In a changing environment species shift their range over time. During the shift, dispersing individuals encounter a new biotic and abiotic environment that theoretically selects for individuals with higher dispersal propensity, lower competitive ability, and lower fecundity rates. These population range dynamics have been investigated though competition-colonization tradeoff studies. Majority of research focuses on a single species and pairwise tradeoffs. Understanding phenotypic plasticity in a dispersal-competition-fitness tradeoff with interspecific interactions will help us understand how dispersing populations react to novel environments.
Using the model species Tribolium castaneum and T. confusum, we assessed the dispersal, competition, and fecundity of wild populations that were collected across the United States. We hypothesized that T. castaneum will have greater competitive abilities and higher fecundity while T. confusum will have greater dispersal propensity. This information will help us address the broader concept that selecting individuals at the range core and edge will lead to different pairwise tradeoffs within the populations with the extent of selection of the two traits being limited by the addition of a third trait. For example, when high dispersing, low fecundity populations are exposed to competition, dispersal ability will be lower and fecundity higher compared to the single tradeoff.
Results/Conclusions
As hypothesized, wild populations of the two species show different dispersal propensity, competitiveness, and fecundity. T. confusum disperses further than T. castaneum (T4=5.02, p=0.003). Adults of both species placed in a patch do not exclude more individuals of the other species (T3=1.48, p=0.12). Both species show intraspecific competition based on density dependent dispersal, but differently. T. castaneum displays a positive density dependent emigration relationship. T. confusum has density independent emigration; however, the distance travelled is positively correlated with density (F1,13=23.44, p<0.001). Lastly, initial populations of T. castaneum have a higher fecundity than T. confusum (T4=5.65, p=0.002). These results support the theorized pairwise tradeoffs with the weaker competitor T. confusum having greater dispersal propensity and lower fecundity. Additionally, the two species have opposite phenotypes within both dispersal-competition and dispersal-fecundity tradeoff models, which has not been addressed cohesively in empirical studies.
This data represents the original core population of an ongoing experiment examining the phenotypic selection of individuals at the interior and edge of a range. Offspring of this generation will be selected for superior and poor dispersal propensity, competitive ability, and fitness for ten generations to assess the dispersal-competition-fitness tradeoff over time.