Uncovering the Effects of Prescribed Fire on Vernal Pool Amphibians

Fire is landscape disturbance that can do great things for resident organisms. Certain plants and animals are adapted to cope with or even thrive in the earlier successional habitat created by this blistering disturbance. Serotinous pine cones that open following a fire, oak trees with thick bark protecting them from the heat of the fire, or grasses taking advantage of the nitrogen released in the post-fire soil. Small rodents like mice and chipmunks dive into burrows to protect themselves from the direct effects of a burn, only to re-emerge in a scorched world that will be filled with food, grasses and acorns, within a year. Other species like snakes and lizards may survive the fire and find a new forest with more sunlight reaching the forest floor and greater basking opportunities. Forestry managers have begun to reintroduce this natural disturbance back onto the landscape in the form of a controlled burn. And whereas some species may benefit from this disturbance, other species may not fare so well to the disturbance itself or the post-fire landscape. For many species, it is unclear how they will respond to prescribed fire.

 

Post-fire landscape.

The charred landscape may affect smaller trees, but the thick bark of Oak trees provide ample protection.

The effects of prescribed fire on vernal pools and vernal pool amphibians remains largely understudied.  Some amphibian species like Spotted Salamanders, Jefferson Salamanders, and Wood Frogs rely on vernal pools as an essential habitat where their eggs can be deposited and larvae can develop in the presence of a plethora of food and absence of fishy predators.  These pools disappear each summer only to refill with winter rain and snow melt, just in time for spring migrating salamanders and frogs which lay their eggs among their submerged branches and vegetation.  The eggs and larvae of amphibians can be highly sensitive to changes in water chemistry and temperature.  As fire changes the landscape around a vernal pool, it may also influence characteristics of a vernal pool. Reductions in forest canopy may allow more light to reach the vernal pool and increase amphibian larvae growth rates.  Run-off from the burnt forest floor may also increase alkalinity within the vernal pool.  Following a series of prescribed burns in Florida, Clay Noss and Betsie Rothermel found a slight increase in vernal pool water pH; however, this change did not affect their focal species, the Oak Toad.  So, would a similar change be expected from prescribed fires in the forests of Pennsylvania?  Would amphibians native to our vernal pools respond in similar ways to the Oak Toad?

 

Jefferson Salamander eggs attached to a submerged branch.

Wood Frogs mating within a vernal pool. Large swollen eggs are present in the background as well as a freshly laid egg mass to the right.

These are just a few of the questions that I am looking to answer with a couple research projects I began this spring.  Luckily, I have the assistance of a fantastic undergraduate, Michaleia Mead, who will stay on after she graduates this spring and turn some of these projects into her Masters thesis.  For our first project, we began sampling the water chemistry (pH, dissolved oxygen, conductivity), temperature, and physical characteristics of a series of vernal pools with differing burn histories.  We are also measuring the canopy cover over the vernal pools and the amount of UV-B radiation that may reach the water surface.  UV-B is known to cause detrimental effects on amphibians in high enough doses. We want to see if vernal pools in an oak dominated forest respond to prescribed fires in similar ways to the vernal pools of Florida.  We will sample invertebrate and amphibian abundance and diversity within these vernal pools.  Do we see a change in community composition of a vernal pool as characteristics are altered by prescribed fire?

 

Vernal pool located in a post-burn landscape. This tract of land was burned in 2014. Note the charred trees in the background and reduction in understory vegetation.

To accompany the field projects, we began to raise Wood Frog tadpoles in the lab under different pH and UV-B conditions.  We will determine if these changes affect tadpole development and survival.  Additionally, we will compare corticosterone levels among tadpoles from different treatments to determine if certain treatments lead to more stressed tadpoles.  Even if tadpoles survive and develop under certain conditions, developing under stressful conditions can result in increased energy expenditures and decreased fitness.  This could have implications beyond the vernal pool if recently metamorphed Wood Frogs have lower energy reserves.

50 tadpoles are placed in each tank with varying amounts of pH and UV-B.

Our experimental set-up manipulates water pH and amount of UV-B exposure, while controlling room temperature.

So stay tuned as Michaleia and I update everyone on the progress of our studies!

Me scouting out vernal pools in previously burned landscapes.

Michaleia working hard in the lab as she cares for her tadpoles.

But Do They Make Omelets?

Short answer: Nope. Fire ants like their lizard eggs raw. Our recently accepted paper in the Journal of Herpetology “Invasive Fire Ant (Solenopsis invicta) Predation of Eastern Fence Lizard (Sceloporus undulatus) eggs” shows that fire ants do indeed eat fence lizard eggs in a natural setting, but, despite being known as “fire” ants, these hymenopterans haven’t quite mastered the art of cooking their food prior to chowing down. Get your non-final, still with a couple of mistakes, pre-print copy online (paywalled) or download one from this blog (posted for personal use of our readers courtesy of SSAR)!

