Density dependent factors are the aspects of the environment that limit a population's growth regardless of the density of the population. So in this case, the answer is hurricanes.
When describing a community, biologist would discuss every population that lives in that area.
On a very simple level, the answer should be that archaebacteria developped in climates, atmospherical conditions, temperatures, physical and chemical environments that were in some ways different from those that characterize Earth today. So, it is reasonable that planets with conditions like those of Earth when archaebacteria developped might bear some form of life similar to archaebacteria, or evolved from those organisms.