THE INFLUENCE OF PREDATION ON THE COMPOSITION OF FRESH‐WATER ANIMAL COMMUNITIES

Abstract

SUMMARY1. The introduction of Salmo trutta into an artificial pond was followed by great reduction in the numbers of tadpoles, certain beetles and Notonecta, all species to be seen in the open water. Of the species that sheltered more securely in the plant cover, the effect on some was a curtailment of their range; on others there was little reduction of range or numbers, particularly the numbers at the end of a generation. This was attributed to self‐regulation of numbers and the creation of a reserve from which losses due to predation by fish on larger specimens could be replenished.2. Changes in the number of fish in stretches of a stony stream exerted little effect on the Ephemeroptera, but records indicate that Gammarus and fish are rarely numerous in the same stretch.3. The most abundant invertebrate carnivores in the fishpond wait for prey to come to them; of two others, a leech swims well but has poor seizing organs, a caddis‐larva the reverse. The amount consumed by such predators falls rapidly as the prey becomes scarce. Moreover the main source of prey for the common predators is from the small Crustacea which are abundant only in summer and reproduce quickly. This prey thus has properties that prevent much reduction of numbers by predation.4. On the stony substrata of Lake District lakes, Asellus and Planaria are numerous where conditions are productive, Ephemeroptera and Plecoptera elsewhere. The absence of these insects from productive places is attributed to predation.5. Planaria cannot move fast and have no efficient seizing organs, but compensate to some extent by laying trails of slime in which prey becomes entangled. As Asellus grows, its chances of being overpowered by Planaria decrease. Planaria, therefore, feed regularly only when prey is abundant. When it is scarce they rest, and they are able to withstand starvation for a long period.6. Planaria are preyed upon extensively by Odonata, newts and Plecoptera, and the first two keep them out of weedy ponds. The last may keep their numbers low in streams and perhaps also on stony shores of unproductive lakes, though here scarcity of food is important too. In productive lakes predation on flatworms is slight.7. Protozoa exhibit three relationships between predator and prey, two of which have been seen in larger organisms. Prey avoid predation in cover. Predators cease activity when prey becomes scarce. Prey occurs in isolated colonies which when found are destroyed by the predator, but generally not before some individuals have dispersed and founded new colonies.8. Only small invertebrates can survive predation by fish in the open water. Many also reproduce rapidly as long as conditions are favourable and enter a resting stage when they are not. Intense predation may eliminate large species. In some ponds in the Colorado mountains salamanders eliminate a large carnivorous copepod, which enables small Cladocera to survive. Absence of the copepod and presence of Cladocera of suitable size for it to feed on enable a Chaoborus larva to co‐exist with the salamander.9. Small planktivorous fish occur in the open water of some African lakes. The great size of these lakes probably makes possible the co‐existence of small fish and their predators, but also makes investigation of the relations between the two difficult.10. When species not previously present have gained access to lakes, the numbers of the native species of fish have often been greatly reduced. The exact nature of the relationship between newcomer and native has, however, not been established because other factors have been varying, observations have been scanty, or records have not been made for long enough.11. In temporary and very small bodies of water predation is mainly by invading individuals that were reared somewhere else. Characteristic organisms are phyllopods in impermanent pools and mosquito larvae in both types of water, two groups that feed in the open and away from cover, an activity possible only where predation is slight.