2003 Program
(Student name highlighted)
Effects of land use history and environmental factors on fern distribution in the Luquillo Mountains, Puerto Rico
Gina V. Collignon and Jess Zimmerman
Reed College
University of Puerto Rico, Rio Piedras
We studied the effects of environmental factors (soil type, topography, and slope) versus the effects of different land use histories on fern distribution within tropical forests of the Luquillo Mountains, Puerto Rico. Land use histories include clear felling, coffee farming and selective logging (Thompson et al. 2002). Our project served as a direct extension of the Thompson et al. 200 study, which documented the long-lasting and dominant effects of land use history on tree species composition within the same area of Puerto Rico. Unlike trees, the fern community within the LFDP is dominated by two species (Thelypterus deltoidea and Blechnum occidentalis). An ordination analysis used to group fern species distribution showed that land use history and the presence or absence of T. deltoidea and B. occidentalis were the major factors determining fern community similarity. We found that land use history does not have a significant effect on fern density or species richness, but land use history does have a significant effect of fern species composition.
Past land use effects on litter fauna in the Luquillo Experimental Forest
Zach Danek, Xiaodong Yang, and Matthew W. Warren
Pennsylvania State University
University of Puerto Rico, Rio Piedras
This study looks to determine whether the past land use of a section of tropical rain forest in Puerto Rico as a coffee plantation continues to affect the litter fauna communities there today. Plant litter samples were taken along two transects, with half of each transect in old forest, and the other half in the former plantation. Litter samples were sorted to their three horizons: Oi, Oe, and Oa. Arthropods extracted from the litter were sorted out to their orders and counted. Analysis showed significant differences in the number of orders and Shannon Index values of each site, with a greater diversity being supported by the former plantation. Coleoptera larvae, however, were the only group that showed significant differences between the two sites. Total amount of litter at both sites was nearly equal, though the former plantation had much more of the Oa and Oe layers, while the forest had a greater mass of Oi. The differences, though, were not quite significant. This study adds to the building body of evidence showing that anthropogenic disturbance of tropical rain forest continues to affect the land for many years.
Effects of shrimp on mayfly growth by controlling algal resources in Quebrada Prieta, Puerto Rico
Christopher Gilmer, Lournet Martínez-López, and Alonso Ramírez
Kutztown University of Pennsylvania
University of Puerto Rico, Rio Piedras
Atyid shrimps found in the Luquillo Experimental Forest play a critical ecosystem role in the tropical stream environment. Their foraging activities provide services that benefit other benthic macro- and microfauna. This study examined how these activities in Quebrada Prieta, through reduction of sedimentation, make algal food resources available to mayflies, affecting the growth rates of these organisms. Results of growth experiments showed no significant differences between treatments, and no significant relationship to sediment content or levels. Algal resources were not a key factor in determining the nutrient supply and growth of mayflies, as no significant levels of chlorophyll A were measured. Results of this study allowed alternative food resources and feeding strategies of mayflies to be considered. The importance of species conservation was illustrated through the concept of nutrient cycling in the community, as well as the ecosystem.
Effects of shrimp on mayfly abundance by controlling food resources in Quebrada Prieta
Lournet Martínez-López, Christopher Gilmer, and Alonso Ramírez
University of Puerto Rico, Rio Piedras
Kutztown University of Pennsylvania
Research has shown that shrimp affect negatively aquatic insects and algae, fine benthic organic matter and sediment cover. We observed the effects of Atya lanipes and Xiphocaris elongata, on mayfly food resources in Quebrada Prieta. We examined how and why mayfly abundance was affected by the presence or absence of shrimp. We hypothesized that with shrimp exclusion, mayfly abundance will be less than control due to sediment accumulation. It was predicted that in the absence of shrimp, sediment accumulation would prevent mayflies from reaching their main food resources. Second, the shrimp exclusion plus sediment removal will result in intermediate mayfly abundance between exclusion and control. We predicted that the periodic sediment removal would maintain an intermediate availability of mayfly food resources. An electric exclousure technique was used to exclude shrimp from reaching tiles. From each tile we analyzed organic and inorganic matter, chlorophyll a and insects family and abundance over time. There was a significant difference in organic and inorganic matter among treatments. There were not measurable amounts of chlorophyll a over tiles. There was a significant difference on mayfly abundance along days, but not among treatments. We proposed that the lack of differences between treatments on mayfly abundance was related to the small size of the stream or to the short duration of the experiment. In terms of food resources, the lack of significant accumulations of algae over the tiles, suggests that there was not a major reason for mayflies to chose one treatment over the other. We propose that while mayflies prefer clean substrates, they were food limited due to the low amounts of algae in the stream bottom. Therefore, food limitation might be more important than shrimp predation/disturbance on explaining mayfly dynamics.
Distribution of Neritina virginea in streams of Puerto Rico
Dani Newcomb, Juan Felipe Blanco-Liberos, and F. N. Scatena
University of Michigan
University of Puerto Rico, Rio Piedras
University of Pennsylvania
Neritina virginea is a species of migratory snail found in the lower elevations of the rivers of Puerto Rico. Adults lay eggs in freshwater that are later washed down to the estuary. Juveniles begin development in the estuary and then migrate upstream in the rivers. This study looked at both the habitat and microhabitat distributions of N. virginea in the low to mid elevations of the Rio Espiritu Santo and Rio Mameyes in Northeast Puerto Rico. Variation in snail density between riffles and pools was examined. Substrate roughness, mean velocity, and depth were measured at each quadrat. Fifteen quardrats were placed randomly in a pool and riffle on each of the rivers on each of the three sampling days. Snail size and density were also analyzed. It was found that snail densities were higher in riffles, while snail size was higher in pools. Snail density was dependent mainly on density and velocity. At the microhabitat level, snails were found mainly on the sides of rocks, and seldom on the tops of the rocks. A greater number of small and medium sized snails were found in riffles. Predation pressures may influence the distribution of N. virginea, but velocity and competition may also be key factors.
