July 9, 2014

decline in population of some birds and a neonicotinoid pesticide

Nature published a study reporting a correlation between declines in farmland bird populations in the Netherlands and use of the neonicotinoid pesticide imidacloprid. Roundup comments accompanied this before the headlines analysis.

Title, Date of Publication & Journal

Declines in insectivorous birds are associated with high neonicotinoid concentrations

Nature, 9 July 2014

Claim supported by evidence?

The paper supports the claim that use of neonicotinoid pesticides is associated with declines in the population of some bird species

Summary

• The study used standardised data on bird populations collected over a long period of time
• A limitation is results are from a single country (The Netherlands)
• As with all observational data, correlation cannot prove causation (though here, a causal relationship seems likely)

Study Conclusions

The study found that areas of The Netherlands with high concentrations of imidacloprid in surface water high greater rates of decline in a variety of bird species than areas with low concentrations of imadacloprid.

The association was found in appropriate statistical analyses and was highly significant overall (P < 0.0001). In areas with high imadacloprid concentrations, bird populations declined by 3.5% on average annually.

In addition to the significant overall effect, statistically significant inverse relationships between imidacloprid and population growth in individual species were found in 6 of the 15 species studied (after appropriate statistical adjustment for multiple comparisons). None of the species showed the opposite effect.

The authors suggest that the relationship between neonicotinoid concentrations and bird decline is causal. Although observational data like this can never prove causality, the claim of causality here seems plausible. This is partly because of the careful way the authors investigated alternative explanations for the relationship (see below), and partly because the results are exactly what we would expect from biological plausibility.

There is good evidence that use of neonicotinoids harms insect populations (see references below). The birds observed in this study are insectivores, and so it is to be expected that if insect populations decline, then insectivorous bird populations would decline as a result.

It is also possible that imidacloprid may have direct toxic effects on birds, though the authors suggest that the loss of food source when insect populations decline is a more likely explanation for the effects seen in birds. However, their study does not provide any data to distinguish between those possibilities.

Strengths/Limitations

Data on bird populations were collected in a standardised way over a long period of time in The Netherlands, so the study had a robust dataset to track bird populations.

The authors were careful to exclude alternative explanations for the association between neonicotinoids and bird population decline. They found no such relationship between location and population decline in a time period before imidacloprid was used, thus ruling out a long-term trend. They also adjusted for land use in a multivariate analysis.

A limitation is that the research is from a single country, so we cannot be sure that the results would apply outside The Netherlands. However, it seems reasonable to believe that countries with similar insect and bird populations would behave similarly.

References

There is abundant literature on the harmful effects of neonicotinoids on beneficial insects. A small selection is provided below:

Krupke CH, Hunt GJ, Eitzer BD, Andino G, Given K (2012) Multiple Routes of Pesticide Exposure for Honey Bees Living Near Agricultural Fields. PLoS ONE 7(1): e29268. doi:10.1371/journal.pone.0029268
Cresswell, James E. “A meta-analysis of experiments testing the effects of a neonicotinoid insecticide (imidacloprid) on honey bees.” Ecotoxicology 20.1 (2011): 149-157.
Henry, Mickaël, et al. “A common pesticide decreases foraging success and survival in honey bees.” Science 336.6079 (2012): 348-350.
Alaux, C., Brunet, J.-L., Dussaubat, C., Mondet, F., Tchamitchan, S., Cousin, M., Brillard, J., Baldy, A., Belzunces, L. P. and Le Conte, Y. (2010), Interactions between Nosema microspores and a neonicotinoid weaken honeybees (Apis mellifera). Environmental Microbiology, 12: 774–782. doi: 10.1111/j.1462-2920.2009.02123.x

Any specific expertise relevant to studied paper (beyond statistical)?

I have no professional expertise in this area beyond statistical, though I have been following the literature on neonicotinoids and insects for some time as a matter of personal interest.

Before The Headlines is a service provided to the SMC by volunteer statisticians: members of the Royal Statistical Society (RSS), Statisticians in the Pharmaceutical Industry(PSI) and experienced statisticians in academia and research.  A list of contributors, including affiliations, is available here.