Trophic Cascades in a Snail Shell

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. 

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Picture 1: The right shows an urchin barren, while the left shows a healthy kelp forest.

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.

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Table 1: These are the four food webs we set up.

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Picture 2: One of our tanks with seaweed, sea stars, and snails.

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.

To learn more about trophic cascades visit here.

Picture 1 from https://friendsoftheseaotter.files.wordpress.com/2012/04/urchinkelpcomparison.jpg

They see me Trawlin’, they Hatin’

The vast majority of life in the ocean is really, really small. So small, that by scooping up a single cup of seawater you encounter a vast menagerie of planktonic creatures that far eclipses even the most prodigious zoo. Despite their generally modest size and limited mobility, plankton play an important role in the ocean and are some of the most captivating animals around.

 As we cruised down Grappler Inlet near Bamfield we saw bald eagles, kingfishers and ducks, but the real biological boon was flowing all around us. Our goal was to compare the plankton communities from different depths and locations within the inlet (the mouth of Grappler is saltier than the interior). We measured conditions at the sites, and trawled a plankton net behind the boat to collect organisms. Plankton, by definition, cannot swim against the current so they are relatively easy to capture.

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Bio 326 students examining the tiny riches of the sea in Grappler Inlet. Photo by Sharon Kay.

Some plankton need sunlight to produce food and are found in shallow or clear waters, while others hide in the dark depths to avoid predators. Some plankton prefer fresher water while others like it saltier. Some travel up and down in the water; this micro migration plays out daily as some animal zooplankton hide during the day and rise at night to feed on other zooplankton and plant-like phytoplankton . Because we sampled during the day we expected to find more of the larger zooplankton lurking in the deeper water.

Sampling the water yielded a cornucopia of planktonic species. We found predominantly copepods and a few phytoplankton at the surface while the deeper samples had higher numbers of crab larvae, shrimp, mussel larvae, and snail larvae, to name a few. Surprisingly, many large, charismatic ocean animals start their lives as humble plankton! As we predicted, there was a higher number of larger zooplankton in the deeper samples, and the saltier of the two deep samples held the tigers of the plankton world: arrow worms. 

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A small shrimp from the deep trawl. Photo by Brianna Cairns.

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The arrow worm, or chaetognath, is a fierce planktonic predator.

Perhaps humble is a misleading moniker for plankton, because they punch above their weight across the globe. The majority of the oxygen we breath, around 70%, is produced by organisms in the ocean. Plankton are also food for many larger creatures, meaning some of the largest animals on the planet are sustained by some of the tiniest. We may not be able to easily see them without a microscope, but we owe a tremendous debt to plankton for all they do for us and the planet. Plankton are tiny heroes, deserving tremendous appreciation for their contributions to the environment.

The Tara Expedition traveled the world’s oceans to study the incredible diversity of plankton, and is a great place to find more information: http://oceans.taraexpeditions.org/en/m/science/results/eukaryotic-plankton-diversity-in-the-sunlit-ocean/