Recovery of biodiversity in European streams has slowed
Our hydrobiologist, Petr Pařil, was part of an international team of around one hundred scientists working on a very extensive comparative study on the development of biodiversity in European running waters, published in the summer of 2023 in the journal Nature. Previously, we briefly reported on the article, entitled ‘The recovery of European freshwater biodiversity has come to a halt’, and in this article we present more detailed conclusions from the study. In a concurrent study, another team of hydrobiologists from the Department of Botany and Zoology studied trends in river biodiversity on a smaller regional scale, using so-called reference streams in the Czech Republic. In the second part of this article, a member of this team, Marie Zhai, gives us an update on the team’s findings.
Freshwater systems are significantly threatened by a decline in biodiversity
It may come as a surprise to many that freshwaters support a disproportionately larger proportion of biodiversity than other ecosystems, despite occupying less than 1% of the Earth’s surface. At the same time, these habitats are most threatened by biodiversity decline. Why? For the hundreds of years, the running waters have been exposed to enormous levels of anthropogenic pressure. Rivers are now affected by regulations, amelioration work, damming and changes to drainage basins and the overall character of the landscape.
The scope of the cited study, which set out to assess how the species composition of European river communities has changed over the past 50 years, was unique, even on a global scale. As its co-author, Petr Pařil states, “we collected data from existing research on more than 1,800 locations from 22 countries across Europe, making this the largest such study that has taken place in Europe. I was invited to work with the team four years ago. The dataset included over 26,000 samples obtained over two years, with 714,000 individuals of 2,650 taxa collected and cleaned. The review process alone in Nature took almost a year and a half. A great deal of effort was devoted to harmonising such a diverse set of data originating from so many projects as comparability was essential, both in terms of species differentiation and environmental parameters. About 100 authors participated in the study in total. At the same time, it drew our team into further research, which we are working on now”, said Petr, when describing his collaboration on the article.
Why was biodiversity increasing until 2010?
A study previously published in Nature reported that scientists have observed a surprisingly large increase in species diversity in running waters over the past 50 years (1968−2019); this has been especially true since 1990, when both the number of species and individuals were seen to increase in river communities by an average of around 1% per year. At the same time, the so-called functional diversity, i.e. the variety of different functions of species within a given community, was also growing significantly, by around 2.5% per year. Of especial note was the return of some pollution-sensitive insect species, such as mayflies, stoneflies and caddisflies, to streams, though their increase in diversity remained lower than that of other aquatic invertebrates. The main cause of this increase in biodiversity was most likely an improvement in water quality due to better wastewater treatment (as required by EU legislation), as well as river revitalisation efforts. On the other hand, restoration of communities under dams, in warmed drainage basins and where there is a significant influence from agriculture or flow regulation has taken place more slowly.
What are the possible causes of the slowdown in biodiversity recovery since 2010?
The study cited above also showed that, after 2010, the recovery trend lost momentum (i.e. the biodiversity growth curve flattened), despite continued revitalisation and improved wastewater treatment. The study also tried to explain possible reasons for this trend.
One possibility is that biodiversity recovery is more-or-less complete; however, this is difficult to assess due to the lack of quality data from the period before 1968. We do not know what the state of biodiversity was originally, i.e. before the period analysed by the cited study. It is likely that the benefits of past measures, such as revitalisation and water treatment, have already been exhausted. The fact that new stressors have appeared, such as climate change, microplastics, pesticides, fertilisers, nutrient input, salinisation and/or drug residues in the water, may also have played a role. Non-native species, which represented almost 5% of all species and 9% of all individuals captured in the study, may also be a further cause of the slowdown in biodiversity recovery.
As the dataset used in the study covers a large area of Europe and is based on a wide range of methodologies, the study could not always pick up local trends. According to Petr, “there were places where, even in this study, species have been declining for a long time. Furthermore, non-native species have invaded 70% of the locations monitored. It was also not possible to distinguish the influence of various anthropogenic stressors from that of climate change, in the data. Therefore, it was necessary to work on regional scales within individual states, where we could analyse individual variables in more detail and determine possible causes for the observed trends much more precisely. Just such a long-term study was prepared for the Czech Republic by my colleagues”, he concludes.
How is the situation with biodiversity in Czech streams?
Marie Zhai's article, entitled ‘Climatically promoted taxonomic homogenization of macroinvertebrates in unaffected streams varies along the river continuum’, which was recently published in the journal ‘Scientific Reports’ by the publisher of NATURE, focused on just this question. The team followed longer-term trends in the development of biodiversity along the river continuum, i.e. all the way from the narrow upper reaches to the wider lower reaches of large rivers. The goal of the study was to analyse long-term biodiversity development in locations minimally influenced by humans (so-called reference streams), which filter out the influence of most human activities and make it possible to focus purely on the effects of climate change and the character of the river landscape, from small foothill streams to larger lowland rivers.
