COS 79-5
The immediate and long-term consequences of multiple introductions for colonizing individuals
Understanding the fate of founding individuals introduced to novel environments is fundamental to ecology and conservation. Whether introductions are accidental (e.g. biological invasions) or deliberate (e.g. reintroductions of rare species, biological control releases), the degree to which colonization events are successful has significant implications for natural resource managers and society. The most reliable predictor of establishment success is propagule pressure, a concept that combines the total number of founding individuals and the number of founding events. However, it is not clear which of these components of propagule pressure is more important for population establishment, how their relative importance might shift depending upon temporal heterogeneity in the recipient environment, or what their long-term consequences are for population persistence and adaptation. We used Tribolium flour beetles as a model system and introduced 917 populations into stable or fluctuating environments, censusing each population for 10 generations. We held the total number of founders constant at 20 and asked how different numbers of introduction events affected the establishment, persistence, and adaptation of experimental populations. Our design models organisms with non-overlapping generations, and allows us to experimentally test how varying the number of introduction events affects colonizing individuals initially and in the long-term.
Results/Conclusions
In this system, 20 founders established a population with high probability (99.78%), but many small introductions (4 individuals, 5 times) established significantly larger populations than other introduction types (e.g. 20 individuals, 1 time; 10 individuals, 2 times). Additionally, introducing 10 individuals on 2 occasions resulted in a significantly higher extinction probability than other introduction types, suggesting two disadvantages for this introduction scenario: 1) introductions may not be large enough to avoid inherent costs of small population sizes, and 2) there may be too few introduction events to gain the benefits of repeated inputs. Counter to expectations, these patterns held in both stable and fluctuating environments despite that fluctuations resulted in considerable variability in mean population growth rates. Our results reveal that, when the total number of founding individuals is limited, a sustained introduction effort through time is more important in promoting population establishment and persistence than the size of the initial introduction event. For invasions, this highlights the importance of preventing further introductions, even for established species. For conservation and biological control, this suggests emphasis should be placed on increasing the number of (re)introduction events, rather than increasing the size of those events.