Most people really don't know that much about the mighty wildebeest, so I asked Mark, the F.O.W.'s head biologist to enlighten the masses.
Wildebeests
A 1.3-1.6 million (as of 1989) member migratory population of wildebeest roams the Serengeti, and occasionally the adjacent Masai Mara reserve in Kenya and several other smaller game reserves (Murry 1990). Adult wildebeests weigh about 125kg and contribute more than 60% of the animal biomass of the Serengeti-Mara ecosystem (Frank et al. 1998). The wildebeest plays a major role in the function and structure of the entire ecosystem and is the keystone species (Sinclair 1990).
There are two species of wildebeests, blue (Connochaetes taurinus) and black (Connochaetes gnou). They are in the Bovidae family. The blue wildebeest is most frequent in the Serengeti-Mara, while the black wildebeest is mostly restricted to southern Africa, except where it has been moved to game reserves. The two species differ in body and horn shape, but are still related enough to produce hybrids (Corbet, et al.1994). Fossil evidence suggest that the two species diverged about 2 million years ago. Allozyme and mtDNA analyses of blue and black wildebeests suggest a slightly more recent split (Corbet, et al. 1994).
Wildebeests are found in large herds, often with members of other species such as zebra and Thompson’s gazelles. Wildebeest population apparently reached its maximum allowable level in 1977. The population was increasing from 1961 until 1977, possibly due to the disappearance of a virus called Rinderpest and increased dry season rainfall in the early to mid 70’s, when it leveled off and stabilized at about 1.34 million (Sinclair et al. 1985). Rinderpest was brought to Africa from Asia in the 1880’s and killed off many cattle, buffalo, and wildebeest until a cattle vaccination effort succeeded in ridding the Serengeti of the virus in 1962 (Sinclair 1990).
Given the poor food supply of the Serengeti, it might seem strange for competing species to mix in the same herd. An analysis of data from population censuses from 1958 to 1986 showed no evidence of interspecific competition between wildebeests and Thompson’s gazelles (Dublin et al. 1990). The interspecific competition hypothesis first suggested by Sinclair predicted that Thompson’s gazelles numbers would have declined when wildebeests became limited by food supply, but no such decrease was observed (Dublin et al. 1990). The results of at least two studies instead supported the intraspecific competition hypothesis for the limitation of wildebeest population size in which the population stabilizes from density dependant dry season mortality due to shortages in food supply, and subsequent undernourishment (Sinclair et al. 1985, Dublin et al. 1990).
There is a large body of evidence that documents the wildebeest’s ability to shape the environment in which it lives. Wildebeests can influence plant establishment, growth, and reproductive success, which leads to plant evolution and results in increased grazing efficiency (McNaughton 1984). The best food for large herd forming grazers in the Serengeti is short grasses. Taller grasses generally contain less protein concentration, more fiber, and are harder to digest (Murry 1990). When in densely packed herds, grazers select for short grasses because they are more likely to survive heavy grazing (Hartvigsen McNaughton 1995). Plants also naturally respond to grazing by increasing lateral growth (Lewin 1985). The dense herds occur in the wet season when rain is more frequent and small areas of grasslands are capable of producing enough food to feed large numbers of grazers (McNaughton 1984). Wildebeests concentrate in the southeastern region during the rainy season where short grasses dominate the Serengeti (Murry 1990). The result is a doubled plant biomass concentration (McNaughton 1984). Another study reported plant biomasses of 4.3 mg/cubic cm in short, 1.2 mg/cubic cm in medium, and 1.9 mg/cubic cm in tall grasslands of the Serengeti (Sheer, 1998). Shorter grasses grow much thicker than tall grasses, allowing wildebeests more food per bite. Improved food resources as a result of heavy grazing has not been observed anywhere else outside of the Serengeti. All other studies have shown deleterious effects of heavy grazing on food availability, so the Serengeti appears to be the exception rather than the rule (Westoby 1985).
Wildebeests also influence soil conditions by promoting the cycling of diet-enhancing nutrients. The soil has been shown to be richer in Na (not required by plants) and N (essential to both plants and animals) in areas where grazing by wildebeests is common than in areas where grazing does not occur (McNaughton 1997). The Serengeti soil has a high level of urease activity that allows the ecosystem to conserve N at a very efficient rate (McNaughton et al. 1997). Increased Na intake is critical for lactating females and young (McNaughton 1997). Plants in areas with high animal densities showed a three fold increase in Na levels, 80% increase in Al and Fe, 50% increase in P, 40% increase in Mn and Pb, and a 10-23% increase in levels of Ca, Mg, and V, relative to areas not supporting animals (McNaughton 1988).
There are several theories for why wildebeests form herds. Studies have shown that individuals gain better nutrition from food lawns (McNaughton 1984). Another major factor is the dilution of risk of predation (Lewin 1985). Risk of predation declines with increasing herd size. Since most successful hunts are limited to one kill, the odds are small that any one given individual will be killed. There are several factors that influence the decreased predation risk. Predators may have a hard time focusing on one individual to hunt and be constantly distracted. Wildebeests may benefit from forming mixed herds in which they are the dominant species because there is some evidence that predators will often look for individuals that stand out from the crowd because they are easier to concentrate on. As group size increases, the proportion of individuals on the outer edges of the herd where risk is higher decreases. With a larger group size, there are more eyes to scan the area for predators and as group size increases the time necessary for scanning decreases and leaves more time for grazing. Also groups can sometimes mount a successful defense against predators, which sometimes occurs with wildebeests fighting off lion and hyena attacks (Scheel 1993).
There are several theories for why wildebeests migrate. Since longer grasses have less protein and are harder to digest, lactating females may require shorter grasses, migration helps escape predation from species confined to territories such as lions, wildebeests don’t like sticky ground, avoidance of tsetse flies, and reduction of grazing competition (Murry 1995). Wildebeest are found in areas of low annual rainfall during the wet season and in areas with higher annual rainfall in the dry season which may lead one to believe that the wildebeests are avoiding wet ground (McNaughton 1990). There has been little experimental evidence for most of these theories. However, a lot of experimental evidence supports the theory that wildebeests migrate in response to cyclic nutritional requirements of Ca Mg Na P Cu and N (McNaughton 1988, 1990, McNaughton et al. 1997, Frank et al. 1998). Grasses of the dry season ranges in the north are low in Na, Ca, P, Mg, so it is possible that the wildebeests leave the areas with highest rainfall during the wet season for drier areas with increased levels of vitally important nutrients (McNaughton 1990).
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