Food webs: SO MUCH MORE THAN JUST WHO EATS WHOM! Food webs are exceptionally complex, demonstrating the interconnected effects of predator populations, herbivore populations, and primary producer populations. Changes in organism numbers higher up on the food chain result in population differences at each of the levels below it; this is called a trophic cascade. Keystone species are animals for which changes in their population causes particularly large changes throughout the trophic cascade, suggesting that the ecosystem relies on them for stability.Here is a video on another trophic cascade dear to our west coast hearts, featuring a very charismatic keystone species!
Through trophic cascades, even though predators (for example sea stars) don’t directly feed on primary producers (for example sea lettuce), they do effect primary producer abundances. How? By effecting the herbivores that eat the primary producers! Meet this week’s favourite herbivore, the common periwinkle snail!
So how do predators like sea stars change the feeding patterns of herbivores like the common periwinkle, and subsequently the abundance of primary producers like sea lettuce? First, and most obviously, sea stars might eat the periwinkles! If there are lots of sea stars eating periwinkles, they might decrease the periwinkle population. With less grazing periwinkles around, you might see an increase in sea lettuce growth! This route of interaction between predator and primary producer via herbivore populations is called density mediated indirect interactions – because predators are changing the number (or density) of herbivores in an area. Another less obvious way sea stars might effect the amount of sea lettuce is that their presence might change the behaviour of the periwinkle snails. This change in behaviour could be eating more or less of the sea lettuce, either increasing or decreasing its growth in an area. Effects of predators on primary producer abundance through changes in herbivore behaviour are called trait mediated indirect interactions.
In the wild, where and when there are sea stars, there is typically more algae growth. We wanted to find out whether this was because density mediated or trait mediated indirect effects. We set up tanks with 1) just sea lettuce, 2) with sea lettuce and periwinkle snails, and 3) with sea lettuce, periwinkle snails, and a sea star. We weighed the sea lettuce, snails, and sea stars separately at the beginning of the week, and then at the end of the week. Sure enough, there was much less sea lettuce left after a week when snails were in the tank alone, than when snails were in the tank with sea stars! Before the study, we thought for sure that this would be because the sea stars ate the snails, causing more sea lettuce surviving after the week through density mediated interactions. However, the weight of the snails in each tank was the same at the start of the week as at the end of the week – NONE OF THEM WERE EATEN! Therefore, sea stars were just changing the behaviour of the periwinkles, causing them to eat less. Even though the sea stars weren’t preying on the snails, the snails appeared to be too scared to eat their salad.