Ecosystem Engineering Effects of Native and Non-Native Consumers in a Subtropical Spring-Fed River Ecosystem

Abstract

Ecosystem engineers can directly and indirectly modify the availability of resources, alter or create habitats, affect nutrient cycling, and disrupt energy flows, and modify trophic linkages. The San Marcos River is home to a wide range of both nonnative and native consumers that affect ecosystem function and processes. Invasive armored catfish (Loricariidae: Hypostomus) are of concern due to their high grazing behavior and potential to redistribute large amounts of benthic sediment. Native big claw river shrimp (Macrobrachium carcinus) in the San Marcos River are large omnivorous and predatory consumers who have the potential to influence nutrient recycling and movement of sediments. The first part of my study I conducted a replicated 2 x 2 factoral design stream channel experiment to determine the individual and interactive effect of these two species. The presence of catfish decreased primary producer periphyton, sediment, leaf litter decomposition and catfish also affected nutrient ratios of particulate matter. In contrast, shrimp had little effect on ecosystem processes. The presence of shrimp affected sediment and they exhibited higher mass-specific N excretion rate than catfish. The presence of shrimp seemed to mediate the direct affect of catfish on ecosystem processes. Fish have the ability to affect nutrient availability and primary production through excretion. The second part of my study was to examine excretion rates and ratios of invasive catfish and other abundant fish species from the San Marcos River. Fishes exhibited substantial variation in body nutrient composition (N and P), ranging from 2-8% P and 8-18% N. Fishes also exhibited a large range in mass-specific P and N excretion rates and ratios of N:P excreted. Body P%, Body C:P and N:P significantly differed among the 6 fish families studied. Hypostomus sp. exhibited the highest %P. Body % N and %C, and body N:P and C:P was significant when fish were grouped by species. Again, Hypostomus sp. exhibited among the lowest N:P and C:P. Similarly loricariids exhibited the lowest mass-specific P excretion rates and high N:P excreted nutrients. This is most likely because they sequester P through their bony armored-like plates. Variation in nutrient recycling mediated by phylogenetic constraints on stoichiometry, suggest that species of fish can influence ecosystem-wide recycling.