NEW VIDEO: Trophic Cascade of the Purple Marsh Crab

Over the past several weeks, I have been working on a new video for CreatureCast. Today, I’m pleased to be able to share it with you! This latest video illustrates the power of trophic cascades. Trophic cascades exist in a number of diverse ecosystems. They occur when the population of one organism changes, and thereby alters the dynamics of an entire food web.  This CreatureCast features one such cascade, which has caused the rampant die-off of cordgrass in Cape Cod salt marshes. It follows the story of the “purple marsh crab”, Sesarma reticulatum, whose numbers have skyrocketed due to the removal of their predators. Along the way, we are reminded that our actions can have amplified and unforeseen consequences on ecosystems and organisms whom we may have never even encountered.

All of the illustrations and artwork in the video are original, and the music is used with permission from Oyster Boy.


CreatureCast – Sesarma from Casey Dunn on Vimeo.

Stephanie Yin, from Mark Bertness’s lab ( at Brown University, tells the story of how marshes in New England have died back as populations of the crab Sesarma have grown.

Visit for more stories about the unexpected world of Biology.

The hand-drawn animations were photographed at the Brown University Science Center ( Music by Oyster Bay (


7 thoughts on “NEW VIDEO: Trophic Cascade of the Purple Marsh Crab

  1. this is nice, and nice-looking blog so far too 🙂 but as i was watching this video i was thinking, how do you(or the speaker) know that this “destruction” is not just part of evolution, so therefore not really destruction at all?

    part of evolution, of course, is that everything must be strong enough to survive “outside” forces…

    • Hi, thanks for stopping by, and leaving your question! The problem here is one of scale. Evolution is a long-term process; it happens over several generations. The human activities, such as overfishing, that trigger this trophic cascade in Cape Cod are occurring at a rapid rate – one that can overwhelm the entire system before nature even has time to respond with its checks and balances.

      It’s similar to global warming. Earth has processes that, over geological time scales, can neutralize inputs of carbon into the atmosphere. However, we are releasing atmospheric carbon into the system at rates that are unsustainable, and that do not allow nature to equilibrate in the way it wants to.

      In these salt marshes, the loss of cordgrass has substantial implications. Salt marshes provide an important refuge for many marine species, sequester carbon from the atmosphere, buffer the terrestrial landscape from storm impacts, and filter nutrients and wastes from entering the ocean, just to name a few services. Moreover, these events can then trigger positive feedbacks that further aggravate the degradation of salt marsh ecosystems and the marine habitats they protect.

      Ultimately, yes, evolution does select for individuals that can better survive environmental challenges. However, it has its limits, and we cannot assume that natural selection can act as a panacea for the impacts our activities have on our environment. We must keep in mind that evolution acts over natural timescales – not the ones that we impose.

      That is not to say, however, that short-scale evolution may not also be occurring here. There is some evidence to suggest that cordgrasses are evolving defenses that make them unpalatable to Sesarma herbivores. It would be interesting to see if that triggers an evolutionary “arms race” between these cordgrasses and crabs. However, as I said earlier, I think it is extremely dangerous to think that evolutionary processes can be a cure-all for our actions. There are several instances, in numerous and diverse systems, of humans causing damage to ecosystems at rates that completely surpass nature’s capacity to re-establish balance. And when it comes down to the balance of ecosystems, I would think it’s always better safe than sorry!

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