Showing papers by "Brian J. Enquist published in 2005"

Plant allometry, stoichiometry and the temperature-dependence of primary productivity

Abstract: Aim While physical constraints influence terrestrial primary productivity, the extent to which geographical variation in productivity is influenced by physiological adaptations and changes in vegetation structure is unclear. Further, quantifying the effect of variability in species traits on ecosystems remains a critical research challenge. Here, we take a macroecological approach and ask if variation in the stoichiometric traits (C: N: P ratios) of plants and primary productivity across global-scale temperature gradients is consistent with a scaling model that integrates recent insights from the theories of metabolic scaling and ecological stoichiometry. Location This study is global in scope, encompassing a wide variety of terrestrial plant communities. Methods We first develop a scaling model that incorporates potentially adaptive variation in leaf and whole-plant nutrient content, kinetic aspects of photosynthesis and plant respiration, and the allometry of biomass partitioning and allocation. We then examine extensive data sets concerning the stoichiometry and productivity of diverse plant communities in light of the model. Results Across diverse ecosystems, both foliar stoichiometry (N : P) and ‘nitrogen productivity’ (which depends on both community size structure and plant nutrient content) vary systematically across global scale temperature gradients. Primary productivity shows no relationship to temperature. Main conclusions The model predicts that the observed patterns of variation in plant stoichiometry and nutrient productivity may offset the temperature dependence of primary production expected from the kinetics of photosynthesis alone. Our approach provides a quantitative framework for treating potentially adaptive functional variation across communities as a continuum and may thus inform studies of global change. More generally, our approach represents one of the first explicit combinations of ecological stoichiometry and metabolic scaling theories in the analysis of macroecological patterns.

Yes, West, Brown and Enquist"s model of allometric scaling is both mathematically correct and biologically relevant

Abstract: The WBE theory shows how the quarter-power scalings of metabolic rate and many other biological attributes have their origin in the fractal-like designs of resource distribution networks. These designs are based on three simple principles: (1) a space-filling network that branches hierarchically to supply all parts of the three-dimensional body; (2) body-size invariant terminal units, such as capillaries or leaf petioles; and (3) minimization of the energy and time required to distribute resources. The WBE model of the mammalian cardiovascular systems additionally shows quantitatively and realistically how the scalings of the structure and hydrodynamics solve the problem of distributing blood from a beating heart through elastic hierarchically branching arteries to body-size invariant capillaries. The model correctly predicts not only the scaling parameters and absolute values of many characteristics of mammalian cardiovascular systems that have been measured by biomedical researchers (see Table 1 in WBE), but also the values in the hypothetical numerical example proposed by K & K (see Table 1, below). By applying the fundamental principles listed above to other resource supply networks in different taxa of organisms, the WBE model explains the origin of the ubiquitous quarter-power scaling exponents that have puzzled biologists since the 1930s (e.g. Kleiber 1932; Peters 1983; McMahon & Bonner 1983; Calder 1984: Schmidt-Nielsen 1984).

Allometric growth, life-history invariants and population energetics

Abstract: Population and community level processes must be at least partially determined by variation in the body sizes of constituent individuals, implying quantitative scaling relations can be extended to account for variation in those processes. Here we integrate allometric growth and life-history invariant theories, and use this approach to develop theory describing the energetics of stationary populations. Our predictions approximate, with no free parameters, the scaling of production/biomass and assimilation/biomass ratios in mammalian populations and work partially for fish populations. This approach appears to be a promising direction and suggests the need for further development of the growth and life-history models, and extensions of those theories.

Organismal size, metabolism and the evolution of complexity in metazoans

Abstract: Questions: What is the macroevolutionary relationship between body size, number of cell types and metabolism? Furthermore, why does the relationship between body size and the number of cell types hold between major metazoan clades but not within closely related taxa? Mathematical methods: Expand the allometric relationship between size and metabolism to include (1) the energetic costs of supporting an increased number of cell types and (2) the phylogenetic constraints governing the number of cell types Key assumptions: An increase in organismal size selects for additional cell types This is due to biophysical constraints and transport demands The increase in cell types allows the organism to perform new functions The extra cell types also require more intercellular networks Therefore, the amount of energy required per unit of body mass should increase with the number of cell types Phylogeny may also constrain the number of cell types within taxa This constraint will limit the number of cell types to be approximately constant within a bauplan (a unique organismal form comprised of an anatomical and physiological design) Predictions: Organismal size should be positively correlated to the number of cell types across metazoan taxa However, this relationship will not hold within clades due to energetic and phylogenetic constraints The energetic constraint leads to a positive correlation between the number of cell types and metabolic intensity (the mass-specific rate of energy processing standardized to a given body size) across metazoan bauplans Available data support these predictions Metabolic intensity is positively related to the number of cell types in metazoan clades

PERSPECTIVES Allometric growth, life-history invariants and population energetics

Abstract: Population and community level processes must be at least partially determined by variation in the body sizes of constituent individuals, implying quantitative scaling relations can be extended to account for variation in those processes. Here we integrate allometric growth and life-history invariant theories, and use this approach to develop theory describing the energetics of stationary populations. Our predictions approximate, with no free parameters, the scaling of production/biomass and assimilation/biomass ratios in mammalian populations and work partially for fish populations. This approach appears to be a promising direction and suggests the need for further development of the growth and life-history models, and extensions of those theories.