Why are there so many kinds of planktonic consumers? The answer lies in the allometric diet breadth

12 pages, 3 figures, 2 tables
In an attempt to explain 'Why are there so many kinds of animals?' G. E. Hutchinson highlighted the food web context to suggest that diversity of primary producers should allow consumer richness to be maintained as a result of their adaptive foraging. Co-existence of consumers is then made possible when species differ in body size and thus only a minor diet overlap occurs. All these ideas are still major topics in ecological research and some have been re-examined in order to provide mechanistic explanations of species richness versus connectance relationships in food web structure. The effect of body size as a determinant of diet, jointly with the assumption that individuals are adapted to switch their diet in order to maximise energy gain, have been combined in recent years to develop the Allometric Diet Breadth Model (ADBM). This model, successful for plankton communities, enables us to determine the specific resource-consumer links and then evaluate the diet breadth and test whether the diet overlaps. Here, we apply the ADBM to infer the feeding linkages within a freshwater planktonic community of a Spanish oligo-mesotrophic lake and three spatial partitions of it. ADBM treats phytoplankton species and bacteria as resources and each consumer species (ciliates, rotifers and crustaceans) as both consumers and resources. We applied ADBM to water-column integrated- and single-layered plankton communities to test the importance of the diet on structuring the plankton. If a given pair of species that co-occur in the whole vertical community overlap their diet more than when they occur in the three layers separately, this means that they will never co-exist and are hence overdispersed (segregated). Not all species pairs that have a weak diet overlap when belonging to the whole water-column community co-exist in water-layered communities. Hence, the richer, whole water-column community would then have lower diet overlap than spatially segregated communities. Therefore, the hypothesis of diet breadth of Hutchinson (The American Naturalist 93: 145-159, 1959) explains community structure throughout the water column, and its deviations may be forced abiotically
The authors would like to thank the Spanish Ministry of the Environment and Ministry of the Science and Innovation for projects 81/2005, CGL2006-02891 and CGL2006-2346. This research was also supported by the Consejería de Innovación, Ciencia y Empresa of the Junta de Andalucía (Project P07-CVI-02598)