Abstract
Predicting whether individuals will colonize a novel habitat is of fundamental ecological interest and is crucial to conservation efforts. A consistently supported predictor of colonization success is the number of individuals introduced, also called propagule pressure. Propagule pressure increases with the number of introductions and the number of individuals per introduction (the size of the introduction), but it is unresolved which process is a stronger driver of colonization success. Furthermore, their relative importance may depend upon the environment, with multiple introductions potentially enhancing colonization of fluctuating environments. To evaluate the relative importance of the number and size of introductions and its dependence upon environmental variability, we paired demographic simulations with a microcosm experiment. Using Tribolium flour beetles as a model system, we introduced a fixed number of individuals into replicated novel habitats of stable or fluctuating quality, varying the number of introductions through time and size of each introduction. We evaluated establishment probability and the size of extant populations through seven generations. We found that establishment probability generally increased with more, smaller introductions, but was not affected by biologically realistic fluctuations in environmental quality. Population size was not significantly affected by environmental variability in the simulations, but populations in the microcosms grew larger in a stable environment, especially with more introduction events. In general, the microcosm experiment yielded higher establishment probability and larger populations than the demographic simulations. We suggest that genetic mechanisms likely underlie these differences 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 reintroductions of desirable species rather than increasing the size of those introduction events into harsh environments.
Highlights
Colonization is the ecologically fundamental process of population establishment in an unoccupied location, and it underlies the past, present, and future distributions of species
In simulations and a microcosm experiment, we evaluated the outcome of introducing 20 total individuals to one of two environmental contexts, varying the number of introduction events used to distribute those individuals through time
Recent work shows that adaptation to the novel, harsh environment from standing variation is possible in this species within a similar timeframe as our experiment (Hufbauer et al, 2015; Szűcs, Vahsen, et al, 2017), and likely explains the greater establishment probability and population sizes in the microcosm compared to expectations derived from demographic simulations which do not include adaptation
Summary
Colonization is the ecologically fundamental process of population establishment in an unoccupied location, and it underlies the past, present, and future distributions of species. Models that hold the total propagule pressure constant agree that multiple, small introductions distributed across space will lead to greater establishment probability compared to a single, large introduction when Allee effects are weak (Grevstad, 1999; Haccou & Iwasa, 1996; Schreiber & Lloyd-Smith, 2009). Both modeling and empirical approaches have generated conflicting views on how an introduction regime affects colonization success when introductions are distributed through time, a situation in which individuals from later introductions interact both demographically and genetically with individuals from previous introductions. We evaluated establishment probability and population size over seven discrete generations to ask: (a) does colonization success increase with more introduction events or with more individuals per introduction event?, and (b) does the effect of the introduction regime on colonization success depend on whether the recipient novel environment is stable or fluctuating through time?
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