![]() For instance, we still do not know whether responses to high population densities are conserved across taxa or to what extent population density during development has carry-over effects to adulthood and offspring that mediate eco-evolutionary processes. Yet, we still do not have a deep understanding of the far-reaching effects of early life conditions on individual life-history traits (Lindström 1999). Thus, the implications of density-dependent effects during development are many. Interestingly, population density at the larval stage can lead to faster adaptation to novel resources through increased intraspecific competition (Bolnick 2001), a factor that can increase the ability of populations to adapt to changing environments as well as population range shifts (Lawrence et al. Therefore, the larval stage is paramount for resource acquisition, with density-dependent effects being particularly notable. Adults have limited scope to compensate for poor developmental conditions later in life, particularly in traits such as body size because adults do not moult (Belles 2011). In non-social holometabolous insects in particular, competition for resources at the larval stage is known to underpin changes in larval growth as well as adult morphology and fitness (Yang 2001). In insects, population density at larval stage modulates resource availability during development that can have both negative and positive effects on development and fitness (Applebaum and Heifetz 1999). As a result, early life conditions can induce long-lasting effects on fitness and population dynamics, shaping populations’ evolutionary trajectories, species distribution range and extinction risks (Criscuolo et al. On the other hand, social interactions may benefit individuals through cooperative feeding (Denno and Benrey 1997), predator defence (Breviglieri and Romero 2019), or potentially by sharing beneficial microbial communities that are horizontally transmitted between individuals in the group (idea developed in this paper).Įarly life conditions influence fitness traits and life-history trade-offs (Nijhout 2015 Stearns 1982). This is because high-density increases intraspecific competition which limits per capita resource acquisition (Klepsatel et al. Individuals from low-density populations often display higher expression of fitness-related traits (e.g., higher fecundity, larger size) compared to individuals from high-density populations (Dey and Joshi 2018 Prasad and Joshi 2003). Population density is a key factor affecting life-history trait expression and trade-offs (Mueller et al. In summary, this review provides an integrative framework of density-dependent effects across biological levels which can be used to guide future research in the field of ecology and evolution. Next, we argue that host-microbe interactions are yet an overlooked biological level susceptible to density-dependent effects and propose a conceptual model to explain how density-dependent effects on host-microbe interactions can modulate density-dependent fitness curves. ![]() We then classify the biological levels upon which larval density-dependent effects can be observed followed by a review of the literature produced over the past decades across major non-social holometabolous groups. First, we provide a functional definition of density to navigate the terminology in the literature. Here, we review the literature on larval density-dependent effects on fitness of non-social holometabolous insects. Better understanding the scope and generality of density-dependent effects on life-history traits of current and future generations can provide useful knowledge for both theory and experiments in developmental ecology. ![]() Larval density-dependent effects can modulate the expression of life-history traits not only in the larval and adult stages but also downstream for population dynamics and evolution. In holometabolous insects, the larval stage is particularly susceptible to density-dependent effects because the larva is the resource-acquiring stage. ![]() Population density modulates a wide range of eco-evolutionary processes including inter- and intra-specific competition, fitness and population dynamics. ![]()
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