Does competition or facilitation regulate migrant ungulate populations in the Serengeti? A test of hypotheses

Summary Interspecific competition and facilitation have both been proposed as processes promoting species separation and co-existence in African ungulates. In one group of grazers on the Serengeti plains, comprising wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni), these processes have also been suggested to regulate the populations. Censuses of these populations over 20 years have shown changes that allow a test of which, if either, process regulates population numbers. Wildebeest numbers have levelled off as a result of intraspecific competition for food following a five-fold increase due to release from disease and an increase in food supply. Zebra numbers have remained stationary throughout the same period. Gazelle numbers have declined in the last 10 years. These results are contrary to the facilitation hypothesis, which predicts that wildebeest numbers should not have increased if those of zebra did not, and that gazelle numbers should have increased since the wildebeest population increased. The gazelle results are consistent with the interspecific competition hypothesis, but the zebra results are contrary to it. We propose an alternative hypothesis that predation regulates the zebra population, and we suggest ways of testing this.     

Possible causes of decreasing migratory ungulate populations in an East African savannah after restrictions in their seasonal movements

In many areas in Africa, seasonal movements of migratory ungulates are restricted and their population numbers decline, for example in the Tarangire region, Tanzania. Here, agriculture restricts migration of ungulates to their wet season ranges. We investigated whether low forage quality or supply are possible causes of population decline of wildebeest and zebra when access to these wet season ranges is restricted and migratory herds have to reside in the dry season range year-round. We simulated grazing through a clipping experiment in the dry season range during the wet season. Clipping negatively affected forage supply and had a positive effect on forage quality by increasing proportions of live and leaf biomass as well as nutrient concentrations in the leaves. However, increase in forage quality in the dry season range due to grazing was not as such that requirements of wildebeest during the wet season, when females are lactating, could be met. We conclude that low forage quality in the dry season range during the wet season could cause the decrease in migratory ungulate populations in the Tarangire region. With this study, the necessity of protecting wet season ranges from expanding human activities to safeguard migratory systems is supported.     

Bottom-up and top-down processes in African ungulate communities: resources and predation acting on the relative abundance of zebra and grazing bovids

African ungulate populations appear to be limited principally by their food resources. Within ungulate communities, plains zebras coexist with grazing bovids of similar body size, but rarely are the dominant species. Given the highly effective nutritional strategy of the equids and the resistance of zebras to drought, this is unexpected and suggests that zebra populations may commonly be limited by other mechanisms. Long-term research in the Serengeti ecosystem and in the Kruger National Park suggests that zebra could be less sensitive to food shortage, and more sensitive to predation, than grazing bovids: if this is a general principle, then, at a larger scale, resource availability should have a weaker effect on the abundance of zebra than on grazing ruminants of similar body size (wildebeest and buffalo), and zebras should be relatively more abundant in ecosystems where predators are rare or absent. We test these expectations using data on 23 near-natural ecosystems in east and southern Africa. The abundance of wildebeest is more closely related to resources than is that of zebra; buffalo are intermediate. We show that hyena densities are closely correlated with those of lions, and use the abundance of lions as an index of predation by large predators. The numerical response of lions to increases in the abundance of their prey was linear for mesoherbivores, and apparently so for the three species alone. Finally, the abundance of zebra relative to grazing bovids is lower in ecosystems with high biomasses of lions. These results indicate that zebras may commonly be more sensitive to top-down processes than grazing bovids: the mechanism(s) have not been demonstrated, but predation could play a role. If it is true, then when numbers of the large mammalian predators decline, zebra populations should increase faster than buffalo and wildebeest.     

What limits the Serengeti zebra population?

The populations of the ecologically dominant ungulates in the Serengeti ecosystem (zebra, wildebeest and buffalo) have shown markedly different trends since the 1960s: the two ruminants both irrupted after the elimination of rinderpest in 1960, while the zebras have remained stable. The ruminants are resource limited (though parts of the buffalo population have been limited by poaching since the 1980s). The zebras resource acquisition tactics should allow them to outcompete the ruminants, but their greater spatial dispersion makes them more available to predators, and it has been suggested that this population is limited by predation. To investigate the mechanisms involved in the population dynamics of Serengeti zebra, we compared population dynamics among the three species using demographic models based on age-class-specific survival and fecundity. The only major difference between zebra and the two ruminants occurred in the first-year survival. We show that wildebeest have a higher reproductive potential than zebra (younger age at first breeding and shorter generation time). Nevertheless, these differences in reproduction cannot account for the observed differences in the population trends between the zebra and the ruminants. On the other hand, among-species differences in first-year survival are great enough to account for the constancy of zebra population size. We conclude that the very low first-year survival of zebra limits this population. We provide new data on predation in the Serengeti and show that, as in other ecosystems, predation rates on zebras are high, so predation could hold the population in a predator pit. However, lion and hyena feed principally on adult zebras, and further work is required to discover the process involved in the high mortality of foals.     

