Exploring the relationship between insecticide use and Sympetrum frequens population declines: An integrated causal inference approach

Abstract

Abstract: Distinguishing correlation from causation when evaluating factors threatening wild populations of organisms is crucial for the implementation of effective conservation measures. However, few studies in the field of conservation ecology have applied a causal inference framework to evaluate wild populations. In this study, we applied an integrated causal inference approach to explore the relationship between insecticide use and population decline, using the dragonfly Sympetrum frequens (Selys), which is among the most common dragonflies in Japanese paddy fields, as an example. In the late 1990s, S. frequens populations declined sharply in many regions. The main cause of these declines is suspected to have been the use of systemic insecticides such as the neonicotinoid imidacloprid and the phenylpyrazole fipronil, which were widely used in rice seedling nursery boxes. These insecticides were introduced immediately before the S. frequens population decline began, and subsequent laboratory- and field-based analyses have shown them to be highly toxic to dragonfly nymphs and other invertebrates. However, a causal relationship between insecticides and the decline of S. frequens has not been systematically determined, mainly due to the limited availability of quantitative data on species population size and habitat characteristics over the period of decline. Given these limitations, we applied five different approaches to investigate the relationship between insecticide use and S. frequens populations, as follows. First, we conducted a review of evidence based on currently available information, followed by the application of a statistical causal inference model using available insecticide usage and dragonfly monitoring data. Next, we conducted a field experiment to assess the effects of a novel insecticide on S. frequens and evaluated the effects of climate warming as a potential alternative explanation for the decline of S. frequens. Finally, we performed a mechanism-based evaluation of the effects of each factor using a population model. Our results suggest that the sharp population declines of S. frequens in the late 1990s were caused by the combined effects of highly toxic insecticides and habitat degradation due to its conversion to well-drained paddy fields.