Posted by: Adam Kay | January 23, 2014

UST Biology student research in “the most biologically intense place on Earth”

This January, our class – Introduction to Field Ecology (BIOL 211) – is conducting research at sites across Costa Rica. Our first research site was on the Osa Peninsula near Corcovado National Park (we have a post about our accommodations here). At the site, the students were responsible for coming up with a project idea, developing an experimental design, executing the project, and creating a presentation. They had to do all of that in just over two days. Many of these students have little experience with ecology, and none of them had ever been to the tropics. Here are abstracts and pictures from their incredible work:

Communication behavior in the endangered scarlet macaw

Jorgen Kvaal, Erik Sathe, and Taylor Schuweiler

Communication among conspecifics can improve foraging and anti-predator success. Many birds communicate with conspecifics through vocalization while living in groups of two or more. However, the function of conspecific communication likely depends on behavioral context. In this study, we studied vocalization patterns of scarlet macaws (Ara macao) near Corcovado National Park on the Osa Peninsula of Costa Rica. Scarlet macaws are endangered, but the Osa may support the largest macaw population (800-1200 individuals) in Central America. Here, we compared macaw vocalization patterns in flight vs. while perched; we also tested whether vocalization frequency varied with group size. We predicted that call frequency would be higher for individuals flying than for those perched because of the greater need for activity coordination in flight. Similarly, we predicted that individual call frequency would increase with group size because of the increased need for coordination in larger groups. We found that, as predicted, call frequency was significantly higher while an individual was flying than while perched. We also found that per capita call frequency was dependent on group size while flying but not while perched. Interestingly, we also found a significant difference in mean call frequency between the most vocal and least vocal individuals within a group, particularly while birds were perching. These patterns suggest that call-response behaviors are critical for organizing flight patterns, which likely help in navigation and anti-predator success.

Predation-induced responses in tropical hermit crabs

Jake Anderson, Matt Boehm, Theo Larson, and Morgan Reeve

712Predator-prey interactions have led to the selection for widespread defense mechanisms in prey species including specific morphological features, habitat usage, ability to evaluate threat intensity, and other behavioral adaptations that balance the risk of predation against the reward of resources. Coenobita compressus (hermit crabs) are well-suited for studying defense mechanisms because their threat response – the retraction of their bodies into their shell – is easily observable. To assess the extent in which C. compressus defensive mechanisms vary across conditions, we tested how shell size, threat intensity, and the presence of food affect time spent in shell after a simulated threat.  We found that time spent in shell after disturbance was significantly longer for crabs with larger shells (r2 = 0.256, df = 49, p < 0.0002). Time in shell was also longer when the perceived threat was more intense (a physical tap vs. a hand waved over the shell) (t = 4.73, df = 41.82, P < 0.001). In contrast, crabs emerged sooner from their shells after threat if food was present vs. in trials with no food present (t = 5.6, df = 25.513, P < 0.001).  These results indicate that hermit crabs can modify defense responses depending on conditions, a flexibility that likely increases their survival in the face of predation.  Future studies could further examine the use of morphological features and habitat use with relationships between habitat and shell color selection.

Sorii ‘bout it: The influence of light intensity on reproductive structures of fern species in the tropics

Sam Carpenter, Kate Hudson, and Ryan Merry

Energy balance between growth and reproduction is a complex aspect of an organism’s life strategy. For plants, reproduction is often restricted by energy availability, especially in low light conditions. However, certain types of plants appear to be adapted and thrive in moist low light conditions. Ferns, for example, rely on moisture for much of their reproductive cycle. As a result, they are often found in shaded, wet areas. Due to this moisture dependence, it is possible that ferns have adapted to low light tolerance out of necessity. Alternatively, the need for moisture may limit fern success in high light areas due to increased drying rates. If moisture is the limiting factor then reproduction should be less in moist, shaded areas than in moist, sunny areas. To test these predictions, we determined the ratio of reproductive to sterile fronds in individuals of three fern species located bordering the Carate River near Corcovado National Park, Costa Rica. We found that fern reproductive investment was strongly positively correlated with light availability (r2 = 0.85, p <0.0001) across all three species, and negatively associated with soil fertility (p < 0.05). These results provide evidence that light is the limiting factor in fern reproduction when moisture is abundant and that ferns in shaded areas in this habitat may be facing sub-optimal conditions due to competition.

Explaining biodiversity in freshwater pools in a tropical biodiversity hotspot

Megan Nichols, Zac Novaczyk, Jordyn Prell, and Meg Thompson

Biodiversity is an important indicator for the health of an ecosystem. However, the factors controlling biodiversity in ecosystems are still incompletely understood. The Osa Peninsula in Costa Rica is considered a biological “hot spot”, containing 2.5% of the world’s biodiversity. Understanding threats to this biodiversity is particularly important given the growth of oil palm plantations, cattle ranches, and urban activities in this region. Here, we examined how disturbance affects biodiversity using aquatic invertebrates in waterfall pools of the Carate River. We conducted surveys of the pools to quantify species richness, abundance, and diversity and compared these measures to pool size and dissolved oxygen levels. We found that pool volume was negatively correlated with aquatic invertebrate species richness (r2=0.23, p=0.034) and species diversity (r2=0.40, p=0.003); dissolved oxygen content was also negatively correlated with diversity (r2=0.38, p=0.004). Together, these results indicate that species diversity was highest in shallower pools out of the main flow of the river, perhaps because these pools were subjected to less disturbance than the turbulent main. Similar to human-related effects in the Osa, our results suggest that disturbance in streams may significantly reduce biodiversity.

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