At their most simple, body-size patterns depend on phylogenetic and evolutionary constraints, on energetic constraints, and on interactions with the habitat structure and with co-occurring species.
As regards phylogenetic and evolutionary constraints, body-size patterns are in some way dependent on existing biodiversity and its evolutionary basis. Each taxon performs best under a fixed set of conditions and has bioengineering constraints on its performance. For example, insects cannot be too large and birds cannot be too small; therefore, although both of them can take advantage of a 3D space, the complete spectrum from insects to birds has 'dome-like' distributions which incorporate the bioengineering constraints of the two groups of species.
As regards energy constraints, metabolic theory gives a general explanation of body-size patterns in terms of energy and temperature constraints on metabolism and the intrinsic properties of energy partitioning. Metabolic theory sets the theoretical expectations of body-size patterns, under the assumption that they are basically driven by simple energy constraints
As regards interactions, clearly populations interact with their environment and with co-occurring species. 'Textural habitat architecture' and 'body-size-mediated coexistence' hypotheses have been proposed to explain the abiotic and biotic components of interaction regarding its influence on the observed body-size patterns.
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