The biomass of game animals in Nairobi National Park, 1960–66

The large animals of Nairobi National Park have been censused intermittently once a month from mid-1960 until the end of 1966. Of the 23 species discussed, the density per kilometre has been determined accurately for the following: Grant's gazelle, Thomson's gazelle, water-buck, impala, wildebeest, hartebeest, eland, buffalo, warthog, giraffe, zebra and ostrich. From these figures the biomass has been calculated for each species each year. After a peak biomass of 12,775 kg/sq. km during the drought of 1960–61 the population declined to and remained fairly level at about 5690 kg/sq. km from 1962 to 1966. This figure includes about 1050 kg/sq. km of cattle and sheep. The large predators account for only about 1.4 % of the total biomass of wild animals while removing about 15.5% from this group annually.     

A seasonal feast: long-term analysis of feeding behaviour in the spotted hyaena (Crocuta crocuta)

The feeding behaviour of the Talek clan of spotted hyaenas in the Masai Mara National Reserve, Kenya, was monitored continuously for 7 years. Talek hyaenas adapted to large temporal variations in prey abundance by being opportunistic predators. During the first half of the year, the hyaenas fed on resident ungulates, and their diet consisted mainly of topi and Thomson's gazelles. Upon arrival of the migratory herds of wildebeest and zebra from the Serengeti, Talek hyaenas switched to feeding on the wildebeest which provided them with a superabundance of food for about 3 months. After the migratory animals returned to the Serengeti, Talek hyaenas experienced a period of reduced prey abundance due to the temporary dispersion of resident ungulates. At this time hyaenas hunted the few remaining wildebeest, and also increased their use of the remaining resident animals. Although Talek hyaenas were generally opportunistic in their feeding behaviour, they did exhibit clear dietary preference for larger prey species, particularly wildebeest. Finally, carrion comprised only 5% of the biomass consumed by Talek hyaenas, the lowest proportion of carrion in the diet of any Crocuta population studied to date.     

Movements of the migratory wildebeest population in the Serengeti area between 1960 and 1973

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.     

High variation and very low differentiation in wide ranging plains zebra (Equus quagga): insights from mtDNA and microsatellites

Patterns of genetic differentiation in the plains zebra ( Equus quagga ) were analysed using mitochondrial DNA control region variation and seven microsatellites. The six morphologically defined subspecies of plains zebra lacked the population genetic structure indicative of distinct evolutionary units. Both marker sets showed high levels of genetic variation and very low levels of differentiation. There was no geographical structuring of mitochondrial DNA haplotypes in the phylogenetic tree, and the plains zebra showed the lowest overall differentiation recorded in any African ungulate studied so far. Arid-adapted African ungulates have shown significant regional genetic structuring in support of the Pleistocene refuge theory. This was not the case in the zebra, and the data are discussed in relation to the impact of Pleistocene climate change on a nonbovid member of the savannah ungulate community. The only other species showing a similar absence of genetic structuring is the African buffalo ( Syncerus caffer ), but this taxon lacks the high levels of morphological variation present in the plains zebra.     

Selectivity in large generalist herbivores: feeding patterns of African ungulates in a semi-arid habitat

Feeding habits of free-ranging wildebeest and zebra were monitored in a semi-arid nature reserve, bordering the southwestern part of Kruger National Park, South Africa. The purpose of study was to distinguish and define the feeding niches of two roughage grazers that occur in similar habitat types. The monthly compositions of diets were evaluated by direct observations of feeding bouts over a period of two years when rainfall patterns were average and animal populations were stable. Other analyses evaluated the standing biomass of grass species in the reserve during the wet summer and dry winter seasons. A considerable overlap of grass species composition was found in the diets of wildebeest and zebra. Ordination of bi-monthly records of the diet composition showed greater variations in scores of grasses in zebra diet in comparison to wildebeest. Seasonal patterns were more apparent in the wildebeest diet. Preference ranking of grass species indicated that zebra diet remained constant in winter and summer. Wildebeest diet however, alternated with seasons, showing high preferences during the winter months for grass species which were rejected during summer. The combined assessment of results from three separate statistical methods analysing temporal patterns and preferences in diet composition revealed contradictory trends. The solution, however, relied on the initial assumptions posed. Hence, wildebeest and zebra are essentially generalist feeders which show a limited amount of preference in their choice of diet.     

