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To investigate bushmeat hunting sustainability and harvest offtake in the Serengeti, a wide variety of models have been used. Although a number of species are harvested for bushmeat in the study-area, previous models have focused on wildebeest (Hilborn Sinclair 1979, Pascual Hilborn 1995, Barrett Arcese 1995, Hilborn et al. 1995, Hilborn Mangel 1996, Pascual et al. 1997, Mduma et al. 1998, Holdo et al. 2010), while Metzger et al. (in press) modelled buffalo population dynamics.
To capture the multi-species nature of this system, my model incorporates resident species (topi and impala), which are harvested all year-round (Hofer et al. 1996), as well as wildebeest; the wildebeest migration is associated with higher hunting participation in the western corridor (NW Serengeti; Barnett 2000, Loibooki et al. 2002, Holmern et al. 2007) but wildebeest are relatively protected from hunting during the wet season (Campbell et al. 2001). The selection of these two resident species was based on: data availability; habitat (snaring takes place mostly in the woodlands and thus resident species of the woodlands, such as topi and impala, are more likely to be harvested than resident species of the plains, such as the Thomson's and Grant's gazelles; Rentsch et al. in prep); information on bushmeat consumption and preference (impala and topi have been reported as frequently harvested and topi is among the preferred species for its taste; Ndibalema Songorwa 2007); and prey behaviour (for example, buffalo and eland were not used as they are considered to be easier to capture due to their herding and escaping behaviour, respectively; Ndibalema Songorwa 2007, T. Sinclair, personal communication). Additionally, although biological information is available on wildebeest, impala and topi, the amount of information and its quality vary between species; these three species provide an interesting case-study to evaluate management strategies under uncertainty due to incomplete biological knowledge.
Bushmeat hunting in the study-area is conducted all year-round but the number of hunters has been suggested to increase drastically during the dry season when the wildebeest migration passes through the western corridor, close to villages (Barnett 2000, Loibooki et al. 2002, Holmern et al. 2007). Poaching is mainly done by snaring and, despite generalized opinions that hunting in the area is mainly non-selective (Campbell Hofer 1995, Holmern et al. 2002), species and gender selectivity, either accidental or intentional, have been suggested (Hofer et al. 1993, Arcese et al. 1995, Campbell Hofer 1995, Holmern et al. 2006, Ndibalema Songorwa 2007). Holmern et al. (2006) also suggested that, where law is poorly enforced in the Serengeti, large herbivores, such as topi and wildebeest, might be mostly harvested through snaring, while mid-sized herbivores, such as impala, are actively stalked by hunters.
Some potential considerations:
a) hunting selectivity (species and gender);
b) resident quotas for bushmeat hunting (currently, all sex- and age-classes for topi and wildebeest but only adult males for impala; allowed from 1st July until 31st December);
c) quotas for trophy hunting (currently, adult males and females for topi and wildebeest but only adult males for impala; allowed from 1st July until 31st December);
d) seasonal variation in hunting strategy and effort.
Impala (Aepyceros melampus)
Impala are mid-sized sexually dimorphic antelope (males 60-65 kg, females 40-45 kg), widely distributed through wooded grassland and open woodland areas of eastern, central and southern African savannahs (Jarman Jarman 1973). They are intermediate (or mixed) feeders, predominantly grazing during grass growth, and browsing at other times (Estes 1992). The main movements of these sedentary ungulates may include going from their wet season ranges to locations near permanent water during the dry season (Dasmann Mossman 1962) or moving along the soil catena according to season and forage quality (Jarman 1979). Daily movements have been described to be up to 3 km in the dry season and 0.95 km in the wet season (Estes 1992). In the Serengeti, impala occur in the woodlands and have annual home ranges of 100 to 1000 ha (Jarman Sinclair 1979).
A detailed description of the social behaviour and reproduction of the impala is given by Jarman (1974) and Jarman Jarman (1973). Impala reproduce polygynously and their populations are composed of large groups of females (â€œfemale clansâ€Â), groups of bachelors and lone territorial males. About one-third of adult males hold territories; these territory holders generally exclude all other males while allowing female herds to move freely along several territories. Female clans group females and young and vary in composition and size, ranging from 6 to over 100 females. Age at which young males are evicted from the female society ranges from 5 to 15 months old in eastern and southern Africa, respectively. Bachelor groups have an age-based hierarchy and include sub-adults and non-territorial adult males; these groups may be forced to occupy marginal areas (Jarman Jarman 1973). Jarman and Jarman (1973) described differences in male survival rates between eastern and southern Africa populations, which they suggested could be related to young male's eviction age from female clans due to differences in the proportion of the year during which adults males are effectively territorial.
The degree of territorial behaviour in impala populations is linked to the breeding season and climatic regime, and impala have a flexible social organization. Territoriality is seasonal and its duration differs among impala populations; during the dry season, when territories are frequently not maintained, males and females associate relatively freely in mixed herds. In East Africa, the extended breeding season leads to maintenance of territories for longer periods than in Southern Africa. For example, collapse of territoriality does not happen in all years or in all regions in the Serengeti, where territoriality may disappear for a few weeks at the end of the dry season (Jarman Sinclair 1979). Although territoriality may affect impala population structure and population reproductive potential (Jarman Jarman 1973), this has received little attention and, to my knowledge, no empirical information from impala populations is available; this will not be incorporated in the model.
Impala breed throughout the year in the Serengeti with an uneven distribution of impala births; two-fifths of births result from conceptions in April and May while conception rates at the end of the dry season are very low (Jarman 1979, Jarman Sinclair 1979). The peaks of births are in the dry season (37% born in June-August) and early wet season (52% born in October-January; Sinclair et al. 2000). Ogutu et al. (2008) also described October-December to be the main peak of newborn abundance in the Maasai Mara National Reserve. Females produce their first young at two years of age and no twins are produced (Dasmann Mossman 1962). Based on Fairall's (1983) study of 860 impala females over 8 years in South Africa, female fecundity is assumed to be 70% in the first year of breeding and 90% in successive years. Males begin reproducing as they mature and gain territories in their fourth year (Estes 1992). Sex-ratio at birth is 1:1 but adult sex-ratio is approximately 1M:2F (Fairall 1983); adult ratios of up to 1:20 have been proposed to increase the production of impala populations without decrease in fecundity (recommended ratio was 1 male for 5-10 females; Fairall 1985, van Rooyen 1994) although the potential consequences of sustaining these levels in impala populations have rarely been considered (Ginsberg Milner-Gulland 1994).
Jarman and Jarman (1973) studied impala populations in the Serengeti during 10 years and their work was used to derive detailed age- and sex-dependent information on survival (Table 1). Sinclair (1995) presented similar results of juvenile survival in the Serengeti and Owen-Smith and Mason (2005) investigated impala survival in South Africa's Kruger National Park. A summary of their estimates is presented in Table 1.
Table 1. Summary results of impala survival analyses