6 Grazers and browsers
A good deal of the discussion so far has been related to animals that eat leaves in the form of grass and other herbaceous plants, the grazers, but this is not the only type of plant food. Also available as food are the leaves of trees and bushes. These form the diet of the browsers.
There are some important differences between grass and other herbaceous plants as food and the leaves of trees and bushes as food. Consider two of these: (a) how much might be available in a given area; and (b) how they might each be affected by drought. Write a few sentences about each of these differences, on the basis of what you already know about grasses and suchlike and about trees with leaves, and from LoM and the TV programme.
(a) Grass tends to occur in large continuous expanses and grows close to the ground. Trees often occur in more scattered clumps and there may be some distance between one clump and the next. Also the leaves are at different heights above the ground, from low-growing bushes to tall trees.
(b) Grass (and other herbaceous) plants generally have short roots and so the growth is very dependent on the amount of water in the soil, and therefore on the local rainfall. If the soil dries up, the grass very soon withers and dies, though the roots probably remain viable in the soil, enabling the grass to regrow when the rains return. Trees, on the other hand, have deep roots which can obtain water from far below the surface and so some retain their green leaves for longer in times of drought and others are evergreen.
There are many types of tree in Africa that provide food for a variety of species of browser, of which one is the acacia.
Watch the TV programme again from 23.42-30.35 and make notes in answer to the following questions.
(a) Which animals are shown feeding on the acacia trees? Are they all competing for the same leaves?
(b) What is the likely benefit to the acacia of having these different animals browsing on its leaves?
(a) You should have observed a gazelle, called a dik-dik, feeding on the lowest leaves, then the impala (an antelope) feeding slightly higher up. Another antelope, the gerenuk, is able to stand on its hindlegs to eat leaves from nearer the top of the tree and, of course, giraffes can reach even higher. Elephants have the ability to stand on their back legs to reach high branches and they can use their bulk to push over the whole tree, so that they can browse on the leaves at the very top of the tree, which even the giraffes cannot reach. Thus five species are using the same tree for their food but each is eating the leaves at a different height.
(b) The acacia produces tough seed pods, which are eaten, but the seeds are not digested by the browsers. The seeds are deposited in their faeces at some distance from the parent tree, thus spreading the seeds across a wide area and allowing the acacia to increase its range.
Despite the seed-dispersal benefit, the acacia tree is under constant risk of losing its life-support, the leaves, which are vital for producing its own nutrients by means of photosynthesis. How plants protect themselves against attack by the plant predators is the topic of the next section.