Population model analyses of the combined effects of insecticide use and habitat degradation on the past sharp declines of the dragonfly Sympetrum frequens

In recent decades, many dragonfly species have become threatened with extinction. For example, populations of Sympetrum frequens, one of the most common dragonflies in rice paddy fields in Japan, decreased sharply around the late 1990s in many regions. Although previous studies suggested that the use of systemic insecticides (particularly fipronil) was likely a major cause of the decline, agronomic factors other than insecticide use and the combined effects of both have been not examined. Here, we developed an S. frequens population model using survival rate parameters associated with the farmland consolidation rate, midsummer drainage, area of crop rotation and abandoned rice paddies, insecticide use, and summer temperature and analyzed the effects of each factor on population dynamics by numerical simulations. Our population models substantially reproduced the past sharp population declines of the dragonfly in three regions. Numerical simulations using hypothetical parameters did not always suggest that the use of systemic insecticides is a sufficient cause of the sharp population declines, as the declines did not occur if the farmland consolidation rate remained at lower levels (before the 1980s). On the other hand, our findings suggest that the use of insecticides with high toxicity is a necessary cause of the sharp population declines, as the declines did not occur when simulated toxicity levels were lower than those actually used. Overall, our numerical simulations suggest that the sharp population declines of S. frequens around the late 1990s were caused by the combined effects of insecticide use and farmland consolidation, in which rice paddies are converted to well-drained paddy fields. Conservation planning for dragonflies needs to account for the combined effects of habitat degradation and use of insecticides. [Display omitted] •Populations of Sympetrum frequens in rice fields declined sharply in the late 1990s.•We analyzed causes of the declines by numerical simulations using a population model.•The model reproduced well the past sharp population declines.•Habitat degradation and highly toxic insecticides greatly affected population decline.•Our results suggested the sharp declines were caused by the combined effects of both.