Contrasting patterns of food and macronutrient limitation in the field among co-existing omnivorous carnivores

1. Carnivores are often food and/or macronutrient limited in their natural habitats, but whether they are limited mostly by protein or lipid is still a matter of controversy. As many predators and carnivorous scavengers also include plant material in their diet (omnivory), carbohydrate limitation is also possible. 2. The authors used a recently described double-test procedure to test for food and macronutrient limitation in five co-existing species of predators and omnivorous scavengers from Uruguay (two crickets: Gryllus sp. and Miogryllus verticalis; two cockroaches: Blatta orientalis and Blaptica dubia and a harvestman: Acanthopachylus aculeatus). 3. The authors found that the two crickets and one cockroach were food limited and one cockroach was non-protein (lipid) limited in the field. The harvestman showed a dramatic change in food limitation between two dates separated by only 3 weeks, but was non-protein (lipid) limited over both dates. From all species, the harvestman showed the highest self-selected intake of carbohydrates (27%), indicating a high level of omnivory. In contrast, the two cockroaches selected a surprisingly low proportion of carbohydrates (13–14%), while the crickets were intermediate (14–19%). The authors hypothesize that these omnivores are more carnivorous in the wild than expected from studies of laboratory populations. 4. Though individuals were collected from the same habitats and all species self-selected macronutrient ratios characteristic of omnivorous carnivores, they showed different patterns of food and macronutrient limitation, reflecting species' niche segregation and individual differences in foraging success.