More than just density: the role of egg number, food volume and container dimensions in mediating larval competition in Drosophila melanogaster

Flies of the genus Drosophila have been extensively used as model systems to study competition. Experiments using larval crowding in these species have furthered our understanding of competition ecology, stress-adaptation, density-dependent selection and population dynamics. Historically, larval competition has been induced by crowding the larvae in high density cultures, compared to low-density controls. However, recent studies have shown that two larval cultures having the same total eggs per mL food density, with different absolute quantities of eggs and food, can differ greatly in their density-specific fitness functions. Similarly, populations adapted to two types of cultures achieving the same density through different means, can also evolve different traits. Thus, it is clear that there is more to larval crowding than just eggs/food density, which has until now been the benchmark variable for quantifying larval crowding in Drosophila studies. In the current study, we explore the consequences of implementing crowding in different ways, using a three-way factorial experiment with egg number and food volume, cast into different food column heights or cylindrical vials with different diameters. We find that not just the same density, but cultures having the same egg number and food volume combination but experienced in food columns of varying height and diameter can have very different pre-adult survivorship and development times. We further propose a new variable for quantifying larval crowding - effective density, which is the density within the larval feeding band, a volume of food close to the surface, with access to air, wherein a majority of the larvae feed. We show that effective density is a much better predictor of the outcomes of competition than the popularly used total eggs/food density.