Sections:
(notes partially contribution by the members of the Ecology class 98!)
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Geographical location: Tropical rain forests are located mostly in temperate as well as tropical regions, around the equator, and in the past on the super continent Pangea. Historical data: How old are the forests? The oldest forests have been evolving for almost 200 million years, during the age of the dinosaurs when the disturbances in forest [ like drought ] eventually stopped. After the last disturbance, the forest eventually encountered warm moist climate again. More forest area grew on the supercontinent known as pangea. *In the old forest, there was no flowering plants, but during the next 100 million years, flowering plants evolved from fern-like ancestors and established relationships with pollinating insects. * More adaptable species which could advantage of drier, sunny spots or those how had the genetic ability to 'evolve' better in competitive couplings took over in the canopy. Ancient survivors like the tree ferns still remain in localized areas from an earlier time. |
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Concept of climate and seasonality: Temperatures generally fall between 23 - 27 deg C, with a greater daily than monthly range. In other words, there is no strong seasonality of temperature. Rainfall tends to be highest near the equator, where the sun's evaporative power causes high evapotranspiration, and rising air cools and then sheds its moisture. Evergreen forests are replaced by deciduous forests as precipitation becomes seasonal. Wherever dry periods are several months or longer in duration, leaves are shed as the dry season takes hold, providing a winter-like appearance. Leaves reappear in anticipation of or with the onset of the rains. (A dry month is one where evapotranspiration exceeds precipitation.) The growing season is thus shortened, and so forest productivity is less than in the evergreen forests of the more humid tropics. *Although we think the tropics to be constant climatically, seasonal changes although more limited as noted above do affect the life cycles of many insects and act to help regulate population numbers. Plants of the tropical rain forest receive annual rain in excess of 80 in/year during the rainy season, one to two times that received by temperate forests. |
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Diversity and species richness : The incredible plant species diversity that supports an equally diverse herbivore diversity in the TRF is well documented. Although the TRF's cover only 7% of the earth's surface area, they contain up to 50% of the world's species. What causes this prodigious concentration of biological diversity? How do we explain such high biodiversity? Many reasons have been given, some of which are briefly explained below. Probably no one 'reason' can account for the great number of species that coexist, but a combination of several is more likely... * In the tropical rain forest, there is greater amount of direct solar radiation year round which enables the sustained growth of many species. As photosynthesis is light limited, with each canopy layer diminishing the amount that makes it to the layer below, the greater the initial radiation at the top layer the more layers that can be established below... * Shortages of soil nutrients are often patchy in the tropics which may actually encourage evolution of species which survive 'better' under different nutrient shortages. See more on soil quality in the section on soils. * The persist pressure by insects on plant life has selected plants which can evolve chemical defenses. In evolving these defense system, such plants encourage the insects that can to develop alternative means to counter the plant defenses. The insects, therefore, become specialist herbivore for that particular plant and a long-term coeveolutionary battle continues, with individual plant species supporting specific coadapted herbivores . * If a given plant species become too populous, they attract attention by virtue of their availability and become 'attractive' to a greater number of herbivores. Increased attack rates eventually results in increased herbivory damage to the point that the species is no longer as successful or common. Thus more species can coexist with less dominant species around. * When there are environmental disturbances in the canopy such as storm related tree-falls, then corresponding species - smaller trees, epiphytes and climbers will grow in this open area. * The number of herbivore species increases from temperate area into tropics and particularly in lowland tropical rain forest . This could be because there hasn't been t much severe disturbance in that tropical zone. However, the carnivores that feed on the species do not increase proportionally. It is probably because the more species of insect present on any one host , the less individuals there are of any one species- hence it does not favor generalist carnivore or a specialist parasitoid because none is equipped to feed on various insects.
It seems likely that the forests of the tropics have gone through many cycles of fragmentation and reunion, due to the same global climate changes that gave us repeated bouts of glaciation at higher latitudes. These repeated episodes of forest retreat and advance would provide numerous opportunities for forest-dwelling species to be isolated into separate small populations. This in turn allows genetic change and evolutionary specialization to occur. During wetter periods associated with interglacials (time during the glacial retreats), forests would expand and rejoin. If the dry periods correspond roughly to periods of glacial advance, they would have lasted 50 to 100 thousand years, which likely is sufficient time for evolutionary divergence. This process has happened over and over, in conjunction with the many cycles of glaciation that the earth has experienced in the last several million years." |
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Still other potential explanations exist... Strong biochemical competition between plant species which discourages dominance by any one species, Time, -the fact that the tropics just has been there physically for such a long time with out the direct stress of major glaciations or mountain building periods, documented immigrations of more northern species during times of glaciation ( ie. NA species into Mexico and southward) some of which could either coexist in low densities or evolve rapidly enough to eventually to co=compete with existing members, We know that parasites play a major role in population regulation whether plant or animal. It is possible that in a site where there is little causes for parasites to temporally retreat( as up in the N with cold weather), that a strong coevolution battle between plants and their parasites ( including fungi, bacteria, viruses) may prevent few plant species from dominating the scene. |
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Coevolution of plant/animal: * A great deal of mutualism and symbiosis appears to exist in the TRF *Incentive for plant and animal relationship because of need for effective pollinators - with fewer plants of any one species, there is a greater dependence on animals to either move pollen or seeds from plants a mile apart. Example of
useful parasites. Cowbird in Latin America lays eggs in
the nests of caciques and oropendolas. Cowbird chicks
share with host chicks. Cowbirds eat some of the
parasitic botflies in the nest. They also pick the
botflys off the hosts. When nests are near bees or wasps,
birds chase cowbirds away, but when nest alone they allow
cowbirds.
Nutrient Cycling in TRF's: for more information see soils & cycling. * The forest trees here store most of the nutrients internally and have evolved means to scavenge nutrients from any forest vegetation or tissue that has fallen from above. Example: There are TRF's in the Amazon Basin that have trees that have adapted and survive in acidic soil where the calcium level is so low that snails cannot survive for lack of shell substrate. * Around the Amazon Basin, the soil is naturally infertile with nutrients of the topsoils leached away as a result of the climate. Certain TRF trees have adapted to the problem by possessing a spongy mat of tangled roots above the ground to absorb any nutrients prior to leaching. |
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