Ecological studies of tropical forests have been most intensively carried out in Central America, especially in Panama and Costa Rica, but these forests do not have the greatest biodiversity. Tropical rainforests with the greatest biodiversity are found in the Peruvian Amazon, followed by those in West Malesia. But those American forests with the richest flora and fauna have been relatively understudied because of difficulty of access and work conditions due to geographical and social factors. In contrast, forests in West Malesia are comparatively easy to access and have been intensively studied.
West Malesia is the center of biodiversity of Southeast Asian terrestrial ecosystems and rather uniform in flora and environments. Although central-east Java and Bali have clear dry seasons, other areas have heavy rainfall, during which rainfall exceeds evaporation plus transpiration throughout the year, except in El Niño years, when Southeast Asia suffers extraordinary droughts of a few to several months. Researchers who pay attention to issues of species richness—origin, mechanisms of coexistence, effects on ecological functions—have favored West Malesian forests as research sites.
From Classification to Species Composition
In the 1950s and 1960s, the central issues of research in plant ecology were the processes of vegetation succession—from grass to shrubs to fast-growing softwood trees, climaxing in true hardwood forest—and the classification of vegetation in relation to environmental factors. Despite great differences in floristic composition among tropical rainforests in America, Africa, Asia, and North Australia, general features of forests and stages of succession were found to be common under similar climatic conditions. Relationships between forest types, stages of succession, and environmental conditions including climate were described in detail. Major findings on these topics were summarized by Richards (1952). In West Malesia, Ashton (1967, 1976) studied forest ecology using this framework, although he subsequently carried out ecological studies using the new frameworks described below.
In the 1970s, attention shifted to mechanisms for maintaining the species composition of different types of forests—in other words, why and how do forests at the “climax” stage remain stable? A textbook by Whitmore (1975) included both descriptive works using the 1960s framework of typology of tropical rainforests of Southeast Asia (especially West Malesia) and studies under the new framework concerning mechanisms for maintaining species composition. Gap dynamics and the regeneration niche are the most important concepts developed to understand this topic.
Gap dynamics plays an important role in forest regeneration. Gaps in the forest canopy are created by tree falls occurring by chance that alter light conditions on the forest floor. Buried seeds, newly dispersed seeds, or shade tolerant seedlings that can live in the understory for many years start to grow as soon as gaps are formed. Different species recover depending on the specific conditions of the gaps, including shape, size, and longevity. Thus, the frequency of gap formation and the types of gaps determine species composition of forests.
Niche means the place and role occupied by an organism in a community. The importance of the regeneration niche in explaining the species composition of forests was pointed out by Grubb (1977)—differing responses to the environment among species can contribute to species coexistence. The species composition of forests is also related to the spatio-temporal heterogeneity of environmental conditions. Mature plants have flexibility and can survive under a wide range of environmental conditions. On the other hand, younger plants have much less flexibility. Different responses to the environment among species is most clearly shown in the processes by which young plants grow and forests are regenerated.
Mechanisms of maintaining species composition had usually been expressed in equilibrium models. Connell (1978) stated that most biological communities are not in equilibrium because of frequent disturbance. Species diversity is highest when medium scales of disturbance occur. Too frequent or strong disturbances cause extinctions of many species, but if disturbances are too rare or weak, communities are dominated by a single species, causing other species to become extinct. Ecologists have differed on the issue of equilibrium, but theoretically it was not difficult to evaluate the effects of frequency or intensity of disturbance by introducing disturbance effects into equations describing the dynamics of communities. Theoretical studies using computer simulations show that medium scales of disturbances do increase species diversity.
In the later 1970s and the 1980s, the ecosystem became an important concept in forest ecology. The network of interactions among organisms and physical/chemical conditions was first called an ecosystem by Tansley (1935). Understanding the stock, flow, and distribution of materials and energy is the main objective of ecosystem ecology, essentially the movement of nutrients and carbon. The ecosystem concept was first applied to aquatic systems, and the framework was later expanded to terrestrial systems, including tropical rainforests. The second edition of Whitmore’s textbook (1984) described how the ecosystem model is used. Kira (1978) and his coworkers also made great contributions.
