Predicting the fate of individuals colonizing novel habitats is an elusive but critical goal in fields as diverse as invasion biology, biological control, climate change-induced species range shifts, and reintroductions of rare species. Propagule pressure, which comprises the number of introduction events (propagule number) and the number of individuals per introduction event (propagule size), consistently correlates with a greater probability of population establishment. It is unclear which component, propagule number or propagule size, is more important for establishment, or under what environmental conditions their relative importance may shift. We used 917 independent Tribolium flour beetle populations in a microcosm experiment to disentangle the importance of the different components of propagule pressure. In a factorial design, we held the total number of introduced individuals constant (20) and varied the number of introductions used to distribute them (1, 2, 4, or 5 events) into stable or randomly fluctuating novel environments. Counter to expectations, we found no effect of environmental stability on extinction probability or time to extinction. We also found that several, small introduction events resulted in the lowest extinction probability and the longest time to extinction. We propose that continuing introductions provided low amounts of gene flow that were critical to alleviating inbreeding depression and/or reducing allelic loss by drift in the incipient populations. Our results speak to the importance of preventing future introductions of invasive species (even those that are already established), and using sustained efforts to establish biological control agents or reintroduce desirable organisms to their former range.