Upon Graham Blandy’s death in 1962, 700 acres of his 900 acre estate were donated to UVa so that students could learn about farming. The focus of Blandy Experimental Farm has since shifted away from agriculture to scientific research. There has been an undergraduate research program at Blandy for students interested in ecology and environmental science since 1992. There are woodlots, successional fields, croplands, pastures, ponds, and wetlands at Blandy allowing for diverse research projects. In addition to being a research field station for UVa’s department of environmental science, Blandy is the State Arboretum of Virginia.
Twelve students from colleges and universities across the country participated in the internship program. Most were seniors, and there were a few juniors. Most were majoring in biology. Eleven were involved in the REU program, and one was sponsored by the Foundation of the State Arboretum to do a horticulture project. We each received a $3000 stipend and $750 for research equipment. We lived on site in the estate’s old slave quarters. The East wing, where I lived, dates from around 1830. There is laboratory space on the first floor and four double bedrooms on the second floor. The central and West wings were added in the 1940’s and contain more bedrooms, a computer lab, offices, a kitchen, laundry, dining room, and a branch of the UVa library. The goal of the REU internship was to familiarize us with field ecology research. Each student carried out a research project with the assistance of a mentor. Professors from UVa and other universities with backgrounds in biology, ecology, and environmental science served as mentors. The UVa professors had been associated with the program for many years, but usually different professors from other universities participate each year. The entire research process was performed from writing a proposal to doing the experiment and finally writing up a final paper, making a poster, and giving a talk. The internship’s objectives matched my goals for the summer. I have considered pursuing a career in ecological research, so this internship served as a wonderful opportunity to experience that kind of work.
During the first week at Blandy we had various tours and talks on topics such as hypothesis testing and statistics to get us prepared for the program. Each of the seven potential mentors gave a presentation on their past research and possible projects for the summer. After listening to the presentations, I decided to work with Dr. Patrick Crumrine from Longwood University on predator-prey interactions between dragonfly larvae and freshwater snails. He did a study with tadpoles, dragonfly larvae, and snails in a simplified freshwater community when exposed to pesticides. When he submitted the manuscript for publication, the reviewers asked for more information on the interaction between dragonfly larvae and snails – thus the origin of my research project. I chose to work with Patrick because his project interested me particularly because it dealt with animals and many of the other mentors worked with plants. During the second week we wrote our research proposals, and at the end of the week, we gave a ten minute presentation about our research project. Our peers and the mentors then had ten minutes to ask us questions. The majority of the remaining time was spent designing and carrying out my research project. At the end of the summer there was a forum during which each person gave a fifteen minute presentation. Each person also wrote a final paper and made a poster about their research.
In addition to carrying out my research project and helping other people in their research, we participated in some fun activities during the summer. Every Wednesday night a speaker was brought in to give a talk on their research and afterwards we would have a potluck dinner. The program coordinators hosted career talks and organized trips to the Smithsonian Conservation and Research Center and Mountain Lake Biological Station, UVa’s other field research station. There was a concert series at Blandy at which we helped. We also participated in the kid’s camps at Blandy. I took the kids to the ponds, helped them collect organisms, and told them about some of the stuff they found. During the summer we also had the opportunity to go canoing and hiking.
Ecological research is really cool but it is very demanding. It takes a lot of time to carry out a good project. Often one must face adverse conditions while collecting data. This could involve rain, heat, or our favorite, deer ticks. Counting snails six hours a day four days a week for three weeks can get pretty tedious, and it isn’t kind to the knees. But all the hardships are worth it to be able to spend all day outside in beautiful environments learning about fascinating organisms to contribute to ecological knowledge.
Predation greatly affects the structure and functioning of communities by forcing choices between predator avoidance and feeding or reproducing. Snails and dragonfly larvae are often very abundant in pond communities and are important components of a healthy pond ecosystem. Dragonfly larvae, opportunistic predators, and snails occupy the same habitat, therefore, dragonfly larvae may prey on snails. However, the interaction between these organisms is not well understood. Snails are only occasionally identified in studies of larval odonate diets because dragonflies would extract the snail from its shell and soft-bodied prey often go unnoticed.
