An experimental test of the ecological mechanisms driving density-mediated carry-over effects in a seasonal population

Carry-over effects occur when past experience influences current individual performance. Although variation in conspecific density in one season has been shown to carry over to influence dynamics in the following season, the proximate ecological mechanisms driving these effects are unknown. One hypothesis is that high density decreases food availability, resulting in poor physiological condition, which in turn compromises performance the next season. Alternatively, high conspecific density could also lead to a high degree of antagonistic interactions, decreasing the amount of time individuals spend foraging. To investigate these hypotheses, we applied a factorial design where both conspecific density and per capita food availability during the non-breeding period were independently manipulated in seasonal populations of common fruit flies (Drosophila melanogaster Meigen, 1830). Individual condition at the beginning of the breeding period was influenced by per capita food availability but not density during the previous non-breeding period. In contrast, reproductive output was most strongly influenced by the interaction between per capita food availability and density in the previous non-breeding period, such that populations that experienced high non-breeding densities and low food availability had the lowest reproductive output. However, the strength of this effect was relatively weak. Our results demonstrate how environmental and social conditions in one part of the annual cycle can carry over to influence individual performance in subsequent periods. Key words: body condition, Drosophila melanogaster, food availability, fruit fly, population density, reproductive output, seasonal interactions. Les effets reportes se produisent quand l'experience passee influence la performance presente des individus. S'il a ete demontre que les variations de la densite de conspecifiques durant une saison ont une influence sur la dynamique durant la saison suivante, les mecanismes ecologiques proximaux a l'origine de ces effets ne sont pas connus. Une hypothese veut qu'une forte densite reduise la disponibilite de nourriture, ce qui se traduit par un faible embonpoint qui, lui, compromet la performance durant la saison suivante. Une autre explication serait qu'une forte densite de conspecifiques pourrait aussi mener a un degre eleve d'interactions antagonistes, reduisant du coup le temps affecte par les individus a la quete de nourriture. Afin d'evaluer ces hypotheses, nous avons applique un schema factoriel dans lequel la densite de conspecifiques et la disponibilite de nourriture par individu durant la periode internuptiale etaient manipulees de maniere independante dans des populations saisonnieres de drosophiles (Drosophila melanogaster Meigen, 1830). L'embonpoint individuel au debut de la periode de reproduction etait influence par la disponibilite de nourriture par individu, mais non par la densite durant la periode internuptiale precedente. En comparaison, la plus forte influence sur l'efficacite de la reproduction etait l'interaction entre la disponibilite de nourriture par individu et la densite durant la periode internuptiale precedente, l'efficacite de la reproduction etant la plus faible dans les populations caracterisees par une forte densite internuptiale et une faible disponibilite de nourriture. L'intensite de cet effet etait toutefois assez faible. Nos resultats demontrent comment des conditions ambiantes et sociales durant une partie du cycle annuel peuvent avoir des effets reportes qui influencent la performance des individus durant des periodes subsequentes. [Traduit par la Redaction] Mots-cles : embonpoint, Drosophila melanogaster, disponibilite de nourriture, drosophile, densite de population, efficacite de la reproduction, interactions saisonnieres.
Introduction Carry-over effects occur when an individual's past experience explains its current performance (O'Connor et al. 2014). In seasonal environments, there is growing evidence that carry-over effects play an important [...]