Past studies by Dr. Mark Bertness have shown that crabs and mussels by themselves can have positive effects on plant growth – most likely because crabs can reduce the stress of low oxygen in the sediments by building their burrows, and mussels can add nutrients to the sediments.
Figure 3 from Mark Bertness's 1984 Ecology study illustrating the positive effects of mussel presence (white bars) on Spartina
Table 3 from Mark Bertness's 1985 Ecology study. Fiddler removal has a negative effect on Spartina in the marsh flat, but not the marsh edge.
Although both fiddlers and mussels occur together in the field, no studies have looked at how the combination affects the plants. Are the positive effects of each species by itself doubled? Or are they redundant with each other? Do crabs somehow reduce the positive effect of mussels, or vice-versa? How many crabs or mussels do you need to get a positive effect on Spartina? These are some of the questions that we hope to answer with our experiment.
Our new deck at FSUCML.
But first, we had to get everything set up. There were several long and hot days of shoveling sand into our “mesocosms” (10 gallon buckets) – many thanks to Robyn, Chris, Althea, and all the others who took care of that task! Then there was another day spent transplanting the Spartina.
Chris, Randall, and Robyn work to transplant Spartina from the greenhouse to the mesocosms.
Finally, it was time to add the fiddlers and mussels, and everything began!
Mussels nestled among the Spartina stems in one of our experimental mesocosms
Althea and Chris have been leading the charge on this experiment, and they’ve spent a lot of time getting to know (and identify) the fiddler crabs. All in all, a pretty fun study organism!
Althea working to identify fiddler crab species.
We’ll continue the experiment another month and then measure the height and density of the plants in each treatment to see if there are any differences. Once this experiment is complete, we’ll set up a separate one asking somewhat of the converse question – are two enemies (periwinkle snails and grasshoppers) worse than one? We’ll keep you posted.
Randall’s research is funded by the National Science Foundation.
The small cages in the photo above were used in an experiment I conducted to study California oysters. The insanely large cages in the photo below are from an experiment designed for our insanely large biogeographic oyster study.
While we had planned to install only 18 of these cages along the Atlantic coast of Florida, my crew wound up installing 70 cages over about six weeks. How did we reach such inflation in the number of cages and amount of digging? Well, it mainly stemmed from my ignorance of this area and the St. Johns River, which happens to dump a lot of sediment around oyster reefs. Because this sediment is deep and flocculent, it’s dangerous and almost impossible to work in. In fact, I may design a new study to analyze how oyster reefs manage to keep themselves above this ever-growing mud pit. I digress.
Relative to the abundance of these un-workable oyster reefs, mudflat areas suitable for our new experiment (i.e., near oyster reefs and firm footing) are quite rare. It was our luck (for better or worse, as you will soon read), we stumbled upon a sufficiently and suitable mudflat north of Jacksonville. After three days of hard digging, we managed to create large cages ready to support our experimental treatments. Suspecting that this site seemed too good to be true, we left the cages to fend for themselves for a week. If we returned to discover no problems, then we would proceed with the experiment.
On to St. Augustine- fitting the theme of bigger not always being better, our gargantuan stone crabs burrowed out of cages we had installed there. Even worse, cages without stone crabs were coming out of the ground because they were not dug in deep enough. The stone crab problem represents another example of why I should always run pilot experiments before attempting anything ambitious. Unfortunately, I have not learned this lesson yet. Or, I seem to periodically forget it.
Because I lacked the time to run such a pilot experiment, I ditched the troublesome stone crabs. We then awoke at dawn for the next three days to re-install cages (see the video below) in an over-kill sort of way. For this task, we took digging deep to a whole new level. Nothing was going to get inside or out of these cages without our permission. You can see how much deeper the cage bottoms extended into the ground by looking at the same cage pre- and post- renovation.
Having weathered the St. Augustine mishaps, we confidently headed back to Jacksonville to assess those cages. Upon arrival, I was subjected to a horrific scene: three days of hard labor undone by high flow conditions.
