SummaryThis paper presents a description of the temporal and spatial distribution of rainfall within the Serengeti Ecosystem, Tanzania. Ecosystem and Regional patterns are discussed with respect to synoptic processes, vegetation zonation, phases of primary production and the distribution and migrations of large mammals.

Summary Data on the monthly distributions of the Serengeti migratory wildebeest population between June 1960 and May 1973 have been analysed. It has been shown that the population performs an annual migration, spending the wet season on the Serengeti plains, then moving west towards Lake Victoria at the beginning of the dry season, and later north to the northern extension of the Serengeti National Park or into the Masai Mara Game Reserve in Kenya. The wildebeest return to the plains as soon as the rain begins, but if this is delayed they may move south‐west from the northern areas into the corridor first. Annual differences in the pattern of the migration can be correlated with differences in rainfall. The rainfall affects particularly the timing of the movements of the wildebeest on and off the plains, and the extent to which they utilize the northern areas. A large increase in the size of the population over the years studied has resulted in a greater utilization of the northern areas. It has been shown that about half the total range of the wildebeest population is outside the boundaries of the Serengeti National Park, and it is therefore important that these areas should be protected if such a large population is to survive.

All three species of parasites found in sticklebacks from Priddy had overdispersed distributions, similar in shape to host–parasite distributions described by other authors.The distributions of both Echinorhynchus clavula and Diplostomum gasterostei were fitted closely by the negative binomial. The values of k were similar for both species, being always less than unity; the Diplostomum distributions were more overdispersed and therefore had higher values of p.The Schistocephalus distributions were mostly described by the log normal, but the high percentage infection and relatively low overdispersion meant that the fit was often not close. There was also some truncation at the upper end.The relationship between the parameters of the negative binomial (p and k) and the shape and parameters of the original distribution have been investigated, both theoretically and with respect to the distributions of the stickleback parasites.In general p varied as the overdispersion and k as the percentage infection.The advantages of an overdispersed distribution to the host and parasite populations are discussed and related to the life cycles of the parasites.I am most grateful to Dr H. D. Crofton for his help and encouragement during this study, and to the staff of the Computer Unit, University of Bristol for their assistance with programming problems. This work was supported by a Science Research Council research studentship and a NATO studentship.

A population of three-spined sticklebacks, Gasterosteus aculeatus L., from Priddy pool in Somerset was found to be infected with three species of parasites: Schistocephalus solidus (Miiller, 1776), Diplostomum gasterostei Williams, 1966, and Echinorhynchus clavula (Dujardin, 1845) nee Hamann, 1892. Pennycuick (1971 described the effects of the parasites on the growth rate and breeding of the fish, and the seasonal variations in the incidence and intensity of the infection. Here the differences in the sizes of the infections in sticklebacks of different sexes, ages and sizes are discussed. MATERIALS AND METHODSA total of 1863 sticklebacks was collected in 27 samples taken at 2-to 4-week intervals between October 1966 and April 1968. Details of the methods of catching and examining the fish, and of the habitat, are given elsewhere (Pennycuick, 19716).The following data were obtained for each fish: sex, total length, total weight, weight without Schistocephalus; total weight of Schistocephalus, number of plerocercoids and weight and length of each; number of Diplostomum in the right and left eyes; number of male and female Echinorhynchus, length of each worm, and presence or absence of shelled acanthors in the females. For fish in the 12 samples from 21. viii. 67 onwards the ages were determined by examination of the otoliths, using the method of Jones & Hynes (1950). RESULTSFour age groups of sticklebacks, 0-3, were distinguished; fish in group 0 were less than 1 year old, those in group 1 between 1 and 2 years, and so on. No fish older than 4 years was found. The fish were also classified according to length, in groups of 5 mm interval, and according to weight, in groups of 0-2 g interval. The lengths of the fish ranged from 16 to 72 mm and the weights from 0-2 to 3*13 g.The percentages of fish infected with the three parasite species were calculated separately for fish of each sex, age and length. The percentages of fish of different weights infected with Schistocephalus were also found. The mean numbers of each parasite were calculated for each sex, age and length of fish and also the mean * Present address:

A population of three-spined sticklebacks, Gasterosteus aculealus, from a pond in Somerset was sampled about every three weeks for 19 months, a total of 1863 fish being collected. Three species of parasites were found in the fish: Schistocephalus solidus, Diplostomum gasterostei and Echinorhynchus clavula.Seasonal changes in the incidence and intensity of the three parasitic infections are described. In general the infections increased in summer and autumn and decreased in winter and spring, although in Schistocephalus the differences in the level of infection between the two years were greater than between seasons.The largest increases in the Schistocephalus and Echinorhynchus infections occurred in August and September and in the Diplostomum infection in May and June. These increases could be related to the feeding activity of the sticklebacks and to the abundance and level of infection of the Cyclops, Asellus and Limnaea pereger populations.Reduction in the sizes of the infections was by predation and death of heavily infected fish in the case of Schistocephalus and Diplostomum and mainly by death of individual worms in Echinorhynchus.These mechanisms are shown to be adapted to the life cycles of the parasites.I am most grateful to Dr H. D. Crofton for his help and encouragement during this investigation and to Dr R. A. Avery for useful discussions and advice. This work was supported by a Science Research Council research studentship and a NATO studentship.

The effects of Schistocephalus solidus, Diplostomum gasterostei and Echinorhynchus clavula on a population of Three‐spined sticklebacks from a pond in Somerset were studied. Schistocephalus was present at the highest level of infection and had the greatest effects on the sticklebacks. It caused the fish to weigh less than uninfected ones of the same length, as shown by the lower condition factor, and to grow more slowly; and it delayed the sexual maturation of fish of both sexes and prevented many from breeding. Diplostomum also had a significant effect on the condition of the sticklebacks and all three parasite species probably caused death or predation of heavily infected fish. The quantitative effects of the parasites on the condition, length and weight of the fish were calculated; seasonal changes in the condition factor could then be seen, and the growth rates for both length and weight calculated.

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