The Turnover Ratio in Production Ecology of Freshwater Invertebrates

The turnover ratio of freshwater benthic invertebrates, expressed as the ratio of a cohort's production to the mean standing crop, has been observed to be relatively constant, about 2.5 to 5, with a mode of about 3.5. Turnover ratios were computed from Allen growth-survivorship curves under various combinations of hypothetical conditions to determine the theoretical range. The effects of varying curve shape, initial individual weight relative to maximum, final population size in numbers relative to initial population, and growth pattern were tested with several series of Allen curves. With moderate variation in these factors around the most probable conditions, the theoretical turnover ratio varied from about 3 to 4 for aquatic insects, but it is probable somewhat larger for crustaceans. Turnover ratios were also considered as equal to instantaneous growth rates computed over an entire single life cycle for several invertebrate species. These were similar to those obtained with the Allen curves, although about 1 unit larger on the average.