At this point, you may be asking yourself, “Why should I care if fire ants eat fence lizard eggs?”, so I’ll discuss the impetus behind this project. Previous research in the lab had given us a good idea of how fire ants can impact juvenile and adult fence lizards: they are found frequently on the ground by fire ants, stung if they don’t run away, and can be envenomated when eating fire ants. BUT we knew incredibly little about what impacts fire ants might have on one unexplored life stage of fence lizards: eggs! And unlike juveniles and adults, these eggs can’t flee or twitch when attacked by fire ants; they remain in a nest for 55-70 days (depending on site and temperature), during which time they might be vulnerable to fire ants. Additionally, fence lizards often (though not always) prefer sandy sites with low canopy cover, where sunlight can warm the nest, exactly the type of microhabitat beloved by fire ants. And fence lizards build their nests 4-8 cm underground, right at the same depths where fire ants construct their underground foraging tunnels. We surmised that fire ants might come into contact with fence lizard eggs with some frequency, and, if fire ants ate the eggs, this might have a large impact on fence lizard populations.

Fire ants are known to eat the eggs of other reptiles, including those of snakes, turtles, and some lizards (such as anoles). To determine if fire ants were physically capable of eating fence lizard eggs, Jill Newman (a former lab undergrad who just started her master’s at Clemson…wooooooo!), Tracy, and I designed a small experiment. We presented fence lizard eggs to captive fire ant colonies and observed them penetrate the eggs in less than 30 minutes…a rather dramatic response! However, we also wanted to see whether fire ants might eat fence lizard eggs under more natural conditions. To address this, Jill dug holes in the ground to the depth of fence lizard nests and placed 12 eggs near fire ant mounds overnight. Upon examination, 11 of the 12 eggs had been punctured and eaten in less than a day!

The next summer, I designed a follow-up experiment to learn more about this type of predation. Specifically, I wanted to know how many eggs fire ants might eat, how quickly they might find them, and whether any environmental variables, like distance of a fence lizard nest from a fire ant mound, might affect predation. To address this, I started by collecting fence lizard eggs. A LOT of fence lizard eggs (over 150!…I resisted the urge to make my own omelet).

I couldn’t, however, just bury the eggs in the ground and check them after 24 hours (as Jill did) to answer my question about how long eggs might survive…I had to be (a little) creative. After reading about a similar problem faced by Kurt Buhlmann and his solution when he wanted to monitor turtle eggs, I developed a method allowing me to monitor the eggs daily without disturbing them (which might attract fire ants and increase predation).

For each nest, I dug a hole into the ground and sunk into it a clear, capped acrylic tube. I carefully replaced the sandy soil around each tube and placed six eggs (a small, but reasonable size for a fence lizard nest) next to the tube. I inserted a small piece of plastic transparency above the eggs and then carefully covered the whole arrangement with the soil. The transparency prevented soil from entering between the eggs and the acrylic tube, and, by lowering a video camera into the tube, I could count the eggs and see if they were being attacked by ants (or other predators). At each “nest” I also measured the amount of canopy cover and the distance to the nearest fire ant mound.

Diagram of nest tube with camera for viewing eggs underground.

…aaaaand what one of them looks like in the ground.

Each day, for up to 20 days, I monitored the nests and recorded if all the eggs were present. If I found ants attacking the eggs, I waited a few hours to let them eat the eggs (or at least let them make a start) and then dug up the nest to catch and identify some of the ants. I can definitively say that fire ants do not like to be disturbed when they are in the middle of a meal! Because of our innovative setup, it only took about 3 minutes a day to monitor each nest (check out a couple of examples below).

In our trials, we found that 24% of nests were attacked by fire ants within 20 days. Extrapolating this to the full incubation period of fence lizards using a mathematical model, we estimated that up to 61% of fence lizard nests are in danger of being preyed on by fire ants. We also did not find any relationship between how far nests were from fire ant mounds and how likely they were to be eaten. Given the high densities of fire ants at many areas in the Southeast, it seems likely that fire ants prey on a substantial portion of fence lizard nests in the wild. Of course, we know that fence lizard populations where we do our research are not in danger of disappearing. Fence lizards are doing fine even in the face of this predation, which is great news, and suggests that survival in other parts of a fence lizard’s life or high reproductive output may allow them to persist in fire ant invaded areas. In the future, I am aiming to build mathematical population models to understand the impacts of egg predation by fire ants, and see how this predation may affect populations over the long term.

One other point of note is that, for many species of southeastern herps that are declining, such as kingsnakes or southern hognose snakes, fire ants are often suggested as a culprit without any definitive proof. My project suggests that fire ants can indeed prey on large portions of the nests of some species, but also shows that one species is doing just fine even when fire ants may be making a buffet of about half of its nests. Moving forward, I would recommend that lab and field trials like those we’ve done be used to pinpoint if fire ants are indeed a threat to the eggs of these other species, and, if so, what proportion of nests are at risk.