Physa and Planorbella are two common pond snails that are preyed upon by many diverse organism and inhabit the same areas as many species of dragonfly larvae. I studied predation of two Planorbella size classes and one Physa size class.
Belostoma are known predators of snails. Some snails were exposed to Belostoma to enable me to compare these treatments to those with dragonfly larvae.
I studied snail predation by two size classes of Anax junius and one Pachydiplax longipennis size class. Anax is characteristically larger, faster, and more aggressive than Pachydiplax.
Most dragonfly larvae utilize an ambush method of foraging by remaining still and waiting for prey to come within reach of their labium. In addition, some larvae stalk their prey, but this usually occurs at night. Dragonfly larvae usually detect prey visually by movement or shape. They may also use chemoreceptors or mechanoreceptors.
When faced with a predator, snails may alter their life history or behavior. Morphology also plays a key role in snail survival. Snails can alter their life histories by suppressing some reproductive behaviors or changing growth rates to enhance survival. Snails tend to be less vulnerable to predators if they are large in size or have a narrow aperture. The defensive behavior of snails varies according to the identity of the predator and the morphology of the snail. Snails may crawl out of the water, move to the surface of the water, bury in the substrate, or hide in vegetation. Crawling out is the most costly avoidance behavior as snails then lose time feeding or mating and may risk additional predation or desiccation. Surfacing and refuge use are favored in long-term exposure. Burial may be the most effective behavior to avoid visual predators such as dragonfly larvae.
The first question I examined was: How strongly does each predator prey upon different sizes and species of snail? I determined this by measuring snail survival in the presence of Belostoma, large Anax, small Anax, Pachydiplax, and no predator. The second question I examined was: How do the different sizes and species of snail respond to the different predators? Growth, reproductive output, and avoidance behavior was examined to determine the degree of response.
I hypothesized that Belostoma , large Anax , small Anax , Pachydiplax , and the control treatment would have the greatest predatory effect respectively. Predation would be greatest in Physa , small Planorbella , and large Planorbella respectively. Snails surviving exposure to predators would have smaller aperture widths and greater average wet masses, lengths, and widths than surviving snails of the corresponding size or species not exposed to predators. The reproductive output of snails subjected to predation would be less than the reproductive output of the corresponding class of snail not subjected to predation. Snails in the presence of predators would perform avoidance behaviors more often than snails without predators. Amount of avoidance exhibited would be proportional to degree of predation risk. Burying, surfacing, using plants, and crawling out would be the most commonly performed avoidance behaviors respectively.
Three snail classes were crossed with five predator treatments in a full factorial design with six replicates. Snails and predators were collected from ponds at Blandy Experimental Farm. Mesocosms were used to study the interactions between snails and predators. 14 liter, plastic containers were set up in the Oak Grove and designed to mimic the organisms’ natural environment. Rinsed sand was added to the tubs to a depth of 1cm and 9L of well water and 1L of pond water filtered to remove macroorganisms were added to the tubs. The pond water promoted the growth of periphyton and algae. A microscope slide was inserted in each tub in a standard orientation so that abundance of periphyton could be studied at the end of the experiment. The tubs were left alone for one week to allow periphyton to grow.
Meanwhile, I measured the snails to be used in the experiment. The wet mass of ten snails was measured for each tub. Dimensional measurements were taken on each snail.
After allowing the tubs to sit for 9 days, a standard amount of rinsed coontail vegetation and ten snails were added to each tub. The next day one predator was added to each predator treatment. Shade structures were used to prevent other organisms from disturbing the mesocosms.
