Blog by RSPB's Head of Monitoring Conservation Science, Richard Gregory and UCL's Postdoctoral Research Associate, Charlie Outhwaite discuss their new paper out today in Nature, Ecology and Evolution. 

Biodiversity loss is increasingly hitting the headlines with growing recognition of the vital role it plays, underpinning people’s livelihoods, food production, wellbeing, and their local and national economies. Added to which, healthy species populations and healthy ecosystems will be the forefront of our fight to combat damaging climate change.

And yet our understanding of biodiversity loss, even in the UK, with its rich tradition of biological recording, is surprisingly fragmented and patchy.  We know an awful lot about the trends of our charismatic megafauna, the vertebrates, mostly birds and the mammals, and some vibrant insects, such as the butterflies, but, until now, much less about the ‘little guys’. It is often the little guys that quietly knit our ecosystems together and allow them to function properly. The little guys are important and play a big role.

In our new study, just published in Nature, Ecology and Evolution, we examined for the first time, occupancy trends in over 5000 species, in 31 different taxonomic groups. This included an array of different species, from ants, bees, bugs, moths, dragonflies, beetles, ladybirds, spiders and flies, to lichens and mosses.

The trend patterns we found, for the period 1970 to 2015, proved to be both complex and surprising (figure 1). Firstly, taking all the invertebrates as a group (n=538), their occupancy has declined, much as you might expect given species losses we see in popular groups, like the birds. But three of the other groups we examine, the freshwater species (n=318), the insects (n=3089), and the lichens, mosses, liverworts and hornworts together (n=1269), their occupancy has increased.

Figure 1: Composite estimates of average annual occupancy of four groups of species. Values are scaled to 100 in 1970. Coloured lines show the average response as the geometric mean occupancy and the shaded area represents the 95% credible intervals of the posterior distribution of the geometric mean. n denotes the number of species contributing to each group. Uncertainty for each year is 593 expressed relative to the 1970 baseline.

The most striking response was in the freshwater species. This group showed a strong decline between 1970 and the mid-90s and since then has recovered back to 1970 levels.  This U-shaped response is noteworthy, and it suggests that this group is recovering after experiencing large declines. Intriguingly, the upturn in the trend coincides perfectly with the implementation of the European Urban Wastewater Treatment Directive. This EU legislation was designed to clean up our waterways and remove harmful pollutants from our rivers. At face value, (and we have not tested this experimentally) it seems to be working, at least in respect of our species group.

The other notable pattern is a strong and steady upsurge in the occupancy of lichensmosses, liverworts and hornworts, since 1970. This group is known to be very sensitive to air pollution and have suffered as a result of this in the past. Here again policy, in the form of the Clean Air Act and other measures, has led to a reduction in air pollution, and it looks like these delicate species may have benefitted from those improvements in air quality over time. 

Taken together, these two examples seem to demonstrate the positive role well-designed regulation can play in supporting the recovery of nature.

Lichens, as a whole, have seen an increased occupancy.

Overall, our study shows that species occupancy appears to be increasing in many of these less well studied groups in the UK and that is quite surprising given established declines in other animals and plants.  However, there are some important caveats. One is that change in species occupancy is not the same as change in species abundance (which is a standard measure of species populations). They each capture different facets of biodiversity change and are not equivalent - and will not necessarily behave in the same way. 

Added to this, we need to remember that the average change in occupancy across many species can hide important information and many individual species in the groups we looked at, had declined steeply. For example, we show that occupancy of freshwater molluscs has declined significantly in the UK, even though the freshwater group as a whole has increased.  Finally, when we look in detail at the average occupancy trends of individual groups, many show signs of decline in the last decade (including the insects as a group, and other invertebrates, such as the wasps, bees, hoverflies, twirler moths, soldier flies, and soldier beetles) so we mustn’t be complacent, and our monitoring efforts must continue.

Freshwater pearl mussel (Margaritifera margaritifera) is one species still on the road to recovery, and will benefit from continued water quality protection.

Our study takes us a step closer to understanding the status of UK biodiversity by exploring patterns of change for speciose groups that have previously been neglected. We demonstrate that occupancy can be a valuable tool for assessing trend patterns over time when abundance data are not available. More broadly, we show how data collected by expert citizen scientists can be used to advance our knowledge of nature and in that way help us better manage its recovery.

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