Student-led ecology projects in a remote tropical forest
Lab courses in Biology provide students with training in many aspects of the scientific process, but students rarely have the opportunity to come up with and develop their own research questions. The Introduction to Field Ecology course, which is organized almost exclusively around student-led projects, is an exception. The course travels to sites in Costa Rica for a month. Students spend a couple of days at each site making observations which they then use to generate a question about the ecology or behavior of organisms at the site. They then develop an experimental design, collect data to test predictions, and present their findings to the group. It is a great opportunity to employ their creativity, inquisitiveness, and effort to the process of discovery.
We recently finished a week-long stay at Corcovado National Park in SW Costa Rica. All of the students did amazing work. Below are abstracts from each of their projects. Videos and pictures from their studies are embedded.
Relationship between social structure and lifestyle habits in co-existing monkey species in Corcovado National Park
Amy Niemela, Mark Painter, Leah Ruhland, Matt Scott
Much of the world’s biodiversity is in tropical forests. Similar species often coexist in the same location, but the factors that allow for their coexistence are often unclear. Monkey species in Neotropical forests are known to have a variety of distinct behaviors and social organizations. In some species, the alpha male fills the role of a lookout when the rest of the troop is foraging, while in others, each individual looks out for itself. Social structure can affect movement patterns through the canopy in a similar way in regards to how mothers caring for young respond to challenges when moving from tree to tree. To test potential relationships between social organization and foraging or movement habits, we performed an observational study in Corcovado National Park on three coexisting monkey species: spider monkeys (video), howler monkeys, and white-faced capuchins (video). We measured forager vigilance by comparing the amount of time spent surveying surroundings to the amount of time focused on foraging. We then compared vigilance among species in both the presence and absence of an alpha male lookout. We measured movement patterns by counting the number of gaps in the canopy from one tree to another that an individual crossed in the observed time, as well as the number of times that the individual jumped in the same period. We found that capuchins, howler monkeys, and spider monkeys had significantly different levels of vigilance when foraging. In addition, individuals foraging in the presence of an alpha male lookout were significantly less vigilant than individuals in troops where the lookout role was not clearly defined. Capuchin troops never employed a lookout and individuals spent much of their time being vigilant. In contrast, howler monkeys always had an alpha male lookout and individuals were rarely vigilant. Half of observed spider monkey troops had a lookout, and individual vigilance was significantly higher in the absence of a lookout. In regards to movement patterns, mothers faced a similar number of canopy gap-crossing situations as non-mothers. However, mothers jumped at a significantly lower frequency than non-mothers across all species. This result suggests that, in any species, the role of caring for young affects risk-taking behavior when traveling through the canopy. Overall, we conclude that there is a clear relationship between community roles and foraging and movement patterns in these three monkey species.
Size-based differences in territorial displays and activity levels in the common basilisk (Basiliscus basiliscus) in Corcovado National Park
Cory Birkestrand, Katelyn Bojan, Liz Chambers, and Jordan Goetting
Competition for territories containing prime resources is common behavior for many organisms. Some species maintain territories by displaying aggressive behaviors while others must actively search for resources. The common basilisk (Basiliscus basiliscus) is a Neotropical basking lizard found in lowland areas near rivers and streams. Male basilisks are territorial, holding and defending their territories by exhibiting aggressive head bobbing and chasing behaviors. In this study, we examined the relationship between size, distribution, activity level, and territorial display of the common basilisk. We predicted activity levels and territorial displays would be higher in smaller basilisks because small basilisks compete for territory with lizards of all sizes, whereas larger basilisks typically only compete with basilisks of similar size. We conducted 11 30-minute observations over a 3-day period (see videos of Cory here, of Katelyn here, and of a small basilisk here). We classified basilisks by size (small, medium, or large), number of head bobs, and number of dashes for each observed basilisk. We found that small lizards were more active (p<0.0014) and exhibited more territorial displays (p<0.0001) than medium or large lizards. It was also found that small and large basilisks generally occupy distinct areas within each habitat. Our observations suggest that small lizards may have higher activity levels and exhibit more territorial displays because they must move around and compete with basilisks of all sizes for territory and resources, whereas large basilisks have larger, more established territories. Large basilisks only take action to defend their territory when others of the same size challenge them. Also we suggest that small and large lizards occupy distinct areas within their habitat because large basilisks are more dominant, and therefore occupy the most preferable and protected areas, whereas smaller subordinate basilisks are forced to occupy marginal areas of the habitat.
Trade-off Between Safety and Food Acquisition in Terrestrial Hermit Crabs
Maya Peters, Tyler Abrahamson, Braedon Wieseler
Animals often face a trade-off between acquiring food and staying in safety. However, the nature of this trade-off may differ among individuals with different characteristics, and it may also differ across environmental conditions. Here, we investigate foraging-safety tradeoffs for terrestrial hermit crabs (Coenibita compressus) (video of crabs eating coconut) at Corcovado National Park, Costa Rica. This species is found across a gradient from exposed beach, to a beach-forest transition zone, to the margins of the forest. We examined whether crab behavior suggests they face a trade-off between acquiring food and defense, and we determined whether the response to this tradeoff depended on crab size and environmental conditions. We tested defense response by using a stick to mimic a bird attack (birds are major predators of this species), and we compared these responses in the presence and absence of food. Our main results were that larger crabs spent more time in their shell after a simulated predator attack, but only in the absence of food. In addition, crabs without food were particularly slow to emerge from their shell after facing a threat in the forest or on the beach. Because crabs were most abundant at the forest-beach transition zone, our results suggest that predation threat is a major factor determining the distribution of hermit crabs in this environment.
Size and food acting as determinants of interactions in the freshwater shrimp, Macrobrachium americanum
Tom Langer, Evan Nolander, Dan Oseid
Why do some organisms exhibit territorial behavior, while others do not? One explanation is because the benefits of being territorial depend on ecological conditions. Macrobrachium americanum is a freshwater shrimp common in lowland tropical rainforests and size-dependent behavioral interactions among juvenile M. americanum can easily be observed in low-flow pools. Based on general observation, we classified juvenile M. americanum into three categories of size, small, medium, and large. Observational and experimental data were collected from eight pools in two streams found in Corcovado National Park and analyzed by viewing densities and numbers of interactions between sizes in the presence and absence of food. Data were analyzed using a two-way ANOVA and Tukey’s HSD test. We found that size of shrimp (F1,5=21.94, p<0.0001) significantly affected the number of interactions among shrimp; the most common type of interaction was medium-sized shrimp interacting with small shrimp. Interaction frequency also generally increased when we added food to natural pools (F1,1=18.73, df=1, P<0.0001) In addition, there was also a significant size-by-food interaction (F1,5=2.77, P=0.0228) because interactions involving medium sized-shrimp became more frequent relative to other interactions in the presence of added foods. Our results suggest costs and benefits of territorial interactions differ among different sized individuals in this population. Future work should determine the ecological factors that account for these differences.