Thursday, August 5, 2010 - 1:50 PM

COS 106-2: Time, temperature, and species interactions within a duckweed-herbivore diamond food web

Ian T. Waterman, University of Nebraska-Lincoln and Chad E. Brassil, University of Nebraska.

Background/Question/Methods
In a food web of two prey under shared predation, ecological theory predicts negative indirect effects between prey populations. Compared to a single prey system, the presence of another prey item numerically increases the predator population which then has a greater negative impact on the other prey; an indirect interaction between prey species termed apparent competition. These effects have been shown repeatedly in field and lab studies. In addition, interactions between communities of organisms are impacted by a variety of extrinsic variables, such as temperature. The question remains how direct and indirect interactions within a diamond food-web change when a temperature regime favors the growth of one prey item over the other. In a single predator/two prey food-web favoring a higher growth rate of one prey over the other, the more fecund prey is predicted to produce a greater negative indirect effect on the other prey.            

Results/Conclusions
I examined these questions in an artificial aquatic community composed of waterlily aphids (Rhopalosiphum nymphaeae) and one or two strains of duckweed, Lemna minor and Spirodela polyrrhiza. These experimental populations of duckweed vary in their response to temperature. At 20°C the two species do not differ in growth rate, but at 29°C S. polyrrhiza produces more fronds per unit time than L. minor.  Our first experiment showed a significant positive indirect effect on both prey species after a single generation in temperatures averaging 20°C, but no indirect effect on either species at average temperatures of 29°C. That is, prey grew faster under predation at lower temperatures when another prey species is present than alone under predation at the same temperature. A second experiment of the same design was run to contrast single generation dynamics with multi-generational dynamics. Our prediction was that short term positive indirect effects would become negative over time, as aphids respond numerically to greater amounts of duckweed. Results indicate reciprocal negative indirect effects between duckweed populations at the lower temperature. However, at the higher temperature, S. polyrrhiza had a greater negative indirect effect on L. minor than L. minor had on S. polyrrhiza