Why might an ecologist use a food chain for one study and a food web for another?

It depends on the area of his interest. Behind our consciousness, humans and other living things participate in a natural systematic transformation of energy from one to another through the intake of food. This is presented by the food web. A food web is a network of food chains. Therefore, if the ecologist wants to focus on a more specific relationship, he would look at the food chain. If he wants to look at the bigger picture to study the extent of the ecosystem, he would study the food web.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.

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