Experimental investigation of top-down effects on the food web of a fish-less temporary freshwater pond

A.K. Magnusson and D.D. Williams. Division of Life Science, University of Toronto at Scarborough, Scarborough, Ontario, Canada M1C 1A4

We tested whether invertebrate predators have the potential to influence community structure in a habitat in which abiotic factors prevail. The study was carried out in a small, temporary pond (Ontario, Canada), in which beetle and dragonfly larvae were the main predators. An experiment was set up using large metal enclosures (2.5 m diameter). In each of two enclosures, a total of 30 beetle and 120 dragonfly larvae was added during nine of the 13 weeks the pond held water. In two other enclosures, aerially colonizing insect predators were prevented from entering by means of a fine mesh screen. Two enclosures served as controls. Most measured water chemistry parameters did not differ among treatments, but phytoplankton abundance rapidly doubled in the predator-addition enclosures. Preliminary data indicate that predator-additions resulted in a 70% lower macroinvertebrate density compared with the controls, especially affecting bivalve (−86%) and chironomid (−82%) abundance. Total invertebrate abundance was lower also in the enclosures in which aerial colonization was repressed, primarily due to 62% fewer beetles and 97% fewer chironomid larvae. Although, further analysis is needed to understand the underlying mechanisms behind these findings, the preliminary data suggest a potentially important top-down effect influencing temporary pond community structure.