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We're facing the problems of the world one biome at a time. Meet the crew...
Updated: Wednesday, 14 March 2007 3:49 PM PDT
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We're facing the problems of the world one biome at a time. Meet the crew...
Climate: The climate is warm with seasonal rainfall. Most grasses pollinate in the wet season. This is when fire is less likely to destroy young plants. Their productivity is limited by water. There is roughly 250-1600mm precipitation annually, particularly during the wet summer months. Generally there are 3 months of low precipitation, which constitute the dry season. Some woody species may lose their leaves in the dry season to conserve water loss.
Soil: Fine soils are characteristic of grassland because it retains the water near the surface where it is readily available to grasses. Also the soil is nutrient poor, so Nitrogen producers have a competitive advantage over other plants. In the grassland there are four main types of soils: Oxisols, Alfisols, Entisols, and Vertisols. Oxisols are red soils due to their high iron content; they are finer and wetter than others. Alfisols contain higher quartz content than Oxisols, due to a combination of calcification and lateralization, and therefore they are grainier. Entisols are sandy soils which are less fertile than the rest because they have low phosphorous content. Vertisol is highly acidic and has the consistency of clay so it will crack in the dry months.
Geographic Distribution: Grasslands can form on any continent between 10-12 degrees latitude where they can be maintained by fire. In South America the largest grassland area is between southern Brazil and Paraguay. In Central America grasslands are primarily human-made. Slash-and-burn techniques tear down tropical forests. Infertile soils that once were tropical forests contribute to grasslands because with slow growth rates by trees, there is an increased probability that fire will reset the growth,thus making the fast growth rate of grasses advantageous. Southeast Asia and inland Australia also have large grassland where dry winter months make it difficult for large trees to grow. Africa has the most extensive savannah, which has been made popular by pictures of the Serengeti.
Fauna: Large grazers and browsers are the predominant feature of the grassland. Typical examples include: giraffes, elephants, zebra and kangaroo. They have extensive regional migration patterns following the grasses and rain. Of the non-migratory species, termites are a keystone element. They decompose cellulose and move nutrients throughout the soil. On a larger scale, vultures eat carrion and recycle nutrients back into the food web.
What Is Grazing?
A grazer is a type of animal, usually nomadic, whose diet is composed of grasses shrubs, and other such plant material. Grazers include animals such as cows, goats, and sheep. As you have likely seen for yourself, grazers spend all day wandering around eating plant material¿GRAZING.
<http://eclectech.co.uk> <www.cow.gr>
What Role Does Grazing Play in Tropical Grasslands?
Herbivory and grazing are integral parts of the Tropical Grassland (AKA-Savannah) biome, where grasses and woody plants such as trees and shrubs coexist, but compete with one another for space. Historically, grazing by native herbivores at moderate levels in such places as the Serengeti has helped to maintain the balance between grasses and woody plants. How you might ask? The more obvious and direct mechanisms are quite simple. By grazing on the grasses, herbivores help to keep grass populations in check so that the Savannahs don¿t become completely overrun with grasses, while at the same time, trampling of small seedlings by larger, and less nimble, herbivores such as elephants help to keep tree and shrub populations at bay as well (Breckle,177).
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Grazing also plays an important role in the hydrologic and nitrogen cycles within Savannah ecosystems. By reducing grass biomass above ground, and subsequently removing transpiring surfaces, grazing helps to conserve soil moisture. The less grass there is transpiring above ground, the less water will be lost from the soil through evapotranspiration, resulting in more water within the soil to be utilized for new plant growth. What does grazing have to do with the Nitrogen cycle in these ecosystems? The answer is in the dung! As the famous children¿s book states, ¿Everyone Poops,¿ and herbivores are no exception to the rule. Some of the Nitrogen consumed by herbivores while grazing is released back into the soil through their dung and urine, converting it from an unusable source (in the old grass), into a source that is readily available to new growth (Mcnaughton, 284).
<www.brooklyn.cuny.edu>
Many scientific studies have been conducted to investigate the relative importance of grazing within tropical grassland communities, in fact, these studies have shown that in areas where minimal grazing has occurred, plant productivity and species diversity is significantly smaller than in areas where a moderate amount of grazing has been allowed; productivity as much as doubling in regions where a moderate amount of grazing took place, proving that grazing is an important process in maintaining optimum levels of species diversity and productivity in grassland regions (McNaughton, 698).
When Does Grazing Become a Problem?
