Robert H. Peters

Relationships between body size and some life history parameters

SummaryPatterns in life history phenomena may be demonstrated by examining wide ranges of body weight. Positive relationships exist between adult body size and the clutch size of poikilotherms, litter weight, neonate weight life span, maturation time and, for homeotherms at least, brood or gestation time. The complex of these factors reduces rmax in larger animals or, in more physiological terms, rmax is set by individual growth rate. Comparison of neonatal production with ingestion and assimilation suggests that larger mammals put proportionately less effort into reproduction. Declining parental investment and longer development times would result if neonatal weight is scaled allometrically to adult weight and neonatal growth rate to neonatal weight. Body size relations represent general ecological theries and therefore hold considerable promise in the development of predictive ecology.

LINKING PLANKTONIC BIOMASS AND METABOLISM TO NET GAS FLUXES IN NORTHERN TEMPERATE LAKES

Plankton communities in oligotrophic waters are characteristically dominated by the biomass of heterotrophs, including bacteria, micro-, and macrozooplankton. It has been generally assumed that these inverted biomass pyramids are the direct result of high specific production rates of phytoplankton and a tight coupling between producers and consumers. There are, however, at least two alternative hypotheses: (1) heterotrophic biomass turnover is much slower in oligotrophic than eutrophic systems; and (2) oligotrophic planktonic communities are significantly subsidized by allochthonous organic matter. In this study we assessed these hypotheses by establishing the relationship between plankton biomass structure (partition between auto- and heterotrophs), plankton function (plankton primary production and respiration) and whole-lake gas (O2 and CO2) fluxes in 20 temperate lakes that span a large range in primary production. We show that the balance of phytoplankton production and community respiration (P/R rat...

The effect of body size on animal abundance

SummaryAlthough it is a commonplace that small animals are more abundant than large ones, few attempts have been made to quantify this and none for non-mammalian species. This study uses estimates of animal density and body mass culled from 12 journals published between 1961 and 1978 to test and extend Damuths relationship between population density and body size of herbivorous mammals. In general, his analysis is supported, for density usually declines roughly as W-0.75 and poikilotherms maintain higher densities than homeotherms. However the residual variation is higher than Damuths regressions might suggest and significant differences exist among animal groups. In particular, birds maintain much lower, and aquatic invertebrates much higher abundances than a general curve for all species would suggest. Carnivores are significantly rarer than herbivores. These relationships may be used to compare the average relative contributions of species of different size to community structure and function. Such relations also provide a necessary basis both for more complete empirical analyses of the determinants of animal abundance and for the construction of more realistic conceptual models in theoretical ecology.

Density-body size relationships in local aquatic communities

Studies of density-body size relationships report generalities in the structure of communities and suggest patterns of energy use among species of different sizes. Most density-body size relationships, however, have been developed from data on populations throughout the world and are expected, therefore, to provide only an approximate description of any specific community. In this study, we test 1) whether log-linear density-body size relationships can also be used to describe local communities and 2) whether these relationships vary systematically with environmental conditions. Our analysis is based on population densities of phytoplankton, zooplankton, zoobenthos and fish species measured in 18 lakes worldwide. Mean annual population density (D, individuals m -2 ) decreases log-linearly with increasing species body size (M, μg fresh mass; log D = a + b x log M), with slopes and intercepts which vary among lakes (b = -1.10 to -0.74, a = 4.5 to 6.9). In contrast with studies which focus on only one group of organisms (e.g. birds, insects, mammals), found that local communities are well described by density-body size relationships (r 2 = 0.78-0.98) when more complete communities are considered. The slopes of density-body size relationships measure the relative density of small to large species, and tend to be shallower (lower relative density of small species) in lakes near cities, but more steeply negative (greater relative density of small species) in deep, highly productive lakes which flush rapidly. The intercepts measure average population density and are positively related to primary productivity. Such differences in size structure among aquatic communities are large enough to suggest shifts, for example, in the relative energy use of small and large species.

Population density and community size structure : comparison of aquatic and terrestrial systems

The size structure of aquatic communities is generally measured using size spectra, an approach which is tedious or inapplicable in benthic and terrestrial communities. This has inhibited comparison of size structure of aquatic and terrestrial communities. This study uses an approach more common among terrestrial ecologists to develop a general density-body size relationship for lacustrine communities, based on mean annual population densities for dominant species of phytoplankton, zooplankton, zoobenthos and fish measured in 18 lakes worldwide. Overall, mean annual population density (D, individuals m -2 ) decreases log-linearly with increasing species body size (M, μg fresh mass) as D = 4 x 10 5 . M -0.89 (n = 280, r 2 = 0.92), although the exponent appeared smaller (-0.55 ± 0.04) within broad taxonomic groups (algae, invertebrates). We found that density-body size relationships for dominant species are quantitatively similar to size spectra, a pattern which suggests that density-body size relationships may provide an interesting alternative to size spectra for the prediction of ecosystem processes. These relationships also suggest that aquatic species reach, on average, 6-60 times higher densities than terrestrial species, depending on their body size and on their thermoregulatory system (ectotherms vs endotherms). The implications of these differences in size structure for size-related patterns of energy use and other processes depend on which physiological groups (unicells, ectotherms, endotherms) are being compared.

