restricted access III.5 Indirect Effects in Communities and Ecosystems: The Role of Trophic and Nontrophic Interactions
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III.5 Indirect Effects in Communities and Ecosystems: The Role of Trophic and Nontrophic Interactions Oswald J. Schmitz OUTLINE 1. Introduction 2. Mechanisms causing nontrophic effects 3. The nature of indirect effects in communities 4. The nature of indirect effects in ecosystems 5. Direct and indirect effects in context Species in ecological communities interact directly with another species through consumer–resource, competitive, or mutualistic interactions. Whenever three or more species are engaged in such interactions, we see the emergence of indirect effects in which one species affects another through a shared, intermediary species. Indirect effects can reinforce or counter direct effects and lead to interesting emergent properties. This chapter explores some of the myriad ways that indirect effects emerge in communities and ecosystems. Through the use of selected examples, it shows why consideration of indirect effects is critical to a complete understanding of species interactions in ecological systems. GLOSSARY direct effect. The immediate impact of one species on another’s chance of survival and reproduction through a physical interaction such as predation or interference food chain. A descriptor of an ecological system in terms of the feeding linkages and energy and materials flows among major groups of species (plants, herbivores, decomposers, carnivores) indirect effect. The impact of one species on another’s chance of survival and reproduction mediated through direct interactions with a mutual thirdparty species nontrophic interaction. A direct interaction that changes the behavior, morphology, or chemical composition of a species in response to the threat of being consumed trophic interaction. A direct interaction involving the consumption of a resource species by a consumer species 1. INTRODUCTION Imagine a herd of wildebeest grazing on a Serengeti plain. Imagine now that a prowling lion—a threat to their life—comes into their vicinity. This causes them all to stop feeding and look up in vigilance to see what the approaching predator will do. The wildebeest are nervous and tense, ready to flee at any sign of attack. Yet they are reluctant to flee because that would mean giving up feeding in a highly nutritious patch of forage, one of a few such high-quality patches currently available within a vast landscape. The resources in the patch are especially favored because they will enable the wildebeest to maximize their resource intake for growth, survival, and reproduction. The wildebeest face a critical choice: do they flee from the predator and give up the valuable food resource or do they stay and risk being captured? This choice is faced by individuals of every species of animal during the course of their daily existence. Nevertheless, the fear factor motivating this choice surely must be short-lived. After all, things will go back to normal once the predator has left or it has subdued the one victim out of the many comprising the herd, right? But the reality is, ‘‘No, not exactly.’’ The critical question here is: What is considered ‘‘normal’’? Often the presumption is that once the predator has left, the threat disappears, and animals can resume feeding on their resources with little worry. But ecological science has revealed that this tends not to be the normal case. Instead, many individuals live in a chronic state of vigilance brought about by the fear of being captured. Ambush predators can lie in wait for long periods of time. Individuals that let down their guard and move within the vicinity of any predator lying in wait have a high likelihood of being the predator’s next victim. Prowling predators can hunt in groups, so foraging individuals that do not regularly scan their surrounding environment may find themselves trapped. Normal, in many cases, means living in situations that pose continuous risks of being a predator ’s next victim. The above vignette of wildebeest daily life on the plain encapsulates several key ecological concepts. First, because wildebeest are both consumers of their plant resources and at the same time resources for other consumers—their predators—they are inherently part of an ecological food chain. Their role in that food chain is identified by the kind of consumptive interaction, or technically trophic interaction, in which they are engaged. Because they eat plants, they belong to the herbivore trophic group. Their predators , because they eat herbivore prey, belong to the carnivore trophic group. By extension, species that consume mineralized nutrients and CO2 in order to photosynthesize carbohydrates belong to the plant trophic group. To the victim (i.e., plants fed on by herbivore; herbivores fed on by carnivores), these...