Today marks the publication of a special issue of the journal Philosophical Transactions Royal Society B., comprising 17 scientific papers, tackling the knotty issue of “Detecting and attributing the causes of biodiversity change, edited by Eden Tekwa, Andrew Gonzalez, Damaris Zurell and Mary O’Connor. One of those papers is led by RSPB and here the lead author, Prof. Richard Gregory (Head of Monitoring, Conservation Science), sets the scene and describes the new findings.
Detecting biodiversity change is not easy because the term ‘biodiversity’ is hazy and encompasses different facets of nature. It can be measured using a range of different metrics, some changing in parallel, others in opposition. Detecting change is not easy and going the next step to identify why it is changing is even harder.
The next problem is data. Some popular groups of species are well known and well assessed in some places, others are poorly known and/or not assessed in many places. Much of the information we have is biased in this way, towards some places, some types of wildlife. There are practical reasons for this, but it makes it difficult to understand how and why biodiversity is changing - and how best to respond. For conservation, it is a big issue.
To illustrate, although many studies provide strong evidence of severe and accelerating biodiversity decline, with increasingly hard-hitting headlines, some have questioned the strength of that evidence and argue that there has been little biodiversity change. Knowing whether biodiversity is or is not declining feels fundamental.
But our knowledge of birds in Europe, built upon the endeavours of thousands of ornithologists over many years, is exemplary. Our new paper takes advantage of two quality datasets on breeding birds in the UK and the EU, to look at how bird numbers and biomass has changed over several decades.
We compiled data on 445 species of native breeding birds in the EU (~86%). We modelled species’ trends through time at a national scale and EU level and examined attribution by associating bird species’ trends with species’ characteristics, like their body size and habitat choices.
Change in abundance
We demonstrate significant change in the bird assemblages with substantial reductions in overall bird abundance with losses concentrated in a relatively small number of abundant and smaller sized species. By contrast, rarer and larger-sized birds had generally fared better.
We estimate the total number of native breeding birds in the UK to have declined by 38 million individuals (19%) between 1966 and 2018, and the total abundance in the EU to have declined by 550 million individuals (17%) between 1980 and 2017 (figure 1). The largest population declines occurred in those birds associated with human-modified habitat, including intensive agriculture and urban habitats.
Figure 1. Estimated number of individuals of native breeding bird species (millions) in (a) the UK from 1966 to 2018, and (d) EU from 1980 to 2017. Overall net change (green shading) in individuals for the UK (b) and EU (e), derived from total increases among species with positive trends (pale blue) and total decreases among species with negative trends (darker blue shading). Frequency distribution of average annual growth rates for species in the UK (c) and EU (f), the dashed vertical lines are medians.
Population losses were skewed towards a small number of the most abundant birds in UK and EU. House Sparrow, Tree Sparrow, Starling, Willow Warbler and Skylark were common to both sets (figure 2). Similarly, a small number of bird species feature in those increasing in number. Wren, Woodpigeon, Blackcap, Chiffchaff, Robin and Goldfinch, are common to the increasing species sets in both cases (figure 2).
Figure 2. Change in abundance of birds (millions of individuals) in (a) UK from 1966 to 2018, and (b) EU 1980 to 2017.
Population trends were positively correlated with species’ body mass, with trends in climate suitability, and varied with species’ abundance, migration strategy and niche associations. This shows that each of these factors was important in determining species trends.
Change in biomass
Surprisingly, the biomass of birds had increased slightly in UK and was stable in EU, indicating a considerable change in community structure (figure 3). The stability in biomass, despite bird declines, tells us that birds with relatively larger mass were tending to increase, while smaller-bodied species were tending to decline.
Figure 3. Estimated total biomass (tonnes) of individual birds of native breeding species in (a) the UK from 1966 to 2018, and (b) the EU from 1980 to 2017, with shaded 95% credible intervals.
The common and the rare
The loss of common species is a concern because it suggests changes to ecosystem structure and possibly functioning. The per capita influence of common bird species on ecosystem services will vary, but their numerical dominance means that changes in their populations may have large implications for ecosystem services. The scale of losses we estimate was very large. There is strong evidence to show that the main driver of bird declines in common birds in Europe is agricultural intensification driven by farm policies.
House Sparrows have seen significant declines in the UK and EU © Ben Andrew (rspb-images.com)
In contrast, many rare and scarce, often larger birds, have fared comparatively well. In the EU, the Birds the Habitats Directives provide legal protection to priority species and habitats and have been shown to help target birds. We believe that these EU nature policies explain why rarer and larger birds are doing well because they are targeted by those actions. This is a good news story for conservation.
Although our data is good quality it is restricted to birds and to Europe. We aren’t easily able to generalise our findings, although similar trend patterns have been seen in North American birds. We urgently need better abundance data for birds and other taxa in more places.
Conclusions
We show substantial biodiversity change and abundance loss in European birds over the last 50 years, though numbers have stabilized in recent decades. Patterns of change varied with the species’ body size, their abundance, the habitat they occupy, and with trends in climate suitability. Further, we show that community structure has changed. We don’t fully understand what drives such wholesale changes, although agricultural intensification and climate change are known to be key drivers.
We emphasize the need for care in measuring and interpreting biodiversity change given its multifaceted nature and because different metrics can provide quite different insights.
Reference: Gregory RD Eaton MA Burfield IJ Grice PV Howard C Klvaňová A Noble D Šilarová E Staneva A Stephens PA Willis SG Woodward ID & Burns F. (2023) Drivers of the changing abundance of European birds at two spatial scales. Phil. Trans. Roy. Soc. B. https://doi.org/10.1098/rstb.2022.0198
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