Interactions between individuals, species, or functional groups in ecosystems have been classified in various ways. One of the oldest terms for a cooperative interaction between two species is symbiosis. Another term for interactions that benefit the interacting species is mutualism. Boucher (1985) has classified mutualisms into several categories:
1. Nutrition/digestion: increasing the availability of nutrients, energy, and water to one or both of the interacting species. An example is mycorrhi-zae, an association between plants and a fungus through which the fungus obtains carbon for energy, and the plants gain an increased ability to take up water and nutrients from the soil.
2. Protection for one or both interacting species. Insects that live on or in a plant and derive energy from the plant may protect the plant in which they live. When an animal begins to browse a protected plant, the insects attack the intruder.
3. Pollination of some plants depends upon the actions of specialized insects and birds. The latter benefit from the mutualism by obtaining energy from the plant's nectar.
4. Seed dispersal: the spreading of seeds away from the parent tree by birds, bats, and small mammals that eat fruits increases the probability that some seeds will survive and grow.
An important nutrition/digestion mutualism in the tropics is the interaction between termites and fungi. Some species of termite carry material back to their nests where it is decomposed by fungus. The fungus is then used as a food source by the termite colony (Wood 1978). Leaf cutter ants also cultivate similar fungal gardens on leaf fragments (Stradling 1978). The mycorrhiza/tree-root association is also extremely important in the tropics, as it contributes to high efficiency of nutrient cycling critical for tropical forests where there is a high potential for nutrient loss through leaching (Jordan 1985).
Plant-animal mutualisms in which the animals provide protection for the plant are one of the most easily observed mutualisms in the tropics. In some cases an ant colony lives in some hollow or hollowable part of the plant and is directly or indirectly fed by the plant (Janzen 1985). The ants defend the plant against mammalian herbivores by swarming over and stinging the herbivores. As many as 90% of the trees in the Peruvian Amazon have some relationship with protective ants (Marquis and Dirzo 2002). Certain rattan palms have a swollen wooden ligule which houses defender ants. When a large herbivore brushes against the palm, the ants rush out and beat their mandibles on the hollowed ligule. In West Africa, ants inhabiting trees of the genus Barteria are capable of stinging an elephant and can numb smaller mammals for several days (Whitmore 1990).
The importance of mutualistic interactions for seed dispersal and pollination appears to increase from temperate to tropical climates. For example, there are no nectarivorous or frugivorous bats north of 33°. Extrafloral nectar glands on plants drop off drastically between the northern limits of the neo-tropics in northeastern Mexico and Texas. The characteristics of the physical environment may play a role in this pattern. At higher latitudes, where there is a much higher average wind velocity, many plants are wind-pollinated and have wind-dispersed seeds. Trees at high latitudes often do not allocate any energy to mutualistic interactions with animals, since animal vectors are not involved at any stage in their life history. In contrast, in tropical rain forests where wind-pollinated species are rare, many species of animals are sup ported by pollen, nectar, and fruits (Orians et al. 1974). Dispersal of seeds away from the parent tree is an important function that helps to ensure germination of the seeds and survival of the species (Janzen and Vazquez-Yanes 1991). Seeds that are concentrated close to the parent tree may be more likely to be eaten by herbivores (Janzen 1970). Dispersal by mammals, ungulates, bats, birds, and even fish is common in tropical forests (van der Pijl 1972; Whitmore 1990).
Many of these mutualisms have arisen through co-evolution. For example, in the case of pollination, a newly arisen species of tree might have the ability to be pollinated by a variety of insects or birds. However, one particular species of bird might have proven more successful in pollination than another species, and so, through selection, the tree modified its flower so that it would accommodate only the beak of that particular species of bird. The bird species, in turn, discovered that this particular species of tree offered more nectar than other species and began to restrict its visits to this one species. Through the passage of evolutionary time, the mutualism became obligatory. Neither the tree nor the bird can survive without each other.
Sometimes the interaction is more complicated. A tree may depend upon one species of bird or insect for pollination and on another species for seed dispersion. An insect may depend upon one species of plant as a host for its larval stage and on another for its mature stage (Gilbert 1980). Often, the result of this process may be more diffuse than a simple one-to-one relationship, that is, a species may be dependent on more than one other species for its survival (Whitmore 1990). The complicated food-web mutualisms in tropical forests render the species particularly susceptible to local extinction following disturbances such as logging, even if logging is carefully carried out (Box 2.3).
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