The allometry of the weight of fruit on trees and shrubs in Barbados

SummaryBranch sampling of branch diameter and fruit crop on 22 species of Barbadian trees and shrubs provided sufficient data to build regressions between plant size and fruit crop weight. Orchard plants bear much more fruit than wild, feral or garden plants of similar size, but this difference disappears in multiple regression of fruit crop weight (F in g, fresh mass) on branch or stem diameter (D in cm) and individual fruit weight (W in g): F=22D1.2 W0.57. This explains 89% of the variation in F and successfully predicts crop weight for wild tropical and temperate trees and shrubs, but underestimated the crops on commercial, temperate, fruit trees by an order of magnitude. Comparisons of crop weight for feral, wild, and garden plants (Ff) using a simple regression Ff=47D1.9 show that crop weight is a minor load relative to branch weight for larger branches. Although fruit crops represent a declining proportion of total plant weight as plants become larger, the crops become larger relative to leaf and twig weight and in this sense, reproductive investment increases in larger plants. Finally, our equations, combined with the self-thinning rule, suggest that stands of large species of fruit plants produce more fruit per unit of land area than stands of small ones.

Useful Concepts for Predictive Ecology

Scientists deal with two classes of ideas. The first are theories, predictive or falsifiable statements about the universe. The second are concepts which, by definition, are unfalsifiable. Both types of ideas are necessary for a scientist, but only theories comprise a science. Nevertheless, in ecology, concept and theory are confused to the point that non-predictive ideas impede scientific advance. In this essay, I will try to distinguish between the roles of theory and concept, indicating the very real shortcomings of a science which is too rooted in concept. I will suggest that we set aside the bulk of our ecological concepts in order to concentrate on theory building and modification and I describe some small steps towards a predictive ecology.

Empirical relationships between the element composition of aquatic macrophytes and their underlying sediments

A simple view of the role of rooted macrophytes in element cycling sees them as pumps retrieving buried elements from the sediment profile. To investigate the relationship between the elemental composition of plants and sediments, we analysed published data for 39 elements. The best general model explained 84% of the variance of the log of plant element concentration: LPE = - 0.81 + 0.90 Log Sediment Element (ug/g dry wt.) − 0.12 Sediment Organic Content (ug/g drt wt.) + 0.67 Atomic radius (nm) (r2 = 0.84; n = 39)This close relationship between the concentrations of an element in plant tissues and in the underlying sediment indicates that acquatic plants do not differ markedly in element composition from the sediments in which they grow. T-tests between mean residuals indicated that these aquatic plants do not discriminate between essential and nonessential elements. Model II regression analyses showed no difference between the slopes of the functional relationships for individual elements and that of the general model. When the elements were separated into three groups (alkali, transition and related metals, and halogens), Log Sediment Element accounted for 75–96% of the variation in LPE. Element physicochemical parameters were also significant independent variables explaining an additional 3–12% variation in LPE. The relative importance of the independent variables differed for the three groups of elements.

Leaching and early mass loss of boreal leaves and wood in oligotrophic water

Following immersion in water, allochthonous litter undergoes aprocess of substantial leaching that is difficult to quantify yetimportant to exclude from analyses of the role ofmacroinvertebrates in subsequent breakdown. Laboratory experimentswhich measured the aqueous release of total phosphorus anddissolved organic carbon from undried leaves (deciduous andconiferous) and woody debris (twigs and bark) revealed that theperiod of leaching is a prolonged process developing over weeks.Immersion of litter from 6 species of riparian trees in 4oligotrophic Canadian Shield lakes demonstrated that undried leaveslost 6 to 18% of their mass after 2 wk, and woody debrisexperienced 0.2 to 27% mass loss after 7 wk. Studies concernedwith quantifying the role of macroinvertebrates in the breakdown ofallochthonous litter in lentic water should therefore disregardsuch mass losses.

The Effects of Analytical Variations on Estimates of Phosphorus Concentration in Surface Waters

ABSTRACT Systematic variations in methods for the storage, digestion and measurement of lake water samples for phosphorus determinations show that these methods are very robust. Frozen samples can be kept for at least one year without any detectable change in total phosphorus, [TP], but the concentrations of P subfractions may change. Variations in the amount of oxidant (0.05 to 0.8 g potassium persulphate/40 ml sample), in the duration (10–90 min), or pressure (100 vs 200 k Pa) of oxidation, and in the amount of sample lost in autoclaving had no effect on estimated [TP]. The exact composition of acid molybdate “mixed reagent” also proved not to be critical, for when each of the components was varied in turn, similar results were obtained with 5 to 10 g ammonium molybdate, 2.5 to 20 g ascorbic acid, 0.025 to 0.4 g potassium antimony tartrate and 55 to 100 ml sulphuric acid per liter of mixed reagent. No significant difference was obtained by varying the amount of mixed reagent from 1 to 7.5 ml/40 ml sampl...