Ecosystem ecology offers great possibilities to applied science. When vegetation is changed by humans, how are the stock, flow, and distribution of materials and energy changed? We need to answer this question in order to predict environmental changes on the local and global scale. However, such predictions will require coordination among scientists in several research fields, including earth scientists. Without this coordination, the contribution of ecosystem ecology to applied science remains only a possibility.
More specifically, ecosystem ecology of human-affected vegetation could be applied to the problem of sustainability of shifting cultivation and other agriculture. To generalize from case studies, however, that shifting cultivation is sustainable (or not sustainable) would be meaningless and even dangerous. The role of ecologists is to prepare methodologies that can be used for various investigations, not to generalize results from specific cases.
In roughly the same period, population ecology using sophisticated statistical methods was developed. Insects were intensively studied because their population dynamics are easy to observe, but plants are also suitable for study because spatial distribution and age/size/class distribution are easy to detect. However, tropical rainforests with rich species diversity were difficult to study because the population density of each species is too small to be statistically treated. Despite this difficulty, a great study was completed by Hubbell and Foster (1990) using a large forest plot of 50 ha in Panama. Their study was developed into the unified neutral theory in 2001, discussed below. Later, two large forest plots were studied in West Malesia, one in Pasoh in peninsular Malaysia, and another in Lambir in Sarawak, Borneo. These plots contain much higher species diversity than the plot in Panama and are expected to provide a useful comparison with the earlier study.
In the 1980s and 1990s, evolutionary ecology became the most active field in tropical forest ecology. Since evolutionary ecology was established by Hamilton (1964), the behavior of insects has been the most intensively studied, because they are easy to treat and exhibit diverse behaviors including genetically programmed sociability. Study of the evolutionary ecology of plants became popular more than fifteen years later than the study of animals. Plant reproduction (including pollination, defense of ovules against herbivores, and seed dispersal) received the most attention. Because the processes of reproduction of tropical plants include various interactions with animals, evolutionary ecology of tropical rainforests often deals with co-evolution.
Many studies have been done of co-evolution in tropical forests: Janzen (1970, 1971) proposed interesting hypotheses about plant-animal interactions. Bawa (1990), who studied in Central America and West India, was a leader in evolutionary ecology research on the reproduction of tropical plants. In West Malesia, after Appanah’s pioneer works in Pasoh (1981, 1985), a research project in Lambir organized by Inoue made significant contributions (Momose et al. 1998 and Sakai et al. 1999, for example). Books by Turner (2001), Inoue (1998), and Yumoto (1999) introduce recent achievements, the latter two in Japanese.
Detailed observations and field experiments are necessary to study the reproduction of tropical plants and their interactions with animals. To observe the canopy, systems of towers, walkways, and cranes were constructed. These are also utilized in studies of plant physiology, because they enable the measurement of photosynthesis, respiration, transpiration, and other responses of plants to environments of the forest canopy.
Evolutionary ecology does not seem to be directly “useful” in the sense of contributing to economic growth or providing technical solutions to environmental or social problems. However, it has a fundamental role in the conservation of biological heritage—as a historical science in biology—because it deals with the causes of evolutionary events that have been identified by paleontologists. Why are historical sciences fundamental to conservation? Just as studies of human history assign value to certain locations and processes, evolutionary ecology assigns values to certain ecosystems, biological communities, and species (as well as higher and lower taxa) that enrich our culture.
In the later 1990s and 2000s, biodiversity has become an important key word. How biodiversity has been created and maintained is now the central issue of ecological studies of tropical rainforests. For this question, neutral theory vs. interactive/competitive theory is a central dispute.
Competitive theories have the longest history. According to the “competitive exclusion principle,” species with identical niches that place equal demands on the same resources cannot coexist. When multiple species are coexisting, their realized niches (ranges of utilized resources under competition) do not overlap (Hutchinson 1957). The theory of niche differentiation succeeded in explaining the coexistence of similar animals. For forest plants, the degree of demand for light, soil moisture, and nutrients are examples of niche axes along which resources used by different species are divided. However, we have yet to detect a large number of niches along many axes corresponding to the huge number of plant species found in tropical rainforests.
