Food Web Tritrophic Interactions

What are the consequences of food web dynamics and trophic cascades for crop production and food security? For the most part, empirical research indicates that insect herbivore populations are regulated by both natural enemies (top-down effects) and food resources (bottom-up effects). Moreover, natural enemies of herbivores are not top predators in most agro-ecosystems, and they typically are attacked by another tier of pathogens, parasites and predators. While the above examples emphasize interactions between populations of two species, most naturally occurring communities, including agro-ecosystems, are much more complex. One observes more species participants, a greater variety of interactions among coexisting species, and the need to consider multiple trophic levels (Rosenheim, 1998). In a sense, combinations of species interactions often interact in tritrophic comparisons. Moreover, relationships among species become more difficult to assign as when species both compete and interact as predators and prey (intraguild predation). Effective biological control may be disrupted by natural enemies of the effective biological control agent itself (Rosenheim, 1998), or food plant quality may be altered by plant pathogens such that insect herbivores do better or worse depending on conditions. In food webs, top-down and bottom-up effects from species interactions "collide," introducing a variety of indirect effects that cannot be predicted let alone understood, unless all participants are both recognized and considered in analyses. Finally, there is an important spatial element that dictates the strength of species interactions and the persistence of species participants in a community, a point not covered in detail here but certainly important for understanding effects of insect pests on crop, fiber, and forage production. This complexity provides a broad array of possible outcomes. To the degree that webs of interactions are affected by climatic conditions and influencing the outcome of sets of often nonlinear interactions, global climate change may have remarkable effects on agricultural pests. Unfortunately, the outcomes may not be predictable. Above examples indicate how species interactions can be erased or even reversed as physical conditions of the environment change. What does this do to community-level interactions in which the end results are highly reticulated? A couple of examples indicate possible outcomes to illustrate the complexity of the problem.

A classic example that highlights the importance and complexity of predicting interactions between trophic levels is the trophic cascade, a process in which indirect interactions among species are often more important to the final outcome than direct interactions (Schmitz, 1997, 1998, 2003). In a trophic cascade, natural enemies affect plant production indirectly by controlling abundance of herbivore populations. Any changes in the interaction between herbivores and natural enemies, such as the addition of hyperparasites or predators that control natural enemies of herbivores, alter primary production. Trophic cascades are ubiquitous in both natural and managed ecosystems. In terrestrial ecosystems, food webs can be highly reticulated, with many species occurring and interacting within and between trophic levels. Consequently, it is essential that the ecological balance sheet be carefully drawn to make accurate predictions, although some recent work indicates that complex food webs often collapse into more tractable food chains from the standpoint of assessing trophic cascades.

Omnivory provides another example of complicating processes within arthropod communities in agro-ecosystems. Omnivores feed on both plants (herbivory) and other consumers (predation). Big-eyed bugs, Geocoris punctipes (Het-eroptera: Geocoridae) prey on eggs of the corn earworm, Helicoverpa zea (Lepidoptera, Noctuidae) and pea aphids, Acyrthrosiphum pisum (Homoptera, Aphidae), in addition to plants. The big-eyed bugs need a mixture of both plants and insect prey for normal development. In controlled experiments in the presence of high-quality plants (lima bean pods), big-eyed bugs fed more on the bean pods, which reduced the number of prey taken by them, and pea aphids reached much larger size (Eubanks and Denno, 2000b), presumably having more potential to damage crops. However, the critical prediction that herbivore populations would be more abundant under field conditions when lima bean plants had pods was not upheld; pea aphid populations were lower. The presence of high-quality plant parts attracted more big-eyed bugs to the field, thus increasing predation pressure overall on aphid populations, even though per capita pressure from each predator was less. Increased prey densities did not have the same attractive effect to big-eyed bug populations (Eubanks and Denno, 1999). To further complicate the interaction, big-eyed bugs preferentially selected mobile pea aphids over corn ear-worm eggs, even though the corn earworm eggs were significantly more nutritious and were required to complete development (Eubanks and Denno, 2000a). In the field, however, the presence of lima bean pods was the greatest predictor of big-eyed bug density, and was responsible for maintaining bug populations during low-density periods.

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