Nutrient composition of the prey's diet affects growth and survivorship of a generalist predator.

It is well known that secondary chemicals produced at one trophic level may affect organisms at subsequent levels of the food chain. Effects of nutrient supplements may also propagate through trophic levels, but the mechanisms here are less clear. We tested the hypothesis that predators can be affected by the nutrient composition of the prey's food. Wolf spider (Pardosa amentata) hatchlings were raised ad libitum on fruit flies (Drosophila melanogaster) that were cultured in poor basic medium with additions of different nutrients. These additions strongly affected the performance of the spiders. Growth rates increased when additions consisted of 19 different amino acids or fatty acids+cholesterol or commercial dogfood. Survival increased in spiders reared on fruit flies from cultures containing 19 amino acids or methionine or dogfood. The addition of dogfood increased spider growth and survival more significantly than the addition of any single nutrient group alone. Adult female flies from the dogfood culture were significantly heavier than females from the basic culture. The nutrients added to the fruit fly media were thus able to create biological effects at both the second and the third trophic levels. To test whether nutrients passed to the predators through the gut content of the prey, we included a treatment where the spiders were fed flies that had been starved for 48 h in order to empty their guts. Gut emptying of the flies did not reduce the positive effects of the enriched fruit fly media, i.e. the nutritional benefits were not due to nutrients that passed directly through the guts of the flies. Since the nutrients added to the fruit fly media were separated from the spiders that benefited from them by two trophic transformations, this phenomenon was a true tritrophic interaction.