Life and science at the Smithsonian Environmental Research Center

Mangrove Tracking: Letters from the Field

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Mangroves are on the move. Read first-hand testimonies from scientists tracking the trees in Florida’s tropical forests, where climate change is forcing two ecosystems to collide. A summer 2012 project of the Smithsonian Environmental Research Center and the University of Maryland.

Researcher Mike Lehmann makes his way through dwarf-form mangroves in the Gulf of California. (C. Johnston)

As you approach stands of mangroves in Florida, you’re likely to notice a few things. They form expansive forests along protected seashore (usually in lagoons and estuaries) that often grow thick and tall overhead, providing welcome shade where the three species (black, white and red) intermingle. In the cool of their shade, they are clearly teeming with life; the constant pop of snapping shrimp ricochets around their oyster- and barnacle-encrusted roots while crabs and insects scurry along their branches.

These mangroves are different. There are no scurrying crabs or snapping shrimp or prominent rocks of oysters. Most of the insects have gone inside; their only traces are burrows and cocoons made in the safety of stems and rolled leaves. The blinding sun and gusty wind make it starkly obvious that the shady, protective canopy is only waist-high. The cactus on the rocky slope in the background gives it away: These mangroves are in Baja California, Mexico.

Spotted-winged grasshopper, one of two insect herbivores the team tested to see if they would eat mangrove leaves. (Alex Forde/UMD)

After spending five weeks working indoors as a research intern at the University of Maryland in College Park, walking out into the salt marsh at the Guana Tolemato Matanzas (GTM) Reserve in Florida was a welcome change of scenery. The sky was a crystal clear blue, egrets and herons soared overhead, and crabs scuttled haphazardly on the sand as we waded into the cordgrass, ready for a hard week of field work.

My mentor, Alex Forde, and I were there conducting experiments for his dissertation and for my internship project. This whole summer we had been studying plant resistance to herbivores, so we were excited to document interactions between leaf-eating insects and black mangrove trees (Avicennia germinans) in Northern Florida salt marshes.

Over the past several decades, climate change has allowed black mangroves to move north along the Florida coastline. As a result, they are invading salt marshes and coming into contact with novel herbivores that are not common in mangrove forests further south. Depending on the behavior and food preferences of marsh herbivores, these species may affect how fast mangroves spread into salt marshes and where the trees are able to survive within marsh landscapes. Therefore, we wanted to test (1) whether salt marsh herbivores will eat mangrove leaves when marsh plants are also available, and (2) if salt marsh herbivores show a preference for leaves of different ages or for trees growing in different habitats.

From the moment I arrived at the Smithsonian Marine Station, I quickly became fascinated by the hundreds of anoles I had seen sunning themselves on both the brick walls of the more developed areas and mangrove trees of the state parks. As an intern, I spend five to six days a week meandering through mangrove stands and gazing at black mangrove flowers to document pollinators and other floral visitors. But after seeing over six anoles on just my first day in the field (and after several failed attempts to catch one and observe it up close) I decided to find out what role these lizards could be playing in the mangrove ecosystem.

Home for most species largely depends on climate: temperature, light and rainfall. With changes in global climate trends, many plants and animals are expanding their geographic limits poleward. However, not all species in a community respond to these changes in the same way.

Organisms often differ in the type and timing of their responses to environmental changes. Sometimes, animals expand more quickly than the habitats they’re used to. When this happens, these organisms are forced to colonize unfamiliar habitats where they often face numerous challenges, like resource shortages and never-before-seen predators.

So how do these animals alter their behavior, resource use and reproductive strategy to succeed in their new habitats? I aim to explore just that question by studying the range expansion of the mangrove tree crab Aratus pisonii into salt marsh habitats.

Mangrove tree crabs are native to Florida and abundant throughout Floridian mangroves, where they are the dominant herbivores of fresh mangrove leaves. Like mangrove trees, they have slowly begun moving northward. But the crabs are moving faster than the mangroves—and they’ve begun to invade salt marsh territory. They can be found crowding onto isolated dwarf mangroves nestled amidst cord grass, as well as in salt marshes with no mangroves in sight!

Because mangrove tree crabs in their native habitats rely heavily on mangroves for food and shelter, their habitat shift into salt marshes also causes a diet shift that can impact their growth, survival and reproduction. By focusing on the range expansion of mangrove tree crabs into salt marshes during my time at the Smithsonian Marine Station this summer, I hope to shed light on what exactly is enabling this species and countless others to successfully expand their range into new environments.

When studying major ecological changes, like the movement of entire species or ecosystems, we often have to sample across large geographic areas. This means lots of road trips!

Taking a moment to reflect after a field day that started at midnight. (Cora Johnston)

Starting nearly two months ago, I began my own road trips along the coast to survey the larval crabs that are washing ashore in swarms. Crabs typically recruit (leave the open ocean as larvae to join adult populations in coastal habitats) in a few brief but frenzied weeks in late spring and early fall. Therefore, I’ve been busy hopping between sites to gather as much data as I can while the crabs are abundant. Unfortunately, this means that my schedule, like the crabs’, depends on moonlight and tides. I’ll wake up around midnight, drive until the wee hours of the morning, and then sample the incoming tides by moonlight until wrapping up and moving to my next site as the sun rises. I then load up a kayak and spend the day paddling around collecting larger crabs (though still far too tiny to eat) from deep in each habitat to compare to the larvae I find riding the currents at night.

