Such organisms differ drastically from neighbors even short distances away, because the typical bottom-dweller is adapted to temperatures around 0 C and--rather like us--to water that is not suffused with hydrogen sulfide. Travel between distant hydrothermal vents is impossible for many species, which cannot tolerate the frigid water and lack of chemosynthetic materials. Hence, some chimneys are isolated enough that they have become virtual Galápagos islands of the deep--closed communities of genetically-isolated species found only in that particular ecosystem.
So one of these vent communities can form and operate more or less independently for eons. Bacteria covert hydrogen sulfide and methane into organic compounds (the undersea equivalent of photosynthesis) from within their symbiotic tube worm and giant mollusk hosts, small crustaceans eat the bacteria, larger crustaceans eat the smaller crustaceans, and the cycle of life goes on.
But it turns out that hydrothermal vents operate more or less at the whim of sub-seafloor volcanic systems. So all it takes is one undersea volcanic eruption to divert the flow of magma and shut down the vent overnight. And then it's all over for the vent community--everybody's dead within days. Kind of makes you think.