The pattern of lion predation in Nairobi Park

The feeding habits of lions in Nairobi National Park have been observed between 1968 and 1972. Nairobi Park is briefly described and the herbivore and predator populations are discussed. The relationship between the changing status of the prey population for the period 1968-72 and the feeding habits of the lions is examined and compared with data from earlier years. Selection ratio (preference ratio) for three of the most important prey species is more closely adjusted to the abundance of these species in the total ungulate population than was the case prior to 1968. Only warthog and eland form an exception. Predation on wildebeest shows the most marked decrease although this species has been increasing during the last 5 years. Wildebeest, kongoni, zebra and warthog have provided 80% of total kills throughout the study on the basis of numbers killed. On this basis, warthog has the highest preference rating. However, the largest contribution to the lions' diet expressed in kg is by kongoni, followed by zebra, wildebeest, eland and warthog, in that order. Kongoni and wildebeest contribute most during the first half of the year and zebra and eland during the last 6 months. Selection for males is significant in zebra and eland and highly significant in kongoni, whilst only in zebra is selection for juveniles evident.     

Predicted impact of barriers to migration on the Serengeti wildebeest population

The Serengeti wildebeest migration is a rare and spectacular example of a once-common biological phenomenon. A proposed road project threatens to bisect the Serengeti ecosystem and its integrity. The precautionary principle dictates that we consider the possible consequences of a road completely disrupting the migration. We used an existing spatially-explicit simulation model of wildebeest movement and population dynamics to explore how placing a barrier to migration across the proposed route (thus creating two disjoint but mobile subpopulations) might affect the long-term size of the wildebeest population. Our simulation results suggest that a barrier to migration--even without causing habitat loss--could cause the wildebeest population to decline by about a third. The driver of this decline is the effect of habitat fragmentation (even without habitat loss) on the ability of wildebeest to effectively track temporal shifts in high-quality forage resources across the landscape. Given the important role of the wildebeest migration for a number of key ecological processes, these findings have potentially important ramifications for ecosystem biodiversity, structure, and function in the Serengeti.     

The migration of zebra and wildebeest between Tarangire National Park and Simanjiro Plains, northern Tanzania, in 1972 and recent trends

In 1972, four aerial censuses were carried out to assess the annual migration of zebra and wildebeest between Tarangire National Park and Simanjiro Plains. About 6000 zebra and 10,000 wildebeest were in the Plains in the middle of the rainy season, in April. During the dry season in August the animals were concentrated in the Park. The migration from the Park to the Plains started at beginning of the rains, in November/December. Recent censuses by Tanzania Wildlife Conservation Monitoring (TWCM, 1991, 1995) indicate that an estimated 23,000 zebra and 11,000 wildebeest migrate into the Park from Simanjiro and other wet season areas. Encroaching cultivation is a threat to the migration corridors and sustainability of the ecosystem . Providing benefits from wildlife to communities around the park would safeguard the future of the wildlife.     

Trends in populations of elephant and other large herbivores in Gonarezhou National Park,Zimbabwe, as revealed by sample aerial surveys

For 30 years, regular aerial surveys in Zimbabwean protected areas were funded, designed and executed primarily to estimate elephant numbers. Other large herbivores were recorded, even though some species were not easily seen from the air in savannah woodlands. Population estimates for species other than elephant provided indices of abundance that could be used to determine temporal trends in population size. This study tests for significant trends in the abundance of large herbivores in Gonarezhou National Park, assuming that data from aerial sample surveys designed for elephant also provide accurate estimates of real trends in the populations of other herbivores. For each species, the exponential rate of population change was calculated using weighted regression, with the variance of this rate based on the sampling variances of the population estimates. Significant population trends were detected for eight species. Before the 1992 drought, elephant number was held approximately constant by frequent culls, but afterwards, it increased at a mean annual rate of 6.2% (confidence limits 4.0% and 8.6%). Elephants in cow herds increased at 7.3%, significantly faster than elephants in bull herds (?0.5%). Buffalo, eland, kudu, nyala, waterbuck, wildebeest and zebra all increased in number, after population declines during the drought.     

