Graduate Thesis Or Dissertation
 

Direct effects and tradeoffs affect vegetative growth and sexual reproduction in an invasive seagrass experiencing different disturbance regimes

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  • Disturbance has both direct and indirect ramifications that can influence species abundance, distribution, and ultimately the diversity found within a community. As a result, we might expect disturbance to play a particularly important role in the ability of non-native species to proliferate outside of their native range. In practice, disturbance has been shown to facilitate invasions indirectly by reducing competition with established species, but less is known about the direct effects of disturbance on potential invaders. It is generally assumed that the direct effects of disturbance will be negative, but an exemption to this exists if disturbance results in tradeoffs that favor the allocation of resources to sexual reproduction. To examine this possibility, I considered the direct effects of disturbance and the relative importance of tradeoffs on the invasion dynamics of the dwarf eelgrass, Zostera japonica Aschers. & Graebn., across different sediment disturbance regimes in Yaquina Bay, a North American Pacific coast estuary near Newport, Oregon. The basis of my thesis work is a conceptual model in which tradeoffs in resource allocation between reproduction and vegetative growth in plants results in increased sexual reproduction at intermediate levels of disturbance. My hypothesis is that as disturbance severity increases, vegetative biomass declines until there is complete mortality of the population. Moreover, I suggest that flowering biomass will increase with disturbance to some intermediate level but then decline as disturbance severity increases. This increase in flowering biomass is the result of a tradeoff in which individuals reallocate resources from asexual to sexual reproduction as disturbance increases the mortality risk and reduces the fitness of clonal progeny. To test this model, I conducted two years of monthly monitoring at six sites representing the potential for different sediment disturbance regimes, and also conducted a field experiment at three sites representative of low, moderate, and high sediment disturbance. The observational study showed sediment disturbance was strongly site dependent, with three sites having high sedimentation, one site having moderate sedimentation, and two sites having low sedimentation. In the observational and experimental study, there was a negative linear relationship between the abundance (percent cover and biomass) of Z. japonica and sedimentation among the sites. Vegetative growth showed a negative linear response to increased sedimentation, with approximately 50% mortality seen at sediment deposition rates of 1 cm per month, and nearly 100% mortality seen at rates above 3.5 cm per month. The experiment also showed that flowering and seed production responded unimodally to increased sedimentation, and they were greatest at sediment deposition rates of roughly 0.75 cm per month, and effectively inhibited at rates above 1.5 cm per month. My results suggest that the unimodal response of this non-native eelgrass to disturbance is the result of tradeoffs in resource allocation favoring sexual reproduction and the production of seeds, which is potentially an escape from increasing disturbance severity. This type of tradeoff might result in an increased likelihood that Z. japonica could reach new sites, and, as has been shown in other invasive grasses, could possibly result in an increase in genetic diversity that would allow this species to successfully proliferate across a wider range of environmental conditions. As disturbance regimes change due to anthropogenic causes, it is important to document these tradeoffs and determine whether they could contribute to making communities more susceptible to invasion and thus of greater management concern.
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