Learning is a powerful tool for improving efficiency of resource use (see Chapter 3). Learning reduces the effort wasted in unsuccessful trials (see Fig. 3.15). Learning to distinguish appropriate from inappropriate prey (e.g., search image), to respond to cues associated with earlier success, and to improve foraging tech-

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Bioelimination of 51Cr (left) and 85Sr (right) by the cockroach,

Gromphadorhina portentosa with (solid blue circles) and without (open green circles) the associated mite, Gromphadorholaelaps schaeferi. 51Cr has no biological function and its elimination represents egestion; 85Sr is an analog of Ca and its elimination represents both egestion (regression lines similar to those for 51Cr) and excretion of assimilated isotope (rapid initial loss). This insect appears to assimilate and begin excreting nutrients before gut passage of unassimilated nutrients is complete. From Schowalter and Crossley (1982) with permission from the Entomological Society of America.

nique greatly facilitates energy and nutrient acquisition (Cunningham et al. 1998). Honey bees represent the epitome of resource utilization efficiency among insects through their ability to communicate foraging success and location of nectar resources to nestmates (F. Dyer 2002, J. Gould and Towne 1988, Heinrich 1979, von Frisch 1967).

B. Tradeoffs

Allocation efficiency often is optimized by adaptations that generally tailor insect morphology, life histories, or behavior to prevailing environmental conditions or resource availability. For example, synchronization of life histories with periods of suitable climatic conditions and food availability reduces the energy required for thermoregulation or search activity. Bumble bee, Bombus spp., anatomy optimizes heat retention during foraging in cool temperate and arctic habitats (Heinrich 1979). Davison (1987) compared the energetics of two harvester ant species, Chelaner rothsteini and C. whitei, in Australia and found that the smaller

C. rothsteini had lower assimilation efficiency but higher production efficiency (largely in production of offspring) than did the larger C. whitei. Chelaner rothsteini discontinued activity during the winter, perhaps to avoid excessive metabolic heat loss, whereas C. whitei remained active all year.

Selection should favor individuals and species that acquire and allocate resources most efficiently. Males that defend territories when the time or energy spent on this activity interferes with mating and reproduction are less likely to contribute to the genetic composition of the next generation than are males that sacrifice territorial defense for mating opportunities under such conditions (Schowalter and Whitford 1979).

However, as discussed earlier in this chapter, female pierid butterflies oviposited preferentially on more conspicuous hosts, a more energetically efficient search strategy for the adult, but these hosts were less suitable for larval development than were less conspicuous hosts (see Fig. 3.10) (Courtney 1985, 1986). Similarly, females of the noctuid moth, Autographa precationis, preferentially oviposit on soybeans, rather than on dandelions, perhaps because the shape of dandelions is a less effective oviposition stimulus, although larvae show a marked feeding preference for dandelions (Kogan 1975). Matsuda et al. (1993) modeled the effects of multiple predators on antipredator defenses and concluded that increasing defense against one predator comes at the expense of defenses against another. Hence, conflicts of interest among metabolic pathways, strategies, or life stages often reduce overall efficiency of resource use. Resource allocation by insects, as well as other organisms, reflects tradeoffs among alternative strategies, even between life stages.

Heinrich (1979) evaluated the tradeoffs among various allocation strategies seen among bees. Some bee species begin producing queens and drones (offspring) concurrent with colony development (i.e., production of combs and workers), whereas other bee species achieve large colony sizes before producing queens and drones. The first strategy yields immediate, but small, returns because of the competing activities of workers, and the second strategy yields no immediate returns but eventually yields much larger returns. In addition, workers must weigh the cost of foraging from particular flowers against the expected nectar returns, especially at low temperatures when nectar return must be at least sufficient to maintain high thoracic temperatures necessary for continued foraging. Because different flowers provide different amounts of nectar, bees tend to forage at flowers with high yields over a range of temperatures but visit flowers with small nectar rewards only at high temperatures. Similarly, bees must weigh the benefits of foraging at various distances from the colony. Bees will fly several kilometers, given adequate floral rewards, but respond quickly to indications of declining nectar availability (e.g., leave an inflorescence or patch after encountering empty flowers).

Heterogeneous habitats force many herbivores and predators to expend energy searching for scattered resources. Many individuals will be unable to maintain energy or nutrient balance under such conditions. By contrast, abundant suitable resources reduce costs of searching for, or detoxifying, resources and facilitate maintenance of energy and nutrient budgets. Frequent encounters with predators, especially when combined with low availability of food resources, may restrict the time an individual can spend foraging and increase the expenditure of energy to avoid predators, reducing net energy acquisition and potentially leading to inadequate energy balance for survival.

Survival of individuals and species represents the net result of various traits that often conflict (Carrière et al. 1997). Environmental changes, especially rapid changes occurring as a result of anthropogenic activities, will change the balance among these tradeoffs, affecting the net result in various ways. Warmer global temperatures may improve energy balance for some arctic species but increase respiration loss or time spent seeking shade for other species. Ecosystem fragmentation will require greater energy expenditure for sufficient foraging and dispersal, thereby impeding movement of intolerant species over inhospitable landscapes. Some species will benefit from changes that improve overall performance (e.g., survival and reproduction), whereas other species will decline or disappear.

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