PARENT SESSION
Poster Session 33: Trohpic Structure
Thursday, August 11, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Regulation of food-web structure in the Sarracenia microecosystem.

Ellison, Aaron^*,1, Gotelli, Nicholas², Bledzki, Leszek³, Butler, Jessica¹, ¹ Harvard University - Harvard Forest, Petersham, MA² University of Vermont, Burlington, VT³ Mount Holyoke College, South Hadley, MA

ABSTRACT- The carnivorous plant Sarracenia purpurea forms pitchers that fill with rainwater and capture arthropod prey. Prey is broken down within the pitchers by an aquatic food web, which excretes mineralized nutrients that are used for plant growth and reproduction. In New England, an alternative source of nutrients for S. purpurea is rainwater, which contains substantial concentrations of inorganic nutrients (N and P) that are immediately available to the plant. We experimentally tested the hypothesis that the living plant itself mediates the structure of its food web. We also asked whether the effects of the plant on the food web are altered by deposition of inorganic nutrients and the presence of the top predator in the food web. In a controlled field experiment, we severed the connection between the plant and its food web by inserting closed plastic tubes into pitchers and observed assembly of the food web. Experimental controls consisted of pitchers without tubes and pitchers with perforated tubes. We added NH₄NO₃ (8 levels), additional prey (2 levels), and the top predator (Fletcherimyia fletcheri) to the three tube treatments, resulting in an orthogonal four-way design. Food web saturation - an index of the presence or absence of all possible species in the web - was 10-20% lower in isolated tubes than in perforated tubes and unmanipulated controls. There was no interaction between tube treatments and NH₄NO₃ level. However, in the presence of F. fletcheri, increasing NH₄NO₃ led to decreased saturation of food web. In contrast, in the absence of F. fletcheri, increasing NH₄NO₃ led to increased saturation of food web. These results illustrate that pitcher plants are not inert containers, but interact biologically with their associated food web. The experiment also shows that top-down and bottom-up effects interact to control food web structure.

Key words: Sarracenia purpurea, food web, keystone predator, nutrient deposition