Water Mite Predation on Planktonic Cladocera: Parallel Curve Analysis of Functional Responses

A water mite, Piona exigua, is the dominant invertebrate predator in some New Zealand lakes. To establish the extent to which this predator has an impact on the populations of its prey species, we have analysed the effect of prey availability on quantitative estimates of predation. We describe Piona functional responses generated under a range of conditions and compare them using 'parallel curve analysis' and a maximum likelihood method of estimating curve parameters. This method overcomes the difficulties of comparing parameters across different combinations of predator and prey type. Rates of predation on a range of cladoceran prey show 'thresholds' of prey size at which rates change abruptly. These thresholds are specific to each size of predator. Females, males and nymphs of Piona exhibit their highest predation rates on Daphnia carinata, Ceriodaphnia and Chydorus, respectively. These rates are relatively insensitive to temperatures above 15°C. Predation rates of adult Piona on Daphnia < 1.5 mm in length were similar to those of fourth instar Chaoborus at the same temperature and prey density.