Hydrocotyle Dissertation


Biological invaders are widespread and can alter population dynamics and community structure of native ecosystems. Substantial habitat alteration by nonindigenous species can additionally affect the surrounding community. Aquatic plants are particularly invasive, especially in areas that are modified by humans. Water hyacinth (Eichhornia crassipes) is a floating aquatic plant that is non-indigenous to the Sacramento/San Joaquin Delta, California. A common native that functionally occupies similar habitats as hyacinth is pennywort (Hydrocotyle umbellata). Based on the utilization of such habitats by invertebrates and fish, my main scientific question was: Has hyacinth modified the invertebrate assemblage structure and fish-invertebrate food web as compared to pennywort? To assess this, I sampled invertebrates in hyacinth and pennywort and analyzed fish diets in the surrounding area at three sites in the Delta during 1998 and 1999. I also took measurements of leaf density, root structure, dissolved oxygen and temperature. Ecological differences between hyacinth and pennywort were linked to habitat architecture. Coupled with the management challenges of hyacinth, its ecological modifications make it an even more influential invader.



This study examined the benefits associated with resource sharing among interconnected ramets spanning a soil salinity gradient. Clones of Hydrocotyle bonariensis, a rhizomatous dune perennial, expand into salt marsh communities from surrounding upland dune systems in coastal North Carolina. In rhizome-severing experiments conducted under both field and laboratory conditions, Hydrocotyle was shown to proliferate ramets under saline conditions, provided that these ramets were connected to other ramets growing in nonsaline conditions. Ramets that benefited from resource integration did not appear to be affected by local salt exposure in that these ramets were morphologically similar to those grown under nonsaline conditions. Supporting ramets incurred no net cost in terms of biomass or ramet production, but there was an increased percent allocation to roots and rhizomes. Ramets grown in saline conditions without the benefit of clonal integration showed high mortality and produced little or no net clonal growth. It is likely that the acropetal movement of water allowed Hydrocotyle clones to ameliorate the heterogeneous saline conditions associated with coastal environments.


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