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AP Enviro



Unit 1

1.11 Food Chains and Food Webs

3 min readjune 11, 2020

Harrison Burnside

Sumi Vora

"Food chains give us a clear-cut picture of who eats whom. However, some problems come up when we try and use them to describe whole ecological communities. For instance, an organism can sometimes eat multiple types of prey or be eaten by multiple predators, including ones at different trophic levels. This is what happens when you eat a hamburger patty! The cow is a primary consumer, and the lettuce leaf on the patty is a primary producer.To represent these relationships more accurately, we can use a food web, a graph that shows all the trophic—eating-related—interactions between various species in an ecosystem. The diagram below shows an example of a food web from Lake Ontario. Primary producers are marked in green, primary consumers in orange, secondary consumers in blue, and tertiary consumers in purple." - From Khan Academy Describing the Image

Source: Khan Academy

Movement of Matter 

The earth is a closed system; matter can never be removed or created. Instead, matter moves across the earth in cycles called biogeochemical cycles. These basic cycles are essential for supporting life on Earth, but when they are disrupted, it can cause matter to get “stuck” or move too quickly between stages. 

Response to Disturbance 

Feedback Loops 

Natural systems try to remain in a steady state by responding to changes in inputs and outputs. Changes in inputs and outputs are called feedback. There are two different types of feedback: negative feedback loops and positive feedback loops.
Negative feedback loops are more common -- they occur when a system responds to change by trying to return back to its original state or by decreasing the rate at which the change is occuring. For example, if drought causes a lake to evaporate, then the lake will decrease in surface area, leading to less and less evaporation. On the other hand, when there is more precipitation, the lake will increase in surface area, which will increase evaporation. The lake therefore always tends to stay the same size. 
Scientists are more concerned with positive feedback loops, in which change always continues to increase. For example, increased global temperatures and pollution lead to ice caps melting. As ground is exposed and smog settles on the ice, the ground becomes darker in color, which absorbs more sunlight and contributes to even more melting. It is very difficult to mitigate the effects of positive feedback loops. 
The words positive and negative can be confusing, but they don’t mean that one loop is bad and the other is good. In fact, negative feedback loops usually indicate healthy environments. Instead, remember that positive feedback loops cause more change, whereas negative feedback loops cause less change

Ecosystem Resistance and Resilience 

We can determine how susceptible ecosystems are to damage by discussing ecosystem resistance and resilience. Ecosystem resistance measures how much a disturbance affects the functioning of an ecosystem. The more resistant an ecosystem is, the more it will be affected by changes. On the other hand, ecosystem resilience measures how quickly an ecosystem is able to recover after a disturbance. Just like organisms, the more resilient an ecosystem is, the better it will be able to support its inhabitants. 
Not all disturbance is bad -- disturbances in ecosystems can lead to increased productivity and maintain species diversity. Natural wildfires, for example, can help to eliminate dead plants and add nutrients to the soil. There should be a happy medium between disturbance and stability in order to create the healthiest ecosystems. The intermediate disturbance theory states that when ecosystems experience intermediate levels of disturbance, they will have higher species diversity than those that don’t experience that much disturbance. This theory works because when there are very low levels of disturbance, competition increases and the weaker organisms die off. Small levels of disturbance allow other organisms to have a more equal chance to survive.

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Unit 2: The Living World: Biodiversity
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