US Long-Term Ecological Research Network
State-structured dynamics of an invertebrate predator
A dramatic increase in Chaoborus abundance following experimental acidification of one basin of Little Rock Lake, WI., provided an opportunity to examine the factors regulating Chaoborus population dynamics in a relatively productive acidified lake. I examined the hypotheses that the observed increase in Chaoborus abundance was driven by a change in 1) food limitation for early instars, 2) fish predation on later instars, and 3) invertebrate predation on early instars. Neither food limitation nor fish predation were important constraints on Chaoborus abundance in Little Rock Lake. Food availability for first instars increased during the acidification; however, available prey exceeded the food requirements of first instars by a large margin. The fish community was relatively insensitive to acidification. Bioenergetics estimates of Chaoborus consumption by rock bass, the most acidsensitive fish species in the lake, were ex1remely small when compared to Chaoborus production. In contrast to the relatively weak effects of food limitation and fish predation, invertebrate predation was an important constraint on Chaoborus abundance. Chaoborus abundance increased dramatically when abundance of the copepod predator Mesocyclops declined to near zero in the treatment basin. Furthermore, upon the recovery of Mesocyclops, Chaoborus abundance returned to pre-acidification levels. In acidified lakes, increases in Chaoborus abundance have frequently been attributed to changes in the fish community. Thus, my finding that an invertebrate predator can create a juvenile bottleneck for Chaoborus represents a departure from conventional wisdom on Chaoborus population dynamics in acidified lakes.
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