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| Rainforest Diversity |
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CANOPY STRUCTURE The canopy system characteristic of tropical rainforests further increases diversity by creating new niches in the form of new sources of food, new shelters, new hiding places, and new areas for interaction with other species. In fact, it is estimated that 70-90 percent of life in the rainforest is found in the trees. One of the best examples of a canopy niche which multiplies diversity are the epiphytes, many of which form tiny ecosystems of their own. The tank bromeliads of New World forests can hold over eight gallons (two liters) of water in catchments formed in their stiff, upturned leaves. These pools of water serve as nurseries for frog tadpoles and insect larvae specifically adapted to life in this tiny obscure niche, and provide water for millions of other canopy dwellers. Over 28,000 epiphyte species are known to science, although many more have never been catalogued. In addition to epiphytes, other plant species including lianas and creepers, create new means for ground-dwelling animals to access the resources of the canopy. Many of the ground-dwelling animals of the temperate zone, like porcupines, kangaroos, anteaters, earthworms, and crabs, have moved up into the canopy in tropical regions. AREA The size of a habitat is another factor in the great diversity of the rainforest. Area increases diversity because a larger plot is likely to have more habitats, hence niches, to support a greater variety of species. In addition, many species require a large range for adequate prey or seed forage. The basis for this idea was set forth by MacArthur and Wilson in The Theory of Island Biogeography (1967) using small islands in the Florida Keys. Soon after the work was published, research focused on whether island biogeography could be applied to fragments of habitat. Evidence for this concept was found in an experiment devised by Thomas Lovejoy in the late 1970s. The experiment was known as the Minimum Critical Size of Ecosystems Project and measured ecosystem decay in forest patches ranging in size from 2.5 acres (1 hectare) to 2,500 acres (1,000 hectares). During the late 1970s the Brazilian government was encouraging widespread clearing of rainforest by offering tax incentives to landowners. However, in an area known as the Manaus Free Zone, just north of the Amazonian city of Manaus, the government required that 50 percent of the forest on a developed area must be saved. Lovejoy used this stipulation for his experiment, convincing landowners to leave their required forest patches in neatly cut squares. The experiment, today known as the Biological Dynamics of Forest Fragments Project, found that the most seriously degraded forest with the least diversity were the smallest, one- hectare reserves, while the reserves that retained the most diversity were the ones of the largest area. In the smaller reserves, drying winds reached the interior, affecting tree species and resulting in more tree falls. Gaps in the canopy allowed more sunlight to reach the forest floor, further altering the understory microclimate and causing changes in the makeup of resident species. Larger herbivores left the patches since the limited number of trees could not provide sustenance, soon followed by predators, which could not cope with the loss of prey. The loss of predators caused an imbalance in the food chain, and the populations of small herbivores and omnivores increased, adding pressure on forest seed banks and impairing the reproducing ability of forest trees. Troops of army ants could not be supported by meager forest patches and they too left, along with the bird, butterfly, and other insect species that depended on the troop. Shade- loving plants and animal species died off as more sunlight penetrated the diminished canopy, and "gap" species, like vines and certain bird and insect species, proliferated. These losses continued to set off a chain reaction that caused profound changes in the system, eventually resulting in its collapse. Similar experiments carried out around the world have yielded similar results (although in some cases diversity among certain groups may actually increase). The colonization of forest patches by forest- edge species, light-gap specialists, and savanna species can counter the loss of species less tolerant of the changed forest and maintain the diversity of the patch. In some cases, forest fragment diversity may hold steady, but overall (global) diversity declines as some unique species lost from the forest patch are not replaced. Floor- dwelling species appear more affected by forest fragmentation than canopy species. Declining biodiversity in accordance with decreasing land area is an important trend to consider for conservation (see section 10). In global studies, larger forest patches lost fewer of their species. Diversity declined but at a rate and to a degree inversely proportional to the size of the patch. In other words, the larger the patch, the more organisms survived and were successful in reproducing. Thus these experiments demonstrated that the area of an ecosystem directly affects biodiversity. SOILS The soils of a rainforest affect the diversity of the forest. Although nearly 70 percent of tropical rainforest exists on poor acidic soils, it retains its fertility in a large part thanks to nutrient recycling and other processes. However, in some areas, soils are so poor that only a limited number of tree species can grow (though these forests are still highly diverse by temperate standards). One example is the so-called "white-sand" or "blackwater" forests that grow on rocky, sandy soils. Some of these forests grow on nothing but rocks and the roots of other trees. Trees that grow under these conditions tend to be species with tannins in their leaves, which in turn, turn local rivers into "blackwater" rivers. The bitter tannins in their leaves limit insect populations, thus reducing the number of animals the forest can support (insects serve as a major food source for larger animals in most rainforests). These "blackwater" forests are self-perpetuating, since the "blackwater" rivers that result from the decay of their leaves only make the soils more acidic and prevent other tree species from growing on the already nutrient-lacking soils. Forest tree diversity, and hence total diversity, may also be reduced in forests with soggy soils like those of the igapò or "swamp forest." The limited number of tree species like Cecropia and palms that can tolerate these wet soil conditions means that these few trees species tend to dominate these areas. Subsequently only the animals that feed on their fruits, leaves, and seeds are abundant in these areas. New research suggests rich Amazon soils High-diversity forests are often found on nutrient rich—sometimes volcanic— soils that are well-drained. These forests are frequently found in areas protected from major disturbances like strong wind and regular flooding. Review questions:
[full photo version] Continued: Short-term Variation, Ice Ages, Ecotones Bibliographic citation for this page Other pages in this section:
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