If the population of crabs becomes extinct in a food web, it is likely that the population of the species that prey on crabs or depend on them for food would decrease. Conversely, the population of the species that crabs prey on would increase.
For example, in a food web that includes crabs, fish, and sea birds, if crabs become extinct, the population of fish that crabs feed on may increase. This increase in the fish population may then lead to a decrease in the population of the small organisms that the fish feed on.
On the other hand, if crabs are preyed upon by sea birds, then the population of sea birds may decrease if they are unable to find alternative food sources. This, in turn, may impact the population of the organisms that the sea birds feed on, as they may not be consumed at the same rate as before.
Overall, the extinction of any species in a food web can have significant impacts on other populations within that web, and can cause ripple effects throughout the ecosystem.