Foodies and Food Webs

We all like food. But there are always factors that influence how much we eat on any particular day. Sometimes we eat too little or too much. Worried about all the papers you have to get done before the end of term? You might forget about eating or the alternative, stress eat and eat too much. These factors could be called trait-mediated indirect interactions because they change our behaviour and in turn our behaviour affects how much we eat. On the other hand, density-mediated indirect interactions would involve a more extreme example – a coyote devours the bunny that eats the vegetables in your garden. Here the coyote indirectly affects the abundance of vegetables in your garden by eliminating the bunny. Similar phenomena occur in food webs in the sea. In last week’s lab, we explored a food web including crabs, sea stars, snails and green algae.


Figure 1- An illustration of the food web explored in this experiment. Sea stars and crabs are both predators of snails.

Four different manipulations of the food web were set up. The first, was a tank with only algae, the second included snails and algae, and the third included snails, algae and a sea star. There were snails, algae and water containing chemical cues from a crab in the last treatment. Half of these tanks were kept at low temperature and the other half at high temperature to explore the effects of temperature on species interactions within the food web. After 7 days, the wet weights of the algae, sea star, and snails in the tank were recorded to determine the amount of feeding on each level of the food web. Then behavioural experiments to examine the crawling speed of snails in response to predation threats were conducted. Next a choice experiment was conducted- whether or not snails fed on algae in the presence of the sea star, or in crab chemical cue water without the sea star.

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Green algae, a sea star and a snail in the experimental set-up for a choice experiment. Will the snail feed in the presence of the sea star?

There were higher amounts of algae remaining in the high temperature compared to the low temperature conditions in the sea star treatment. Also, in the high temperature treatments, sea stars gained more weight, suggesting increased consumption of snails. Snail biomass was lower in the sea star treatments and about the same in the control and crab treatments. In the choice experiment, snails fed less in the presence of a predator, but feeding did not depend on the type of predator. In conclusion, these results lend more support to density-mediated indirect interactions between the predators and the algae in high temperatures. I found these results to be surprising! I was expecting a trait-mediated interaction between the sea stars and algae because it seemed the sea stars did not eat many of the snails. Nonetheless, I definitely learned that marine food webs are more complex than they look!

To learn more about density and trait mediated interactions and marine food webs check out the following article and video!


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