In the latter case the resistance is caused by dissipation of mechanical energy when the configuration or the direction of flow is sharply changed, by the formation of vortices and secondary flows as a result of the flow breaking away, by the centrifugal forces, etc.
Why do animals depend on plants. Plants give animals the food and shelter they need. Herbivores, which are plant eating animals, rely on plants to provide them with a regular source of food. They also provide shelter to may animals, both large and small.
Animals need the sun because plants need light, herbivores eat the plants, and carnivores eat the herbivores. Therefore,if there is no sun, there is no plants, if there are no plants, there are no herbivores, if there are no herbivores, there are no carnivores.It's like a domino effect.