situ have on maintaining structure. The second, on the other hand, explores the "rules" responsible for creating structure, i.e. assembly mechanics. Recently, an assembly rule, the conservative assembly rule (Sugihara, 1982), was proposed to explain certain topological regularities in community structure. The rule states that species invading a community will invade more readily when they overlap competitively with single guilds rather than multiple guilds. As a test of this hypothesis, two sets experimental microcosm communities were invaded with an identical species set, which included Hydra littoralis, Cyclops vernalis, Moina sp., and Cypris sp. However, these microcosms received the species with two different invasion orders, one conservative, one non-conservative. The conservative assembly hypothesis would predict that species invading non-conservatively should be less persistent. The results of this experiment indicate that, in this case, conservatively invading species were no more persistent than those invading non-conservatively. Apparently, the impact of predation in these systems was so strong that the competitive effects upon which conservative assembly is dependent were overridden. These results should be viewed as a thought provoking counter example which will help us to refine the conservative assembly concept, rather than an outright refutation of the hypothesis.