Recent high-profile studies showing insect declines have hit the mainstream media and been widely discussed online, often with dramatic talk of “insect armageddon”.
That insects are in decline, overall, should come as no surprise to anyone in the UK or neighbouring countries. Butterfly Conservation helped lead the way in demonstrating unambiguous declines in British butterflies1,2 and moths3,4 back in the early years of this century. The most recent official Government trends, derived from the UK Butterfly Monitoring Scheme, show that the relative abundance of habitat specialist butterflies (26 species combined) dropped by 77% since monitoring began in 1976, while wider countryside butterflies (24 species) decreased by 46%. The last assessment of the total abundance of nocturnal moths in Britain showed a 28% decrease over a 40-year period, although the trend was much more severe (40% decrease) in the southern half of Britain5.
In this context, not to mention the much-publicised concerns about bee declines, it was surprising that the recently reported major decrease in biomass of flying insects reported from German nature reserves6 gained so much media and public attention.
Much more significant, shocking and newsworthy, in my opinion, is a study published last autumn7. This also showed dramatic declines of insects (and other arthropods such as spiders), as well as their vertebrate predators. However, the big difference is that this research was undertaken in a protected rainforest in Puerto Rico, with no significant human impact since the 1930s, rather than in the highly modified, intensively managed and often polluted landscapes of north-west Europe.
The study, in the Luquillo rainforest, consistently found massive decreases in a range of samples since the 1970s. The biomass of arthropods caught in sticky traps at ground level decreased by 60 times in January samples and 36 times in July samples, while sweep-net samples showed decreases of eight times in January and four times in July. Monitoring of invertebrate abundance in the forest canopy from 1990-2010 also showed significant declines. Decreases in both biomass and abundance were found in all of the main insect groups in the samples, showing serious declines across the whole community.
Given the absence of direct human impacts (e.g. habitat destruction, logging etc.) in the forest, the researchers explored correlations with climatic factors. They concluded that climate change, particularly a significant increase in mean maximum annual temperature, had caused the changes in arthropod abundance and biomass. This could reflect an uphill shift in species ranges, as has been found in studies of butterfly8 and moth9 communities elsewhere, rather than a fundamental decrease in population size across all altitudes in the rainforest, but more research is required to address this.
In parallel with the declines in insects, the scientists also documented significant declines in a range of vertebrate predators, including lizards, frogs and birds. Among common birds in the rainforest, the rate of decline was significantly correlated with their dietary dependence on insects – ranging from the Ruddy Quail Dove, which does not eat insects and which did not decline at all, to the entirely insectivorous Puerto Rican Tody, which decreased by 90% between 1990 and 2015.
If this study is representative of changes to insect populations in protected tropical habitats (driven by human-induced climate change) then, given the severe impacts of direct human activities such as intensive agriculture already demonstrated in temperate countries, perhaps “insect armageddon” isn’t an exaggeration.
Associate Director Recording and Research, Butterfly Conservation
Follow Richard on Twitter - @RichardFoxBC
- Warren, M.S., Hill, J.K., Thomas, J.A., Asher, J., Fox, R., Huntley, B., Roy, D.B., Telfer, M.G., Jeffcoate, S., Harding, P., Jeffcoate, G., Willis, S.G., Greatorex-Davies, J.N., Moss, D. & Thomas, C.D. (2001) Rapid responses of British butterflies to opposing forces of climate and habitat change. Nature 414: 65–69. doi:10.1038/35102054
- Thomas, J.A., Telfer, M.G., Roy, D.B., Preston, C., Greenwood, J.J.D., Asher, J., Fox, R., Clarke, R.T. & Lawton, J.H. (2004) Comparative losses of British butterflies, birds, and plants and the global extinction crisis. Science 303:1879–1881. doi:10.1126/science.1095046
- Conrad, K.F., Woiwod, I.P., Parsons, M., Fox, R. & Warren, M. (2004) Long-term population trends in widespread British moths. Journal of Insect Conservation 8:119–136. doi:10.1023/B:JICO.0000045810.36433.c6
- Conrad, K.F., Warren, M., Fox, R., Parsons, M. & Woiwod, I.P. (2006) Rapid declines of common, widespread British moths provide evidence of an insect biodiversity crisis. Biological Conservation 132:279–291. doi:10.1016/j.biocon.2006.04.020
- Fox, R., Parsons, M.S., Chapman, J.W., Woiwod, I.P., Warren, M.S. & Brooks, D.R. (2013) The State of Britain’s Larger Moths 2013. Butterfly Conservation and Rothamsted Research, Wareham, Dorset.
- Hallmann, C.A., Sorg, M., Jongejans, E., Siepel, H., Hofland, N., Schwan, H., Stenmans, W., Müller, A., Sumser, H., Hörren T., Goulson, D. & de Kroon, H. (2017) More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS One 12:e0185809. doi:10.1371/journal.pone.0185809
- Lister, B.C. & Garcia, A. (2018) Climate-driven declines in arthropod abundance restructure a rainforest food web. Proceedings of the National Academy of Sciences 115: E10397–E10406. doi:10.1073/pnas.1722477115
- Wilson, R.J., Gutiérrez, D., Gutiérrez, J. & Monserrat, V.J. (2007) An elevational shift in butterfly species richness and composition accompanying recent climate change. Global Change Biology 13:1873–1887. doi:10.1111/j.1365-2486.2007.01418.x
- Chen, I-C., Shiu, H-J., Benedick, S., Holloway, J.D., Chey, V.K., Barlow, H.S., Hill, J.K. & Thomas C.D. (2009) Elevation increases in moth assemblages over 42 years on a tropical mountain. Proceedings of the National Academy of Sciences 106:1479–1483. doi:10.1073/pnas.0809320106