Parsing propagule pressure: Simulated and experimental disentanglement of introduction 1 size and number of introductions for colonizing individuals 2

Colonization of novel habitats is more likely with increasing propagule pressure— the total 21 number of individuals introduced to a site. Two key components of propagule pressure are the 22 number of introduction events and the size of those introduction events. It is unclear which of 23 these components is more important for colonization success, or under what environmental 24 conditions their relative importance might shift. Using demographic simulations paired with a 25 Tribolium flour beetle microcosm experiment, we introduced 20 total individuals into replicated 26 novel habitats of stable or fluctuating quality and manipulated the number and size of 27 introduction events used to distribute them through time. After seven generations, we evaluated how different combinations of the number and size of introductions affected establishment 29 probability, size of established populations, and whether effects depended on the variability of 30 the recipient environment. We found no effect of biologically realistic environmental 31 stochasticity on establishment probability or size of established populations in the demographic 32 simulations. However, there was strong evidence that establishment probability was enhanced 33 with more, smaller introductions. In the microcosm, we similarly found no effect of 34 environmental stochasticity on establishment probability, but unlike the simulations found that 35 populations that established were larger in the stable environment, especially with more 36 introduction events. The microcosm experiment yielded greater overall establishment probability 37 and larger populations compared to the demographic simulations. Genetic mechanisms likely 38 underlie these differences in results and thus deserve more attention in efforts to parse propagule 39 pressure. Our results highlight the importance of preventing further introductions of undesirable 40 species to invaded sites, and suggest conservation efforts should focus on increasing the number 41 of introductions or re-introductions of desirable species rather than increasing the size of those 42 introduction events.