Posted by: Adam Kay | January 18, 2016

Ecological research in Rara Avis, Costa Rica

Our January-term course, BIOL 211: Introduction to Field Ecology, focuses on independent projects. Our students are sophomores, juniors, and seniors. Only a few have taken an advanced ecology course. Students did everything for their projects – they came up with the idea, developed the design, executed it, and presented their results. They had ~5 days to work on projects from start to finish.

Our first site, Rara Avis Nature Reserve, is a mid-elevation site on the Atlantic slope of the Cordillero Central in Costa Rica. The site receives ~8.3m of rain each year. The 357-hectare reserve is mostly secondary forest, but it borders a large national park (Braulio Carillo).

Here are the students’ abstracts:

A Tad too Close for Comfort

Jessica Brown, Madison Gonsior, Sara Osborne, Quinn Whiting

Tadpoles face predation from many organisms, including birds, crabs, frogs, and other tadpoles. As a result, tadpoles may often sacrifice foraging opportunities to stay safe. However, specific strategies used to avoid predation may differ among habitats with different predation risks, inducing different tradeoff priorities. In this study, we compared predation avoidance behaviors among tadpoles at Rara Avis, a mid-elevation tropical rainforest.  We tested behaviors in tadpoles from two habitats, the Attelobus river (hereafter = “river tadpoles”) and smaller streams in the forest (“pond tadpoles”). River tadpoles inhabit exposed pools with rocky substrate; pond tadpoles inhabit pools under closed canopy with muddy substrate that is easily disturbed. We used a series of experiments to test whether river vs. pond tadpoles differed in (1) hiding behavior in a controlled, undisturbed environment, (2) hiding behavior in a controlled, disturbed environment, (3) reaction distance (how close a simulated predator can get to a tadpole before the tadpole flees) in natural habitats, and (4) escape responses in natural habitats. We found that, in a controlled environments, river tadpoles hid under rocks more often than pond tadpoles in both undisturbed (t=3.879, DF=9.471, p=0.003) and disturbed (X2=16.576, DF=1, p<0.001) conditions. In natural environments, reaction distance for river tadpoles was 3 times less for river tadpoles than for pond tadpoles (t=3.272, DF=17.99, p=0.002), but river tadpoles traveled farther than did pond tadpoles once they were disturbed by simulated predator attack (F1=6.278, p=0.017). In contrast, pond tadpoles in the natural environment were more likely than river tadpoles to hide (X2=15.268, DF=1, p<0.001). We speculate that sediment, which only exists in pond habitats, is a key driver of these behavioral differences. Sediment could signal predation risk for pond tadpoles that would allow for greater reaction distance. Sediment could also provide hiding locations for pond tadpoles (but not river tadpoles), reducing their need for longer escape distances. Regardless of the mechanism, our results suggest that environments select for different anti-predator strategies in this system.

Bat sh*t crazy

McKenna Reid, Jenny Walz, Ethan Ridgewell

Animals often damage plants to create nest sites. Although this damage suggests that this type of interaction is parasitic (i.e., one party benefits at the expense of the other), it is possible that animals provide other benefits to plants such that the relationship is mutualistic (i.e., both parties benefit). One way that nest animals could benefit plants is by providing excreting waste products that provide concentrated nutrients for plants to use. Here, we studied an ideal system for exploring a possible mutualism between a nest-making animal Uroderma bilobatum (the Common Tent Bat) and its host plant (a common palm, Geonoma sp.). U. bilobatum form tents by making incisions in the leaves of Geonoma palms. We compared palm height, number of leaves, distance from water, the extent to which the plant is surrounded by other vegetation (protected), and chlorophyll levels between palms with bat tents and palms without bat tents. We found that leaves with bat tents had similar chlorophyll levels as leaves on the same plant without bat tents (t=0.284, df=7, p=0.784); this result suggests that bat tents have little effect on plant photosynthetic capacity. A few results suggest that bat presence is associated with higher quality Geonoma palms. First, leaf chlorophyll in undamaged leaves on bat-associated plants had almost significantly higher leaf chlorophyll levels than did plants without bat tents (t=1.929, df=12.1, p=0.078). Second, a multiple logistic regression model including plant height, number of leaves, distance from water, and protection showed that bats were associated with significantly taller plants (X2 = 7.796, df = 1, p = 0.005). We also found a significant interaction between plant height and number of leaves (X2 = 5.084, df = 1, p = 0.024) – bats were only associated with shorter plants if those plants had an exceptionally large number of leaves. There are two interpretations for these results. First, the “bats as parasites” explanation suggests that bats select higher quality plants to provide them with better protection. Alternatively, the “bats as mutualists” interpretation suggests that bat presence increases the quality of plants. The presence of significant amounts of nutrient-rich bat guano under bat tents suggest that the “bats as mutualists” explanation warrants further study.

Studying Mammalian Pack Behavior in Coatis

Brennan Arendt, Evan Keil, Emma Squires-Sperling

Within mammalian packs, individuals of different ages often have distinct role. For example, in whale pods, the eldest female plays the role of the matriarch; teaching younger individuals how to survive. Coatis, a member of the raccoon family, are found throughout Costa Rica and travel in packs. Although packs consist of individuals differing in age and size, there is still much to learn about how these factors affect behavior of animals living in pristine environments. We studied age-related behavioral differences in coatis in Rara Avis Nature Preserve, a secluded mid-elevation location bordering Braulio Carrillo National Park in central Costa Rica. Our test group was a pack of coatis that regularly fed at a compost site close to the dining center at Rara Avis. We predicted that older (larger) individuals would be more vigilant compared to younger coatis because they provided parental care. Younger coatis were predicted to be more adventurous than adult coatis because they were less sensitive to predation risk. Furthermore, we predicted older individuals would feed less, walk more and be more alert compared to younger coatis and that tail positions would not differ between age groups. These hypotheses were tested by baiting and observing the coatis at the compost site and then taking pictures of individuals at 1-min intervals. We found that adults showed more vigilance (proportion time spent with head raised) than younger coatis; the difference in vigilance was nearly statistically significant (df = 22, p = 0.051). In contrast, tail positioning did not differ significantly between age groups (p = 0.634). Although preliminary, our results suggest that coatis in remote locations show division of labor within packs based on individual age.

The ecological niche of an endemic species

Zach George, Nick Hable, Katie Hoffman, Zach Mader

Endemism, when species are only present in a specific geographic location, is relatively common for species in biodiversity hotspots like the rainforests of Costa Rica. Understanding the specific traits of endemic species is particularly important for conservation, especially given climate changes occurring in many areas. In this study, we studied the niche breadth (range of conditions in which a species is found) of the stained-glass palms (Geonoma epetiolata), a species endemic to mid-elevation rainforest habitat near Rara Avis Nature Reserve in central Costa Rica. We compared plant abundance, soil moisture, and canopy cover above plants for both stained-glass palms and fishtail palms (Geonoma spp.), a close relative to Geonoma epetiolata. We predicted that stained-glass palms would be more abundant than fish tail palms because they were specifically adapted to this location. We also predicted that stained-glass palms would occur in narrower soil moisture and canopy openness ranges than fish tail palms because of their specific requirements associated with endemism. We found that fish tail palms were substantially more abundant than stained-glass palms, indicating that the endemic species did not have a competitive advantage in this system. We also that fish tail palms had a 45% wider range of soil moisture levels and were more likely to be found across a range of canopy covers than stained-glass palms. Our results suggest that stained-glass palms are extremely vulnerable to local extirpation because of their limited abundance and narrow niche. Given that the population at Rara Avis is one of the few remaining populations in Costa Rica, our results suggest this species should receive additional conservation attention.


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