471
record, and other reasons of uncertainty that the three beetle lineages did not extend
to earlier than the late Early Jurassic (Toarcian Stage), and thus were not considered
as parasitoids before the MMPR (Fig. 11.19).
11.6.2 The Trophic Cascade and Resource Concentration
Hypotheses of Food Webs
A food web consists of a network of trophic interactions among species that specifically identify the consumers and the consumed within a local community (Loreau
2010; Price et al. 2011). With very few exceptions (e.g., Dunne et al. 2014), wellpreserved biotas in the fossil record have not been rigorously analyzed for their
food-web structure. For well-preserved biotas, modern or fossil, two competing
hypotheses would be important for understanding major changes in food-web trophic structure through geologic time (Matson and Hunter 1992). The first option
that describes a food web is the trophic cascade hypothesis (Carpenter et al. 1985;
but see Polis and Strong 1996) in which carnivores control herbivores in a local
biota, and in so doing regulate plants for consumption by herbivores. The trophic
cascade concept often is described as top–down control by higher trophic levels of
lower trophic levels. The alternative to the trophic cascade is the resource concentration hypothesis (MacArthur and Levins 1964; Schmitz et al. 2000), in which the
trophic structure of plants, herbivores and carnivores is competitively dependent on
access of plants to resources that allow for growth. The resource concentration
hypothesis frequently is mentioned as bottom–up control by lower trophic levels
(autotrophs) of higher trophic levels (Oksanen et al. 1981; Hunter and Price 1992;
Power 1992). There is evidence that changes in plant traits, induced by shifts in
parasitoid attack, can have major and lasting effects on the food web (Bukovinszky
et al. 2008). Both hypotheses have major ecological consequences for the beginnings of parasitoidism during the mid Mesozoic (Fig. 11.19).
The trophic structure of food webs can be affected significantly depending on
whether consumption of arthropod prey is conducted by predators or by parasitoids.
Compared to predators, parasitoids typically are more speciose, possess greater trophic complexity, have higher host specificities, and their arthropod hosts often
remain capable of accommodating additional parasitoid individuals (Lafferty et al.
2006; Price et al. 2011; Dunne et al. 2013). These characteristics indicate that parasitoids are much more efficient, on average, than predators in top–down regulation
of the food webs. By contrast, resource limitation to plants often has a negative
effect on food-web structure by limiting bottom–up energy available to plants, their
herbivores and their consumers (Bukovinszky et al. 2008). A relevant study of this
latter phenomenon is the Brussels sprouts (primary producer) to aphid (herbivore)
to primary parasitoid (primary consumer) to secondary parasitoid (secondary consumer) food web (Bukovinszky et al. 2008). In this food web (Fig. 11.20), sourceplant quality has major, bottom–up, cascading effects across trophic levels that is
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution
record, and other reasons of uncertainty that the three beetle lineages did not extend
to earlier than the late Early Jurassic (Toarcian Stage), and thus were not considered
as parasitoids before the MMPR (Fig. 11.19).
11.6.2 The Trophic Cascade and Resource Concentration
Hypotheses of Food Webs
A food web consists of a network of trophic interactions among species that specifically identify the consumers and the consumed within a local community (Loreau
2010; Price et al. 2011). With very few exceptions (e.g., Dunne et al. 2014), wellpreserved biotas in the fossil record have not been rigorously analyzed for their
food-web structure. For well-preserved biotas, modern or fossil, two competing
hypotheses would be important for understanding major changes in food-web trophic structure through geologic time (Matson and Hunter 1992). The first option
that describes a food web is the trophic cascade hypothesis (Carpenter et al. 1985;
but see Polis and Strong 1996) in which carnivores control herbivores in a local
biota, and in so doing regulate plants for consumption by herbivores. The trophic
cascade concept often is described as top–down control by higher trophic levels of
lower trophic levels. The alternative to the trophic cascade is the resource concentration hypothesis (MacArthur and Levins 1964; Schmitz et al. 2000), in which the
trophic structure of plants, herbivores and carnivores is competitively dependent on
access of plants to resources that allow for growth. The resource concentration
hypothesis frequently is mentioned as bottom–up control by lower trophic levels
(autotrophs) of higher trophic levels (Oksanen et al. 1981; Hunter and Price 1992;
Power 1992). There is evidence that changes in plant traits, induced by shifts in
parasitoid attack, can have major and lasting effects on the food web (Bukovinszky
et al. 2008). Both hypotheses have major ecological consequences for the beginnings of parasitoidism during the mid Mesozoic (Fig. 11.19).
The trophic structure of food webs can be affected significantly depending on
whether consumption of arthropod prey is conducted by predators or by parasitoids.
Compared to predators, parasitoids typically are more speciose, possess greater trophic complexity, have higher host specificities, and their arthropod hosts often
remain capable of accommodating additional parasitoid individuals (Lafferty et al.
2006; Price et al. 2011; Dunne et al. 2013). These characteristics indicate that parasitoids are much more efficient, on average, than predators in top–down regulation
of the food webs. By contrast, resource limitation to plants often has a negative
effect on food-web structure by limiting bottom–up energy available to plants, their
herbivores and their consumers (Bukovinszky et al. 2008). A relevant study of this
latter phenomenon is the Brussels sprouts (primary producer) to aphid (herbivore)
to primary parasitoid (primary consumer) to secondary parasitoid (secondary consumer) food web (Bukovinszky et al. 2008). In this food web (Fig. 11.20), sourceplant quality has major, bottom–up, cascading effects across trophic levels that is
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution