Insect decline – concerning news for biodiversity

Insect decline – concerning news for biodiversity

Insect decline has been big news this summer. The alarming notice of a decrease of nearly 80% of the insect fauna within 30 years was discussed all over the media. What followed was quite predictable: trivialisations and counterattacks on the scientists. However, the decline is well documented and definitely not trivial. What may be less clear are the causes for this decline. There are the usual suspects – agriculture and habitat loss – but data are missing. And of course: the situation is a bit more complex than the number of 80% may suggest.

This number originates from the Krefeld Entomological Society (Entomologischer Verein Krefeld, short EVK)  in Northwestern Germany. Since 1985 the members collect insects at more than 100 sites in Northrhine-Westfalia, as well as all over Germany and some neighbouring countries. They always use the same kind of trap (a Malaise trap designed for their purposes) and a standardized protocol. By this, the data are very well comparable for all sites. They compare the samples in diversity and abundances and ID a part of them for closer examination. In addition, they weigh the samples. This results in a very detailed image of diversity and abundance of different insect species at their examination sites. One of these sites is the Nature Reserve Orbroicher Bruch, near Krefeld. In 2013, they compared the weight of samples from that year with former samples from 1989. And discovered that 2013 the overall weights for that year in two traps was 77 and 79.3% lower than in 1989. In the different months, the differences were quite diverse, the highest decrease was in July, though no month of 2013 reached the level of 1989.

Long-term monitoring on insect decline

This news triggered a hot discussion in Germany. However, the data are much more complex than these numbers alone suggest. In the publication describing these findings, the authors themselves say that they cannot give clear answers on the causes, but suggest the changed land-use in the Reserve. Habitat loss and/or degradation is indeed a probable cause for insect decline and biodiversity loss in general. It is mainly attributed to intensified agriculture. And here is where the counterattacks come in. The German Farmers Association criticized that these data were not valid for whole Germany and that more data were necessary. Streets and settlements consume 66ha a day and farmers would do more for biodiversity if only the bureaucracy wasn’t so bad.

This is right, but also simplifying. First of all, there are much more data than these cited numbers. The work of the EVK is a real treasure of data for entomologists and ecologists. The samples are all in the society’s collection together with all the accompanying documents. Such a data pool is quite unique. Universities and other research institutions in general don’t have the resources and personnel for projects that last longer than 3-5 years. In the case of slowly advancing biodiversity loss this is not enough. Of course the data from a single site are not transferrable to whole Germany or even worldwide. But these are not the only data pointing in this direction. There are also other indicators for a the loss of the insect fauna.

pollinator, biodiversity, insect decline

A survey in Bavaria shows big declines in moths and butterflies.

More studies pointing in the same direction

In these days, the German Wildlife Foundation published a preliminary report on a moth and butterfly survey in Bavaria. Also this monitoring documents a steep decline after the 1980s. Compared to this period, today there are 50% less night-flying moth species with much lower abundances. Day-flying meadow species declined 73% compared to the 1970s. Also this survey observes the biggest decline in agricultural landscape. However, it may be even much worse: monitorings from the IUCN on 203 insect species record a worldwide decline of 33% of the species (as reported here). In addition, there are differences depending on the insect group: moths and butterflies are less affected than beetles, bees, wasps and ants or even othopterans (grasshoppers, locusts, crickets etc.). Some insect groups often are less popular in surveys, so we don’t know how they are doing worldwide. In the data from the EVK, hoverflies were especially affected. These insects are important pollinators as adults and the larvae of many species are natural enemies of aphids.

pollinators, hoverflies, decline, biodiversity

Hoverfly feeding on male willow flower. These insects’ abundances decreased heavily in Northwestern Germany.

Data on insect decline vary not only between insect groups, but also between different localities. Therefore, the objection from the German Farmers’ Association about transferring the Krefeld data to whole Germany is correct. Also their concern about the huge data gap. However, this is no reason to trivialize the existing data and to not assume responsibility.

Requests on how to stop insect decline

The farmers are also right to complain about the bureaucracy for implementing measures to help insects. A first step may be to make this easier and also to motivate farmers to use these tools. I’m not sure (opinion, not fact!), how many farmers are aware of the services they get from insects, or biodiversity in general. In addition to pollination, there are pest control, soil formation, clean water and so many more. However, the existing measures for helping insects in agricultural landscape should be used and improved. Most importantly, we need more data! This doesn’t mean to wait until we have understood the whole phenomenon – it may be too late by then. But to increase the efforts in monitoring the insect fauna also to know better how to help the most endangered ones. Flowering strips near fields are fine, but they may not help the right species. In addition, another “endangered species” are taxonomists. There are only few people who really know the insects by name and are able to detect the changes in their populations. The work of societies like the EVK by this gains even more importance considering this aspect.

There may not be a unique cause for the insect decline. But, this doesn’t mean we cannot work on the factors we already have hints and/or solid data they may play a role. A common demand of conservationists is to reduce the “too much” in agricultural management. Too much monotony in the landscape, too much fertilizer and, this is the most prominent, too much pesticides. In fact, the “good practice” should be an integrated use of pesticides. Meaning as much as needed, but as little as possible. Or, how the participants of the Hymenopterist Conference in Stuttgart last year put it in their resolution:

We acknowledge that modern agriculture has to take measures to optimize the yields and minimize pests. […] We see the challenge in developing sustainable agricultural practices that do not harm biodiversity.

I couldn’t agree more and wished the whole discussion had this tone.

Note on 28/08/17:

I made some corrections after the kind mail from Martin Sorg from the EVK, referring the sample comparison and the landuse in the reserve. However, the main argument of this post remained untouched.


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