B

Figure 6.7. (A) This graph shows how combining the vegetation growth curve (GC) with a type II or type III (in this case) functional response can result in two alternative stable states. Vdenotes vegetation mass. VL is the lower stable equilibrium, Vt is a transient unstable equilibrium, and Ve is the high stable equilibrium. (B) This graph shows how dual stability can arise through changes in foraging pressure. (After Noy-Meir 1975 and Thornley 1998.)

Stocking density, 104 x nanimals (sheep ha-1)

Figure 6.7. (A) This graph shows how combining the vegetation growth curve (GC) with a type II or type III (in this case) functional response can result in two alternative stable states. Vdenotes vegetation mass. VL is the lower stable equilibrium, Vt is a transient unstable equilibrium, and Ve is the high stable equilibrium. (B) This graph shows how dual stability can arise through changes in foraging pressure. (After Noy-Meir 1975 and Thornley 1998.)

for example, management. This dual stability is temperature dependent. As seen in figure 6.7B, the bifurcation disappears as the temperature increases.

Noy-Meir assumed that grazers defoliated plants in a deterministic and continuous manner. Schwinning and Parsons (1999) relaxed these assumptions and considered the grazing process as discrete bites in a spatially heterogeneous environment, with selection behavior by the animal, and used a more realistic plant growth function. Most importantly, they also considered how the herbivore's behavior alters and responds to the spatial heterogeneity in the pasture. Figure 6.8 shows that the animals' foraging behavior can indeed have

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Figure 6.8. Dual stability domains for different patch selection decisions. Patch encounter occurs randomly, and there is no extra cost associated with patch rejection. The defoliation fraction is the fraction of the standing biomass that a grazer removes before moving on to the next patch or feeding station. (A) Foragers probabilistically reject patches that are > 0.2 kg/m2. (B) Foragers probabilistically reject patches thatare < 0.12 kg/m2. (After Schwinning and Parsons 1999.)

a strong effect on the dynamics of the herbivore-pasture interaction. Dual stability is less likely when animals reject patches of high biomass, but more likely when animals reject patches of low biomass. Schwinning and Parsons (1999) point out that there are several detailed and spatial analyses of foraging (e.g., Farnsworth and Beecham 1999; Grunbaum 1998) that take into account the costs of different foraging strategies, but fail to realistically incorporate resource regeneration (but see Hutchings and Gordon 2001).

Schwinning and Parsons (1999) suggest that progress now depends on the merging of spatially explicit approaches that consider the dynamics of the interaction between herbivores and plants (for more on this subject, see Parsons et al. 2001) with developments in foraging theory. I would agree. In order to adopt a truly dynamic view of herbivore foraging behavior, we must understand the dynamics of plant growth. We need to become experts, not just on animal behavior, but also on plant growth and metabolism (or find someone who is and work very closely with them!). To understand foraging behavior in herbivores, we must understand the entirety of the plant-animal interaction.

6.10 Summary

In this chapter, I present a view of herbivory based on four interacting behaviors: habitat choice, diet choice, intake rate, and foraging time. We can view each ofthese behaviors as resulting from objectives and constraints, both environmental and physiological. Sometimes it is useful to think of one or more of these behaviors as a constraint, but at other times our analysis will be better served if we frame these behaviors as decisions. Most importantly, this complex of interacting behaviors must be seen as inducing change in the plants and responding to these changes in a dynamic fashion.

Although there are clearly many gaps in our understanding ofthe answers to each of the four big questions, I want to reiterate two areas that need further attention. One important frontier is the integration of the four basic behaviors. Under which circumstances does one action take priority over another, and can we develop a sufficient understanding to predict this? We need to develop mathematical models that incorporate all four big questions, and we need to use these models to generate and experimentally test predictions about how the four questions interact in various environmental circumstances. This is the area of decision making with multiple objectives discussed in section 6.8. The second frontier, which I discussed in section 6.9, is the dynamic aspect of the plant-animal interaction. We need models that integrate population dynamics with foraging behavior, and we need experimental tests of these models. A worthy, but as yet unfulfilled, goal is to develop a dynamic understanding of even a simple herbivore-plant system.

My final comment is a plea for openness and integration. I have attempted, perhaps too superficially, to integrate literature from a variety of fields because I think that trading ideas and perspectives leads to unexpected gains. While there are many well-written reviews on aspects of this chapter's material, they are invariably limited to particular taxonomic groups, and in many cases even more limited than that. A true integration of vertebrate and invertebrate foraging behavior, across applied and basic science disciplines, would yield great benefits. Hmmm, that sounds like a topic for another book . . .

6.11 Suggested Readings

E. A. Bernays and R. F. Chapman's Host-Plant Selection by Phytophagous Insects (1994), chapters 4 and 5, provide an excellent introduction to herbivorous insect foraging behavior. This book is a little dated now, but serves as a good starting point for the field. In The Ecology and Management of Grazing Systems, edited byJ. Hodgson and A. W. Illius (1996), chapters 5 (Laca and Demment),

7 (Ungar), and 9 (Murray and Illius) provide a good starting point for larger vertebrates. For a much broader view of herbivory, see Herbivores: Between Plants and Predators, edited by H. Olff, V. K. Brown, and R. H. Drent (1999). Parsons and Chapman (2000) offer an excellent discussion of the dynamic nature of the plant-animal interaction. O'Connor and Spash (1999) is a good starting point for the field of economic valuation, and Gustafsson et al. (2003) and Louviere (1988) provide details on conjoint analysis.

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