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.
Organisms obtain energy in different ways. But all energy traces down from the sun. Most plants get energy by absorbing sunlight in their chloroplasts by the means of photosynthesis, however there are some carnivorous plants, such as the Venus Flytrap. Herbivores eat the plants and get energy from them. Then carnivores eat the herbivores. This is the food chain. So, all organisms are in the food chain, and most get energy from eating food, either plant or animal.