Location: Barro Colorado Island
Dates: 1/10/17- 1/13/17
Lecturers: student group projects!
We’re heading to Barro Colorado Island for a couple of days for lectures, to explore around the island, and to do a group project! Normally, everyone groans at the mention of having to work on a project, but I was definitely looking forward to it. Hands-on learning and getting a little dirty is what I enjoy the most! After hearing a lecture from a few visiting scientists, we split up into three groups, based on our research interests, and start brainstorming.
Scott Mangan and Erin Spear were our research mentors and did a fantastic job of getting us on the right track by allowing our curiosity and ambition to guide the project, while reminding us to consider the feasibility of the project (we would only have an afternoon and the following morning to do all of our measurements and data collection…). Ultimately, we decided to observe woody plant seedlings (seedlings with a woody stem) in two different microhabitats. We observed shade vs. gap environments to study the influence it has on the type of damage on the seedlings: herbivores, pathogens, both, or none.
The shaded habitats have canopy cover and more humidity, whereas the gap habitats (areas with no overlying canopy cover) receive more direct sunlight and are drier. Based on our preliminary literature review, we discovered that more herbivory occurs in gap species as a result of these plants focusing their effort on growing quickly, and less on chemical defenses and developing the chlorophyll to give the leaves a darker green color (this is why the youngest, newest, expanding leaves are lighter green, white, or red compared to the rest of the plant). The drier environment with more UV sunlight significantly reduces pathogen activity in the gap environment. In contrast, more pathogens are founded in the shaded areas as a result of the higher density of seedlings, as well as a humid environment, both of which are more favorable to pathogen growth and dispersal.
So, our hypotheses were as follows:
H0: There is no difference in damage type related to habitat type
HA: There is more herbivory damage in gap habitats and more pathogen damage in shaded habitats
Therefore, we would expect to see our data form a graph like this:
As a side note, I would like to briefly discuss a major concept that we learned about during our research: The Disease Triangle. It serves as a conceptual model to illustrate the interactions between the environment, the host, and an infectious (pathogen) agent and can predict the expected outcomes in plant health and mortality. In short, it explains the give-and-take of what external influences may be applied to the plant seedling before it is killed, and it takes a Goldilocks-and-the-three-bears amount of everything being just right to favor the plant’s mortality.
For our project, we did a paired test of 0.5×0.5m quadrats in nearby (~10m apart) shade and gap habitats and measured the number of woody seedlings, type and proportion of damage (herbivory, pathogen, or none), and the general classification of the size of the gap (small, medium, large). For the data analysis, we did a paired t-test and a two-way ANOVA. The paired t-test was used to measure the difference between the mean of the two groups and because we used the same time to measure control (shade) vs treatment (gap). The two-way ANOVA was then used to test for the two factors (shade vs gap) and damage type (damaged vs undamaged).
- Studying the density of the seedlings indicated that there was not a statistically significant difference in the density of seedlings between the habitat types (p=0.4646).
- Studying the damaged vs undamaged seedlings indicated that there is no interaction between habitat type (shade vs gap) and seedling status (damaged vs undamaged) (p=0.8850).
- Studying the damaged vs undamaged seedlings indicated that there were more damaged seedlings than undamaged seedlings in both the shade and in the gap habitats, which is to be expected and has been observed as a general trend in tropical forests.
- Studying the damage type on the seedlings indicated that there is no interaction between the habitat type (shade vs gap) and seedling status (damage from herbivory vs pathogen) (p=0.6530).
- Contrary to our hypothesis, herbivory was the dominant form of damage in both habitat types. There was not a statistically significant difference in the type of damage between the two habitats.
- There was not a significant difference in the density of seedlings between the habitat types
- In the gap and shaded environments, it appears that the seedlings were affected in the same proportion
- Overall, herbivory is the dominant form of damage in both habitat types
- Fail to reject our null hypothesis of no association between habitat type and damage
- No statistically significant difference in damage was observed between the two microhabitats observed
Reflecting on our experiment, we discussed the potential sampling errors and what we would do to improve the experiment if we were doing it all over again. It was a great experience and we learned a lot from doing these small projects! In conclusion, I am really glad that the ‘group projects’ component was added into the course (they had not done this is the past few years). It forced us to come up with a question, figure out how to conduct an experiment to study that question, to physically go out and collect the data, and how to then analyze the data that was collected, all in an incredibly short time frame. Granted, it came with some complications along the way and we occasionally bickered and butt heads about how to do our experiment, but it was a fantastic experience overall. Plus, I really enjoyed hearing what the other two groups did and to see how they tackled their research projects. I am excited to see how things go for the second set of group projects while we’re out at Bocas del Toro at the end of the course!
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