Observations were made 3 times daily (morning, afternoon, evening), 4 times per week for 3 weeks. The shade structure was removed fifteen minutes prior to observations so I could be sure that the behavior I was observing was not in response to the shade structure removal Behavior was determined by noting how many snails in each tub were crawled out, surfaced, buried, and in the vegetation out of the total number of live snails observed. There was high predator mortality. Whenever a dead predator was observed, it was replaced. This was not problematic, as I was not studying snail predation effect on predators. Rather it was important that snails in treatments with predators be constantly exposed to predation. After the three weeks of behavioral observation, the slides were removed to dry, the number of live snails in each tub was counted, and living snails were measured.
Physa was less hardy than Planorbella as they had high levels of mortality in all treatments, including the control. However, they were also probably the most convenient prey for the predators. They were the smallest of the snails studied, making them more manageable prey for most of the predators. It is likely that survival in Pachydiplax treatments was similar to that in control treatments because Pachydiplax is the smallest and characteristically least aggressive of the odonate larvae studied. Belostoma are known predators of snails, but this was not supported by the results of this study, probably because most of the Belostoma escaped from their tubs. Large Anax were the strongest predators closely followed by small Anax , as expected due to their notorious voracity.
Physa was the snail class most vulnerable to predation so it would make sense that they would crawl-out the most. It is also probable that Physa crawled out more in order to perform gas exchange. Large Anax were the strongest predators and thus exerted the most stress on the snails, so perhaps these snails did not choose to perform the most stressful avoidance behavior, crawling out, in their presence. Dragonfly larvae often use visual cues to hunt by ambush, therefore the afternoon, the time with the most light, may be a peak in this type of odonate foraging. At night, dragonfly larvae tend to be more mobile and use chemoreceptors and mechanoreceptors to actively hunt for food. These two foraging methods have stimulated the peaks in afternoon and evening crawl-out.
Surfacing may have conferred some protection from predators but possibly made snails more vulnerable by increasing visibility. Physa still surfaced because they were the most vulnerable snails and their small size may have made them difficult for predators to see. Due to the increased vulnerability, Physa only surfaced in the presence of the weakest predator, Pachydiplax. The beginning of active night hunting and the cover of dark may explain the high level of surfacing in the evening. Afternoon surfacing would be less favorable due to increased visibility, but if visual hunters were most active then, the benefits of predator avoidance conferred by surfacing may have exceeded the cost of visibility. Surfacing decreased over the duration of the experiment perhaps because by the end of the experiment most of the vulnerable snails were already dead. Alternatively the reduction in snail density towards the end of the experiment may have precluded the need for snails to find food on the surface.
It appears that burying was not used for predator avoidance. Physa , the most vulnerable snail class, buried the least, and burying was constant across time of day suggesting that snails may not have changed burying behavior in response to level of predator activity. As the experiment progressed, food may have become scarcer, so snails may have had to bury in order to find it. However Physa may have searched for food in other locations due to their weak shells or competition may have been so low due to the high mortality in Physa treatments that there was not enough competition to warrant much burying.
Plants may not have been used for predator avoidance either as Physa , the most vulnerable class of snails, used plants the least. Predators often occupied the vegetation, so it is questionable if the vegetation would still serve as refuge for the snail while the predator is resting in it. As food became scarcer, more snails may have been forced to forage in the vegetation or consume the vegetation itself. However, Physa plant use remained constant. Perhaps the high level of mortality in Physa treatments reduced competition for food and negated the need to use the vegetation for food.
All of the larger Physa died. This could be because they were less able to handle predator-induced stress or predators preferred them. The largest Planorbella in the large Planorbella treatments may have died for similar reasons, or small Planorbella may simply have grown faster than large Planorbella .
Mortality was highest in Physa. Snails modified their behavior according to the type and behavior of predators and possibly the abundance of food. Therefore snails tended to perform avoidance behaviors in the afternoon and evening when predators were probably most active. Physa crawled out to avoid predators when it was not too stressful but only surfaced when the benefits of avoidance exceeded the costs of visibility. Burying and plants were not used to avoid predators but may have been important in foraging. Most notably, in this experiment it was shown that large Anax were the strongest predators of snails. Although the experimental system was not very representative of natural systems, these results suggesting large Anax as a potential predator of snails are still important and add to our understanding of dragonfly-snail interactions. The next stage of experimentation could examine dragonfly predation of snails when another prey such as damselfly larvae are present.
