Summary 1. We measured the body sizes (weights or lengths) of animal species found in the food webs of natural communities. In c. 90% of the feeding links among the animal species with known sizes, a larger predator consumes a smaller prey. 2. Larger predators eat prey with a wider range of body sizes than do smaller predators. The geometric mean predator size increases with the size of prey. The increase in geometric mean predator size is less than proportional to the increase in prey size (i.e. has a slope less than 1 on log-log coordinates). 3. The geometric mean sizes of prey and predators increase as the habitat of webs changes from aquatic to terrestrial to coastal to marine. Within each type of habitat, mean prey sizes are always less than mean predator sizes, and prey and predator sizes are always positively correlated. 4. Feeding relations order the metabolic types of organisms from invertebrate to vertebrate ectotherm to vertebrate endotherm. Organisms commonly eat other organisms with the same or lower metabolic type, but (with very rare exceptions) organisms do not eat other organisms with a higher metabolic type. Mean sizes of prey increase as the metabolic type of prey changes from invertebrate to vertebrate ectotherm to vertebrate endotherm, but the same does not hold true for predators. 5. Prey and predator sizes are positively correlated in links from invertebrate prey to invertebrate predators. In links with other combinations of prey and predator metabolic types, the correlation between prey and predator body sizes is rarely large when it is positive, and in some cases is even negative. 6. Species sizes are roughly log-normally distributed. 7. Body size offers a good (though not perfect) interpretation of the ordering of animal species assumed in the cascade model, a stochastic model of food web structure. When body size is taken as the physical interpretation of the ordering assumed in the cascade model, and when the body sizes of different animal species are taken as log-normally distributed, many of the empirical findings can be explained in terms of the cascade model.

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Body sizes of animal predators and animal prey in food webs

The classical approach of limnologists has been to consider the interactions between lake ecosystem components as an unidirectional flow of influence from nutrients to the phytoplankton, to the zooplankton, and finally to the fish, through successive controls by physical, chemical, and biological processes (Straskraba, 1967). The effect of planktivorous fishes on zooplankton and phytoplankton communities was not recognized until the studies of Hrbacek et al. (1961), Hrbacek (1962), Brooks & Dodson (1965) and Straskraba (1965). They showed that (1) in ponds and lakes in the presence of planktivorous fishes the zooplankton communities were composed of smaller bodied species than in those lacking planktivores, and (2) the resulting small-bodied zooplankton communities affected the phytoplankton communities. Although the variability of the phytoplankton response to fish predation showed the importance of other factors (such as nutrient limitation and interspecific competition of algae), these studies emphasized that zooplankton and phytoplankton communities can be affected by the feeding selectivity of planktivorous fishes. During the last two decades, many limnological studies have focused on this dramatic impact of fish on plankton communities. The direct response of zooplankton communities to visual fish predation (i.e. particulate feeding) has been of major interest, whereas the multilevel effects of filter-feeding fish (predation on zooplankton plus grazing on phytoplankton) have been neglected. The objectives of this review are to document fish-plankton interrelationships in order to (1) provide insights into the impact of fish on plankton communities, and (2) outline mechanistic models of planktivory according to the feeding repertory and the selectivity of the fish, the adaptive responses of the plankton, and the environmental conditions. The approach adopted here is based on field and laboratory experimental results derived from the literature on tropical and temperate freshwater (occasionally marine) systems. Four types of planktivorous fish are distinguished: the gape-limited larvae and small fish species, the particulate feeders, the pump filter feeders, and the tow-net filter feeders. For each type of planktivore, the mechanisms of prey selection are analyzed from the point of view of both the predator and the prey. To investigate the main determinants of the predator feeding selectivity, and to discuss its potential effects on prey communities, the predation-act is divided into a sequence of successive events (Holling, 1966): detection, pursuit, capture, retention, and digestion for particulate feeders; and capture, retention, and digestion for filter feeders. The strengths and weaknesses of various measures of selectivity (i.e. electivity indices), as well as their appropriate usages are considered. Available prey selection models and optimal foraging theories are analyzed for the different planktivore feeding modes. Mechanistic models based on Holling's (loc. cit.) approach are proposed for each feeding mode to determine differential prey vulnerabilities and optimal diet breadth. This review has application to several fields, including general ecology, limnology, fisheries management (for example, utilization of planktonic resources, stocking, introduction, or maintenance of natural fish populations), and biological control of the eutrophication processes (biomanipulation approaches). It emphasizes the real need for more knowledge of the feeding selectivity and food utilization of planktivores. It concludes that predator and prey are mutually adapted. Thus, in most cases, study of plankton dynamics and water quality should include the assessment of fish predation and grazing pressures.

