Nutrient regulation by an omnivore and the effects on performance and distribution

Abstract

Omnivores have a unique interaction with their nutritional environment because they have adapted to consume food from different trophic levels. To successfully navigate their variable resources, omnivores must maintain some level of nutrient regulation. To explore the effects of nutrient regulation by an omnivore, I used a salt marsh katydid, Conocephalus spartinae.

To first address the ability of Conocephalus to perform on a wide range of diets and to regulate their nutritional intake, I used artificial diets that differed in relative amounts of protein and carbohydrate (Chapter 1). I found that Conocephalus survival decreased on a high protein diet due in part to a decrease in lipid stores but growth was not affected by diet. In a second experiment Conocephalus showed a degree of nutrient regulation as evidenced by the difference in what they actually ate and the predicted consumption if they had been feeding equally on the diets presented in each treatment. However, I did not find evidence for tight macronutrient regulation.

Next I explored capacity of Conocephalus to regulate their nutrient intake (nitrogen and lipid) when fed naturally co-occurring prey (Chapter 2). I first established that the prey differed in their protein and lipid content and that these differences were related to the size of the prey species. When Conocephalus were fed different prey species individuals showed no differences in either growth or survival. In the final experiment, I found that Conocephalus did show evidence of a degree of nitrogen and lipid regulation because they did not feed equally on all of the prey species offered.

Lastly, I documented the relationship between the ability of Conocephalus to locate plant and prey resources and the effect that these resources have on omnivore performance (Chapter 3). I found that Conocephalus aggregates in areas of high plant quality but that their numbers do not correspond to areas of high prey density. However, I found that katydid growth and survival was enhanced by prey availability but not plant quality.

Overall, I documented how an organism like an omnivore relates to its nutritional environment and how nutrient regulation might affect performance and distribution. Last, I documented the relationship between the ability of katydids to locate plant and prey resources and the effect that these resources have on omnivore performance (Chapter 3). I found that katydids aggregate in areas of high plant quality but that their numbers do not correspond to areas of high prey density.