Chapter 8, Bugs and Drugs, discussed the mechanisms plants use to defend themselves and how those mechanisms affect the evolution of the organisms that eat them. Plants use chemical defenses called secondary compounds to discourage herbivore predation. Many of these compounds are natural insecticides and they are innocuous to large animals such as humans. In fact, humans use many secondary compounds as ingredients in pharmaceuticals, herbs, spices, and fragrances. The same chemical in basil, for example, that gives it its delicious flavor mimics the growth hormones of insects and severely impacts their ability to function.
There are, however, limits to the use of secondary compounds. Herbivores evolve to combat the toxicity of plants. The plants, in turn, continue to evolve to make their defense mechanisms stronger. Plants can only up the ante so much or else they risk poisoning themselves with their own defensive chemicals. In addition, secondary compounds require energy and resources, namely nitrogen, to manufacture. Nitrogen is also important to growth so some plants sacrifice defense for growth. This is the case for plants growing in nutrient deficient soil, such as acid bogs, or in the rainforest where plants must grow rapidly to compete for light. The balsa tree, for example, does not invest in secondary compounds but in rapid growth instead. Insects may consume as much as 40 percent of the tree’s leaf area but the tree continues to grow and grow.
In ecosystems such as tropical white sand forests, trees do not have the luxury of sacrificing defense for growth. The soil in white sand forests is so course and porous that nutrients leach rapidly away. Plants cannot afford to lose leaf matter the way the balsa tree can because they have very little hope of recovering those nutrients from the soil. Instead, they invest heavily in secondary compounds, largely tannins. Tannins are chemical compounds that bind with proteins to make large, complex polymers that hard to digest. Tannins are made from sugar, not nitrogen so plants can afford to synthesize these compounds without jeopardizing their ability to grow. Tannins are very acidic and when they leach from the plants they contaminate water sources and turn rivers and streams black. These black waterways are the equivalent of terrestrial deserts.
The diverse defense mechanisms of plant life diversify herbivores who evolve to combat the defense mechanisms. When the evolution of one species influences the evolution of another, it is called coevolution. One way in which herbivores combat secondary compounds is through mixed function oxidase which is a detoxifying enzyme. It turns fat soluble substances into water soluble substances so the organism can excrete the toxins. Some organisms have evolved to combat the defense mechanisms of a specific part of a specific species of plant. These organisms are called specialists. They do not have to compete with other species for resources, however, their specialization makes it difficult for them to adapt to environmental changes. Other organisms feed on a wide range of plant species. These are called generalists. They have strong digestive systems and they can adapt their diets easily to environmental changes. They have higher concentrations of mixed function oxidase in their guts than do specialists. Mixed function oxidase, however, cannot protect against every secondary compound. For this reason, generalists operate on the principle that dosage equals toxicity. They do not gorge themselves on any one food source but rather graze lightly and then move on. This limits the amount of toxins they receive from each food source and it also prevents poisonous pushback from the plants. Plants increase their toxicity after damage.
Chapter 9, Creeping Socialists, was dedicated to discussing the importance and pervasiveness of ants in the rainforest. Ants make up a large part of the biomass of the rainforest and they come into contact with many different organisms. Colonies of Atta ants may have up to one million workers. Different groups or castes of workers do different jobs. Smaller workers take care of the eggs, medium ones forage for food, and the largest ones, soldiers, protect the nest. If food and resources are short, members of one caste will kill off members of another caste, usually soldiers, that is not essential to the nest. The members of the unessential caste do not resist their slaughter because they are sterile; the best way to protect their gene pool is to die so the nest can survive. The queen is the only member of an ant colony that can reproduce.
The diversity of ants depends on the type of tropical habitat. Neotropical ants thrive in forest regions while temperate ants thrive in open spaces. This is because temperate ants use solar energy from the sun. Ants are scarce in tropical cloud forests, but abundant higher up in the mountains because of the clear air and intense sunlight. In tropical lowlands, ants depend on warmth rather than sunlight for energy.
Ants are territorial about their food. When they find a food source, they leave a chemical trail back to the nest so other workers can come and break down the food source. They must be quick or ants from a different colony will invade the food source, such as the aggressive Azteca ant who use chemical sprays to keep competitors away.
Plants usually benefit from the presence of ants because they eat insects, larvae, and eggs of the plants’ leaves. Plants have developed adaptions to attract ants called extrafloral nectaries, or sources of nectar outside their complicated floral structures. Because ants are territorial of their food, they will protect the sources of sweet nectar.
This relationship is demonstrated between Pseudomyrmex ants and the bullhorn acacia. The ants and the bullhorn acacia have a mutualistic relationship. The plant, which has hollow thorns where the ants live, provides the ants with shelter and a source of food: extrafloral nectaries. It is in the ants’ best interest to protect this plant.
Moraceous trees and Azteca ants have a similar relationship. Moraceous trees have holes in their trunks through which the ants can enter and produce nutrient rich structures called trichilia. The ants defend the tree from herbivory to protect their source of food and shelter. Plants attract ants as an alternative to chemical and mechanical defenses.
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