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In governments, communities are generally organized in a top-down or bottom-up fashion. Either you're a proponent of the 'trickle-down effect' or of grassroots organization. Similarly, in nature, communities may be organized in a bottom-up or a top-down model. What do these models represent? Well, for one thing, it has nothing to do with voting or your political beliefs, since these are models of a more natural rather than human understanding.
Let me pose a little simple scenario for you so you can begin mulling over the two models, the first one being the bottom-up model. Let's say there is some grassland and some deer. The grass, G, provides food for the deer, D. In the bottom-up model, we can say that the flow of influence on community organization goes in one direction, G to D, meaning it flows from a lower trophic level (grass) to a higher one (deer). From the bottom to the top, bottom-up. What this means is that if there is more grass, then deer will increase in number, or biomass, the total mass of living matter in a certain region. However, because the flow of G to D is unidirectional, the abundance of deer will not change the abundance of grass in such a model.
Okay, that was a simple example. Let's solidify our understanding of the bottom-up model with a more complex example. Let's say the soil contains nutrients, N, that helps the grass, G, grow. This, in turn, controls the number of deer, D and this, in turn, controls the number of predators, wolves, W. In the bottom-up model, our flow of influence is thus N to G to D to W. This means that if we want to alter this bottom-up community structure, we have to change the biomass at lower trophic levels in order to affect the higher trophic levels. If we increase the abundance of N, then G should increase and, theoretically, so should D and then W. However, if you decrease W, this should not effect lower trophic levels.
All of this stands in contrast to the top-down model of community organization, where the flow of influence goes in the opposite direction. That is to say, W to D to G to N. If we increase the number of wolves, more deer are eaten. This means more grass can grow since less of it is being eaten by a fewer number of deer. However, this also means nutrients decrease because of the abundance of grass growing and utilizing those nutrients. The top-down model of community organization has been used by ecologists to alter an ecosystem's characteristics. This is called biomanipulation. One example of this is the use of the top-down model to improve water quality without the use of chemicals. For example, to manage the occurrence of detrimental algal blooms, ecologists can remove fish from the ecosystem. This means zooplankton density will increase and this will, in turn, decrease the algal population. That's because fish eat zooplankton that, in turn, graze on algae.
I don't think that algae would be voting for the top-down model of natural government. The top-down model basically says that the influence on community structure flows from the top, the predators, down to the herbivores, then the vegetation, and finally the nutrients. Bottom-up model says that the influence on community organization flows from the nutrients, to the vegetation, then the herbivores, and finally the predators. So, if there's more vegetation then higher trophic levels, like that of herbivores, should increase in biomass, the total mass of living matter in a certain region, as a result.
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Campbell Biology: Online Textbook Help56 chapters | 510 lessons
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