Contents overview <ul><li>What is Blandy Experimental Farm? </li></ul><ul><li>What is their REU program? </li></ul><ul><li>What did I do there? </li></ul><ul><li>What was my research project? </li></ul>
Blandy Experimental Farm - background <ul><li>Graham Blandy donated 700 acres to UVa in 1962 </li></ul><ul><li>Undergraduate ecology and environmental science research since 1992 </li></ul><ul><li>State Arboretum of Virginia </li></ul>
The internship <ul><li>12 participants </li></ul><ul><ul><li>$3000 stipend </li></ul></ul><ul><ul><li>$750 for research equipment </li></ul></ul><ul><ul><li>Housed in estate’s old slave quarters </li></ul></ul><ul><li>Carried out ecological research project with the help of a faculty mentor </li></ul><ul><li>Possible career in ecological research </li></ul>
Structure <ul><li>Week 1: orientation and mentor selection </li></ul><ul><li>Week 2: wrote and defended research proposal </li></ul><ul><li>Weeks 3-9: experiment </li></ul><ul><li>Weeks 10-11: analyzed data, wrote final paper, made presentation at forum, made poster </li></ul>
Fun stuff <ul><li>Weekly seminar and potluck dinner </li></ul><ul><li>Career talks </li></ul><ul><li>Smithsonian’s Conservation and Research Center </li></ul><ul><li>Mountain Lake Biological Station </li></ul><ul><li>Concerts </li></ul><ul><li>Kid’s camp </li></ul><ul><li>Canoe trip </li></ul><ul><li>Hiking </li></ul>
The highs and lows of ecological research <ul><li>Time intensive </li></ul><ul><li>Adverse conditions </li></ul><ul><li>Tedious </li></ul><ul><li>Physically taxing </li></ul><ul><li>Get to spend all day in beautiful, fascinating ecosystems learning about organisms! </li></ul><ul><li>What could be better? </li></ul>
Predator-prey interactions between dragonfly larvae and snails Paige Barlow Blandy Experimental Farm Research Experience for Undergraduates Summer 2004
Background <ul><li>Importance of predator-prey interactions for community </li></ul><ul><li>Lack of research on the possible interaction between dragonfly larvae and freshwater snails </li></ul><ul><ul><li>Interaction is suspected </li></ul></ul><ul><ul><ul><li>Dragonfly larvae are opportunistic, polyphagous predators </li></ul></ul></ul><ul><ul><ul><li>Common habitat </li></ul></ul></ul><ul><ul><li>Inconclusive results from studies of larval odonate diets </li></ul></ul>
Freshwater snails large Planorbella small Planorbella Physa
Giant water bug <ul><li>Belostoma flumineum </li></ul><ul><li>Known predator of snails </li></ul>
Response of snails to predator <ul><li>Alter life history </li></ul><ul><ul><li>Reproductive output </li></ul></ul><ul><ul><li>Growth rate </li></ul></ul><ul><li>Importance of morphology </li></ul><ul><ul><li>Shell length </li></ul></ul><ul><ul><li>Aperture width </li></ul></ul><ul><li>Perform avoidance behavior </li></ul><ul><ul><li>Crawl-out </li></ul></ul><ul><ul><li>Surface </li></ul></ul><ul><ul><li>Bury into substrate </li></ul></ul><ul><ul><li>Hide in vegetation </li></ul></ul>
Questions <ul><li>How strongly does each predator prey upon different sizes and species of snail? </li></ul><ul><li>How do the different sizes and species of snail respond to the different predators? </li></ul><ul><ul><li>Growth </li></ul></ul><ul><ul><li>Reproductive output </li></ul></ul><ul><ul><li>Avoidance behavior </li></ul></ul>
