Body sizes of hosts and parasitoids in individual feeding relationships

• In a natural community of 49 species (12 species of aphids and 37 species of their parasitoids), body lengths of 2,151 parasitoid individuals were, to an excellent approximation, related to the body lengths of their individual aphid hosts by a power law with an exponent close to 3/4. Two alternative models predict this exponent. One is based on surface area to volume relationships. The other is based on recent developments in metabolic ecology. Both models require a changing ratio (in both host and parasitoid) of length to diameter with increasing body length. These changing ratios are manifested differently in the two models and result in testably different predictions for the scaling of body form with increasing size. The estimated exponent of 3/4 for the relationship between individual host body size and individual parasitoid body size degrades to an exponent of nearly 1/2, and the scatter in the relationship between aphid and parasitoid body length is substantially increased, if the average length of a parasitoid species is examined as a function of the average length of its aphid host species instead of using measurements of individuals.allometry | aphids | development | metabolism | weight–length relationsExplaining the size of organisms is an enduring challenge to ecologists and evolutionary biologists (1, 2) to cellular and developmental biologists (3). Ecological studies of the relationship between consumer and resource body sizes (4–8) usually assume that the average body size of a species is an adequate approximation to the size of the individuals taking part in a particular trophic interaction. However, individuals of different size within one resource species may be selectively consumed by different consumer species or individuals of different size within a given consumer species. Vice versa, individuals of different size within one consumer species may selectively consume resource species of different average size or individuals of different size within a given resource species. To understand the relationship between consumer and resource body sizes, it is important that the data correctly represent the body sizes of the consumers and resources involved in the trophic interactions. What are the consequences of focusing on body sizes of consumer and resource individuals vs. average sizes of taxonomic species for understanding feeding relations in natural communities? To answer this question, here we report quantitative field data on body sizes in individual events of parasitism.Animal consumers are often considerably larger than their prey (4), whereas parasites and pathogens are generally much smaller than their resources (5). Solitary insect parasitoids that complete their larval development on or in the body of other living insects, and require just a single host to complete development, lie between these extremes: they are often similar in size to their insect hosts. Parasitoid and host body sizes are well suited to shed light on the role of individual differences in consumer-resource body size relations because the variations in both parasitoid and host body sizes are likely to be of comparable magnitude.Parasitoids are important components of all terrestrial ecological communities. Probably 1–2 million species are parasitoids (9), and they are thus a significant fraction of all species on this planet. As potentially important regulators of their host populations, parasitoids are intensively used in biological control (10). Most prior studies of the body sizes of hosts and parasitoids consider only a single species of host. The few studies (11–14) that consider host–parasitoid size relationships of multiple species have only one data point per species.We studied quantitatively the relationship between final individual aphid host and parasitoid body length in a natural aphid-parasitoid community with multiple species of hosts and parasitoids. The objectives of the study were to (i) describe the relationship between final aphid host and parasitoid body size, (ii) analyze the consequences of focusing on body sizes of consumer and resource individuals vs. average sizes of taxonomic species for the apparent relationship between final aphid host and parasitoid body size, and (iii) offer two alternative explanations for the relationship between final aphid host and parasitoid body size. We hope that future studies will discriminate between these alternative explanations.

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