PT - JOURNAL ARTICLE AU - Michael J Koontz AU - Meagan F Oldfather AU - Brett A Melbourne AU - Ruth A Hufbauer TI - Parsing propagule pressure: Number, not size, of introductions drives colonization success in a novel environment AID - 10.1101/108324 DP - 2017 Jan 01 TA - bioRxiv PG - 108324 4099 - http://biorxiv.org/content/early/2017/02/21/108324.short 4100 - http://biorxiv.org/content/early/2017/02/21/108324.full AB - Colonization of novel habitats is more likely with increasing propagule pressure— the total number of individuals introduced to a site. Two key components of propagule pressure are the number of introduction events and the size of those introduction events. It is unclear which of these components is more important for colonization success, or under what environmental conditions their relative importance might shift. Using demographic simulations paired with a Tribolium flour beetle microcosm experiment, we introduced 20 total individuals into replicated novel habitats of stable or fluctuating quality and manipulated the number and size of 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 probability, size of established populations, and whether effects depended on the variability of the recipient environment. We found no effect of biologically realistic environmental stochasticity on establishment probability or size of established populations in the demographic simulations. However, there was strong evidence that establishment probability was enhanced with more, smaller introductions. In the microcosm, we similarly found no effect of environmental stochasticity on establishment probability, but unlike the simulations found that populations that established were larger in the stable environment, especially with more introduction events. The microcosm experiment yielded greater overall establishment probability and larger populations compared to the demographic simulations. Genetic mechanisms likely underlie these differences in results and thus deserve more attention in efforts to parse propagule pressure. Our results highlight the importance of preventing further introductions of undesirable species to invaded sites, and suggest conservation efforts should focus on increasing the number of introductions or re-introductions of desirable species rather than increasing the size of those introduction events.