Note to self: mudflats are firm because flow is too high to allow sediment accumulation.
Stubbornly, I decided to force my will upon Mother Nature by digging cages in deeper and reinstalling them at locations behind marshes that would presumably buffer flow. Lacking the time to test this new cage installation, we immediately installed experimental treatments. This leap of faith was necessary in order to stay on schedule with the NC and GA teams.
Okay- cages up, reefs in, bells and whistles turned on. Afterwards, I raced back across the state to help two interns on their projects. Halfway back across the state and late on the Friday of Memorial Day weekend, I managed to blow the old lab truck’s transmission. As if getting a tow truck to Lake City at midnight wasn’t hard enough, getting one that would tow our truck and our kayak trailer was highly unlikely. But, taking pity on us, a wonderfully nice tow-truck driver agreed to load the trailer onto our truck.
Meanwhile, team Georgia was also experiencing problems with flow, sedimentation, and misbehaving predators. In short, we were throwing everything at this experiment and making little progress. At this point, ironically, the relative slackers amongst the three teams- the slow-to-start NC team- moved into first place- the horror!
After the passing of one mercifully tranquil week, we headed back to St. Augustine to check on things and collect data on our tile experiment. Interestingly, the experiment was working and we observed some variation in how predators indirectly benefit oysters; the positive effect diminished with latitude.
But then back again to Jacksonville- destroyed cages followed by some extremely colorful language. There should not have been deep pools of water surrounding the cages at dead low tide.
Obviously, it was time to cut our losses by not messing around with this site anymore. As a result, we spent the next three days searching all of northern Florida and southern Georgia to find a new ideal study site: suitable to oysters, no quick sand, firm footing and modest flow. After three days of intensive searching, we can confidently claim that such a site does not exist.
After accepting that this experiment could not be conducted in northernmost Florida, we decided to redirect Jacksonville resources to St. Augustine. There we would conduct a similar experiment that focused on a predatory assemblage unique to Florida: stone crab, toadfish, catfish, and crown conchs. So, nine more cages, nine more experimental reefs, and all the associated bells and whistles were established once again. By this time, my crew felt that they could easily serve in the Army Corps of Engineers.
Although things are now going well and we have a much better understanding of how to initiate this type of an experiment, my general ignorance has kept a Florida State University intern in St. Augustine for 7 weeks after agreeing to be there for only two weeks. Ooopsie!
Stay tuned in for a Hanna update on St. Augustine’s crown conchs and a post from Tanya about the summer madness from a technician’s perspective.
David’s research is funded by the National Science Foundation.
Randall examines an experiment cage as Robyn looks on.
Calling a one year experiment an “era” is probably a bit of an over-statement, but the end of our snail field experiment definitely feels significant. Especially for Robyn, who has traveled to St. Joe Bay at least once a week for the past year to count snails and take other data. And also for the Webbs, who were kind enough to let us put cages up in the marsh right in front of their house and then proceed to show up to check on them at odd hours for the last year! And finally for this blog, because the beginning of the snail experiment was the first thing we documented last summer when we started this project with WFSU. It’s nice to come full circle.
So why, you may wonder, are we ending things now? Is it simply because one year is a nice round number? Not really, though there is some satisfaction in that. The actual reasons include:
(1) The experiment has now run long enough that if snails were going to have an effect on cordgrass, we should have seen it by now. (At least based on prior studies with these same species in GA.)
(2) In fact, we have seen an effect of periwinkle snails, and in some cages there are very few plants left alive for us to count! (And lots of zeros are generally not good when it comes to data analysis.)
(3) Perhaps the most important reason to end things now: it’s become increasingly difficult in some cages to differentiate the cordgrass that we transplanted from the cordgrass that is growing there naturally. Being able to tell them apart is critical in order for our data to be accurate.
(4) The results of the experiment have been consistent over the last several months, which increases my confidence that they are “real” and not simply some fluke of timing or season.
