Light and nutrient conditions determine algal food quality for consumers. Low food quality can decrease consumer growth rates, but herbivores can compensate by increasing their foraging effort. However, the effects of light/nutrient conditions on predator foraging effort have not been demonstrated. Increased foraging effort may result in higher energy expenditure and thus lower fat content, a potential fitness cost of compensatory feeding. We investigated food quality effects on growth rate, larval foraging behavior, and adult fat content in the predatory damselfly Enallagma aspersum. To manipulate food quality, we reared larval damselflies to adulthood in mesocosms under high/low light and high/low nutrients conditions. We also conducted laboratory bioassays of larval foraging behavior to determine a potential compensatory mechanism underlying the effects of low food quality on larval growth rate. Finally, we quantified adult fat content in individuals from the mesocosms. We expected the highest growth rates under high food quality (low light, high nutrients) and the lowest growth rates under low food quality (high light, low nutrients). However, if foraging effort increased at low food quality, growth rates should be similar between treatments. Finally, treatments where larvae spent more time foraging were expected to produce adults with lower fat contents.
Results/Conclusions
Similar growth rates between nutrient treatments suggest growth compensation at low nutrient levels. Growth rates were 30% higher under high light than low light; however, our light manipulation also altered water temperature. We observed similar growth rate differences between individuals reared in the laboratory at the temperatures observed in the mesocosms. Thus, we could not distinguish the effects of light and temperature on damselfly growth rate. However, growth rates did not differ substantially between nutrient treatments, suggesting growth compensation at low nutrient levels. As further evidence for growth compensation, larval damselflies reared under low nutrient conditions showed higher attack and consumption rates (i.e., higher foraging effort) than damselflies reared under high nutrient conditions. Foraging effort did not differ between light levels. In addition, preliminary results of adult fat content suggest that higher food quality (low light, high nutrients) leads to higher fat content than low food quality (high light, low nutrients). Taken together, these results are evidence of predator behavioral compensation (increased foraging effort) and subsequent fitness costs (reduced fat content) at low nutrient levels.