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Abstract
In the tropical headwater streams of Puerto Rico, most ecosystem processes are linked to the consumer assemblages dominated by freshwater shrimps. Shrimp assemblages are known to regulate organic matter processing, nutrient cycling, biomass and production of benthic algae, and benthic organic matter levels. This strong influence on ecosystem-level processes and properties is a result of their diverse feeding mechanisms and high abundance. The effects of shrimps on ecosystem properties and processes at the reach scale are well known through multiple studies using small-scale experimental manipulations to control their presence and abundance. However, their natural distribution and abundance can be determined by geomorphological and anthropogenic features within the watershed, and data describing their ecosystem-level role at larger spatial and temporal scales are scarce. For the first part of my dissertation, I looked at long-term and large spatial scale patterns of shrimp top-down control on benthic resources in two streams with contrasting shrimp densities. My results confirm that shrimp density is a primary factor regulating (a)benthic algae, (b) benthic organic matter, and (c)inorganic sediment in high elevation streams. Secondly, I looked at the landscape scale effects of shrimp extirpations across the island and how this affect the ecosystem processes of decomposition and nutrient cycling. I used reach-scale manipulations and landscape-scale experiments to measure these processes and contrast high-elevation streams located above large dams where shrimps have been completely extirpated and streams without a downstream dam where shrimps are found in high densities and their top-down control remains unaltered. Decomposition rate was significantly higher in all streams where shrimps were present. However, there was no significant difference in nitrogen cycling between streams because of shrimp extirpations. Phosphorus uptake was significantly higher in streams above large dams where shrimps are absent. I was able to show the large spatial and temporal extent of the importance of shrimp assemblages to the function of stream ecosystems in Puerto Rico. This work is a valuable contribution to future management and conservation efforts to restore shrimp populations and ecosystem integrity across the island.