Grazing sounds great right? It maintains the balance between grass and woody plant species that make the Tropical Grasslands unique, it helps to conserve moisture within grassland soil, and it aids in the cycling of Nitrogen and other nutrients. However, as is true with most good things, too much can be just as bad, and in some cases worse, than not enough. Though grazing has been a natural part of Savannah ecosystems for hundreds of years, and grass species have coevolved with herbivores to develop coping mechanisms for grazing, the carrying capacities of Savannah ecosystems are not unlimited. Just as studies have shown that productivity and diversity increase from low to moderate levels of grazing, the opposite has been proven from moderate to high levels of grazing. As grazing levels continue to increase past the optimum point (dependent on species of vegetation), grasses are consumed faster than they can reproduce, not only leaving space for woody vegetation to grow in, but also leaving more water and nutrients for that woody vegetation to utilize in its growth. (Vegten, 3). This process in which woody shrubs begin to take over in grassland ecosystems is known as shrub encroachment, and is becoming a more and more prevalent problem in areas where higher than optimum levels of grazing are maintained over long periods of time. In what areas has bush encroachment become a threat? Areas of prolonged, domestic, cattle grazing. We only have ourselves to blame.
Temperate Deciduous Forests are easily recognized by their-broad leafed trees that become barren in the winter. This is due to the annual cycle of snowy winters and hot summers in this climate. Dropping leaves is an adaptation against freezing in the winter, however, these plants need a sufficiently long growth period of at least 4 months to grow back their leaves and make up for energy spent maintaining themselves in the winter. During the winter, plants may also go through the process of hardening to tolerate lower temperatures without sustaining damage. They do this by stabilizing membranes and increasing protoplasm viscosity. Therefore, these forests can also be known as hardwood forests.
Climate: The climate is influenced by the ocean. This stabilizes the temperature due to water's high the heat of vaporization. It also allows more moisture into the climate causing an extended growth season as well as heavy winter snows. The growing season lasts roughly six months between the winter frosts. There is typically 800-1400mm precipitation annually, gradually increasing closer to the equator.
Geographic Distribution: The deciduous forests transition into grassy steppe in more arid regions closer to the equator and boreal forests further from it. They generally occur in the holoarrctic region, with the exception of southern Chile, and the southern tip of New Zealand. The largest geographic areas of deciduous forest are the China/Japan region, Eastern United States and Central-Western Europe.
Soil: The deciduous forests' soil is very dark with a high organic content, called humus. It can either be the consistency of clay or sponge. The clayey type is called Ultisol. It is waterlogged and highly anaerobic due to a highly under-decomposed content. The other type is Alfisol, which has a low nutrient content because winter water remains in the soil and turns it acidic. Most nutrient cycling occurs through litterfall. Some flora, however, do not drop their leaves, but instead hold on to them in the winter to prevent herbivory from hungry browsers. These plants are called marcescents.
Fauna: Omnivores are the most noticed animal types. They survive principally by foraging, but they can store high caloric content with protein intake for surviving the winter months. Examples include bears, raccoons and jay birds. Insects, however, are the most abundant animal. They are the primary consumer of plants, particularly in the spring when sprouts are non-woody. Trees and shrubs produce defensive chemical compounds to deter foragers away from them.
What is an alien species?
No, it is not some large green slimy being with a big head and weird eyes traveling down from Mars to destroy our planet. It is, however, just as frightening. An alien species or invasive species, is a "species that has been moved into an area and reproduced so aggressively that it has replaced some of the original species." (www.speciesatrisk.gc.ca/glossary e.cfm) Alien species come from our very own planet and attack our very own species. They can be in our lakes and streams, parks and nurseries, or even our own backyards.
Why do we care?
Alien species have the overwhelming potential to destroy habitats, convert them into completely new ecosystems, or cause mass extinctions of native species. Next to habitat destruction, invasive species are the most important cause of species endangerment and extinction. Historically, we have associated invasive species as only being significantly detrimental in island communities. In island communities, invasive species out compete the native species for resources. There are no natural predators that have been able to evolve to suppress their numbers, thus they take over. On islands or island like areas, native species have no where to escape to and no new possible niches to occupy, thus they go extinct. The effects of alien species are often times not even seen, however. Now, we are discovering the invisible "bottom-up" invasions of alien species in non-island ecosystems, such as the effects of invasive species in the soils of deciduous forests.
Invasive Species in Deciduous Forests: (A Hypothesis)
In 1990, scientist Vitousek proposed the possibility of a positive feedback loop caused by invasive species changes in soil composition. He suggested that by changing the natural soil ecology, invasive species could not only cause the acceleration of their own growth, but perhaps even promote further growth by other invasive species.
An experiment was constructed to prove how invasive plants could effect soil composition and how these changes in soil characteristics could affect the natural ecological community. In the experiment, two dominant alien species, Berberis Thunbergii and Microstegium Vimineum, were studied in the under story of three highly infested New Jersey parks, Morristown Natural Historical Park, Worthington State Forest, and Allumuchy State Park. The ancient soils of these three deciduous forests are considered a peinobiome. The lands were never covered by the ocean since they were part of the old Gondwana shield and thus they have suffered from long term leaching and are extremely nutrient poor.
