Indirect effects of largemouth bass on macrophytes through non-consumptive effects on crayfish.

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Non-consumptive effects occur when the presence of a predator alters the behavior of prey species without directly killing and eating the prey (Brown, Laundré, & Gurung, 1999). Under the threat of predation, prey species may alter their foraging behavior, and in some cases may relocate to avoid the predator altogether (Laundré, Hernández, & Ripple, 2010). Through changes in prey behavior non-consumptive effects can alter the function of trophic cascades and have larger impacts on food webs than direct consumption of prey alone (Peacor et al., 2013). As a large scale example of non-consumptive effects, the reintroduction of wolves into Yellowstone National Park has caused the elk population to switch from feeding on small aspen saplings in the river bottoms and lowlands to a diet of alpine grasses and leaves farther up in the mountain valleys. This reduction of elk herbivory in the lowlands has allowed thick aspen stands to spring up along riverbanks and in other areas that were previously grazed down by the elk. These aspen stands provide new habitats along the rivers for many different species and have altered the way other animals move throughout the park (Laundré, Hernández, & Ripple, 2010). A smaller scale example of non-consumptive effects would be the relationship that exists between prairie grasses, grasshoppers, and spiders. In prairies with normal populations of spiders, the diversity of grasses is high and the grasshoppers feed primarily on low growing forbs, because the spiders build webs on the grasses to catch the grasshoppers. Where spiders are absent from the prairie, grasshoppers feed primarily on the grasses and their seeds, which reduces the diversity of the grasses and promotes the growth of more forbs (Schmitz, 1998). This alteration in the plant community of the prairie has implications for the entire food web. The wolf and spider non-consumptive effect examples show the connectivity and interdependence of predator, prey, and plant abundances in communities, and how changes at any trophic level can lead to cascading effects in the others. Though non-consumptive effects have been observed in marine systems, these effects are understudied in freshwater environments. As a model, the presence of predatory fish may cause a change in crayfish foraging behavior that could impact macrophyte communities in streams. Though crayfish are known omnivores, plant material is the primary dietary component for most crayfish and adults can survive on herbivory alone (Hogger, 1988). Crayfish can severely impact the density and diversity of macrophyte beds when their foraging is unchecked (Lodge et al., 1994), especially in cases of crayfish invasion of previously crayfish free water bodies (Feminella & Resh, 1989). In lotic systems, the most abundant predators of crayfish are bony fishes. Crayfish are adapted to detect potential predators through a combination of visual, mechanical, and chemical stimuli, and their chemoreceptive abilities are especially well adapted. If crayfish were to detect the odor of fish that could potentially prey upon them, the non-consumptive effect of this detection would be a change in the crayfish’s behavior. Most likely, the crayfish would seek shelter, move less or more slowly, and would reduce their feeding habits or limit their feeding to nutritionally efficient resources, to minimize time exposed while feeding. If alterations in crayfish foraging behavior do occur as a result of NCEs from fish, then there should be observable differences in the macrophyte community after such changes have taken place. The implications of such a relationship are important to our understanding of stream community dynamics for many reasons, not the least of which is the field of invasion biology. In the United States and elsewhere around the world invasive species are becoming a problem of increasing concern. In lotic systems we already have species of invasive fish (Cyprinus carpoides, Channidae), crayfish (Orconectes rusticus, Procambarus clarkii), and macrophytes (Potamogeton crispus, Myriophyllum spicatum) that are established in the environment. The presence of any one of these invaders can upset the balance of a stream community. An invasive predatory fish could damage native crayfish populations. If an invasive crayfish were introduced it could out-compete native species or rapidly destroy macrophyte beds. An invasive macrophyte could potentially be controlled or eliminated through the action of crayfish herbivory. Therefore, understanding how the relationships among fish, crayfish, and macrophytes interact is important to our future efforts to control or eradicate harmful invasive species
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