Hydrobiologist Petr Pařil was a member of an international team of approximately 100 scientists. They worked on a wide-range comparative study on the development of biodiversity of European water courses, which was published in the summer of 2023 in Nature under the title “The recovery of European freshwater biodiversity has come to a halt”. Some trends in the changes in biodiversity were subsequently studied on Czech (unaffected) reference streams by another team of hydrobiologists from the Department of Botany and Zoology, which included Marie Zhai.
Some people may find it surprising that freshwaters represent a much larger proportion of biodiversity in spite of the much lower area (less than 1% of the surface of the Earth). Indeed, these areas are most affected by the decline in biodiversity because, over the last one hundred years, they have been exposed to human activity at an enormous scale. Rivers, as an example, are affected by regulation, melioration, damming and pollution.
“We collected data from previous research projects covering over 1,800 locations in 22 countries all over Europe. Nobody has ever done anything similar in Europe, and these efforts were unique on the global scale as well. The dataset ultimately included over 26,000 samples and 714,000 individuals from 2,650 taxa after two years of collection and cleaning; the peer-review process in Nature took almost 18 months. A lot of effort was put into the streamlining of a very variable dataset from multiple sources due to their comparability from the perspective of the differentiation of species and habitat aspects. On top of that, this cooperation drew us into other research projects in which we now participate,” says Petr Pařil, commenting on the analysis of how the makeup of species in European rivers has changed over the last fifty years.
A study published in Nature shows that scientists have seen a surprising increase in the diversity of species in water courses over the respective fifty-year period (1968−2019). This is particularly true of the period since 1990, when they saw an increase in the number of species and populations. At the same time, functional diversity, i.e. the variety of different functions of species within communities, was growing (by 2.5% annually). The increase in the number of species and their respective populations was 1% annually on average. Scientists confirmed the return of some species that are incompatible with increased levels of pollution, such as mayflies, stoneflies and caddisflies, even though this increase was still lower than that of other aquatic invertebrates. The reason for the increase in biodiversity was attributed to improved water quality due to wastewater treatment as per EU legislation, or river revitalisation projects. That being said, the process was slower downstream from dams, in warming basins and in areas affected by agriculture or regulation of water courses.
Slowdown of the recovery of diversity after 2010
However, the study also shows that after 2010 the trend in the recovery slowed down (the biodiversity growth curve has flattened), in spite of the efforts in revitalisation and improved wastewater treatment processes. The study aimed to explain the reasons for this trend.
One of the possibilities is the fact that the recovery of biodiversity had simply come to an end, and, possibly, the benefits of the measures introduced years ago, such as revitalisation and wastewater treatment, had exhausted their potential. Newly emerging factors may have also played a role, such as climate change, microplastics, pesticides, fertilisers, nutrients, salinisation or pharmaceutical residua in the water. Non-native species may have also caused the slowdown in the recovery of biodiversity.
Moreover, the study may have failed to notice some strictly local trends. “There are rivers where, even in this study, species have been declining for years. Moreover, there was proof of the presence of non-native species at 70% of these locations. The datasets do not acknowledge the differences between various effects of human activity and climate change. It is necessary to work within individual countries where we can focus on the analysis of individual variables in greater detail and determine possible causes for these trends more precisely. Such study was carried out for the Czech Republic by my colleagues,” Pařil concludes.
What is the situation with biodiversity in Czech waters?
Marie Zhai described long-term trends in the development of biodiversity along the river continuum in an article titled “Climatically promoted taxonomic homogenization of macroinvertebrates in unaffected streams varies along the river continuum”. It was recently published in the journal Scientific Reports. “Unlike the authors of the Europe-wide study, we analysed the biodiversity from smaller streams to lowland rivers separately. It gives us a better understanding of the variety of species in individual parts of the river continuum. We also carefully measured changes in environments, which allowed us to find context between the representation of species and the environment. We found out that the populations of many of our species are increasing, and that the species are spreading to new locations. We were surprised to find out that only a small number of species are declining, mostly cryophiles,” Zhai explains.
Some trends can be observed in the Czech Republic
“The increase in biodiversity observed in unaffected locations in the Czech Republic over the past twenty years was really unexpected. The populations of some species have increased multiple times. It was surprising that invasive species played almost no role,” Zhai notes.
Species with increasing populations are referred to as “winners of climate change”. These winners still outnumber the “losers” in Czech unaffected streams. Only seven “winners” have been spreading to all zones. Most spreading species occupy new locations within the same zone, with exceptions spreading to neighbouring ones. The significant changes in the composition of species are linked to “homogenisation” in individual zones, with community species composition at individual locations becoming more similar.
The changes in biodiversity of unaffected streams are not caused by temperature changes alone. Other significant factors include water flow reduction due to irregular precipitation throughout the year, poor retention and increased evaporation due to higher temperatures during summers. Even though the flow in unaffected streams reduced only slightly, the effect on aquatic invertebrates was still significant. The success of the so-called winners is attributed to their ecological properties, but the overall increase in biodiversity cannot be explained by a single trend..
Further research
The factors described above (temperature, flow, sediments) support each other’s effects, and the overall effect on biodiversity can be caused by synergy, where the individual effects are added or multiplied (or, conversely, suppressed). “We are unable to distinguish the effects of individual factors in our field research. Mostly they occur together. We are continuing our work, and the follow-up project will include detailed questions, such as whether the rising temperatures have similar or identical effects on the winners and losers of climate change. I am hoping to have the answers soon because it will help us estimate the future development of biodiversity and the measures for its protection,” Pařil adds.