1. Although many non-native species arrive in novel environments, only a fraction successfully establish. A crucial factor affecting the colonization process of invading species is propagule pressure. The positive association between propagule pressure and colonization probability is driven both by stochastic dynamics and the 'Allee effect'. Although the role of Allee effects in invading populations is theoretically compelling, they are difficult to quantify in the field because the earliest phases of biological invasions are typically not observed.
2. We conducted parallel studies using two species of bark beetles, Hylurgus ligniperda in New Zealand and Ips pini in North America, to (i) assess the role of propagule pressure on colonization success, (ii) empirically test for Allee effects and (iii) estimate Allee thresholds. We evaluated each of these factors by experimentally testing the effect of propagule pressure (numbers released) on simulated tree colonization success in two different settings: (1) field conditions where beetles could disperse freely and (2) enclosed cages where dispersal loss was prevented.
3. Colonization success was positively associated with numbers released (i.e. propagule pressure) for both species, though colonization occurred at lower numbers for H. ligniperda than for I. pini. A demographic Allee effect was only detected in H. ligniperda and the Allee threshold was smaller when beetles were released 1 m from host billets than when they were released 10 m away. Greater colonization success at lower release densities may explain the invasion success of H. ligniperda worldwide. Higher release densities required for successful colonization may explain why I. pini is a poor establisher.
4. Synthesis and applications. Our results linking invasion failure to small founding population densities generally support the theoretical literature on the role of propagule pressure and Allee effects in biological invasions. Agencies such as the International Plant Protection Committee (IPPC) setting phytosanitary measures such as the International Standards for Phytosanitary Measures No. 15 (ISPM 15) should consider using the colonization thresholds estimated here to limit numbers of colonizing individuals below invasion thresholds. Additionally, agencies conducting trapping efforts at ports-of-entry should be aware of these thresholds and widely report when trapping numbers exceed these thresholds.