ReviewLinking herbivore experience, varied diets, and plant biochemical diversity
Introduction
It is widely purported that ecosystem function depends on biological diversity. Nonetheless, the issue is controversial (Kaiser, 2000), in part because little is known about the biochemical links associated with plant diversity. While we have much to learn about biochemical interactions among plants, and about how herbivores learn to mix their diets to optimize intake of biochemicals, we submit that biochemical diversity increases the potential for resiliency, adaptability, and productivity of ecosystems by increasing options for plants, herbivores, and people. Furthermore, people interested in managing ecosystems can use understanding of biochemical interactions among plants and herbivores to enhance and maintain the biodiversity of landscapes.
The kinds and amounts of nutrients and toxins in a plant or its neighbors determine the effectiveness of a toxin and influence the probability of a plant being eaten (Provenza et al., 2002). All plants contain some levels of toxins so complete avoidance is not feasible. Rather, herbivores must regulate intake of toxins (Foley et al., 1999). Intake of toxic plants depends on the kinds and concentrations of nutrients and toxins; some interactions increase intake, others do not (Burritt and Provenza, 2000, Villalba et al., 2003). How herbivores learn to mix their diets from an array of biochemically diverse plants in turn influences plant biochemical diversity. Associational effects involving plant chemistry and herbivore learning influence coexistence of plants and animals, and can either enhance or diminish biodiversity (Provenza et al., 2002, Villalba et al., 2002c, Villalba et al., 2003).
In this paper, we discuss how plant biochemical diversity interacts with an herbivore’s experiences mixing foods with different flavors, nutrients, and toxins, and the implications for managing animals in confinement, on pastures, and on rangelands.
Section snippets
Uniqueness of the individual
With the advent of statistics in the 20th century, great emphasis has been placed on assessing the response of the “average” animal to a treatment. While the discipline of statistics has advanced our ability to conduct experiments, it also has made variation among individuals a foe to counter rather than a friend to embrace (Provenza et al., 1999). Today, we emphasize means and populations, rather than individuals and variation, while nature and evolutionary processes do the opposite.
Roles of nutrients and toxins
Interactions among nutrients and toxins influence food and habitat selection and mediate plant–herbivore interactions, but they do so in ways we are just beginning to understand. Most studies of nutrient–toxin interactions have been done with rodents and humans. Although many of the principles derived from these investigations apply across species, care must be taken when extrapolating the influence of specific compounds on different species and individuals, both of which respond uniquely to
Theoretical considerations regarding experience and diet mixing
Optimal foraging theory assumes that herbivores forage to maximize energy intake per unit effort (Stephens and Krebs, 1986). According to this theory, foods vary with respect to their intrinsic quality, and an herbivore should either eat or ignore a type of food (the zero-one rule). In other words, a ‘high quality’ food is always better than a ‘low quality’ food. Accordingly, herbivores should not exhibit partial preferences for “poorer” quality foods.
Nevertheless, partial preferences have been
Feeding animals in confinement
Dairy cows, beef cattle, and sheep maximize energy intakes when their diets consist of a mixture of concentrates and roughages, especially when the concentrate portion of the ration greatly exceeds needs. The amount of roughage needed to maximize energy intake depends on the individual, as well as the species, age, and level of production (reviewed by Grovum (1988)). Lactating cows consume less energy from an extremely high energy diet containing mainly grain than they do when the mixture of
Conclusions
We contend that plant biochemical diversity increases the resiliency, adaptability, and productivity of systems by increasing options for plants, herbivores, and people. Biochemical diversity enables the neurological, morphological, physiological uniqueness of each individual in a group to be manifest by selecting diets that optimize its intake of nutrients and toxins. Feeding practices that permit animals to choose among foods create opportunities to reduce costs and enhance performance in
Acknowledgements
This effort was supported by the Utah Agricultural Experiment Station (Project No. 4-20526), and the United States Department of Agriculture Cooperative State Research, Education and Extension Service (Agreement No. 2001-52103-11215).
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