Red flour beetle (Tribolium castaneum (Herbst)) has been a model species for study of influences of demography (birth, death and dispersal) and genetics on range expansion by invasive species. Demography and genetics had important and independent roles in colonization success in nutritionally challenging environment. Inbreeding had negative effects while outbreeding had positive effects on population growth and long-term establishment. Prior adaptation to the environment rivaled the importance of number and size of introductions. Adults emigrated more and further from a high-density than a low-density population. These studies are important to predicting likelihood of establishment, preventing establishment of new pest species and successfully introducing new natural enemies. On a local level, T. castaneum is an invasive species each time it colonizes an uninfested commodity or facility.
Establishment increased with number of founders (43% with 2 to 100% with 32), inbreeding and environment quality reduced population growth, and genetic diversity and environment quality increased dispersal (Szűcs et al. 2014). Establishment increased more over 7 generations with several, smaller introductions than with one or two large introductions (Koontz 2014, Koontz et al. 2018). Establishment was not affected by biologically realistic fluctuations in nutritional quality of food with up to 99% corn flour. Focken (2007) showed that red flour beetles develop faster and reproduce more on wheat flour than corn flour and suggested that larvae may be able to distinguish between particles of corn and wheat in mixtures. Both demography and genetics had important and independent roles in colonization success in nutritionally challenging environment (96.3% corn flour, 3.515 % wheat flour, and 0.185 % brewer’s yeast) over 7 generations in an array of linearly interconnected food patches allowing dispersal between patches (Szűcs et al. 2017a). Inbreeding had negative effects while outbreeding generally had positive effects on establishment, population growth and long-term persistence. Extinction risks depend upon environmental stochasticity, demographic stochasticity, and demographic heterogeneity (Melbourne and Hastings 2008) and variation in spread rates among replicates was high (Melbourne and Hastings 2009). Genetic diversity can overcome predicted high extinction risk in new habitats (Agashe et al. 2011). Spatial evolution has a significant 6% higher mean spread rate and heightened variability in spread rates compared with the shuffled treatment in which spatial evolution was prevented (Weiss-Lehman et al. 2017). Higher dispersal ability and lower intrinsic growth rates evolve at the expansion edge.
Prior adaptation to the environment rivaled the importance of number of introductions and the number of individuals per introduction in determining the establishment success and size of founding populations (Vahsen 2017, Vahsen et al. 2018). Within six generations, evolving populations expanding range into a nutritionally challenging environment grew three times larger and spread 46% faster than populations in which evolution was constrained (Szűcs et al 2017b). Over the course of six generations, genetic rescue by replacing individuals from laboratory cultures with field strain increased population sizes and intrinsic fitness (Hufbauer et al. 2015). Genetic rescue is constrained by genetic load (Stewart et al 2017). Demographic and genetic rescue reduced extinction, and those effects were additive. Individuals, on average, were 21.4% more likely to emigrate (i.e. disperse from the first patch) when dispersing from a high-density than a low-density environment (Endriss et al. 2019). Individuals were also more likely to disperse further. Prior juvenile density strongly mediated the effect of current density on dispersal. Interspecific competition dramatically slows expansion across a landscape over multiple generations (Legault 2017, Legault et al. 2020). Nutritional quality of natal wheat flour and new oat flour habitat also influenced population growth (VanAllen and Rudolf 2013) and emigration (VanAllen and Bhavar 2014). Nutritional quality of natal habitat also can alter outcome of competition in new oat flour habitat (VanAllen and Rudolf 2015, 2016).
Initial studies found that egg or pupal cannibalism by larvae and adults in populations closed to dispersal determined outcome of competition between Tribolium species (Sokoloff 1974). Alabi et al. 2008 determined variation in cannibalism by developmental stage and species. More recently allelopathic chemicals conditioning of flour has been shown to influence life history and may be important in intra- (Khan et al. 2018) and interspecies (Bullock et al. 2020) competition. Fecundity of T. confusum was largely unaffected by flour conditioned by T. castaneum, T. castaneum fecundity was reduced by flour conditioned by either species and conditioning by the other species decreased development times for each species.
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