The Effects of Prey Size Selection by Lake Planktivores

Abstract

Although the biomlass of the planktivorous fish in any lake is but a few per cent of that of the plant and animal plankton, these fish exert a potent influence upon the composition of these lower trophic levels. The vulnerability of the zooplankton to predation, together with the high degree of selectivity exercised by the planktivores in their choice of prey items are responsible for this pervasive effect. Freshwater planktivores appear to prefer large Cladocera, especially Daphnia. Trout and yellow perch, facultative planktivores, switch to non-planktonic food when large Daphnia are not available. Freshwater populations of the primarily marine genus Alosa are obligate planktivores, shifting to progressively smaller zooplankters as the larger, preferred items become scarce. Within each prey category there is strong preference for the largest items. In an experimental observation of Alosa sp. feeding on a small calanoid copepod, the survival time of each instar was inversely proportional to its mean body length. 'Te strong preferences evinced by lacustrine planktivores can shift the competitive balance between the zooplanktonic herbivores, all of which exploit the samie supply of particulate food. Whenever or wherever planktivores are absent, large zooplankters like Daphnia pulex are numerically dominant. When planktivory is more intense the large zooplankters are cropped sufficiently to allow smaller, hitherto suppressed, species to become numerous. The interest displayed by facultative planktivores toward decreasing sizes of prey is considered in relation to the shape-size polymorphism characteristic of most limnetic Daphnia. The effect alluded to in the title relates to determination of the qualitative as well as quantitative composition of the animal plankton of open-water communities of lakes by selective predation of planktivorous fish. The biomass of piscine planktivores may on the average be only a few per cent of that of the combined phytoplankton and zooplankton. Yet, the active selection of prey by these vertebrate carnivores does exert a decisive influence directly upon the composition of the animal plankton, and thus indirectly on the quantity and, perhaps to some extent, quality of the phytoplankton. This effect upon phytoplankton will not, however, be discussed here (see Brooks, in press). It seems advisable to preface this consideration of the significance of selection, e.specially size-selection, with a brief notice of some of the characteristics peculiar to open-water communities, as opposed to those of terrestrial ones. The most striking difference is in the size of green plants relative to the animals that feed upon them. The algae, either individual or colonial, are small. relative to the animals that feed by one or another semi-automatic means of collecting and concentrating these algae. Terrestrial green plants on the other hand are large, often enormous relative to the invertebrate herbivores, and in forests at least are large relative to the vertebrate herbivores as well. These large plants provide terrestrial ecosystems with complex three-dimensional systems of substrata. These substrate systems are of sufficient temporal stability to have permitted the structure and behavior of associated animals to evolve in relation to the characteristics of the substrata. This has resulted in the use of special parts of this threedimensional nexus for feeding, for reproductive behavior, and for seeking protection from beasts on the next higher level of the food web. The tiny algae, while concentrated in the upper, illuminated water layers, determine no temporally 'stable fine-grained spatial pattern within the ecosystem, in relation to which the planktonic herbivores could have evolved behavior patterns. While the phytoplankton may be patterned in space by their various orientations in wind-driven currents, these patterns have a short temporal stability. The wind-generated spiral vortices may