Who eats who is of primary importance in nature. The large-scale impacts of these interactions can be difficult to initially spot, but are in part responsible for the diversity of ecosystems we see around us. Predators consume plant-eating animals, preventing them from eating too many plants. Larger predators eat the smaller predators, preventing them from eating all the plant-eaters. These relationships can be depicted as food webs, where animals and plants are connected by feeding.
A special type of feeding effect is called a trophic cascade: it is when a species higher up in a food web effects the abundance of species lower down by eating their consumers. A well-known example is the sea otters of the Pacific coast. Otters eat sea urchins, keeping urchin numbers low, but when the otters were hunted for their firs their numbers plummeted; in the absence of predators the urchin population grew and ate much of the kelp forests in the region. In a healthy system the otters produce a trophic cascade by eating urchins, which allows kelp abundance to rise.
We wanted to see if we could observe a mini trophic cascade involving snails, seaweed, crabs, and sea stars. Snails graze on seaweed, and both crabs and sea stars feed on snails, so we wanted to see if adding the predators changed the amount of seaweed the snails ate. We used live sea stars to see if the presence of a predator influenced snail feeding, and water with crab scent to see if just being able to sense a predator would lower grazing.
We found that in the presence of sea stars the snails ate significantly less of the seaweed, while the crab water appeared to have little effect on snail grazing. Unsurprisingly, without any predator influence the snails ate the most seaweed.
There are two main types of interference we could see in our experiment. One is density: the predators eat the grazers, thereby reducing the amount of plant material eaten. The other is behavioral: the predators may not eat many of the grazers, but their presence still reduces grazing because the grazers spend time avoiding danger. Our experiment seems to show that the sea stars have a behavioral effect on the snails, because the sea stars did not eat many snails over the course of the test but still lead to lower amounts of grazing.
Our mini trophic cascade belies the complexity of natural food webs. Large food webs can include hundreds of species and thousands of interaction. Food webs are important to study because they can help researchers better understand the forces that maintain healthy ecosystems, and highlight important species to conserve.
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