Most people are well aware of the direct interactions such as predation, herbivory, and competition that occur within different organisms that form food chains. As those food chains mingle amongst each other, they form a larger food web. However, there are different types of indirect interactions that affect the abundance of particular species thereby altering food web structure. Indirect interactions occur when an interaction between two species is mediated by a third species. These effects can be shown via trophic cascades. One of the well-known trophic cascades involve sea otter-> sea urchin-> kelp. Sea otters have an indirect positive effect on kelp by consuming or altering the behaviour of sea urchins thereby reducing the number of urchins feeding on kelp. If the increase in kelp population is due to sea otters eating the sea urchins, it is a density-mediated indirect effect; whereas if it is strictly the presence of the sea otter that changes the urchin’s behaviour and prevents them from feeding on the kelp, it is a trait-mediated indirect effect.
We were curious to see if a similar trophic cascade existed between Littorina littorea (snail) which grazes on Ulva lactuca (algae); and the snail’s two potential predators: Evasterias troschelii (sea star) and Metacarcinus magister (crab).
We set up an experiment involving 4 different environments:
- Ulva + Littorina
- Ulva + Littorina + Evasterias
- Ulva + Littorina + Metacarcinus cue water
We compared the weight of Ulva before and after being exposed in each environment for 7 days. We predicted the snails to graze on the algae but with the presence of the sea star and the scent of crab, they would reduce their amount of feeding or even get consumed in environments with the sea star. As we had predicted, the snails were able to graze more without the presence of predators. They ate significantly less algae with the presence of the sea star, although the crab water was not as significant. Moreover, the sea stars tended to not eat the snails.
Through our results, we were able to see a positive indirect effect of algae and the sea star. Although the sea stars didn’t eat the snails, we still saw a significant decrease in the amount of grazing, meaning the snail’s behaviour was affected by the presence of the sea star. Therefore, we can categorize this effect as trait mediated.
Watch this youtube video for more information about trophic cascades: https://www.youtube.com/watch?v=hRGg5it5FMI