Study ID 312

Stability in a Multi-Species Assemblage of Large Herbivores in East Africa

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Realm: Terrestrial
Climate: Tropical
Biome: Tropical and subtropical grasslands, savannas and shrublands
Central latitude: 3.500000
Central longitude: 35.750000
Duration: 9 years, from 1959 to 1984

117 records

13 distinct species

Across the time series Aepyceros melampus is the most frequently occurring species

Citation(s):

Prins, H. H. T. & Douglas-Hamilton, I. (1990) Stability in a Multi-Species Assemblage of Large Herbivores in East Africa. Oecologia, 83, 392–400.

Methods

The study was carried out on data collected in Lake Manyara National Park (lat. 3º30' S. long. 35º45' E) in northern Tanzania (Fig. 1). Until substantial poaching began in 1986. Manyara had the highest mammalian biomass density in any natural ecosystem (Delany and Happold 1979; Drent and Prins 1987). comparable to the system of Queen Elizabeth National Park. Uganda (again dominated by elephant and buffalo) before severe poaching reduced mammal numbers drastically there in the 1970s. Manyara is situated mainly in the Eastern Rift Valley (960 m above sea level) but partly on the Mbulu Plateau (1600 m a.s.l.) and receives an average of 650 mm of rain annually. Most game in the Park is sedentary but wildebeest and Burchell's zebra show extensive migratory movements into the rest of the Masai Ecosystem (Prins 1987). Estimates were obtained from a variety of sources (see Legend of Table 1) of the total number of large herbivores (heavier than 20 kg) within the present Park boundaries during the last 25 years. The species and nomenclature are present in Table 2. Where data were lacking for a particular species for a given year of census. estimates were made on the basis of figures from the previous and subsequent censuses. The obtained estimates were discussed with park rangers who had been working in the Park at the time of the missing value. Published figures (Table 1) were also discussed with these rangers. and on the basis of these and on the warden's reports (1959 through 1973). the published figures for the hippopotamus population in 1961. 1965. 1967. and 1970 were disregarded and a new estimate was used instead. It should be noted that estimates of bush species (viz. black rhinoceros. bushbuck. and to a lesser extent impala. waterbuck. and olive baboon) because of the limited visibility in their habitat. are undoubtedly less reliable than estimates for plains species (viz. Burchell's zebra. wildebeest. and to a lesser degree warthog) or for other large herbivores (African elephant. Cape buffalo. and giraffe; cf. Prins and Weyer-haeuser 1987). The estimates for reedbuck are quite reliable as the wardens paid special attention to their numbers. Consumption was calculated according to the assumption that the herbivores' daily food intake (dry matter) equals 2.5% of their live body weight (Crampton and Harris 1969; Van Soest 1982. Van Wijngaarden 1985). which compares reasonably well with the published data for food intake of East African herbivores (Delany and Happold 1979). For the calculations the average live weights used for the different species are presented in Table 2. Some of the species in Manyara are considered real grazers. others real browsers but many species are mixed feeders (Hofmann 1973). As we are interested in quantifying the consumption of grass and browse in Manyara. the consumption by species falling in the category mixed feeder had to be divided into grass consumption and browse consumption. For this division we used the data from Table 2. The data we present are gross indices of consumption per unit land in the Park. The acreage of the dry land area of the park has varied over time because of changes in the level of Lake Manyara. After periods of heavy rainfall the lake submerged parts of the grasslands and woodlands but during extended droughts the lake receded after which the surfacing shore was again colonized by grasses (Loth and Prins 1986. Prins and Loth 1988). The real land area of the Park in the different census years as calculated from aerial photographs (Loth and Prins 1986) is presented in Table 1. The pressure exerted on the vegetation by the herbivores is assumed to be equal to their consumption. although this ignores the effects of trampling. which can be quite important (cf. Sinclair and Norton-Griffiths 1979). Five groups of herbivores are distinguished in the analyses. viz. buffalo elephant-as-grazer. elephant-as-browser'. other grazers (i.e. hippopotamus. Burchell's zebra. wildebeest. impala. reedbuck. waterbuck. warthog. baboon. and bushbuck) and other browsers (i.e. giraffe. black rhinoceros. baboon. impala. bush- buck. waterbuck. reedbuck. and warthog); the categories are weighted according to body weight and the proportions grass or browse in the diet (Table 2). Mean-square successive difference testing was used to detect whether a time series was randomly distributed around the mean. or not (Wonnacott and Wonnacott 1977). The constancy parameter s. defined as the standard deviation of the base-10 logarithm of population estimates. was used to assess the stability of the population; when s is smaller than approximately 0.3 the population is considered stable (Ostfeld 1988). It is. however. better to speak of constancy (i.e.. lack of change in a numerical parameter) than of stability. as the latter implies that the population returns to a certain state after disturbance (Orians 1975). For the species assemblage we calculated a stability index. following McNaughton (1978). Thus. we defined n as the number of herbivore species (n = 13). i as the interaction term equalling the mean value of significant Spearman rank correlation coefficients (rj) between population numbers (for P<0.05 and for P< 0.01). and c the proportion of all r. values that were significant. The stability index is calculated as i/(nc) 1/2; when the stability index is < 1.0. the system is stable and when the index is > 1.0. the system is not stable. We did not calculate guild size (nc) (McNaughton 1978). as this does not appear to be a valid calculation (Harris 1979; Lawton and Ralliston 1979).” Large mammals census in Park in Tanzania Unit of abundance = IndCountInt, Unit of biomass = NA