This ecosystem works because of the extremely low metabolism of the SGPs and the near lack of predators on the two continents where they are found. They are in a way extensions of the plants they garden that aggressively shade competitors and deliver additional sunlight that they would not otherwise get. In fact, some scientists have speculated that the ancestral SGP might have evolved from a plant that then lost its ability to photosynthesize and learned how to feed on other plants.
SGPs have also been known to help aerate the soil and defend against herbivores. They defend by the use of spines, venomous hairs, or by directing all available sunlight into the herbivore's face. This has led some herbivores to feed at night and others to give up the use of eyes. Given the very low incidence of true predators on the two continents, this is not much of a disadvantage.
Below are some other doodles I made recently: