Bottom-Up & Top-Down Models of Community Organization

Bottom-Up & Top-Down Models of Community Organization
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  • 00:00 Community Organization
  • 00:27 Bottom-Up
  • 2:24 Top-Down
  • 3:22 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
How can a community be organized? Maybe you've done some community organization yourself, either from the bottom up or the top down. Now, take a look at how this occurs in nature.

Community Organization

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.

Bottom-Up

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.

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