EQUAL CHANCE AND DISPERSAL LIMITATION OF TROPICAL FOREST COMMUNITIES

The equal chance or null community hypothesis is based on the idea that all species are equivalent in terms of their habitat requirements and growth rates, and therefore that every species has an equal chance of inhabiting openings as they become available in the forest.

Basically this hypothesis suggests that local diversity is based on the number of species contributing seeds in the local area, and that tree replacement occurs via random chance. Recent modelling studies indicate that simple null models of tropical forest communities can retain very high species diversity over long periods of time.

Although species in such a null community eventually go extinct via a ‘random walk’ process, the time to extinction is sufficiently long that speciation may act to add new species to the system. For such a null community to maintain very high levels of diversity, one more kind of process is also generally required : namely "dispersal limitation", which refers to the fact that seeds of a given species do not germinate in all possible sites that could potentially be occupied by that species.

The result of dispersal limitation is that many species fail to encounter one another. If dispersal limitation is sufficiently strong, competitive exclusion can be avoided entirely. Recent studies have shown that the absence of plant species at a given site is in fact commonly due to a lack of seed dispersal rather than their inability to compete in that particular area.

TROPICAL DEFORESTATION | Conservation And Sustainable Management Of Tropical Forests

Greatly refined estimates of tropical deforestation have recently been obtained through analyses of changes in forest cover in satellite images. For example, analyses of Landsat imagery covering the Brazilian Amazon indicated an increase in deforested area of 78.000 km2 in 1978 to 230.000 km2 in 1988, or a loss of approximately 6% of the total forested area. Tropical deforestation rates vary greatly across geographic regions, and have shown marked swings over the last decades. Through the 1980s the highest deforestation rates were observed in southeast Asia, but more recently deforestation has shifted to the neotropics and Africa.

In addition to the outright removal of forest, tropical deforestation also acts to fragment landscapes, a pattern of great conservation concern. Tropical forest fragments offer an insufficient amount of habitat for many larger or wide-ranging species of animals, and forest fragments can be seriously degraded by decreased humidity and high wind exposure near edges.

The internal fragmentation of tropical forests caused by selective logging is also a major concern. Studies suggest that low-intensity logging can allow for recovery of primary forest conditions within a couple of decades; however, heavy logging requires a much longer recovery period, and some highly degraded forests may not be able to approach pre-harvest conditions even after hundreds of years.

In many regions construction of logging roads makes forested areas far more accessible to those interested in further exploitation such as subsistence farmers, hunters and fuelwood gatherers. For example, when a commercial logger leaves the concession, subsistence farmers are able to penetrate deeper into the forest than would have previously been the case. Post-logging forest use is becoming increasingly intense due to high population growth rates in many tropical countries.

One partial answer to these difficulties is development of sustainable forestry practices in combination with improved conservation of remaining tropical forests. "Natural forest management" in which gap phase dynamics is emulated by harvesting has been advocated as a means of mitigating losses of diversity and ecosystem function while allowing continued timber harvests. Alternative harvesting practices, such as planning of harvest areas and skid trails, tree marking and directional felling, can be used to reduce the residual impacts of the harvest. Recent studies suggest that such reduced-impact logging in tropical forests can dramatically reduce post-harvest tree mortality. This results in greater retention of forest biomass, increased long-term value of the forest in terms of timber commodities, and more rapid recovery of pre-harvest forest conditions.

NICHE DIFFERENTIATION IN TROPICAL FOREST

Equilibrium hypotheses for the maintenance of tropical diversity generally invoke some form of niche differentiation. This hypothesis is based upon the idea that ecologically similar species are unable to coexist unless they have developed different patterns of habitat distribution and/or resource use. Within this framework, the more specialized the resource requirements of each species are, the more species can be packed into a given habitat. Niche differences among tropical animals are generally related to the type of food resources utilized, or spatial or temporal differences in habitat use.

For example, otherwise ecologically similar animals can differ in terms of height of activity in the canopy, or the time of day they are active. In contrast, all plant species utilize essentially the same set of basic resources, namely: light, water, carbon dioxide, physical space, and nutrients such as nitrogen, phosphorus and potassium.

However, plant species can differ in terms of more subtle ecological characteristics, such as the efficiency of resource use, tolerance of physiological stress, or dependence on specific pollinators, seed-dispersers or root symbionts. Many studies of tropical forest trees have emphasized differences in the "regeneration niche", or the resources and conditions required by seedlings and saplings to successfully establish in the forest. An important distinction is made between ‘pioneer’ tree species that can grow rapidly in large canopy openings or cleared areas, and "late-successional" or "primary forest" tree species that can establish under low light conditions in the understorey. Other kinds of niche differences among tropical trees include ‘structural niche’ differences related to the size reached by adult trees, and differences in ‘habitat preference’ related to soil characteristics and hydrology.

CLIMATE AND BIOLOGICAL PRODUCTIVITY OF THE TROPICS

The warm, wet, and relatively aseasonal climate of the tropics is apparently more favourable for maintaining higher diversity than anywhere else in the world. But why is this the case? One simple idea is that the high solar energy inputs and productivity of tropical regions result in greater numbers of species that can be supported energetically.


