An attraction of food webs is that they cut across the narrow habitat and taxonomic divisions that are still a powerful, restricting force in the development of ecological theory (Lawton 1995). Ideally, food webs could fulfil this goal by representing a random sample of all species found in natural communities. Far from this ideal, ecologists typically ignore parasites and diseases and restrict their study to either parasitoid–host communities or predator–prey communities (Lawton 1989; Marcogliese & Cone 1997). Consequently, habitat and taxonomic divisions still restrict ecological theory. For example, many terrestrial projects have looked at the parasitoid species richness of different host insects and plants (e.g. Askew 1961; Askew & Shaw 1986; Hawkins & Lawton 1987; Memmott, Godfray & Gould 1994; Muller et al. 1999) while ignoring predator species richness or pathogen species richness within the same communities. In contrast, numerous freshwater food webs almost exclusively describe predators while ignoring parasitic and pathogenic trophic interactions (e.g. Winemiller 1990; Martinez 1991; 1993a; Havens 1993).
These discrepancies are largely due to the relative ease of observing trophic interactions in these different habitats. Quantifying parasitoid species richness for a terrestrial herbivore is relatively simple. The herbivore is simply collected from the field and parasitoids reared out in the laboratory. However, it can be extraordinarily difficult to determine predator–prey interactions. The hit-and-run style of insect predation makes it easy to miss a predator–prey interaction. It is also rare for clear evidence to remain of who ate whom, after predation occurs, which further compounds the problem. In aquatic habitats, the trophic resources of predators and herbivores such as fish and zooplankton that engulf their prey whole are relatively easy to observe in gut contents. Beyond ease of observation, divergent subdisciplinary conventions (e.g. terrestrial vs. aquatic ecology) also maintain the appearance of empirical discrepancies between habitat types (May 1983). One of the few trophic hypotheses that bridge predator and parasitoid-biased studies is the assertion that the host ranges of parasitoids are more specialized than those of predators because parasitoid life histories are more intimately tied to their hosts (Price 1980). Here, we present one of the few studies that compares the generality of parasitoid–host and predator–prey interactions in the field.
Hawkins & Lawton (1987) and Hawkins (1988) have demonstrated that herbivore feeding strategy affects vulnerability as measured by the species richness of parasitoids that consume the herbivore. They proposed that the pattern could be explained by differences in the ease with which parasitoids locate hosts in different feeding niches and the degree to which the hosts are protected from predation. Hawkins, Cornell & Hochberg (1997) investigated the mortality factors of 78 species of insect and their data suggest that herbivore feeding biology affected pathogens, predators and parasitoids differently. Although the characteristics that influence the vulnerability of herbivores to parasitoids have received considerable attention, interactions among parasitoids and predators have been much more sporadically studied and it is not clear how mortality due to different types of consumers covary (Hawkins et al. 1997). How and where a herbivore feeds undoubtedly influences its predators (Hawkins & Lawton 1987), but it is not clear whether herbivores escape one type of enemy only to be consumed by another type of enemy.
Herbivore feeding style is one of many factors that affect trophic interactions in insect communities. Body size is also important. For example, large predators eat prey with a wider range of body sizes than do smaller predators (Cohen et al. 1993a). Body size is one of the most obvious features of any organism and one of the most easily measured. One approach to studying the relationship between size and trophic relationships is to measure the ratios of weight or body-length of consumers and their trophic resources in a particular community. This approach has not previously been used on a large insect food web.
In their work on the cascade model, Cohen & Newman (1985) demonstrated that statistically assembled food web matrices, constrained to be upper triangular, generate several of the patterns found in food webs. The cascade model assumes that species can be arranged into a cascade or hierarchy such that a given species can feed only on species below it and itself can be fed on only by species above it in the hierarchy (Pimm, Lawton & Cohen 1991). The original work on the model did not provide an explanation for the proposed trophic cascade, but subsequently body size has been suggested as a likely candidate (Warren & Lawton 1987 and independently by Cohen 1989b). Thus, predators are typically larger than their prey (Vezina 1985) and parasitoids are typically smaller than their prey (Elton 1927). Published food webs largely conform to the assumptions of the cascade model, based on a hierarchy of body sizes. For example, Cohen et al. (1993b) found that about 90% of feeding links involve a larger predator feeding on a smaller prey. However, this is not because there are no exceptions to these generalizations, but because nobody has really studied them (Lawton 1989). Thus, parasitoid sub-webs support the cascade model, as do predator sub-webs, but the body size interpretation of the cascade model has not been tested against a web containing predators, parasitoids and pathogens.
In this paper, we describe a community centred on Scotch broom, Cytisus scoparius, at a field site in England. The data are presented as a food web describing the trophic relations in a community of 154 species: one plant, 19 herbivores, five omnivores, 66 parasitoids, 60 predators, and three pathogens. The data come from published work on these organisms at a single field site. We use the food web to answer five questions: (1) Is the trophic generality of predators greater than that of parasitoids? (2) Does herbivore feeding style affect herbivore vulnerability to predator, parasitoid, hyperparasitoid and pathogen species to the same degree? (3) Does mortality due to predators and parasitoids covary? (4) Does analysing parts of food webs (i.e. webs missing entire groups of consumers) bias estimates of food web statistics? (5) Does a food web containing predators, parasitoids and pathogens conform to the assumptions of the cascade model?