I head off on these adventures wielding stacks of audiobooks, a hefty thermos and lots of pre-labeled jars and data sheets that ease the demands on a weary mind. I munch trail mix to battle the exhaustion and swim to soothe the bug bites. After a few weeks on this schedule, even I find it hard to believe that I will get up at midnight the very next week to start all over again. Luckily, what keeps me coming back is what got me out of bed to do these studies in the first place.

Many insects visit black mangrove flowers, including bumblebees (left) and Pseudomyrmex ants (right). But which pollinators are the most important? (Mayda Nathan)

Introduced species have a bad—and sometimes well-earned—reputation. Brown tree snakes in Guam, mosquitoes in Hawaii, cheatgrass in the intermountain west, and many more invasive organisms have turned native ecosystems upside-down, changing fundamental ecosystem properties like species diversity, nutrient availability, and the size and shape of food webs. Biologists are hard at work learning how to tell when, where, and how a species becomes a successful invader and driver of ecosystem change. (See a recent post on how tricky this can be.)

But how can we make predictions about invaders that are…native?

In other words, what happens when an organism starts to spread out from its native range into adjacent territory—without hitchhiking along with humans? And why does this happen in the first place?

A large adult male Aratus eating a juvenile of its species. (Megan Riley)

As adults, mangrove tree crabs (Aratus pisonii) tend to forage on fresh mangrove leaves in the canopy. As omnivores, they will also prey on nitrogen-rich insects, larvae, and even juveniles of their own species. However, it’s unclear how these organisms balance their nitrogen requirements and other nutritional demands. This summer I have focused my research on addressing this question. Specifically, I am investigating how their diet choices affect their overall health and ability to reproduce.

In order to perform this research, I first have to catch the Aratus! Despite the abundance of these animals in mangrove stands on the Indian River Lagoon, tree crab hunting is a tricky business. Over the course of the summer, I’ve learned a number of tips for trapping the elusive crustacean…

The waters beneath mangroves are teeming with marine life, in part due to the refuge provided by the tangled complexity of their underwater roots. (Cora Johnston)

From salty branches to mucky roots, mangroves are teeming with life. Although many people recognize mangroves as spindly trees emerging right out of the water, it is under the water’s surface that mangroves really come alive for a marine ecologist like me. (That is also where you start to appreciate red mangroves’ apt name.)

Mangrove roots, both dangling from above (prop roots) and growing up from the sand (pneumatophores), not only mine for nutrients and allow for oxygen and carbon dioxide exchange for the plant; they also provide apparently crucial food and refuge for a stunning array of marine species. Fish, worms, crabs, shrimp, barnacles, and many other organisms take shelter among the roots – gluing right to the wood, hiding in crevices, and peering out through the maze. In such harsh intertidal conditions, where waves break, salt builds up, and the sun beats down, the shade and nooks formed by mangroves may be the key to survival for juvenile fish and crustaceans that will someday populate coral reefs and fishing hotspots farther offshore.

Over the coming months, I will be investigating how and why young fish and crustaceans use mangroves and marshes. By understanding the refuge provided by these very different coastal plants, I hope to better understand how the northward march of mangroves will influence the survival, abundance, and composition of marine species utilizing these now changing coastal nurseries.

Juvenile fish and crustaceans find safety in red mangrove roots during the early, vulnerable stages of their life. This young barracuda may be looking for a snack while hiding from larger predators. (Cora Johnston)

Even mangrove tree crabs that spend most of their time foraging in the canopy climb down to the safety of the mangrove roots to shed their old exoskeletons and harden their new ones. (Cora Johnston)

-Cora Johnston is a PhD student at the University of Maryland. This material is based upon work supported by the National Science Foundation under Grant Number 1065098. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

It sometimes seems crazy to be climbing through mangrove stands and wading through large ponds to collect our data, but the sites we explore are chosen for a reason. That reason is two-fold: One, to ground truth satellite imagery so we can map historic and current mangrove distributions. Two, to document the plant communities in places dominated by mangroves, in places where mangrove encroachment is occurring, and in places where mangroves have not yet arrived.

By using satellite imagery from years past, we hope to determine how far mangrove communities have spread in the last few decades. To do this we have to first understand what individual plant species comprise the large areas of vegetation that we can see from the satellites.

Beetle tunnel. This Scolytid beetle has burrowed into a mangrove seedling and lain its larvae inside. (Jake Bodart)

In science not everything goes according to plan. For example, half of your project’s experimental units might die before you start.

In the back of the Smithsonian Research Station here in Ft. Pierce, the mangrove team has built an artificial pond (we call it Lake Simpson) to raise mangrove seedlings that will be used in experiments. However, when we arrived here last month, we noticed that about half of the red mangroves were turning black and dying. It was unclear at first whether these mangroves were dying directly as a result of the artificial habitat (was our pond too hot? Too salty? Not salty enough?), or if the pond was somehow making the mangroves more susceptible to pest insects. We know from other studies that predation by insects can cause a large amount of propagule and seedling mortality.

Upon closer inspection, we decided insects were the culprit. The evidence of insect predation: small bore holes and little piles of frass (chewed up/excreted parts of the plant, a.k.a. insect poop). We decided to sacrifice the seedlings that were clearly infested, and dissect them to see if there were any insects inside.