Structural Variations of Headwater Stream Food Webs Along Geophysical Gradients at the HJ Andrews Experimental Forest

The structure of food webs and how they relate to community stability has been an important debate in ecology. Specifically, predictability of web shape and how it influenced by surrounding landscapes is one of the main goals of such discussions. Headwater streams provide a study template that is of interest to both aquatic and food web ecologists, as headwaters are linked to a myriad of ecosystem services and provide a linear system in which to study the transition of food web structures along natural gradients. This thesis uses network topology from 18 food webs to examine how food webs vary along a headwater stream gradient and how changes in structure relate to geophysical surroundings.

I found that web structure could be strongly associated with both local and landscape scale conditions, including stream slope and basin area. A comparison of structural metrics among the 18 webs show similarities based on basin size and surprisingly were not strongly influenced by geographic proximity. In addition, there were significant structural differences between food webs located in upstream and downstream locations. Importantly, metrics related to stability, such as connectance, omnivory, and average path length, varied systematically from upstream to downstream, eluding to the idea that downstream communities may be more resilient against disturbances. Results from this study can help ecologists understand how structure relates to function and stability of aquatic ecosystems and could aid in forecasting how these communities may change with alterations to surrounding landscapes.