The topic of the costs and benefits of Acacia seed consumption by ungulates was reviewed by Miller and Coe (1993). Preliminary data presented in Miller and Coe (1993) suggested that in the 'inevitable' presence of bruchid beetles it paid Acacia seeds to be eaten by ungulates, since seeds surviving ingestion showed a greater potential for germination compared to uneaten seeds. The results presented in Miller and Coe (1993) were crude since data were collated from a variety of sources. It was suggested that it would be helpful to have comparable data gathered within one ecosystem during one time span, in order to more accurately assess the costs and benefits of Acacia seed consumption by ungulates. A primary aim of this current research was to gather data within one ecosystem over one time period in order to ascertain whether it is advantageous for Acacia seeds to be eaten by ungulates. Acacia undergo sexual reproduction and bear seeds within woody pods (Coates-Palgrave 1984, Coe and Beentje 1991). Within the genus of African Acacia there are two types of pods those that are dehiscent, i.e. the pod splits and the majority of the seeds are dispersed by wind or gravity; and indehiscent species, the pods of which do not split but remain on the tree until removed by browsers or mechanical action (e.g. wind, heavy rain, natural senescence or browsers brushing against them). The seeds of the latter species then remain in the pods until pods are eaten, trodden on, or degraded by fungal attack (Miller 1993). In African Acacias pod consumers which disperse seeds include birds (Glyphis et al. 1981), ungulates, rodents, termites and ants (Miller 1993, 1994a,b, Miller and Coe 1993). The pathway taken by an Acacia seed into the soil can be complex, and involves the interactions of both insect and vertebrate seed predators and dispersers (Miller 1993, 1994a,c, Miller and Coe 1993). These complex interactions have been briefly discussed for a variety of tree species including Acacia spp. by Traveset (1990, 1991, 1992), Sallabanks and Courtney (1992). Bruchid beetles are one important group of insects which attack Acacia seeds. The larvae of bruchid beetles undergo development inside seeds (Southgate 1979, New 1983, Tolsma 1989, Ernst 1992) and can destroy 9-100% of the cotyledons (Ernst 1992). Infested seeds are usually non-viable (Peake 1952, Coe and Coe 1987, Ernst et al. 1990a), although not exclusively so. Visibly infested seeds can germinate, but germinate less than uninfested seeds (Halevy 1974, Lamprey et al. 1974, Miller 1993). The extent to which infested seeds can germinate probably depends on how much of the embryonic tissue has been consumed (Coe and Coe 1987). Bruchids initially attack fresh green Acacia pods on the tree; reinfestation following emergence may then occur on mature, dry pods on the tree or ground (Southgate 1981, Tolsma 1989, Ernst et al. 1990b, Miller and Coe 1993). Attack rates can reach 99% in Africa (Southgate 1979), although lower attack rates are usually reported (Coe and Coe 1987). High levels of attack may be due to pod storage prior to analysis (Ernst et al. 1990ab, Miller 1993). African ungulates consume indehiscent Acacia pods during the dry season when other food sources are rare. Some insect damaged fruits and seeds may be unattractive to herbivorous mammals (Semel and Anderson 1988); however, ungulates do browse on bruchid infested seeds (Janzen 1969, Halevy 1974, Lamprey et al. 1974, Coe and Coe 1987, Miller 1993). Passage of Acacia seeds through the ungulate gut decreases the percentage infestation by bruchids in seeds that survive passage and are egested, possibly by a combination of the action of gut acids upon bruchid larvae (Pellew and Southgate 1984, Coe and Coe 1987, Miller 1993, Miller and Coe 1993), and crushing of infested (and therefore weakened) seeds during gut peristalsis (Miller 1993). A high proportion of viable, uninfested seeds may also be destroyed during gut passage (Miller 1993). Seeds which have not been destroyed by chewing or digestion are subsequently passed in dung (Burtt 1929, Lamprey
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Jul 23, 2004
Oranuch Leelapon + 2
Open Access