Once in about every thousand years, volcanic eruptions create concentrations of atmospheric dust so high that the sky becomes too dark for grass pokemon and plants to grow. For humans and pokemon higher up the food chain, these years have meant starvation, especially in marginal areas such as deserts where plant life hangs in the first place by a thread.
The lone exception to these disasters are the Skorupi, who can spend an entire year in barren deserts without any need for food. When the dust dissipates and spring comes again, plant life will grow back from their seeds and roots, but most of their predators will not. In these years, Skorupi are as common in the desert as Bidoof are in Sinnoh's grass, free from predation and able to poison Cacnea with ease.
It is this resilience which has kept the Skorupi unchanged since the Permian era in a variety of marginal habitats, of which deserts are the most recent, and allowed them to survive countless mass extinctions. They will endure long beyond most wild pokemon, no matter how much man destroys with pollution or industry. Pollution has only an indirect effect on these poison-types; it can wipe out their food, but all it does to the Skorupi themselves is make their venom more potent. If their current food supply, the (in no way endangered) Cacnea, were to be driven to extinction, it would kill very few Skorupi.
The rest would leave the desert – for even small bug pokemon can travel a great distance in a year – and search for something to feed on, for Skorupi are omnivores and not remotely picky. And these pokemon would surely find prey that survived whatever caused Cacnea's destruction, as they have throughout the horrors of eons.