The Nature of Nutrition by Stephen J. Simpson

Author:Stephen J. Simpson
Language: eng
Format: epub
ISBN: 9781400842803
Publisher: Princeton University Press

8.2 INTERACTIONS AMONG ORGANISMS AND THE ENVIRONMENT

The most direct nutritional interaction of all is when one organism attempts to eat another. The outcome that is most commonly considered in classical community ecology is that the predator succeeds in killing and eating the victim, and returning its unused components to the environment in the form of feces. But the interaction might be ecologically significant even if the prey animal is only wounded or escapes unharmed. It might, for example, change its subsequent behavior to avoid the risk of further attack, or the interaction might result in the induction of defensive chemicals and immune responses to prevent or counteract future threats (Karban and Baldwin 1997; chapter 5). Being removed from the environment as a living organism and returned as feces—or being changed in behavior, chemical composition, and defensive armory—may resonate throughout the ecosystem in unexpected ways (Simpson et al. 2010).

This is illustrated by an interesting experiment that we introduced in chapter 1: the study by Hawlena and Schmitz (2010) of the North American grasshopper Melanoplus femurrubrum. This herbivore selects a diet of grasses and herbs and is preyed upon by hunting spiders, such as Pisuarina mira. Hawlena and Schmitz housed grasshoppers in the presence of spiders, which were kept in clear plastic cylinders where they were visible but unable to attack the grasshoppers. Compared with controls that were not exposed to spiders, the grasshoppers selected a diet containing 40% more carbohydrate, whereas protein intake was not affected by the presence of spiders. This shift in the intake target toward a higher carbohydrate to protein ratio was due to grasshoppers having an increased metabolic rate when exposed to spiders: the “fear effect.” There was also a change in the chemical composition of the grasshopper’ feces, which were lower in carbon to nitrogen ratio (C:N) when spiders were around, reflecting the higher metabolic need for carbohydrate. Finally, the body composition of spider-exposed grasshoppers differed, being higher in C:N than controls.

Hawlena and Schmitz calculated that the change in the grasshoppers’ intake target when at risk of spider predation would shift the structure of the natural host plant community as a result of selective foraging by fearful grasshoppers. The changes in body and fecal chemistry and plant community structure would, in turn, affect the chemical composition of the pool of detritus in the ecosystem. We already have seen in the previous chapter evidence of how one important decomposer, the slime mold, responds to nutrient balance, and this would have knock-on effects for soil ecology and plant growth.

Hawlena and Schmitz (2010) concluded that their study “leads to the testable hypothesis that trophic dynamics are neither bottom-up- nor top-down-regulated, but instead may be an emergent property of herbivore regulation of nutrient balance that emanates from the middle of the food chain.” We agree with this interesting point, but would argue that the message is broader than this: trophic dynamics might be an emergent property of regulation of nutrient balance at all trophic levels.

The example of the response of

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