The Effect of Nitrogen Addition on Litter Decomposer Basidiomycete Fungi
Sarah Ward and Jean Lodge
Michigan State University
Forest Products Lab, USDA Forest Service
One of the effects of air pollution is nitrogen addition in ecosystems. In our experiment, we simulated air pollution in a tropical rainforest by regularly fertilizing plots located in the Caribbean National Forest of Puerto Rico. We studied the effect of nitrogen addition on basidiomycete fungi, which are almost exclusively the only fungi that can degrade tough, high-lignin litter. Basidiomycetes grow by sending out fungal attachments to litter and scavenging the litter for nutrients such as carbon and nitrogen, creating mats of litter on the forest floor. This process retains a large amount of the forest’s nutrients and prevents erosion. Our experiment tested the effect of nitrogen addition on the function of basidiomycete fungi. Results showed that fungal mats in fertilized plots grew less over an 18-month period than fungal mats in control plots. The fungi in fertilized plots attached freshly fallen leaves to the litter mat at a slower rate than those in control plots, but only in the higher-elevation set of plots with a relatively high C: N ratio. These results are consistent with previous studies reporting that nitrogen inhibits the production of the delignifying enzymes of basidiomycetes, thus depriving the fungi both of a survival strategy and of a competitive advantage. The results of this study indicate that increased air pollution may lead to greater erosion in tropical forests, and, subsequently, a lower soil nutrient content.
The effects of nitrogen additions on tree growth in the Bisley Experimental Watersheds and the Rio Icacos Basin, Luquillo Experimental Forest, Puerto Rico
Rebecca C. Weatherall and William H. McDowell
University of Hawaii at Hilo
University of New Hampshire
As industrialization and agricultural practices expand in tropical regions, there is growing concern over the impacts associated nitrogen deposition will have on tropical ecosystems. Atmospheric nitrogen deposition rates are expected to grow as much as 200% in the next several years, and there is a lack of research on what effects this might have on forest systems in the tropics. This study addresses the effects increased nitrogen deposition will have on tree growth on an elevational gradient in the Luquillo Experimental Forest (LEF) in Puerto Rico. The data gathered for this study is part of a long-term study designed to look at the overall effects of nitrogen fertilization. The study sites for this research are located in the Bisley Experimental Watersheds (18o18’58”N, 65o44’10”W) and the Rio Icacos Basin (18o16’32”N, 65o47’4”W). The Bisley Experimental Watersheds are a subtropical wet forest system, characterized by Dacryodes excelsa. The area ranges from 250m-300m above sea level, and receives 350cm of rainfall yearly. The Rio Icacos Basin is a high elevation, lower montane rainforest and is characterized by Cyrilla racemiflora. It is located at 620m above sea level, and receives an average of 400cm of rainfall yearly. Treatment in the sites is in the form of ammonium nitrate fertilizer, with a total application of 50kg.ha-1 yearly. Fertilization began in January of 2002 and is performed every six months. It was hypothesized that the two forest types are P-limited, not N-limited, and so the nitrogen additions will not cause increased growth in trees. The findings of this study are consistent with the hypothesis. There was no significant effect of treatment or location on percent growth, change in basal area, or change in woody biomass estimates (p>0.05). While no significant tree growth occurred, studies suggest that additional nitrogen may increase litter production of the trees, and may also be lost to the atmosphere, streams, and groundwater.
Conditioned Allochthonous Inputs and Tropical Freshwater Shrimp Feeding Preferences
Vanessa Welsh and Todd Crowl
Utah State University
Headwater streams with closed canopies rely on organic matter of terrestrial origin (called allochthonous) to provide the energy basis for their food webs. In the tropics, these detrital inputs are constant due to low seasonality, and are thus a continuous energy source for detrital consumers in the stream. Tropical freshwater shrimp, like Xiphocaris elongata, play an important role in the detrital food webs of the Luquillo Experimental Forest stream systems by initially processing organic matter, which makes nutrients available to other consumers downstream. Microbial colonization, or conditioning, is known to make leaf litter more nutritional and palatable to stream insects, but few studies have been done to assess whether or not tropical freshwater shrimp have a feeding preference for conditioned leaves over non-conditioned leaves. This study first analyzed the source, quantity and species composition of allochthonous inputs into the Quebrada Prieta, a tropical headwater stream, to quantify what freshwater shrimp diets with particular conditioning types were composed of using over stream, in stream and bank runoff traps. In terms of biomass, the greatest amount of allochthonous inputs entering the study sites came through in stream transport, which suggests that the majority of leaves in the sites available for consumption have been subject to aquatic conditioning. All of the leaves found in each trap were identified to species and differences in species composition for each of the three trap types were found. The four most abundant species overall were subjected to controlled conditioning for use in the laboratory experiment. In a laboratory experiment, the feeding preferences of Xiphocaris elongata for different types of conditioned leaf litter were analyzed. No significant trends were found in feeding preferences as the conditioning types of each leaf species were all consumed at close to the same rate. This lack of trend suggests that Xiphocaris elongata have no feeding preference for conditioned leaves over non-conditioned leaves.