The Lizard Log

The Langkilde Lab in Action

Backswimmers, Dragonflies, and Newts, Oh My!: How Large and Small Tadpoles Respond to Different Predators

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This week in our undergraduate blogging, we’re featuring a post from Danielle Rosenberg, a senior in the Schreyer Honors College majoring in Veterinary and Biomedical Science and minoring in Equine Science. While at Penn State she has been the Community Service Chair in the Block and Bridle Club and  in charge of planning the largest blood drive on campus. She’s also just applied to veterinary school and  hopes to become an equine surgeon in the future

Last fall, I applied for and received an Eberly College of Science Undergraduate Research Grant to conduct an independent research project on wood frog tadpoles. Last spring, I planned my project with Brad, a graduate student in the Langkilde Lab, and waited for the ice to melt, the frogs to come out, lay their eggs, and for the eggs to hatch. I waited and waited and waited a little bit longer. The past spring happened to be a very cold one, and unfortunately the tadpoles hatched much later than expected which delayed my research plans, but did not deter me from addressing the following questions: “What risks do specific predators impose on tadpoles of various sizes, and how do tadpoles respond to these threats?

Tadpoles are very important components of aquatic ecosystems. They help to cycle nutrients through the ecosystem by feeding on detritus and on phytoplankton and periphyton, which are both primary producers in aquatic ecosystems and provide energy for living organisms. Tadpoles’ role in an ecosystem can be drastically affected by the presence of specific predators. Predators can have a huge effect on an ecosystem by not only eating the prey and decreasing their population density, but by also causing prey to react to the predator’s presence and change their behavior in some way. For example, tadpoles are known to reduce their activity in the presence of predator to avoid detection. Behavioral and morphological changes brought about by predator presence can cause the allocation of resources to change from reproduction to camouflage or defense mechanisms, greatly affecting a population and thus an entire ecosystem.

Tadpoles are prey for a number of insect and vertebrate predators, such as fish, newts, dragonfly larvae, salamanders, and water bugs. Each predator presents a unique level of risk to tadpoles based on their size, speed, effectiveness at capturing their specific prey, etc. and thus can cause varying responses on prey of different sizes. For example, smaller predators may be limited in the size of the prey they can eat (their mouthparts may only be so big!), while larger predators may not have that limitation. In addition, predators feed at different rates and eat varying numbers of tadpoles at a time. Similarly, the risks associated with each predator may change as tadpoles grow. As tadpoles age, they become larger and faster and may be able to escape or avoid some predators.

Wood frog tadpoles (Rana sylvatica) are a model system for this type of study because they are known to exhibit a strong response to predators and play an important role in aquatic ecosystems. For my study, I was looking to determine the susceptibility of tadpoles to various predators based on their size and to see if this influenced their behavioral response. To examine this I observed tadpole behaviors in response to a predator cue and followed-up with predation trials which allowed predators to feed on the tadpoles. I predicted that a more dangerous predator would cause a greater effect on the behavior of the tadpoles at their most susceptible size class.

Three of the different predators used; from R-L: a backswimmer, a dragonfly nymph, and an Eastern Newt

Three of the different predators used; from R-L: a backswimmer, a dragonfly nymph, and an Eastern Newt

The study seemed like it would be pretty straightforward and simple, but science never goes as expected! I started my research at the beginning of summer and had literally thousands of tadpoles at my disposal along with a group of predators that were known to consume tadpoles. I was using backswimmers, newts, dragonfly nymphs, and diving beetles as my predators and wood frog tadpoles as my prey. I would weigh tadpoles and separate them into two different size classes based on the limits I had set. One size class was 100 mg +/- 20%, and the other was 400 mg +/- 20%. I began weighing tadpoles and noticed some mortality of my populations in the lab, so Brad and I decided to move my research project out to the field where the environment might be less stressful on the tadpoles.

I began my research over again, weighing and sorting. Once I had sorted enough tadpoles into the two prescribed size classes, I began to run behavioral trials. For these I needed a predator cue to make the tadpoles think that a potentially dangerous predator was present during trials. I created this cue by allowing each predator type to eat a single tadpole in a small container and then collecting the water the predation event took place in. For the trials, I put 8 tadpoles from one of the size classes into 1800 mL of water and recorded their natural behaviors. I added 10 mL of regular water to the tubs as a control and observed the tadpoles’ movement every three minutes for a half hour and recorded the number of tadpoles swimming at each time interval. I then added 10 mL of the prescribed predator cue to each tub and again recorded the movements every three minutes for a half hour. The pre-predator cue data provided me with a way to detect tadpole behavioral changes due to the presence of the predator cue. I am still in the early stages of analyzing this data, but the preliminary analysis showed a decrease in tadpole activity across all predators and size classes. It did not seem to matter which predator or what size the tadpoles were; it seems as though the tadpoles respond by decreasing their behavior in the presence of any predator regardless of size. This makes sense as reducing activity might be a good way to avoid the attention (and subsequent attack) of potential predators.

An example of the two different size classes of wood frog tadpoles

An example of the two different size classes of wood frog tadpoles

Next, I ran predator trials to determine the susceptibility of each size class to the individual predators. I ran these trials by placing 8 tadpoles of a specific size class into a tub with a prescribed predator. I allowed each predator 2 hours to eat as many of the tadpoles as they desired. At the end of the 2 hour time period, the number of tadpoles remaining was recorded, and I am currently analyzing this data to determine predation rates on the two size classes.

These experiments will help to provide information that can be useful in understanding the workings of aquatic ecosystems and the various effects that predators can have on tadpoles during their various pre-metamorphic stages. Conducting this research was a great experience, and it gave me a new respect for the life of a researcher. While I always assumed that once a method was put in place, the experiment was just carried out, it turns out it does not always go exactly as planned. There are a lot of bumps and holes that need to be worked around and fixed to create a successful project. Some methods that look very simple on the outside probably went through a lot of revisions to get results and conclusions. Researchers need to have a lot of patience and be willing to constantly update and rework their methods. I think it is impressive how much time researchers can put into a project yet can seem so easy and laidback.


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