Prey selection and prey preferences of spotted hyenas <Emphasis Type="Italic">Crocuta crocuta</Emphasis> in the Etosha National Park,Namibia

The feeding ecology of the spotted hyena Crocuta crocuta was studied in the central and eastern part of the Etosha National Park, Namibia. Hyenas mainly hunted migratory ungulates such as springbok Antidorcas marsupialis, zebra Equus burchelli, and blue wildebeest Connochaetes taurinus, but also resident species such as the greater kudu Tragelaphus strepsiceros and gemsbok Oryx gazella. There were, however, major differences in the species most frequently killed by hyenas in central and eastern Etosha. The preferred prey species of spotted hyenas in central Etosha was springbok, whereas regarding the abundance of zebra and wildebeest, these two species were rather avoided. In contrast, the prey species preferred by hyenas in eastern Etosha was kudu. Zebra and gemsbok were taken in proportion to their abundance, whereas wildebeest and springbok seemed to be rather avoided. Differences in prey selection and preferences were also reflected in differences in hunting group sizes. In eastern Etosha, where spotted hyenas frequently hunted larger prey, hunting group sizes were significantly larger compared to those in the center of the park.     

Modelling ungulate dependence on higher quality forage under large trees in African savannahs

In African savannahs, large trees improve grass quality, particularly in dry and nutrient poor areas. Enhanced below-canopy grass nutrients, such as nitrogen and phosphorus contents should therefore attract and benefit grazers. To predict whether ungulates really need these forage quality islands we focused on four grazer species, i.e., zebra, buffalo, wildebeest, and warthog, differing in body size and digestive system. We confronted literature estimations of their feeding requirements with forage availability and quality, observed in three South African savannah systems, through linear modelling. The model predicted the proportion of below-canopy grass that grazers should include in their diet to meet their nutritional requirements.During the wet season, the model predicted that all animals could satisfy their daily nutrient requirements when feeding on a combination of below- and outside-canopy grasses. However, wildebeest, having relatively high nutrient demands, could meet their nutrient requirements only by feeding almost exclusively below canopies.During the dry season, all animals could gain almost twice as much digestible protein when feeding on below – compared to outside-canopy forage. Nonetheless, only warthogs could satisfy their nutrient requirements – when feeding almost exclusively on below-canopy grasses. The other ungulate species could not meet their phosphorus demands by feeding at either site without exceeding their maximum fibre intake, indicating the unfavourable conditions during the dry season.We conclude that grazing ungulates, particularly warthog, zebra, and buffalo, actually depend on the available below-canopy grass resources. Our model therefore helps to quantify the importance of higher quality forage patches beneath savannah trees. The composition of grazer communities depending on below-canopy grasses can be anticipated if grazer food requirements and the abundance of large trees in savannahs are known. The model suggests that the conservation of large single-standing trees in savannahs is crucial for maintenance of locally grazing herbivores.     

Does competition regulate ungulate populations? Further evidence from Serengeti,Tanzania

Summary Changes in populations of several ungulate species in the Serengeti-Mara region of East Africa over the past 30 years suggest several hypotheses for their regulation and coexistence. Recent censuses in the 1980s have allowed us to test the hypotheses that: (1) there was competition between wildebeest (Connochaetes taurinus) and Thomson's gazelle (Gazella thomsoni). This predicted that gazelle numbers should have declined in the 1980s when wildebeest were food limited. Census figures show no change in gazelle numbers between 1978 and 1986, a result contrary to the interspecific competition hypothesis; (2) wildebeest and African buffalo (Syncerus caffer) populations were regulated by intraspecific competition for food. Since both populations reached food limitation in the 1970s, the hypothesis predicted that the populations should have been stable in the 1980s. The results confirm these predictions for wildebeest and the buffalo population in the Mara reserve. In the Serengeti the buffalo population declined 41% over the period 1976–1984. The decline was not evenly distributed over the park, some areas showing an 80–90% decline, others no change or an increase in numbers. The decline was associated with proximity to human habitation; (3) an outbreak of the viral disease, rinderpest, in 1982 may have been the cause of the drop in buffalo population. Blood serum samples to measure the prevalence of antibodies were collected from areas of decreasing, stable and increasing populations. If rinderpest was the cause of decrease there should be a negative relationship between the prevalence of rinderpest and the instantaneous rate of increase (r). The results showed no relationship. We conclude that rinderpest was not the major cause of the drop in buffalo numbers. Elephant (Loxodonta africana) numbers dropped 81% in Serengeti in the period 1977–1986. In the Mara there was little change. The evidence suggests that extensive poaching in northern and western Serengeti during 1979–1984 accounted for the drop in both elephant and buffalo numbers.     