And what are the results? As I mentioned above, snails can have a really dramatic effect on cordgrass, most noticeably when our experimental transplant is the only game in town (i.e., all the neighboring plants have been removed). And not surprisingly, cordgrass does just fine in the absence of snails and neighbors – they’re not competing with anyone or being eaten!
Snails also have a pretty strong effect on the experimental cordgrass transplant (compared to when no snails are present) when all of its neighbors are cordgrass.
Most interestingly, snails do not have a big effect on the experimental cordgrass transplant when some of the neighboring plants are needlerush.
This result is consistent with some of the patterns we’ve observed in natural marshes, where cordgrass growing with needlerush neighbors is taller and looks “healthier” than nearby cordgrass growing without needlerush.
Having decimated the plants in the cage, the snails move towards the tallest structure they can reach- a PVC pipe.
But why? Those snails are pretty smart. They generally prefer to climb on the tallest plant around, because it gives them a better refuge at high tide when their predators move into the marsh. (We’ve shown this refuge effect in the lab – fewer snails get eaten by blue crabs in tanks with some tall plants than in tanks with all short plants.) Needlerush is almost always taller than cordgrass in the marshes around here, so this preference for tall plants means that snails spend less time on cordgrass when needlerush is around. And finally, less time on cordgrass means less time grazing on cordgrass, so the cordgrass growing with needlerush experiences less grazing pressure.
These results – consumer (snail) effects on cordgrass are lower when cordgrass grows mixed with needlerush – are consistent with theory on the effect of diversity, even though in this case we’re only talking about a “diversity” of 2 plant species. And they could be important in the recovery or restoration of marsh areas where snails are causing a large reduction in cordgrass biomass.
The one thing we still don’t know with certainty – how do the snails determine which plant is taller??
I guess that’s the beauty of this job, in that there are always more questions to answer.
Randall’s research is funded by the National Science Foundation.
The new documentary, In the Grass, On the Reef: Testing the Ecology of Fear had a segment on the snail experiment. Watch the full program here. You can also read Randall’s post from the beginning of the experiment, and watch a video, here.
During their visit to FSUCML, Randall took the SciGirls to the small marsh next to the lab. One SciGirl found this fiddler crab carrying her eggs.
This video is part of the WFSU SciGirls project. SciGirls, for those who haven’t heard of it, addresses an unfortunate reality in the world of science- there are a lot more men doing research than women. It’s a problem that needs to be addressed as interest in science as a career has been waning overall. Every Summer, the SciGirls camp takes groups of teen and preteen girls into labs and into the field with scientists. After visiting Dr. Randall Hughes at the Florida State University Coastal & Marine Laboratory last Summer, a couple of SciGirls returned to conduct this interview.
Randall is a good role model for young aspiring female scientists. Aside from the fact that she herself is a female scientist, most of her lab- and that of her colleague Dr. David Kimbro- are females as well. If you’ve been following this blog, you’ve read about Emily Field’s graduate work on seagrass wrack and Kattie Lotterhos’ graduate work in genetics. In David’s lab, we’ve heard from Tanya Rogers, a lab technician who keeps David’s lab organized, and who is crucial in the planning and implementation of their large field experiments. We have more recently started hearing from Hanna Garland, Tanya’s fellow lab tech who is starting graduate school in the fall and who is looking into the abnormal levels of crown conchs on Randall and David’s Saint Augustine reefs. And we have also heard from Cristina Lima Martinez, an intern who comes to the Kimbro lab from Spain to study the Bay Mouth Bar ecosystem.
I’ve come to Saint Augustine to get the last of the footage I need to finish the In the Grass, On the Reef documentary, and we’ve come a long way from where we started from on this blog. One year ago today, this site went live and Randall and David introduced you to their research. The oyster study had just gotten its grant from NSF and we went out with David as he walked out into Alligator Harbor in search of study sites. It was a slow, messy day- but a necessary first step. Continue reading →