Is It Advantageous for Acacia Seeds to Be Eaten by Ungulates?

Abstract

It has been widely reported that herbivory benefits plants by increasing foliage density (Pellew 1983a,b, 1984a, Coe and Coe 1987, Milton 1987, Du Toit 1990) and by overcompensation leading to increased seed output (Paige and Whitham 1987). However this issue is surrounded with uncertainty (Owen and Wiegert 1976, 1981, 1982a, b, Westoby 1985, Belsky 1986, Gordon and Lindsay 1990). By analogy, an interesting proposition is that ungulates also benefit Acacia trees by consuming their seeds, despite the fact that some seeds are damaged and killed in the process. There are two types of Acacia seed 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 seed 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 they are eaten, trodden on, or degraded by fungal attack (pers. obs.). African ungulates consume Acacia seed pods during the dry season when other food sources are rare. Some of the seeds which have not been destroyed by chewing or digestion are subsequently passed in dung (Burtt 1929, Lamprey 1963, 1967, Gwynne 1969, Kingdon 1971, Sinclair and Norton-Griffiths 1979, Pellew 1984b, Coe and Coe 1987). The adaptations of indehiscent Acacia seed pods that appear to favour consumption by ungulates (nutrient content, a rounded form and a strong attractive scent) as described by Gwynne (1969), Coe and Coe (1987), and supported by personal observations provides evidence that Acacia seed pods are attractive to potential ungulate dispersers. Two main questions arise from ungulate consumption of Acacia seeds: