473
indices such as connectance (proportion of possible links among all species that are
realized) and linkage diversity (feeding links among the species of aphid hosts and
the species of their parasitoids) (Bukovinszky et al. 2008). One notable effect was
aphid body size on the nonadjacent trophic level of secondary parasitoids. This
effect began with larger aphids forming larger mummies that resulted in a greater
number and size of mummies in the parasitoid guild, and elevated connectance and
linkage diversity, as indicated by arrow widths (Fig. 11.20). This study emphasizes
that an increase in plant quality causes bottom-to-up cascades across trophic levels
that increase the size and number of herbivores and an abundance of primary and
secondary parasitoids. An opposite condition is where a resource limitation, such as
scarce availability of plant biomass promotes top-to-bottom regulation of trophic
levels within a food web (Hunter and Matson 1992).
11.6.3 The Importance of Parasites and Parasitoids
in Food Webs
Rather than examine the trophic structure of food webs as they exist in nature, discussed above, another approach is examination of the effect that the addition of
parasites have on food web structure. Whereas insect parasitoids, probably are second only to herbivores as the most common lifestyle frequently included as primary
data in food webs, parasites generally are not included (Lafferty et al. 2006). One
reason for this is that parasites do not cause the death of their hosts, and their effects
would be based on interaction strengths, or measures of the intensity or degree of
interactions between two ecologically connected species, which are difficult to
assess (Ings et al. 2009). However, within the last decade, parasites increasingly
have been included explicitly in food webs (Dunne et al. 2013; Lafferty et al. 2008).
Notably, incorporation of parasites in food webs that include predator–parasite and
parasite–parasite links approximately doubles the connectance in food webs
(Fig. 11.21) and changes other food-web indices. These and other, increased foodweb indices include the number of links, nestedness (asymmetry of interactions),
chain length (arithmetic average of the lengths of all chains in a food web) and linkage density (Lafferty et al. 2006). These data show that the increase of parasites in
food webs may have had a supportive and parallel role in propelling the MMPR that
largely was attributable to the evolutionary and ecological diversification of several,
related parasitoid clades (Fig. 11.19). Because of the positive effect that parasites
(and parasitoids) both have for increased connectance and nestedness in ecosystem
stability (Lafferty et al. 2006, 2008), it is quite likely that the MMPR buffered local
food-web structure by inserting top–down control of trophic cascades that balanced
bottom–up control through resource limitation (Labandeira 2015). One relevant
effect of the MMPR shown by the Messel food web study (Fig. 11.22) is that for the
Messel lake part of the web, the highest trophic level consumer was a large crocodile. By contrast, the highest trophic level for the Messel forest part of the web was
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution
indices such as connectance (proportion of possible links among all species that are
realized) and linkage diversity (feeding links among the species of aphid hosts and
the species of their parasitoids) (Bukovinszky et al. 2008). One notable effect was
aphid body size on the nonadjacent trophic level of secondary parasitoids. This
effect began with larger aphids forming larger mummies that resulted in a greater
number and size of mummies in the parasitoid guild, and elevated connectance and
linkage diversity, as indicated by arrow widths (Fig. 11.20). This study emphasizes
that an increase in plant quality causes bottom-to-up cascades across trophic levels
that increase the size and number of herbivores and an abundance of primary and
secondary parasitoids. An opposite condition is where a resource limitation, such as
scarce availability of plant biomass promotes top-to-bottom regulation of trophic
levels within a food web (Hunter and Matson 1992).
11.6.3 The Importance of Parasites and Parasitoids
in Food Webs
Rather than examine the trophic structure of food webs as they exist in nature, discussed above, another approach is examination of the effect that the addition of
parasites have on food web structure. Whereas insect parasitoids, probably are second only to herbivores as the most common lifestyle frequently included as primary
data in food webs, parasites generally are not included (Lafferty et al. 2006). One
reason for this is that parasites do not cause the death of their hosts, and their effects
would be based on interaction strengths, or measures of the intensity or degree of
interactions between two ecologically connected species, which are difficult to
assess (Ings et al. 2009). However, within the last decade, parasites increasingly
have been included explicitly in food webs (Dunne et al. 2013; Lafferty et al. 2008).
Notably, incorporation of parasites in food webs that include predator–parasite and
parasite–parasite links approximately doubles the connectance in food webs
(Fig. 11.21) and changes other food-web indices. These and other, increased foodweb indices include the number of links, nestedness (asymmetry of interactions),
chain length (arithmetic average of the lengths of all chains in a food web) and linkage density (Lafferty et al. 2006). These data show that the increase of parasites in
food webs may have had a supportive and parallel role in propelling the MMPR that
largely was attributable to the evolutionary and ecological diversification of several,
related parasitoid clades (Fig. 11.19). Because of the positive effect that parasites
(and parasitoids) both have for increased connectance and nestedness in ecosystem
stability (Lafferty et al. 2006, 2008), it is quite likely that the MMPR buffered local
food-web structure by inserting top–down control of trophic cascades that balanced
bottom–up control through resource limitation (Labandeira 2015). One relevant
effect of the MMPR shown by the Messel food web study (Fig. 11.22) is that for the
Messel lake part of the web, the highest trophic level consumer was a large crocodile. By contrast, the highest trophic level for the Messel forest part of the web was
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution