Posted by: Adam Kay | January 31, 2014

From plant invasions to army ant defense: J-term research near San Vito, Costa Rica

Las Cruces Biological Station was the second stop for the J-term Biology course in Costa Rica. Las Cruces is a mid-elevation site that receives about 4m of rain a year. The station is connected to a small (~300 hectare) primary forest fragment. It also has the Wilson Botanical Garden – the most famous botanical garden in Central America – filled with rare and endangered plants from the New and Old World tropics. We had about a week at Las Cruces. We asked our students to come up with a project idea and then execute the research during that time. It’s impressive that they were able to do high quality work during such a short period of time. Here are their abstracts (along with some pictures and videos):

Does release from herbivory facilitate plant invasions? A test in a tropical rainforest

Jorgen Kvaal, Ryan Merry, Morgan Reeve

invasive plantsInvasive species are major threats to biodiversity in many ecosystems, including tropical rainforests.  One mechanism that could allow exotic species to invade new ecosystems is if these species face less predation and parasitism in novel areas than in their native habitats.  At Las Cruces Biological Research Station in San Vito Costa Rica, three non-native plant species have escaped from the station’s Wilson Botanical Garden into the native forest and are visibly encroaching on native species.  Here, we tested whether parasitism reduction may allow these non-native species to outcompete native inhabitants.  Specifically, we hypothesized that herbivory and epiphyte loads (=parasitism) would be reduced on non-native than on similar native species, and this reduction in parasitism would lead to higher growth rates.  We paired each of three non-native species with a native species that was present where the non-native species was found, was being pushed to the periphery of the available habitat, and had a growth form similar to that of the paired non-native species. We assessed plants for herbivory and epiphyte cover, and assayed photosynthetic levels and leaf nitrate content (a rough indicator of leaf quality).  We found that parasitism was ~9x greater (main effect of native/invasive status: F1,24 = 12.96, P=0.001), photosynthetic levels were significantly lower (F1,17=20.941, p=0.0003) and nitrate content was significantly lower (F=6.778, p=0.0005) for native plants than for non-native plants. These results suggest that reductions in parasitism result in greater allocation of resources to growth in non-native plants which could in turn allow for a competitive advantage versus native counterparts.

Stayin’ Alive: Predation responses in highly abundant and vulnerable organisms

Jake Anderson, Matt Boehm, Sam Carpenter, Meg Thompson

047Nearly every organism is under extreme selective pressure from predation, and therefore develops specific traits and techniques for defense. Tadpoles in tropical streams are typically under heavy pressure from a variety of aquatic predators. To explain the frogs’ success in the ecosystem despite this vulnerability, we studied the behavioral responses of tadpoles to threat from a common predator, a dragonfly naiad. Previous studies have shown consistent responses to predation threat in a variety of tadpole species. Common responses include reduced activity levels and increased time spent in shelter in the face of predation risk. Here, we tested whether tadpoles actively seek shelter when it’s available and alter activity level in the presence of a predator or the presence of tadpole carnage (a common indicator of predation risk). Unexpectedly, we found that tadpole behavior did not differ significantly when given access to shelter (F1,18=1.528, p=0.231), when directly exposed to predators (F1,18=0.730,p=0.403) nor when exposed to water containing tadpole carnage (F1,18=0.929,p=0.347). We then tested predator effects on tadpole distributions using containers divided with a barrier through which tadpoles but not naiads could travel. With this design, we showed that neither naiad nor carnage presence affected tadpole distribution (X2 = 0.529, df = 1, P = 0.467). These results suggest strongly that tadpoles in this system do not respond behaviorally to predation risk. Although our results could be due to the artificial nature of the simulation methods we used, our methods were consistent with previous studies in other tadpole systems that found anti-predator responses. We suggest that tadpoles in this system may focus exclusively on rapid growth and large population sizes to avoid predation. Further study could also explore neurological constraints on behavioral sophistication.

Bushwhack to the Bivouac

Theo Larson, Megan Nichols, Zac Novaczyk

Social organisms often use division of labor and intricate communication among group members in defense against predators and parasites. The Neotropical army ant, Eciton burchelli, lives in colonies of >1 million workers that vary significantly in size and behavior. Colonies regularly travel across the forest to hunt invertebrates (video), and these raids are known to have coordinated responses in the face of danger (video). When threatened, each E. burchelli worker can release an alarm pheromone (4-methyl-3-haptanone) that induces an aggressive response from nearby colony members. Here, we tested whether the response to alarm pheromone differs among worker size classes (small, large, soldier), and whether pheromones released from each worker size class induces the same aggressive response from colony members. We released alarm pheromone by crushing ant heads of each size class on a target area 5cm from the foraging column. For one minute, we counted the total number and size-class distribution of ants that entered the target area and compared those counts to two controls: 1) the number and size distribution of ants along the foraging column in the absence of pheromone release, and 2) the number and size distribution of ants attracted to an uncrushed ant head near the foraging column. As predicted, we found that the percentage of responding ants that were soldiers (the largest ant caste) was significantly higher in the target area than along the foraging column (F1,60 = 56.422, P < 0.0001). We found that many more ants responded to the crushed head treatment than to the uncrushed control (F1,30 = 71.672, P < 0.0001). However, surprisingly, the percentage of ants that were soldiers did not differ in areas where heads were crushed vs areas with uncrushed heads (F1,60 = 1.8135, P = 0.1719), suggesting that alarm pheromone was not the cue that induced a specific soldier response. We also found that significantly more ants responded when small ant worker heads were crushed than when an equal mass of large ant or soldier heads were crushed (F1,30=71.672, P < 0.0001). Our results suggest that E. burchelli have nuanced alarm threat responses that have not been previously documented.

Evolutionary trade-offs of web capture strategies in tropical spiders

Kate Hanson, Jordyn Prell, Erik Sathe, Taylor Schuweiler

Predators often face a trade-off between capturing prey and remaining protected. How this trade-off has shaped the evolution of predation strategy remains an unanswered question in many biological systems. Spiders in mid-elevation habitat in southern Costa Rica generally employ either a snare (i.e., web-building) or ambush (non-web-building) strategy. In a survey conducted in the Wilson Botanical Garden near San Vito, Costa Rica, we found that snare spiders were more likely to be brightly colored than were ambush spiders (X2 = 8.289, P < 0.004). We also found that spiders active during the day were more likely to be brightly colored than spiders active at night (X2 = 13.115, P = 0.0003). These results suggest that bright coloration is particularly advantageous for diurnally active snare spiders. We then tested two potential advantages of bright coloration: coloration could serve as a warning to potential predators (aposematism) or it could attract and confuse potential prey (mimicry). To test these hypotheses, we did a simulated predatory attack and web disturbance tests. We found that spider response to a predator differed among species (X2 = 7.896, P = 0.0193) and within species, size plays a significant role in determining the spider’s response, with larger spiders tending to respond more frequently (X2 = 4.5826, P = 0.0323). We also found that response to web disturbance did not vary with spider size (r2 = 0.7605, P = 0.2321) and did not differ among species (F2,20 = 0.707, P = 0.507). In the course of sampling, we noticed that snare spider web density decreased substantially after rainfalls. In response, we opportunistically conducted an additional web disturbance test, rain simulation, and found varied spider response, including web consumption and water collection and discard (see amazing video here). From these preliminary results, we conclude that coloration benefits may depend on spider species and body size, and that brightly colored, snare predators may invest significant energy in keeping webs free of water that increases web visibility.

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