“Unlike the pan-European study, we analysed individual parts of the river continuum separately. This gives us a better idea of what is happening to species diversity in individual sections of the stream. At the same time, we carefully measured changes in environmental conditions, allowing us to put changes in the representation of species and changes in the environment into context. We found that many of our species are increasing in number and, at the same time, spreading into new, previously uninhabited locations. Surprisingly, it turned out that, for the time being, only a very small number of species are declining, mainly species that prefer colder conditions”, Marie told us.
European trends are also visible in Czech streams
Some trends confirmed at the European level are also visible in the Czech Republic. According to Marie, “the increase in biodiversity observed in unaffected Czech locations over the past 20 years or so was truly unexpected, with some species having increased in abundance many times over. Another surprise was that invasive species, whose spread is expected due to climate change, are almost uninvolved in this increase in biodiversity; instead, it is almost exclusively native species”. What reasons could there be for native species to increase by so much? Surprisingly, the current increase in temperatures appears to suit most species, at least so far. In comparison, improvements in environmental conditions due to river restoration are unlikely to have played any major role as, in our case, the reference rivers were almost untouched.
Climate change impacts on river species diversity have been significant
“The growth of biodiversity has been positively influenced by an increase in the winter temperature minimum and summer maximum”, said Marie. Thanks to higher winter temperatures, species that did not thrive here in previously colder conditions have now spread to the upper reaches of streams. So far, the increase in summer temperature maxima also appears to have had a positive impact on biodiversity growth. Nevertheless, our hydrobiologists agree that these trends cannot be seen as a positive effect of climate change for the future. At present, it is likely that most species have not yet reach their temperature maximum; the question is, when will they? “So far, the reference streams are resisting major change as their winter water temperatures have only risen by one degree, and summer water temperatures only a little more. Thus, the stream communities are likely to be functioning in a situation of so-called climate debt, i.e. where the change in conditions has not yet manifested itself. However, if water temperatures rise further and exceed a certain limit, it may turn into a major extinction event”, said Marie.
So far, the winners of climate change are prevailing
“We refer to species that are increasing in our unaffected streams as climate change 'winners', and these are generally outnumbering declining species, which we refer to as 'losers'. Nevertheless, only seven 'winning' species have spread to all longitudinal zones over the entire river continuum, most other spreading species only occupying new localities within a single zone, and sometimes only spreading to the neighbouring zone”, said Marie.
Alongside significant changes in the species composition, individual zones have also been subjected to so-called homogenisation, with community species composition at each locality becoming more similar. This phenomenon is usually more common in streams where anthropogenic stressors have had a negative effect, causing the community to ‘narrow down’ to a limited group of tolerant ‘winners’, the more sensitive species being the ‘losers’. It is rather unusual, therefore, to find cases of homogenisation due to the dominance of ‘winners’.
Temperature changes alone were not responsible for the changes in biodiversity observed in the reference streams. A further highly significant factor has been the reduction in water flow in Czech rivers, caused by the unevenness of precipitation during the year, poorer retention of water in the landscape and increased evaporation due to rising summer temperatures. In general, it can be said that, even with our continental climate, the influence of low flows is becoming increasingly apparent. Additionally, the Czech Republic is located at the confluence of three rivers, and almost no water flows to us. Although there has only been a slight reduction in flow in the reference streams, its effect on aquatic invertebrates has been noticeable, partly due to an increase in finer sediments with lower flows, to which some species responded sensitively. In line with this, we attempted to explain the success of the winners in relation to their ecological properties. “There were more species with different preferences. Some species, such as worms or small pea clams, had a positive relationship with fine sediment and/or preferred slower currents and, consequently, were more common. However, there were also some other, previously rarer, species whose success we have no ecological explanation for yet. Overall, the large increase observed in biodiversity cannot simply be explained by a single trend”, Marie said.
Further research is being focused on differentiating the individual influences shaping the biodiversity of streams
The factors described above (i.e. temperature, flow, increase in amount of finer sediment) occur in combination and the resulting effects on biodiversity can be caused by synergistic effects, where individual effects may be added or even multiplied (or, conversely, dampened). As Petr Pařil concludes, “we are not yet able to clearly distinguish the effects of individual factors in field research as, in most cases, they occur together. However, we are continuing our work and, in a follow-up project, we are asking more detailed questions, such as whether rising temperatures will have similar or different effects on the 'winners' and 'losers' of climate change. I hope that soon, we will be able to find at least some of the answers that will help us not only to better estimate the future development of river biodiversity but also to propose targeted measures for its protection”.
