The article discusses what determines the number of plants, herbivores, and carnivores in a community. It explains that the abundance of these different trophic levels is controlled by both the availability of resources and the presence of predators. The strength of these effects depends on the number of species in each trophic level. Trophic cascades show that predators play a crucial role in ecosystems, but it's unclear whether they are more important than resources. These cascading effects can be caused by predators eating prey or by other influences. Natural systems can have anywhere from three to six trophic levels, and it seems that productivity and ecosystem size are the main factors determining this number.