Ecological Armageddon is a bit dramatic. But the message from this paper published in PLOS One is important. The study shows an 82% decline in mid-summer flying insect biomass since 1989 over multiple sites in Germany. Mid-summer is usually peak insect activity, so this is weird.
But every ecological study has a context. This context is described in the Methods section – the most important but least-read section of a scientific paper. For this study, most of the media stories glossed over or overextended the context.
This study doesn’t mean the ‘global ecosystem’ is rapidly collapsing. The data were collected at 63 sites clustered in two regions in the east and west of Germany (see this map (S1 Fig) – the yellow dots are site locations). To quote the authors, these sites are “representative of Western European low-altitude nature protection areas embedded in a human-dominated landscape”. Not the whole world. Not even other kinds of agricultural landscapes in other parts of the world. Most of the media stories missed the opportunity to highlight two key messages from this: (i) we need more long-term monitoring like this from other locations; (ii) to understand these impacts, we need more information on the hundreds of thousands of unknown invertebrates in the world that we know absolutely nothing about where they live, how they benefit humans, and what they need to persist.
Biomass is not the same as abundance. Nearly all media stories mentioned abundance, either in the headline or article text. Biomass is not abundance. Biomass is the collective mass of all insect bodies caught in a trap. Biomass doesn’t tell us whether these insects are pests, pollinators or natural enemies. This study does not provide any data on abundance (the number of insect individuals) or richness (the number of different types of insects). The authors are honest about this, and also that it’s pretty hard to identify millions of insect individuals from 1503 trap samples without lots of funding to pay experienced people to do this. But the media stories didn’t really clarify that without information on abundance and richness it’s hard to make assumptions about environmental effects. It’s also misleading to link this study to the collapse of pollinators, or declines in butterflies etc., as many media stories and tweets have done.
27 years vs. 27 years. To identify a true decline in a taxonomic group at a single location, you need to collect data at that same location every year for multiple (preferably consecutive) years. In this study, over half (59%) of the trap locations were only surveyed for one year during the 27 year period. And only 26 sites were surveyed in multiple (2, 3 or 4) years – these were not all consecutive years. This is an important point to remember, because phrases in media stories like “waded through 27 years of data” or “insect abundance has fallen by 75% in the last 27 years” are slightly misleading (hang on, not to mention the study didn’t even measure abundance). Animal and plant populations naturally vary from year to year, for a variety of ecological reasons. If you only collect data at a site once, it is impossible to know whether that sampling event was a ‘normal’ year or an ‘odd one out’. It’s okay to do this, and many ecological studies do – you can still build knowledge of a site from one year, and you can build knowledge of a region over multiple sites. But without any baseline knowledge of insect populations in the study region, it’s hard to know whether insect populations at each individual site are ‘normal’ or responding to environmental factors like drought, extreme storms, the conditions of the previous winter, or resource pulses. The authors haven’t looked at these factors in this study. I think the most interesting figure is buried in the supplementary material (S4 Fig) – it shows the trends just at the 26 sites that were sampled more than once. You can clearly see that biomass measured in 2013-2016 is a lot lower than biomass measured in earlier years (1989-2006).
‘It must be bad, because this is happening in protected areas’. I couldn’t find any information in the paper about the size of the reserves, although the authors mention toward the end of the paper that reserves were of ‘limited size’. Insect populations can increase rapidly, as most species produce lots of offspring in every reproduction event and some can produce multiple generations in a season. But if essential resources for survival (food and nest sites) are limited, then more individuals will need to disperse to other habitats to survive as the population increases. Small reserves (e.g. a couple of hundred metres square) that are isolated from other reserves will be less likely to have enough habitat resources to maintain insect populations compared to larger, or more connected, reserves. We still know very little about how fragmented habitats influence most insects.
Yes, insects are in trouble. Despite the inevitable media beat-up, this study is an important scientific story. We know that overuse of agricultural chemicals, particularly pesticides, and intensification of agricultural landscapes have negative effects on non-target insects. There are so many more questions that can be asked of this dataset, I hope the researchers have the time and funds to find all these answers. I also hope the story gives much-needed attention to the fact that entomologists and insect ecologists all over the world need more support and funding to answer similar questions in other environments. The only way to identify ‘ecological Armageddon’ is to show that these patterns of decline exist across multiple scales.
© Manu Saunders 2017