Interactive theories predict that interactions among species enable the coexistence of species with similar resource demands. Prey-predator relationships are the most important. If dominant prey species are targeted by predators, rarer prey species competing with the dominant one obtain an advantage and avoid extinction. Recently, Chawanya and Tokita (2002) theoretically established that complicated prey-predator relationships enable the coexistence of competing species. It may be doubted that herbivory has significant effects on plant populations because adult plants in tropical rainforests are seldom killed by herbivores. Herbivory of seeds and seedlings, however, often has a very severe impact. Thus there is a possibility that seed predators contribute to the coexistence of competing species.
Hubbell’s neutral theory (2001) asserts that a number of species respond to environments in the same ways. In statistical terms, variations of response to environments among species are not significantly larger than variations among individuals within species. Under such conditions, the extinction of species occurs by chance and the number of species per area is maintained by migration or speciation (the process of species formation). Species-area curves (curves plotting the number of species appearing within different sizes of areas) are usually S-shaped when both axes of species and areas are log scaled. This matches the predictions of Hubbell’s neutral theory.
Other problems of biodiversity are the relationships between species number and community stability or ecological function. The environmental conditions of tropical regions, especially patterns of rainfall, fluctuate greatly and unpredictably. This is why it is not easy to obtain stable outputs from tropical agriculture. Can biodiversity stabilize such fluctuations? Empirically it has been known that species-rich communities are more stable than species-poor ones. Classically it was believed that the stability of species-rich communities was related to the complicated network of relationships among organisms (Elton 1958). However, this was theoretically refuted by May (1972). Complicated networks often cause instability, although networks of prey-predator relationships may improve the stability of communities (Chawanya and Tokita 2002).
According to recent theories, stability results from having a number of similar competing species within communities, rather than complicated relationships. Yachi and Loreau (1999) theoretically revealed that species-rich communities can maintain high productivity and biomass under fluctuating environmental conditions. Ives and Hughes (2002) unified some previous models predicting that species-rich communities have stable total individual density.
Tropical rainforests consist of several functional groups—meaning sets of similar species sharing the same ecological niches. The species diversity of tropical rainforests is divided into the number of functional groups and the number of species within functional groups. Classically it was believed that a large number of functional groups interacting with each other was related to the stability of communities. However, today it is well known that species diversity in primary tropical rainforests is mostly explained by the number of similar coexisting species within functional groups. According to the recent theories introduced above, redundancy within functional groups observed in primary tropical rainforests results in stable communities.
The same conclusion can be applied to human-affected ecosystems. Traditional agriculture maintained diverse varieties of agricultural crops. The number of varieties of upland rice recognized by farmers is often more than fifty per village and sometimes exceeds 100 (Yin 1994). Such crop diversity can be understood as redundancy that compensates for environmental fluctuations. In contrast, modern agriculture (including “green revolution” agriculture in tropical areas) tries to stabilize outputs by controlling the environment. However, strict control of the environment is sometimes extremely difficult in the tropics; even today, the redundancy strategies of traditional agriculture are still valuable.
The value of ecology
Ecology, as one of the disciplines of biology, has progressed by proposing and testing new theories that range in applicability from the specific to the very general. At the same time ecology is a field science. It must contribute to understanding (and if possible, improving) the areas where fieldwork is carried out. The general questions that ecologists have explored for several decades are helpful in understanding these areas, improving welfare, and enriching culture.
Ecosystem ecology clarifies the material basis of food production and maintenance mechanisms of environmental conditions for human life. Studies of biodiversity are related to ecosystem ecology and provide alternatives to strict artificial control to stabilize environmental fluctuations.
Traditional societies have rich cultures associated with plants and animals. They use the natural environment in many ways and we are often impressed by the rich culture of their myths and tales about it. Today, however, human migration is the most conspicuous trend, and city residents live increasingly distant from the forests. Migrants and city residents have lost the traditional cultures associated with plants and animals. In our present situation, interesting scientific “tales” born of evolutional and other fields of ecology can help create new cultures to recover lost associations with the natural environment.
Momose Kuniyasu is Research Associate at the Graduate School of Asian and African Area Studies, Kyoto University. He wishes to thank Shimamura Tetsuya, also of the Graduate School, for helpful comments.
Appanah, S. 1981. Pollination in Malaysian primary forests. Malaysian Forester 44: 37-42.
Appanah, S. 1985. General flowering in the climax rain forest of South-east Asia. Journal of Tropical Ecology 1: 225-240.
