a. Pollination: Many animals are essential in the reproductive processes of forest plants. Bats are known to be pollinators of more than 300 plant species (many of which are economically important as timber, fuel, fiber, medicine, or dyes). In Southeast Asia, bats pollinate popular forest fruits such as durian, banana and mango (Payne, 1995). Nectar-feeding birds and insects such as beetles, bees, and wasps are even more important than bats in tropical forests. In Panama, birds pollinate between 40% and 80% of forest plants (Karr, 1994).
Some plants share a pollinator species with other plant species, but as the plants of the different species don’t flower simultaneously, competition for the pollinators’ attentions is avoided (and potential hybridization between different species is inhibited). For example, Costa Rican Heliconia species which share the same bird pollinator produce flowers at different times of the year. Some species of trees are pollinated by only one or a few species of thrips, which will not pollinate other species of trees. Plants also have special means to ensure cross-pollination. Individuals of Oroxylum species in Malesia produce nectar at night, but only in tiny bursts, so as to encourage its bat pollinators to visit a number of plants. Figs are pollinated by fig wasps, the females of which become trapped in the fruit forming from the flower they pollinated. These wasps can travel over great distances – more than 14 kilometers – to pollinate trees.
The animal pollinators and the flowering parts of the plants are well-adapted to each other. Flowers pollinated by birds are bright and have watery nectar; bat-pollinated flowers open at night and have viscous nectar, while those pollinated by bees open in daytime and often have bright colors and footholds. Some flowers have potent odors to attract beetles and flies.
b. Seed dispersal: The seeds of many forest plants are distributed by animals. Most of these plants produce fruits which are desirable as food. The fruits of the many fig species (genus Ficus), for instance, are a major source of food for a number of animals, which through their movements, distribute the seeds throughout the forest. Among these animals are bats, monkeys, tree shrews, squirrels, and larger animals such as deer, elephants, wild cattle, and civet cats, all of which consume the fruits and discard the seeds. The seeds are then left to germinate if they are in favorable conditions. In some cases, seeds must pass through the gut of an animal before they can germinate. The gut enzymes break down the tough seed coat so germination can occur. Rodents eat seeds, but often bury some of them for future consumption, and some of the buried seeds may eventually sprout.
The seeds of more than 26 species of trees are dispersed by fish. On Borneo alone, 13 species of fish feed on fruits and are major seed dispersers. Frugivorous birds are also crucial for seed dispersal of a variety of plants. The Sulawesi red-knobbed hornbill, Aceros cassidix, is an effective seed disperser for many of the plant species which it consumes. The male may bring as many as 265 fruits of a single species to its nest during a visit, and seeds of 33 different species of fruits are used to feed the young. The birds can carry seeds as far as one kilometer to the nesting site. Small seeds are passed through the baby birds’ guts; larger seeds are regurgitated in the vicinity of the nest by the young. Discarded and excreted seeds germinate under the nest, where the forest becomes enriched with the plant species preferred by these birds. Interestingly, though, there are few plants of the genus Ficus underneath hornbill nests, although Ficus fruits form more than 70% of the birds’ diet during the breeding season (Kinnaird, 1998).
Animals may also be essential for the dispersal of spores of mycorrhizal fungi. The feces of some Australian rainforest mammals contain the spores of many species of these fungi. Some of these mammals probably ingest the spores while they are foraging for food, and disperse them accidentally. The very high concentration of sporocarps of certain species of fungi in the guts of certain rodents (rat kangaroos [Potoridae], bandicoots [Peramelidae] and others) suggest that these animals seek out the fungi as part of their diets. The scat of the white-tailed rat (Uromas caudimaculatus), for example, contains the spores of many different fungi (Reddell, Spain, and Hopkins, 1997).
c. Forest maintenance: Vertebrates undoubtedly play a role in forest maintenance. Many primates and ungulates eat leaves and shoots, and their selection of food sources may alter the balance among different species of plants. When these animals eat certain species of plants and avoid others, the ones which are consumed will be suppressed, while those not eaten will gain a competitive advantage. Orangutans feed on the shoots and soft stems of plants such as climbing bamboo, a plant which, by invading forest gaps, can hinder the regeneration of forest trees. Therefore a healthy orangutan population will prevent forest gaps from being overwhelmed by bamboos and other non-tree species, and leave room for tree regeneration and succession. Pigs, in their search for food, turn over the soil, and rodents burrow and damage vegetation, but these soil disturbances enhance seed germination. Even when animals burrow in holes in trees, their waste products may provide nutrients for the trees. In a few cases, animals serve as dinner for plants.
d. Forest “webs”: A web is an association of different species. Animals are important parts of tropical forest webs, which can be highly intricate and involve many species – plants, animals, microorganisms. [A discussion of webs is too complicated to enter into here; consult any general biology or ecology text.] One example of a complex web comprises the Passifloraceae family of the Neotropics and the animals associated with it. This plant group consists of approximately 500 species of small trees and climbers, which are fed upon by butterflies, beetles, bugs and moths. One small area may contain ten to fifteen different Passifloracea species, each with its specific associated pollinators and predators (ten to fifteen separate webs). Why don’t these webs compete? In this case, competition is inhibited by several mechanisms. The time of flowering of each species of plant is staggered, and each is pollinated by an animal (bee, bird, bat, or moth), which is “monogamous” to that plant but which may feed on different Passifloracea species at other times of the year. Thus the animals provide an essential link between and among plant webs and are critical for their functioning. As another example of interlocking webs, the various species of euglossine bees of the Americas pollinate 30 to 50 different plant species; any one species of bee may pollinate as many as 12 plant species. Thus, these insects interconnect many otherwise separate webs involving these plants. Humans have not always recognized the critical nature of such webs. Brazil nut trees, indigenous to South American rainforests, require the agouti, a small rodent, which is a seed predator, and which opens and disperses Brazil nut seeds throughout the forest. These trees also require (among other things), bees as pollinators. Brazil nut plantations fail because the pollinating bees need alternative sources of nectar when Brazil nut trees are not in flower. These sources are present in an intact forest but not in a monoculture plantation.
e. Maintenance of plant diversity: It has long been posited that herbivores might affect the distribution of plant species within rainforests by eating the seedlings of abundant plant species and allowing the seedlings of scarcer species to grow. This has recently been demonstrated by comparing two forests in Mexico, one of which had an intact fauna, the second of which had lost almost half of its large mammals to hunting or to the pet trade. The latter forest, Los Tuxtlas, had a dense understory of seedlings of only a few species, while Montes Azules, the former, had a sparser understory but one with great diversity of plant species. Also, when large mammals were excluded from experimental plots in Montes Azules forest, there was a decline in the number of plant species represented in the understory (Kaiser, 2001b).