Biology Reference
In-Depth Information
reasons. These trees and aloes were rare in many parts of Natal, particularly between Hilton
and the Highveld. Could it be because herbivores were no longer eating them? Acacia trees
defend themselves somewhat in that they pump tannins into their leaves and branches and the
bitter tannins then force the kudu to move on to the next tree. Within two minutes the amount of
tannins in Acacia nigrans leaves increases 70%. Indeed, in game farms at least 100 hectares
are required for every 3 to 4 kudu otherwise the kudu starve to death because of the tannins.
The reason for this is that tannins, as in tanning skin to produce leather, bind to the protein in
foods and thus the protein the kudu eats is not absorbed, in a sense starving it to death from
protein deficiency. Tannins are not the only way acacia trees protect themselves. Ants and
acacia have a cooperative interaction (“mutualism”) in that at the base of thorns ants form nests
and the hundreds of thorns may have 100,000 ants. The three types of ants get shelter and
nectar and ants protect trees against elephants and kudu. However, if the trees are fenced off
there are less ant nests, the trees get sick, and a fourth species of less aggressive ants settles in
and does not protect the trees against the invasion of beetles or aphids that are attracted by the
nectar. Thus, it is of interest that the trees need herbivore browsers to remain healthy. It is also
of interest that if the ants are removed, elephants increasingly ate the trees. But once the ants
were reintroduced, elephants ate less of them because the ants climb up an elephant's trunk -
which they dislike. Elephants are also adverse to African bees and if recordings of bees are
played to elephants within 10 seconds half leave and within 80 second most elephants will
have left an area. Hence, in Zimbabwe, apart from planting peppers, bee hives are being used
to shield crops. The calculation of land and vegetation needs for animals has become quite
sophisticated based on research by Norman Owen-Smith from Wits and others.
Within conservation circles and wildlife management there is a concept of carrying capacity
of land. This can be esoteric and obtuse but for those who have an interest in the issues some
of these aspects are summarized below. Owen-Smith's basic concept is that sunlight, water
and soil produces vegetation that in turn is converted (Conversion) into herbivore biomass.
This biomass is lost mostly to predators with feedback loops to the previous stage. In addition,
not all the vegetation is digested. Elephants in particular do not metabolize all of the food they
intake with a large component of the vegetation going to basal metabolism. Animals are often
lost to intrinsic mortality and not to predators because of senescence, disease, starvation and
drought. From these inputs Owen-Smith created a net physiological model of herbivore
biomass population dynamics based on his research with rhino, kudu, and other large
herbivores that he called GMM: Growth, Metabolism, and Mortality. These models have been
widely applied, along with other equations, for managing wildlife population in Southern
Africa. For those who may have further interest in this science, his publications are
recommended but some of the basics are summarized below.
If all herbivores died from senescence and had an average life expectancy of 10 years, about
10% would die each year. However this is pre-empted by other causes of death such as
predation by predators, metabolic costs, food abundance, and disease.
Thus, the equation for mortality: Mq = Mp + z0 + {zMp/G} + (H, C)
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