Here's why:

* Producers: Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to create their own food (glucose) and release oxygen. These organisms are called producers because they create the energy base for the entire food web.

* Consumers: All other organisms in the food web, called consumers, rely on producers for their energy. Herbivores eat producers directly, while carnivores and omnivores eat other consumers that have consumed producers.

* Energy flow: Photosynthesis is the initial step in the flow of energy through an ecosystem. The energy stored in glucose produced by photosynthesis is passed on to consumers when they eat producers, and then further up the food chain as consumers eat other consumers.

Without photosynthesis, there would be no primary source of energy to fuel the entire food web, and life as we know it would not exist.