"How Communities Evolve"
This work presents an overview of our empirical and theoretical research on the trophic organization of biological communities (Mendoza et al., 2004). Some regularities are observed in the analysis of the relationship between the trophic structure (i.e., the distribution of species among feeding groups) in a number of African large mammal communities and the type of ecosystem. Different ecosystem types (e.g., arid communities with sparse vegetation, wooded savannas and evergreen forests) are characterized by specific patterns in the trophic structure of their mammalian communities. Those communities from the same type of ecosystem occupy a specific region of the n-dimensional trophic space defined by the number of species of n feeding groups (i.e., grazers, mixed-feeders, browsers and frugivores among herbivores; flesh-eaters, omnivores and bone crackers among carnivores). In order to explain the origin of these patterns, we propose a theoretical model based on the distribution of energy flows through a three trophic level food chain, with a topological organization based on observed mammalian communities. The main aim of this study is to show that it is possible to obtain a fully dynamic explanation of those patterns. The model is shown to spontaneously define different types of macroscopic structures in community organization, closely related to those observed in mammalian communities. In summary, we suggest that communities are unitary structures with coherent properties that result from the self-organizing dynamic of the whole system. The initial flux of energy into the system is partitioned among different classes of primary producers, on which herbivores with distinct feeding preferences depend, followed by carnivores that again fall into distinct classes according to prey size. The dynamic properties of the whole community reflect constraints on the trophic organization of the system, resulting in stable points that can be grouped in different types of organization despite continuous environmental gradients that determine overall energy flux through the system. This type of dynamic structure fits observed ecosystem organization, and provides a basis for identifying communities as self-organizing units that undergo evolutionary change. The proposed model constitutes a theoretical base for explaining the correlation between different environmental factors and the abundance or diversity of herbivores (Olff et al., 2002). In addition, this model establishes a general mechanism that makes it possible to understand how and why some rules constrain the assembly of the communities (Fox, 1999). In short, the model proposed leads us to see how biological communities can operate in an integrated way, which allows for the acceptance of their changes on large time-scales as evolutionary (Eldredge, 1996). The results of the simulation, moreover, support the view that the discrete nature observed in the structure of mammalian communities arises as a result of bifurcations in parameter space. References Eldredge, N., 1996. Hierarchies in Macroevolution In: Evolutionary Paleobiology (Jablonski, D. Douglas, H. E. and Lipps, J.H. eds.) 3: 43-61. Chicago University Press; Chicago and London. Fox, B.J., 1999. The genesis and development of guild assembly rules. In: Ecological assembly rules: Perspectives, advances, retreats. (Weither, E. & Keddy, P. eds.) 1: 23-57. Cambridge University Press. Mendoza, M., Goodwin, B., Criado, C. Emergence of Community Structure in Terrestrial Mammal-Dominated Ecosystems. Journal of Theoretical Biology (In press). Olff, H.; Ritchie, M.E. and Prins, H.H.T, 2002. Global environmental controls of diversity in large herbivores. Nature 415: 901-904.