Hypotheses <ul><li>1. Belostoma , large Anax , small Anax , Pachydiplax , control have greatest predatory effect respectively </li></ul><ul><li>Predation greatest for Physa , small Planorbella , large Planorbella respectively </li></ul><ul><li>2. a) Predator small aperture width; large wet mass, length, and width </li></ul><ul><li>b) Predator low reproductive output </li></ul><ul><li>c) Predator avoidance behavior proportional to risk: bury > surface > plant use > crawl-out </li></ul>
Methods <ul><li>Set up mesocosms </li></ul><ul><ul><li>Sand </li></ul></ul><ul><ul><li>Water </li></ul></ul><ul><ul><ul><li>Well (9L) </li></ul></ul></ul><ul><ul><ul><li>Filtered pond (1L) periphyton, algal community </li></ul></ul></ul><ul><ul><li>Microscope slides </li></ul></ul><ul><li>Three snail classes crossed with five predator treatments in a full factorial design with six replicates </li></ul><ul><li>Collected invertebrates from ponds </li></ul>
Methods – snail measurements <ul><li>Wet mass (10 snails/tub) </li></ul><ul><li>Shell dimensions </li></ul><ul><ul><li>Aperture width </li></ul></ul><ul><ul><li>Width </li></ul></ul><ul><ul><li>Length with aperture </li></ul></ul><ul><ul><li>Length without aperture (for Planorbella ) </li></ul></ul>
Methods – mesocosms <ul><li>Added vegetation and snails </li></ul><ul><li>Introduced predators 1 day later (1 predator/tub) </li></ul><ul><li>Shade structures </li></ul>
Methods – data collection <ul><li>Made behavioral observations – 3 times daily, 4 times per week for 3 weeks (July 8-July 27) </li></ul><ul><li>Removed slides to dry </li></ul><ul><li>Determined proportion of snails surviving in each tub </li></ul><ul><li>Measured surviving snails </li></ul>
Statistics <ul><li>Two-way analysis of variance </li></ul><ul><ul><li>Survival </li></ul></ul><ul><ul><li>Mass </li></ul></ul><ul><ul><li>Growth </li></ul></ul><ul><li>Two-way repeated measures ANOVA – behavior </li></ul><ul><li>Proportional data (survival and behavior) were angularly transformed to meet the assumptions of ANOVA </li></ul><ul><li>Slides will be analyzed at a later date </li></ul>
Results – survival <ul><li>Low survival for Physa </li></ul><ul><li>Low survival in large Anax treatments </li></ul>p<0.001 p<0.001
Results – crawl-out <ul><li>Physa crawled out when exposed to small Anax and Pachydiplax </li></ul><ul><li>Increased through day </li></ul>p=0.022
Results - surfacing <ul><li>Physa surfaced in response to Pachydiplax presence </li></ul><ul><li>Increased through day </li></ul><ul><li>Decreased through experiment </li></ul>p=0.004 p=0.001
Results – burying <ul><li>Planorbella buried more than Physa </li></ul><ul><li>Did not bury in presence of predator </li></ul><ul><li>Little change through day </li></ul><ul><li>Increased through experiment for Planorbella </li></ul>p=0.004
Results – plant use <ul><li>Planorbella used plants more than Physa </li></ul><ul><li>Increased through experiment, especially for Planorbella </li></ul>p=0.041
Results – growth <ul><li>Average size of Physa shells decreased </li></ul><ul><li>Average size of Planorbella shells increased </li></ul>p=0.006 p=0.008 p=0.003
Limitations of the study <ul><li>Belostoma escaped from tubs </li></ul><ul><li>Physa behavioral proportions may be inaccurate </li></ul><ul><li>No egg masses found </li></ul><ul><li>Wet mass by nature not very accurate </li></ul><ul><li>Minute dimensional differences </li></ul>
Conclusion Belostoma large Anax small Anax Pachydiplax Snail - Physa - large Planorbella - small Planorbella ?