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.
Animals depend on plants for food as plants produce food and energy which animals can't, so, herbivores eat plants to get energy, carnivores eat herbivores and other carnivores for energy and omnivores eat all four, plants, herbivores, carnivores and also other omnivores for energy.
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.