Ashton, P. S. 1967. Climate versus soil in classification of south-east Asian tropical lowland vegetation. Journal of Ecology 55: 67-89.
Ashton, P. S. 1976. Mixed dipterocarp forest and its variation with habitat in the Malayan lowlands: A re-evaluation at Pasoh. Malayan Forester 39: 56-72.
Bawa, K. S. 1990. Plant-pollinator interactions in tropical rain forests. Annual Review of Ecology and Systematics 21: 399-422.
Chawanya, T., and K. Tokita. 2002. Large-dementional replicator equations with antisystematic random interactions. Journal of the Physical Society of Japan 71: 429-431.
Connell, J. H. 1978. Diversity in tropical rainforests and coral reefs. Science 199: 1302-1310.
Elton, C. 1958. The Ecology of Invasion by Animals and Plants. London: Methuen.
Grubb, P. J. 1977. The maintenance of species-richness in plant commuities: The importance of regeneration niche. Biological Review 52: 107-145.
Hamilton, W. D. 1964. The genetical evolution of social behavior, pt. 1, 2. Journal of Theoretical Biology 7: 1-52.
Hubbell, S. P., and R. B. Foster 1990. Structure, dynamics and equibrium status of old-growth forest on Balo Colorado Island. In Four Neotropical Forests, ed. A. Gentry, 522-541. New Haven: Yale University Press.
Hubbell, S. P. 2001. The Unified Neutral Theory of Biodiversity Biogeography. Monographs in Population Biology 32. Princeton and Oxford: Princeton University Press.
Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Symposia on Quantitative Biology 22: 415-427.
Inoue, T. 1998. Seimeino Hoko: Nettai Urin (Treasury of life: Tropical rainforest). Tokyo: NHK library.
Ives, A. R., and J. B. Hughes. 2002. General relationships between species diversity and stability in competitive systems. The American Naturalist 159: 388-395.
Janzen, D. H. 1970. Herbivores and the number of tree species in tropical forests. The American Naturalist 104: 501-528.
Janzen, D. H. 1971. Seed predation by animals. Annual Review of Ecology and Systematics 2: 465-496.
Kira, T. 1978. Primary productivity of Pasoh forest – a synthesis. Malayan Nature Journal 34: 291-297.
May, R. M. 1972. Will a large complex system be stable? Nature 238: 413-414.
Momose, K., T. Yumoto, T. Nagamitsu, M. Kato, H. Nagamasu, S. Sakai, R. D. Harrison, T. Itioka, A. A. Hamid, and T. Inoue. 1998. Pollination biology of a lowland dipterocarp forest I. Characteristics of the plant-pollinator community in a lowland dipterocarp forest. American Journal of Botany 85: 1477-1501.
Richards, P. W. 1952. The Tropical Rainforests. Cambridge: Cambridge University Press.
Sakai, S., K. Momose, T. Yumoto, T. Nagamitsu, H. Nagamasu, A. A. Hamid, and T. Nakashizuka. 1999. Plant reproductive phenology over four years including an episode of general flowering in a lowland dipterocarp forest, Sarawak, Malaysia. American Journal of Botany 86: 1414-1436.
Tansley, A. G. 1935. The use and misuse of vegetational terms and concepts. Ecology 16: 284-307.
Turner, I. M. 2001. The Ecology of Trees in the Tropical Rain Forest. Cambridge: Cambridge University Press.
Whitmore, T. C. 1975. Tropical Rain Forests of the Far East, 1st ed. Oxford: Oxford University Press.
Whitmore, T. C. 1984. Tropical Rain Forests of the Far East, 2nd ed. Oxford: Oxford University Press.
Yachi, S., and M. Loreau. 1999. Biodiversity and ecosystem productivity in a fluctuating environment: The insurance hypothesis. Proceedings of the National Academy of Sciences of the United States of America 96: 1463-1468.
Yin, S. 1994. Senlinyunyu de nonggeng wenhua: Yunnan daogenghuo zhong zhi (A farming culture born out of forests: Swiddening in Yunnan, China). Kunmin: Yunnan People’s Press.
Yumoto, T. 1999. Nettai Urin (Tropical rainforests). Tokyo: Iwanami.