Patch-scale effects of an invasive ecosystem engineer on the structure and function of a eutrophic stream.

摘要

Recent theoretical and technological advances in ecosystem science have dramatically expanded the ways in which scientists can pursue and explore ecological questions. For my thesis research, I integrated the recent theoretical concept of organisms as ecosystem engineers with the relatively recent development of stable isotope tracer tests to ask the question: how does the invasive common carp affect stream ecosystem structure and function? To investigate the structuring role of carp, I measured autotroph seasonal distribution and abundance and macroinvertebrate seasonal abundance and diversity within two stream reaches in Spring Creek, Utah, USA; one with low carp biomass (LCB) and one with high carp biomass (HCB). I installed a series of carp exclosures in the HCB reach to examine the response of the stream to carp exclusion. To explore the effects of carp on stream nitrogen dynamics, I performed a three-week, continuous injection of 15N as ammonium chloride.;The macrophyte and macroinvertebrate community was severely depauperate in the HCB reach compared to the LCB reach. The observed rapid colonization of a relatively abundant and diverse macrophyte and macroinvertebrate community at the carp exclusion sites in the HCB reach not only indicates that carp engineering reduces the abundance and diversity of these communities, but also highlights the importance of the spatial distribution of engineered and non-engineered patches in dictating the temporal scale of re-colonization. Carp engineering had a simplifying effect on stream N dynamics that ultimately limited the uptake and retention capacity of the HCB reach. For example, macrophytes played a dominant role in the N dynamics of the LCB reach by directly assimilating NH4, retaining N rich FBOM, and by providing habitat necessary to support an abundant and relatively diverse macroinvertebrate community that facilitated greater trophic transfer of nitrogen. Conversely, carp reduction of macrophytes in the HCB reach resulted in an overall reduction in areal uptake rates of NH4, reduced trophic transfer of N, and significantly reduced N retention. These results clearly indicate that carp engineering reduces macrophyte and macroinvertebrate abundance and diversity in streams and that N dynamics are simplified in carp engineered patches.