Ecosystem Engineers: Trophic impacts from tropical headwaters to temperate tideflats.

摘要

Ecosystem engineers are organisms that modify or create habitat. While historically the definition of ecosystem engineers has avoided inclusion of trophic relationships (Berke 2010), because of their broad impacts on habitat it may be expected that the presence of an ecosystem engineer might have effects on food web structure and function. This thesis focuses on the trophic impacts of ecosystem engineers and is an outgrowth of the author's fascination with food production by the most disruptive of ecosystem engineers: Homo sapiens. The ultimate question of this research is to understand the ecological consequences of consumption by certain ecosystem engineers in both time and space. In this work, I focus on the pacific oyster ( Crassostrea gigas) and a freshwater shrimp (Macrobrachium americanum). They are of interest not only for their capacity to dramatically impact the systems that they interact with but also because they themselves comprise an important part of our foodshed.;Chapter one focuses on the question of time in oyster consumption: can the impacts of oyster filtration be observed over the movement of one tide? We calculated filtration rates by measuring chlorophyll depletion as water moved over tideflats with aquacultured oysters. The major finding from this work is that published rates of oyster filtration gathered from tank studies when scaled to the size of an oyster bed overestimate the filtration effects of oysters in the field. We found filtration rates of oysters at aquaculture densities to be between 0.35 L g-1 h-1 (shucked dry weight) for on-bottom aquaculture and 0.73 L g-1 h -1 for longline culture.;Using these filtration rates we undertook to answer a question about the scale of ecosystem effects that pacific oysters (C. gigas) in ground culture can impact. Oysters grown shoreward on the tideflat exhibited significantly lower condition than oysters grown channelward on the tideflats even when standardized for immersion time. Our initial hypothesis that this was due to the impact of food depletion caused by oysters channel ward was incomplete to account for the dramatic depletion of chlorophyll (30-40 %) that we measured at shoreward locations. We found that oysters were influencing chlorophyll concentrations over multiple tides due to long water residence time ∼4 tides) on the tideflats.;In chapter 3 the study location shifts from the temperate tideflats to a tropical headwater stream to explore the distribution of an important ecosystem engineer across a gradient of elevations. Freshwater shrimp are both megaomnivores (an organism with broad influences on a system through feeding behavior), but are also ecosystem engineers (Pringle et al. 1993). The freshwater shrimp in question (M. americanum) is an important protein source for many local residents and although residents continue to hunt the shrimp many have anecdotally reported that they have witnessed a decline in its abundance. We found that river shrimp of large size classes are equally abundant across the elevations surveyed, however, the available food resources shift significantly as one moves from lower elevation to higher. In addition, and contrary to literature, we found that river shrimp at the highest elevations are more active during the day than at night. We present preliminary data that suggest this shift in activity is due to a shift in food resources that occurs as one travels upriver.