Monday, August 3, 2009

PS 13-111: The impacts of temperature change on species interactions within a duckweed-herbivore food web

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

Interactions between communities of organisms are impacted by a variety of extrinsic variables.  In a food web of two prey under shared predation, ecological theory predicts negative indirect effects between prey populations.  These effects have been shown repeatedly in field and lab studies.  We hypothesize that temperature change can have an impact on these indirect interactions if the prey differ in growth response to temperature.  When temperature favors the growth of one prey over the other we predict the faster growing prey will have a greater negative indirect effect on the other prey, and vice-versa.  Here I examined the influence changes in temperature have on a community composed of phytophagous flies (Lemnaphila scotlandae) and one or two strains of duckweed (Lemna minor).  The experimental populations of duckweed originated in Memphis, TN and Lincoln, NE.  We expected some degree of regional adaptation of duckweed to the local temperature regimes.  To test this assumption we grew both populations under different temperatures and calculated growth rates.  Growing these communities in microcosm under different temperatures, and having both duckweed populations in the presence and absence of the other population, demonstrated the effect temperature change has on species interactions within this system.          

During the summer of 2008 a mesocosm experiment was executed to test the effects of temperature change on a duckweed-herbivore food web.  Species combinations consisted of one or two duckweed resource populations and herbivorous waterlily aphids (Rhopalosiphum nymphaeae).  These were grown in two different temperature regimes; one averaging 24°C and the other 27°C.  Results did not indicate a significant effect of temperature on duckweed growth, or the presence of apparent competition. The duckweed populations also grew at the same rate with and without another strain of duckweed.  However, aphid populations did not survive and duckweed populations were counted only at the beginning and at the end of the experiment.  During the follow up experiment measurements were taken throughout and the duckweed fly replaced the aphid as the herbivore.  Our previous experiments have shown the flies to have a greater negative impact on duckweed growth.  We used a machine that counts duckweed frond surface area and number.  By also taking duckweed biomass we have multiple metrics of resource population size.