The cover image for our April issue shows a rodent herbivore alongside black mangrove seedlings, in a Florida salt marsh. Author and photographer, Rachel S. Smith, describes how she and her team tracked down this mystery mangrove muncher!
This image relates to recently published research article: “Dead litter of resident species first facilitates and then inhibits sequential life stages of range‐expanding species” by Smith, Blaze & Byers.

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The case of the mysterious mangrove muncher: a marsh detective story

“These bites seem too big to be from a crab, don’t they?”

Examining the partially eaten mangrove propagule, I channeled my inner Sherlock Holmes, determined to use my ecological training to uncover the identity of the mysterious mangrove muncher!

In the prior months, my co-authors and I had been conducting field experiments in the marsh-mangrove ecotone on the Atlantic coast of Florida. With warming temperatures, mangroves are expanding into salt marshes, and we were curious how resident salt marsh species influence mangrove establishment in uninvaded marshes.

In particular, we noticed that water-dispersed mangrove propagules are often stranded with piles of dead salt marsh litter (wrack) in the high marsh, where mangroves then root and grow. This observation inspired us to establish a series of field experiments where we measured mangrove recruitment, retention, rooting, and growth in high marsh microhabitats with and without wrack (detailed in Smith et al. 2021).  

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The search for the mangrove muncher began when we noticed bites on the mangroves in the experimental treatments. The culprit appeared to bite off the tops of mangrove seedlings and then feast on the seedlings’ nutrient-rich cotyledons, especially in the wrack treatments. Along with the large size of the bites, these details didn’t fit the culinary profile of our usual list of crab suspects.

Another clue surfaced when we found rodent nests snuggled into the experimental wrack treatments. But, the case broke wide open when we spotted rodents fleeing the scene after we lifted the wrack treatments to count mangroves.

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Given this new evidence, we added rodents to our suspect list of hungry herbivores and began a series of feeding assays that included both crabs and rodents. We trapped rodents from the field, including the juvenile Hispid cotton rat, Sigmodon hispidus, pictured in the cover photo. In the lab, we offered all suspects mangrove propagules with and without wrack and measured their consumption.

Although we documented some invertebrate bites after 8 days, rodents rapidly consumed mangrove propagules within 6 hours, leaving some of the same bite patterns on the lab propagules that we had previously observed in the field.

As a marsh detective, I’d love to say that the case of the mysterious mangrove muncher is closed, but as an ecologist, I know that every study leads to more questions and the inquiry always goes on. In this case, we offer laboratory evidence that terrestrial rodents can discourage mangrove establishment in the high marsh, especially when wrack is present to serve as refuge. But the enigma of the mangrove munchers continues as we seek to understand how rodents use high marsh habitats in the field and influence mangrove expansion in the process.

Rachel S. Smith University of Georgia, USA & University of Virginia, USA

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You can read the full article by Smith, Blaze & Byers here: Dead litter of resident species first facilitates and then inhibits sequential life stages of range‐expanding species