A rare fight in female plains zebra

We describe a fight between two female plains zebra (Equus burchelli). Plains zebra are ungulates with stable social groups known as harems. Female aggression rarely escalates to the level we observed. The fight immediately followed the birth of a foal to one of the females. The initiating female repeatedly kicked and bit the mother, who reacted aggressively and by guarding her foal. We present hypotheses on the causes underlying this rare event.     

The role of wetlands in wildlife migration in the Tarangire ecosystem,Tanzania

Twenty-two years of rainfall data from six sites, 5 years of animal migration data and 2 years of water quality at 13 sites were explored to quantify the role of water in the Tarangire ecosystem. Inter-annual fluctuations in rainfall were large and not predictable solely from the Southern Oscillation Index. Seasonal fluctuations of rainfall were pronounced, with marked wet and dry seasons. In the dry season, the only drinking water available for wildlife was the Tarangire River and a number of small, scattered wetland-fringed water holes. Their salinity was often high (>8 ppt) and was higher in dry years than in wet years, as well as at the start of the wet season. Water quantity and quality may control the annual migration of wildebeest, zebra, elephants and buffaloes. These animals aggregate in the dry season in areas with the least salty water. The timing of seasonal variations in rainfall is largely predictable and controls annual migration. All wildebeest and most zebras migrated out of Tarangire National Park and into the wider Tarangire ecosystem at the start of the wet season, and they returned into the park in the dry season. Some elephants and buffaloes also migrated in out of the park and a larger resident population remained, whose size may vary inter-annually depending on surface water quantity and quality. The extent of the migration zone may also vary inter-annually.This revised version wa published online in March 2005 with corrections to the issue cover date.     

Black wildebeest seek shade less and use solar orientation behavior more than do blue wildebeest

Many ungulates, including wildebeest, seek shade and orient their bodies relative to incoming solar radiation in order to reduce environmental heat loads. Blue (Connochaetes taurinus) and black wildebeest (Connochaetes gnou), which co-exist artificially in some reserves in South Africa, are thought to adopt different thermoregulatory behaviors to mitigate high environmental heat loads. However, whether or not blue and black wildebeest use different behaviors to reduce heat loads in regions where they co-occur has never previously been examined. We compared the shade seeking and solar orientation behavior of free-ranging blue and black wildebeest in summer at three locations in South Africa where both species co-occur. We found that blue wildebeest exhibited more shade seeking behavior than did black wildebeest at all times of day, at all study sites. Black wildebeest remained in the sun but were more likely than blue wildebeest to orient their bodies parallel to the sun at all study sites, a behavior which reduces the amount of surface area exposed to incoming radiation. Black wildebeest were most likely to employ parallel solar orientation during the hottest times of the day when the sun was not directly overhead (i.e., solar noon ± 1 hour). We thus demonstrate that co-occurring blue and black wildebeest use different thermoregulatory behaviors to reduce high heat loads. It is possible that the lack of shade in the historical distribution of black wildebeest led to selective pressure for reliance on solar orientation. Differences in thermoregulatory behavior can affect species-specific heat loads, habitat use, body mass, fitness and grazing activity. Such differences may also allow blue and black wildebeest to inhabit separate microclimates within the same habitat, provided there is sufficient heterogeneity in vegetation structure, potentially facilitating reproductive isolation.     

Adaptive heterothermy and selective brain cooling in arid-zone mammals

Adaptive heterothermy and selective brain cooling are regarded as important thermal adaptations of large arid-zone mammals. Adaptive heterothermy, a process which reduces evaporation by storing body heat, ought to be enhanced by ambient heat load and by water deficit, but most mammals studied fail to show at least one of those attributes. Selective brain cooling, the reduction of brain temperature below arterial blood temperature, is most evident in artiodactyls, which possess a carotid rete, and traditionally has been considered to protect the brain during hyperthermia. The development of miniature ambulatory data loggers for recording body temperature allows the temperatures of free-living wild mammals to be measured in their natural habitats. All the African ungulates studied so far, in their natural habitats, do not exhibit adaptive heterothermy. They have low-amplitude nychthemeral rhythms of temperature, with mean body temperature over the night exceeding that over the day. Those with carotid retes (black wildebeest, springbok, eland) employ selective brain cooling but zebra, without a rete, do not. None of the rete ungulates, however, seems to employ selective brain cooling to prevent the brain overheating during exertional hyperthermia. Rather, they use it at rest, under moderate heat load, we believe in order to switch body heat loss from evaporative to non-evaporative routes.