Resource Budget

The energy or nutrient budget of an individual can be expressed by the equation

in which I = consumption, P = production, R = respiration, E = egestion, and I -E = P + R = assimilation. Energy is required to fuel metabolism, so only part of the assimilated energy is available for growth and reproduction (Fig. 4.1). The remainder is lost through respiration. Insects and other heterotherms require little energy to maintain thermal homeostasis. Hence, arthropods generally

Growth

Respiration i

Production :

Reproduction

Respiration

Production :

Reproduction

Egestion

Ingestion

Model of energy and nutrient allocation by insects and other animals. Ingested food is only partially assimilable, depending on digestive efficiency. Unassimilated food is egested. Assimilated food used for maintenance is lost as carbon and heat energy; the remainder is used for growth and reproduction.

Egestion

Ingestion

Model of energy and nutrient allocation by insects and other animals. Ingested food is only partially assimilable, depending on digestive efficiency. Unassimilated food is egested. Assimilated food used for maintenance is lost as carbon and heat energy; the remainder is used for growth and reproduction.

respire only 60-90% of assimilated energy, compared to >97% for homeotherms (Fitzgerald 1995, Golley 1968, Phillipson 1981,Schowalter et al. 1977,Wiegert and Petersen 1983). Availability of some nutrients can affect an organism's use of others (e.g., acquisition and allocation pathways may be based on differences in ratios among various nutrients between a resource and the needs of an organism) (Elser et al. 1996, Holopainen et al. 1995, see Chapter 3). Ecological stoichiome-try has become a useful approach to account for mass balances among multiple nutrients as they flow within and among organisms (Elser and Urabe 1999, Sterner and Elser 2002).

Arthropods vary considerably in their requirements for, and assimilation of, energy and various nutrients. Reichle et al. (1969) and Gist and Crossley (1975) reported significant variation in cation accumulation among forest floor arthropods, and Schowalter and Crossley (1983) reported significant variation in cation accumulation among forest canopy arthropods. Caterpillars and sawfly larvae accumulated the highest concentrations of K and Mg, spiders accumulated the highest concentrations of Na among arboreal arthropods (Schowalter and Crossley 1983), and millipedes accumulated the highest concentrations of Ca among litter arthropods (Reichle et al. 1969, Gist and Crossley 1975).

Assimilation efficiency (A/I) also varies among developmental stages. Schowalter et al. (1977) found that assimilation efficiency of the range caterpillar, Hemileuca oliviae, declined significantly from 69% for first instars to 41% for the prepupal stage (Table 4.1). Respiration by pupae was quite low, amounting to only a few percent of larval production. This species does not feed as an adult, so resources acquired by larvae must be sufficient for adult dispersal and reproduction.

ABJE4U Assimilation efficiency, A/I, gross production efficiency, P/I, and net production efficiency, P/A, for larval stages of the saturniid moth, Hemileuca oliviae. Means underscored by the same line are not significantly different (P > 0.05).

Reproduced from Schowalter et al. (1977) with permission from Springer-Verlag.

II. ALLOCATION OF ASSIMILATED RESOURCES

Assimilated resources are allocated to various metabolic pathways. The relative amounts of resources used in these pathways depend on stage of development, quality of food resources, physiological condition, and metabolic demands of physiological processes (such as digestion and thermoregulation), activities (such as foraging and mating), and interactions with other organisms (including competitors, predators, and mutualists). For example, many immature insects are relatively inactive and expend energy primarily for feeding and defense, whereas adults expend additional energy and nutrient resources for dispersal and reproduction. Major demands for energy and nutrient resources include foraging activity, mating and reproduction, and competitive and defensive behavior.

A. Resource Acquisition

Foraging activity is necessary for resource acquisition. Movement in search of food requires energy expenditure. Energy requirements vary among foraging strategies, depending on distances covered and the efficiency of orientation toward resource cues. Hunters expend more energy to find resources than do ambushers. The defensive capabilities of the food resource also require different levels of energy and nutrient investment. As described in Chapter 3, defended prey require production of detoxification enzymes or expenditure of energy during capture. Alternatively, energy must be expended for continued search if the resource cannot be acquired successfully.

Larger animals travel more efficiently than do smaller animals, expending less energy for a given distance traversed. Hence, larger animals often cover larger areas in search of resources. Flight is more efficient than walking, and efficiency increases with flight speed (Heinrich 1979), enabling flying insects to cover large areas with relatively small energy reserves. Dispersal activity is an extension of foraging activity and also constitutes an energy drain. Most insects are shortlived, as well as energy-limited, and maximize fitness by accepting less suitable, but available or apparent, resources in lieu of continued search for superior resources (Courtney 1985,1986, Kogan 1975).

The actual energy costs of foraging have been measured rarely. Fewell et al. (1996) compared the ratios of benefit to cost for a canopy-foraging tropical ant, Paraponera clavata, and an arid-grassland seed-harvesting ant, Pogonomyrmex occidentalis. They found that the ratio ranged from 3.9 for nectar foraging P. clavata and 67 for predaceous P. clavata to > 1000 for granivorous P. occidentalis (Table 4.2). Differences were a result of the quality and amount of the resource, the distance traveled, and the individual cost of transport. In general, the smaller P. occidentalis had a higher ratio of benefit to cost because of the higher energy return of seeds, shorter average foraging distances, and lower energy cost m-1 traveled. The results indicated that P. clavata colonies have similar daily rates of energy intake and expenditure, potentially limiting colony growth, whereas P. occidentalis colonies have a much higher daily intake rate, compared to expenditure, reducing the likelihood of short-term energy limitation.

Insects produce a variety of biochemicals to exploit food resources. Trail pheromones provide an odor trail that guides other members of a colony to food resources and back to the colony (see Fig. 3.14). Insects that feed on chemically defended food resources often produce more or less specific enzymes to detoxify these defenses (see Chapter 3). On the one hand, production of detoxification enzymes (usually complex, energetically expensive molecules) reduces the net energy and nutritional value of food. On the other hand, these enzymes permit exploitation of a resource and derivation of nutritional value otherwise unavailable to the insect. Some insects not only detoxify host defenses but digest the products for use in their own metabolism and growth (e.g., Schöpf et al. 1982).

Many insects gain protected access to food (and habitat) resources through symbiotic interactions (i.e., living on or in food resources; see Chapter 8). Phytophagous species frequently spend most or all of their developmental period on host resources. A variety of myrmecophilous or termitophilous species are tolerated, or even share food with their hosts, as a result of morphological

|_| Components of the benefit-to-cost (B/C) ratio for individual Paraponera

clavata and Pogonomyrmex occidentalis foragers.

Paraponera

Pogonomyrmex

Nectar Forager Prey Forager

Energy cost per m (J m-1) Foraging trip distance (m) Energy expenditure per trip (J) Average reward per trip (J)

356 67

100 1111

0.007

0.09

Reprinted from Fewell et al. (1996) with permission from Springer-Verlag. Please see extended permission list pg 569.

(size, shape and coloration), physiological (chemical communication), or behavioral (imitation of ant behavior, trophallaxis) adaptations (Wickler 1968). Resemblance to ants also may confer protection from other predators (see later in this chapter).

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Detoxification and Weight Loss

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