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MANY species of acacia plants have evolved complex relationships with ants. Among them is the Central American bull-horn acacia, which provides both food and shelter for its protective insect army.
By doing this, they cut the bridges that might be used in an assault by marauding leaf-cutter ants. Only one route remains for these invaders to take — that of straight up the main trunk of the bull-horn acacia. Such a frontal assault is a good deal easier for the acacia ants to repel than would be a flank attack via the vines. |
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| The ants are repaid for their work with
free boarding and lodging. They live in acacia’s
large, hollow thorns, which they enter by chewing into a
weak spot near the tip. In one of them a home is
established for the brood and the queen. The acacia yields nectar from its flowers for the adult ants and a special food for the larvae, which is collected by the worker ants from the tips of its leaflets. This takes the form of tiny, brightly coloured structures called Beltian bodies, which are rich in fats and proteins and provide the larvae with a valuable dietary supplement. Another plant of Central America that attracts a particular species of ants is the giant piper plant. This also offers shelter and tiny food packages rich in protein and fat. The ants live in the hollow curled leaf blades but are too small to provide a defence against leaf-eating insects. So they serve the plant, instead, by picking off fungal spores from the leaves and stems. The relationship between the piper plant and this single species of ant has developed so far that the plant will not produce food bodies when its leaf blades are unoccupied. This is, however, a relationship that one species of beetle has learned to exploit, for the beetle larvae are able to mimic the ants’ secretions so convincingly that the piper plant responds by producing the food packages for them. But in this case, the plant gains nothing in exchange, because the beetles neither tend it nor guard it. In another example, aspen sunflowers grow in the Rocky Mountains of Colorado, where they provide a nuptial couch for picture-winged flies. Having mated, the females lay their eggs on the buds and flowerheads, to ensure that when they hatch, the larvae have a plentiful supply of food. In fact, they eventually eat their way through the flower’s entire reproductive system, devouring its seeds and effectively sterilising the plant. However, the aspen sunflower is not entirely helpless. It attracts and recruits a team of some 25 ants to act as guards. Though the ants cannot catch the flies, they deter them from landing. In return, the plant secretes a rich nectar to feed its sentinels. Another example of a symbiotic relationship is also very interesting. Growing from the top of a tree in Australia, the vivid green leaves of an ant-house plant positively glow with health. And this despite the fact that the plant’s bare roots tap no food source and serve only to anchor the plant in its eyrie. The ant-house plant owes its robust condition entirely to its relationship with ants. It has somehow evolved a ready-made ants’ nest in its swollen stem, complete with nesting chambers, ventilation shafts and walkways. On its outer surface, the plant stem is covered in spines that provide easy footholds for the ants and protect them from predators. Even the entrances to the network of protected tunnels are ant-seized, inaccessible to larger creatures and easy to defend. Although the ants guard their home against leaf-eating and other pests, it is not their most important contribution to the plant’s welfare. When the ants feed insects to their larvae, they deposit the remains of their kill and their own faecal wastes neatly in refuse chambers, where an assortment of microbes breaks the material down into a rich compost. An ant-house plant absorbs this nutritious food through warty structures on the inner surface of the chamber. It is entirely due to this rich supply of fertiliser that these plants manage to survive in their difficult environment. |
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These ants patrol the tree, attacking any
intruding insect that ventures into their domain and
carrying off its remains to feed their young. An even
more unusual aspect of the relationship is the manner in
which the ants tend their home trees. They clear away any
seedlings that germinate around the base, and keep the
soil beneath the tree free from weeds that might compete
for moisture and nutrients. Vines that reach across to
the branches from other trees are also severed.