Adult Body Mass and Annual Production/Biomass Relationships of Field Populations

Abstract

We investigate specific production rates (per unit biomass) of populations using published data on the relation of annual production/mean biomass (P/B). Aquatic and terrestrial invertebrates between the sizes of copepods and clams are emphasized, ranging about 10 5—fold in body mass upon reaching maturity (M s, in kcal to compare with respiratory energy expense) and about 10 2—fold in P/B. Fishes and mammals are briefly treated; phytoplankton is mentioned. For 33 invertebrates living at annual mean temperatures between about 5° and 20°C, M s is shown to be an efficient and precise estimator, or scaling factor, of the annual P/B. The rate declines markedly with M s according to P/B = 0.65M s—0.37. The exponent differs significantly from the —0.25 power of comparative physiology. Most of the measured values of P/B fall within 50 to 200% of predicted values. Much of this variability is associated with the ratio of annual production/annual respiration (P/R): for a given M s, species achieving about half the predicted P/B have P/R ratios of about 0.1; those achieving twice the predicted P/B have P/R ratios of about 1.0. Age upon reaching maturity contributes some variability, with late—maturing (>1 yr) species tending towards a higher P/B. The variability is not significantly correlated with phylogenetic relationships (excepting insects for which P/B might not be mass—dependent), trophic type, major habitat, production rate, or biomass of the populations. The values of P/B of invertebrates living at annual mean temperatures >25° may be elevated over those of temperate species of the same M s, while those of polar forms are depressed. The reasons for a single power function governing the mass dependence of P/B of temperate invertebrates, and for the particular exponent, are unclear; an ecological cause, i.e., mortality, combining with the general size dependence of life processes, is implicated. On the average, the annual specific mortality rate equals P/B and hence also declines by 0.65M s—0.37. Very small metazoans (pelagic rotifers, benthic meiofauna) tend to have an appreciably lower P/B than indicated by the relationship for larger invertebrates. A refuge from predation by being small is postulated which may also apply for phytoplankton. For meiofauna, a power function of mass dependence of P/B with average rates 3—5 times below those of the larger invertebrates is suggested. Annual P/B values of fishes and mammals likewise decline by a power function of M s; the few available data yield exponents of —0.26 and —0.33, respectively. Ecological reasons are again invoked. Values of P/B and the specific mortality rates of temperate fishes seem to be 4—5 times, and those of mammals 20—25 times, higher than those of temperate invertebrates of the same mass.