/pdf/a-review-of-planktivorous-fishes-their-evolution-feeding-4uqtrukvtn.pdf

A review of planktivorous fishes : their evolution, feeding behaviours, selectivities, and impacts

The size-efficiency hypothesis, first proposed in 1965 by Brooks & Dodson (24), has profoundly influenced the direction of research in aquatic ecology since its publication.3 The size-efficiency hypothesis (SEH) is an attempt to explain the commonly observed inverse relationship between the abundances of smalland of large-bodied herbivorous zooplankton in freshwater lakes. The SEH was originally stated as follows:

The Size-Efficiency Hypothesis and the Size Structure of Zooplankton Communities

A 4 year series of field, light-microscope and ultrastructural observations is presented to illustrate biological aspects of the annual cycle of natural Microcystis populations enclosed in Lund tubes. Nine morphological stati, all referable to M. aeruginosa f. aeruginosa , feature at various stages of the cycle. Summer bloom-forming populations originate from vegetative colonial stock that overwinters on the bottom sediment each year, but there is no mass transfer of these colonies to the water column: intensive growth from individual cells in the old colonies leads to the formation of new infective colonies, being stimulated when the bottom water approaches anoxia and light penetrates to the bottom sediments. Growth is slow but the developing populations sustain only minor losses through grazing and settling out, eventually becoming dominant over other species. Allelopathy possibly contributes to this effect. In postmaximal populations, several mechanisms can contribute to net buoyancy loss and a (usually) rapid recruitment of vegetative colonies to the sediments is observed. Hypotheses are advanced to account for the observed behaviour, and some of these have been tested in the laboratory. The apparent physiological flexibility of Microcystis seems well suited to growth and survival in the microenvironments encountered in eutrophic lakes.

On the Annual Cycle of the Blue-Green Alga Microcystis Aeruginosa Kutz. Emend. Elenkin

4 – Functional Aspects of Nutrition and Digestion

The fauna of Lake George is dominated by herbivores, in particular the fish Tilapia nilotica and Haplochromis nigripinnis, and the cyclopoid copepod Thermocyclops hyalinus Digestion, and the assimilation of carbon from algae, by these herbivores were studied in detail It was found that, contrary to previous observations reported in the literature, both fish and zooplankton were able to digest and assimilate blue-green algae The diurnal feeding patterns of the herbivores were examined, and methods devised for assessing, in terms of carbon, the daily ingestion of algae Hence, using biomass data, the total amount grazed by the herbivores was calculated Estimates of carbon requirements at other trophic levels were made, as was an assessment of the required level of net production by the algae for comparison with figures for standing crop and net algal production measured by other means Food selection by secondary and tertiary producers is discussed, and in several species age correlated changes in selectivity were examined Both the major herbivorous species of fish adopt phytoplankton feeding after a period of carnivorous or omnivorous feeding as fry Thermocyclops hyalinus is herbivorous all its life, but the size of particle taken changes with age There are more species of carnivorous than herbivorous fish and these exploit a wider variety of food sources; the few species studied also show changes in food preference with age

Feeding and Grazing in Lake George, Uganda

Ninety-five per cent of the total biomass in the open water of Lake George, a shallow equatorial lake, is phytoplankton. The biomass of this and the other major groups of organisms are given and compared with some figures from other lakes. Apart from the phytoplankton, standing crop levels in Lake George are not high and may be exceeded, although only for short periods, by organisms in temperate waters. Levels in Lake George, in contrast, fluctuate little throughout the year. The specific composition of the open water community is described and shown to be dominated by blue-green algae and three herbivores which utilize them as a food source: one cyclopoid copepod and two fish. The reasons why these and other species make up the open water community in Lake George are discussed and also possible factors which may limit some of the animal populations to the levels observed. Both the phytoplankton and the zooplankton exhibit a concentric pattern of distribution across the lake. Concentrations are highest in the centre and this may be attributable to the circular movements of the water mass which result from the shape of the basin and the wind regime. It might also be correlated with grazing effects since the fish population, which also shows a concentric distribution of biomass, has its highest concentrations in the inshore regions and its lowest in the centre. The distribution and species composition of the benthos is determined primarily by the nature of the substrate. In the open water area the bottom consists of soft, unstable, deoxygenated mud which contains few species. Inshore the substrate is firmer and more varied and carries a greater density and variety of animals. These are preyed upon by a greater variety of fish, particularly species of the cichlid genus Haplochromis, than are found in the mid-lake area. The inshore region is short, due to the regular shape of the lake basin, and narrow, but provides a variety of habitats in contrast to the homogeneity of the mid-lake area. The lake is fringed in many places by papyrus swamp, the Nile cabbage floats on the surface of the water but there is no submerged aquatic vegetation.

https://www.jstor.org/stable/pdfplus/76176.pdf

The Biomass and Distribution of Organisms in Lake George, Uganda

(1969). Lake George, Uganda: A limnological survey. SIL Proceedings, 1922-2010: Vol. 17, No. 1, pp. 284-288.

I. G. Dunn

Papers

Feeding and Grazing in Lake George, Uganda

The Biomass and Distribution of Organisms in Lake George, Uganda

Lake George, Uganda: A limnological survey: With 1 figure and 1 table in the text