The Experiment:
An experiment was conducted to prove how invasive plants could effect soil composition and how these changes in soil characteristics could effect the natural ecological community. In the experiment, two dominant alien species, Berberis Thunbergii and Microstegium Vimineum, were studied in the under story of three highly infested New Jersey parks, Morristown Natural Historical Park, Worthington State Forest, and Allumuchy State Park. Berberis Thunbergii is a woody shrub with dense surface roots. It was used often as an ornamental plant in the late 1800s and thus is found frequently in abandoned farm lands. Microstegium Vimineum is a C4 grass that forms continuous lawns in the under story.
The results of the experiment revealed that the soil below the invasive plants had significantly higher pH than the soil found below native species. The NH4 concentration studied between March and June under Berberis had increased probably due to the early leaf growth of Berberis. The NO3 soil concentrations below both invasive species were also increased for the entire year studied. As the season progressed, the net nitrification rates had increased, thus by midsummer most of the mineralization in the soil below the exotics, was due to nitrification. This high level of nitrification is unusual and detrimental to native plants, like Vaccinium, who have become adept to peinobiome soils. By midsummer, the soils below Vaccinium are found to have higher nitrification rates, however, ammonification also increases. The ammonification increase contributes the largest portion of the soils net mineralization and keeps the soil acidic unlike nitrification.
Berberis Thunbergii (www.habitas.org.uk)
Microstegium Vimineum (www.dnr.wi.gov)
The increased root density of Berberis might contribute to its higher pH.
Soil respiration in the soils below the exotics also increased, adding further nutrients to the soil. Root biomass probably is partially responsible for this as well.
The litter decomposition under Berberis is extremely fast. In fact, up to 90% of the initial plant material is decomposed every year. 17% of the nitrogen remains in the litter after decomposition, thus again increasing the nitrogen nutrient concentration in the normally nutrient poor soil. Microstegium litter decomposes about 40%-60% in the first, but very little after that. The decomposed litter, however, contains almost all of its original nitrogen.
Conclusion:
Deciduous Forest plant species are perfectly evolved and adept to growing in highly nutrient poor soils. Invasive species, like Berberis Thunbergii and Microstegium Vimineum are altering the natural characteristics of deciduous soils, making them more prone to further, successful invasions. The result could mean the loss of the entire deciduous forest ecosystem.
Centris Bees: A Genus in Danger
A notable example of a type of animal whose distribution has been hindered by the deforestation and its subsequent increase in fire frequency are bees of the genus Centris, who utterly depend on the forest and its resources for their survival. Aside from the pollen and nectar they must acquire from flowering plants in the forest, bees have an increased reliance on the structure and composition of the forest to maintain optimal survival conditions. That is, Centris bees create nests in deadwood portions of tree holes and in protected sites near the ground, both of which are destroyed by frequent fires. Additionally, fires also scorch oil-producing plants, which female bees use to maintain the young cells growing in their nests. Excessive fires decrease the amount of oil produced by these plants, limiting the ability of the bees to offer sustenance to their young.
Thus, as a whole, the increased frequency of fires due to deforestation has had drastic consequences on the distribution of Centris bees, whose populations over the past 2 decades have declined by almost 90% in temperate deciduous forests.
Alien Species: A Threat to Forest Diversity
In addition to changing the distribution of the vegetation in temperate deciduous forests, humans have further impacted these ecosystems by introducing alien species, both accidentally and intentionally.
African Honey Bees:
Over the last 2 decades, Mayans in the Yucatan region have imported the aggressive African honey bees as a substitute to the native stingless bees. The African bees produce higher quantities of quality honey, making them a more valuable resource to residents of this region who invest in the honey market.
However, the introduction of a new species has not been without its costs. As a result, there has been a breakdown in the diversity of stingless bees, to a point where they are now rare in the Yucatan, that correlates directly to the presence of the African honey bees. The breakdown of stingless bee populations has been caused by the aggressive nature of the African bees, who attack the passive native bees, thus lowering their diversity.
Red Fire Ants:
On the other hand, some alien species are introduced into foreign environments unintentionally, as is the case with the red imported fire ant, Solenopsis invicta. Over the last 60 years, this species of ants has increased dramatically in North America. However, this has come at a cost, as the presence of fire ants has led to a decreased diversity of other species of ants. Moreover, in the presence of the fire ants, the co-occurrence of many ant species has been random. This is because the invasive ants, foreign to their new habitat, fail to acknowledge the domains of other ant species. Instead, they inhabit land randomly, leading to aggressive behavior between ants that would otherwise not have occurred.
Conclusion:
The presence of invasive species can drastically affect the species composition of native plants and animals, leading to an imbalance in the native species distribution as the indigenous flora and fauna greatly decreased in diversity.
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