Academic journal article Genetics

Genetic Diversity in Introduced Populations with an Allee Effect

Academic journal article Genetics

Genetic Diversity in Introduced Populations with an Allee Effect

Article excerpt

(ProQuest: ... denotes formulae omitted.)

THE amount of genetic diversity in a recently established population is strongly shaped by its early history: While the founder population size determines the amount of genetic variation imported from the source population, the population sizes in the following generations influence how much of this variation is maintained and how much is lost through genetic drift. A phenomenon that strongly affects the early history is the demographic Allee effect, a reduction in the average per-capita growth rate in small populations (Stephens et al. 1999; Fauvergue et al. 2012). Allee effects have been detected in species from many different taxonomic groups (Kramer et al. 2009). Apart from cooperation between individuals, the study subject of the effect's eponym (Allee 1931), they can result from a variety of other mechanisms such as diffi- culties in finding mating partners, increased predation pres- sure in small populations, inbreeding depression, or biased dispersal toward large populations (Stephens et al. 1999; Kramer et al. 2009). In this study, we investigate the population genetic consequences of two kinds of demographic Allee effect: Our main focus is on the strong demographic Allee effect, in which the average per-capita growth rate is negative for popu- lationssmallerthanacertaincriticalpopulationsize,butwe also consider the weak demographic Allee effect in which the average per-capita growth rate is reduced, but still positive at small population sizes (Taylor and Hastings 2005).

Another important process shaping the dynamics of small introduced populations is demographic stochasticity: fluctua- tions in population size due to randomness in the number of birth and death events and in sex ratio (see, e.g., Shaffer 1981; Simberloff 2009). On the one hand, demographic stochasticity-in this case a random excess in birth events- implies that even populations with a strong Allee effect start- ing out below the critical threshold have a positive probability of overcoming the Allee effect and persisting in the long term. On the other hand, demographic stochasticity can lead to population declines or even extinctions in populations with- out an Allee effect or with a weak Allee effect. Note, however, that demographic stochasticity itself does not fulfill our defi- nition of an Allee effect, because it does not influence the average per-capita growth rate of the population (see Dennis 2002, for a comprehensive discussion of the interaction be- tween demographic stochasticity and Allee effects).

If a species is subject to an Allee effect, particularly if the Allee effect is strong, the success probability for any particular small population may be very low. Nevertheless, it may be quite common to see populations that have established in this way. The reason is that with more and more transport of goods around the world, many species are introduced to a location not just once, but again and again at different time points. Eventually, a random excess in the number of birth events may cause one of these small introduced populations to grow exceptionally fast, surpass the critical population size, and become permanently established. Whereas most failed introductions pass unnoticed, the rare successful populations can be detected and sampled. As invasive species, some of them may have substantial impacts on native communities and ecosystems.

Our main question in this study is how expected levels of genetic diversity differ between successful populations that either did or did not have to overcome an Allee effect. Answering this question would help us to understand the ecology and evolution of introduced and invasive populations in several ways. On the one hand, the amount of genetic variation is an indicator for how well an introduced pop- ulation can adapt to the environmental conditions encoun- tered at the new location. Therefore, the Allee effect-if it influences genetic diversity-could shape the long-term suc- cess and impact even of those populations that are successful in overcoming it. …

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