However, it is not entirely clear why a small number, or even one species, could not monopolize most or all of the incoming solar energy. Another idea is that climate stability is the main factor promoting species diversification and coexistence. In the harsher temperate and polar regions, species must be able to tolerate drastic fluctuations in seasonal temperatures. Species occurring in habitats nearer the poles are therefore adapted to a wider range of local environments in order to survive the winter months.

As a consequence, one expects a narrower range of adaptation to environmental conditions and narrower latitudinal and altitudinal distributions in the tropics, a hypothesis sometimes referred to as ‘Rapoport’s rule’. The more limited ranges of species in the tropics may allow for greater ‘species packing’ compared to temperate or boreal regions.

Although this idea has received much research attention, recent analyses give only equivocal support, at best, for Rapoport’s rule. On the other hand, it is clear that the relatively aseasonal nature of tropical rain forests allows for the evolution of highly varied and complex species interactions. This complexity itself contributes to the overall species diversity found in the tropics. For example, "dependent" ecological forms, such as specialist herbivores or predators, only persist in the community if their host species is present.

HISTORICAL IMPORTANCE OF TROPICAL FORESTS IN BIOLOGY

Tropical forests have played a central role in the conceptual development of biology from the time of the major biological expeditions that began at the close of the eighteenth century.

Alexander Von Humbolt initiated the study of plant ecology on his voyages through South America in the early nineteenth century. While climbing Mount Chimborazo in the Andes, Von Humbolt characterized the vegetation changes with climate as he ascended. These early observations on plant distributions provided the foundation of the field of biogeography. The most significant development in biological thinking inspired by tropical forests was the theory of evolution by natural selection. Charles Darwin and Alfred Russel Wallace independently derived this theory as a result of their scientific voyages in the tropics during the mid nineteenth century. Darwin’s inspiration was his exploration of various parts of South America as the naturalist aboard the Beagle beginning in 1831.

Wallace conducted expeditions in both South America (1848–1852) and the Malay Archipelago (1854–1862), where he characterized two sets of fauna distinct to the different parts of the archipelago, a division now known as Wallace’s Line. For both Darwin and Wallace, the high diversity of species in the tropics, and particularly patterns of diversification associated with island groups, allowed an appreciation of evolutionary relationships not apparent in the poorer faunas of the temperate zone. The high diversity of the tropics continues to be an important inspiration and testing ground for ideas in biology, particularly in the fields of ecology and evolutionary biology.

LOWLAND EVERGREEN RAIN FORESTS

The tall, lush evergreen forests envisioned by most when referring to tropical forests are lowland evergreen rain forests. These forests are characterized by canopies with multiple layers of vegetation and the presence of large canopy emergent trees. Lowland evergreen rain forests generally have very high species diversity, with over 1000 tree species per square kilometre found in the richest forests of Amazonia and southeast Asia. Canopy and emergent trees in lowland evergreen forests often have large spreading crowns with a radius of >20m at maturity, can grow to more than 1min girth, and commonly possess plank-like buttresses important in physical support.

 

Beneath the upper canopy layer are smaller understorey trees, treelets, and a layer of herbaceous ground vegetation. Cauliflory and ramiflory are especially common among understorey trees in lowland tropical rain forests. One also generally finds abundant lianas; woody climbers that germinate in the understorey, but possess climbing mechanisms (such as tendrils or hooks) that allow them to use free-standing trees as support structures. Lianas that reach the canopy thus remain anchored to the forest floor.

Also common are epiphytes; plants that live on other plants (most often trees), but which at no point in their life history are rooted in the ground. Orchids, ferns and bromeliads provide many examples of tropical epiphytes, which enhance tropical diversity immensely (epiphytes are thought to comprise 10% of all vascular plants). Another group of plants characteristic of tropical forests are hemi-epiphytes, which germinate in the canopy, as do epiphytes, but produce roots that grow down the trunk of the host tree to become rooted in the ground. The most important group of tropical hemi-epiphytes are figs (many species of Ficus), the fruits of which are an important resource for many vertebrate species.

TROPICAL FOREST FORMATIONS

The main types of tropical forest are distinguished by differences in the distribution of rainfall throughout the year, by elevation, and by soil type. Tropical forests that experience ever-wet conditions with no month receiving less than 100mm of precipitation are generally referred to as ‘tropical rain forests’, although a distinction is also sometimes made between tropical ‘moist forests’ and ‘rain forests’ in a strict sense that receive annual rainfall in excess of 4000 mm.

The two other main tropical forest types, ‘tropical dry forests’ and ‘semi-evergreen rain forests’, experience an annual dry period. In tropical dry forests (also called ‘monsoon forests’) the dry period is severe, and during this most trees drop their leaves in order to reduce water loss. In semi-evergreen rain forests the seasonal drought is less extreme, and a leafless period does not occur to the same extent.

Within these broad moisture regimes, tropical forests are subdivided on the basis of elevation and soil type, and corresponding differences in forest structure. The distinguishing structural characteristics include canopy height, crown layering and the presence (or absence) of different climbers and epiphytes. Tree buttressing, crown shape, leaf structure, and position of flower/fruit formation are other important physiognomic descriptors of tropical forests.

On a global basis the most important types of tropical moist forest include lowland evergreen rain forests, upper and lower montane rain forests, heath forests, peat swamp forests, freshwater swamp forests and mangroves.