The Recovery of European Freshwater Biodiversity Has Come to a Halt
Peter Haase
Senckenberg Society for Nature Research &University of Duisburg-Essen, Germany
Ellen Welti
Smithsonian’s National Zoo and Conservation Biology Institute, USA
Winning article: The recovery of European freshwater biodiversity has come to a halt (Nature, 2023)
“Legislative measures can promote recovery in freshwater ecosystems, but recovery recently stalled thus further action is critically needed”
Freshwater ecosystems, such as rivers, lakes, ponds, and streams, cover only about 2% of the Earth´s surface but harbor an estimated 10% of all known animal species, including one-third of vertebrate taxa, globally. Moreover, they provide clean water, one of the most essential resources required by humans and all life on Earth. However, environmental degradation of freshwater ecosystems as a result of human activity began centuries ago, and sharply accelerated in many countries following World War II, with disproportionally high biodiversity losses. Major ongoing threats to freshwaters that were exacerbated during this period include nutrient and chemical inputs from runoff and aerosols, alterations to the physical shape of flowing waters such as construction of dams and channelization, and climate change. To mitigate elevated pollution loads, measures such as the use and improvement of wastewater treatment plants and desulphurization systems were implemented in the 1970s and 1980s, with clear benefits for water quality. Unfortunately, new stressors such as pesticides and invasive species have emerged, while climate change impacts continue to accelerate. Identifying both how freshwater biodiversity has changed during this period of accelerated human impacts, and the extent to which the reduction of some stressors but increase of others have driven these changes is a key knowledge gap, especially when considering larger areas of study, over extended periods of time. While water quality monitoring that includes sampling of freshwater macroinvertebrate communities (spine-less organisms such as insects, crustaceans and worms) has been adopted by all European countries, major challenges exist when seeking to synthesize these data to inform management actions. Initial challenges included a lack of long-term biodiversity monitoring data, particularly before 2000, and a lack of a unified network of scientists across the continent sharing and comparing data to infer large-scale ecological responses.
To overcome these issues and provide insights into how mitigation policies and stressors have shaped freshwater biodiversity, we launched a continent-wide call for long-term collection of data sets that document and track changes in the abundance and diversity of macroinvertebrates over time. This resulted in 1,816 long-term data sets from 22 European countries and a collaborative network of 96 habitat mangers and scientists. On average, data sets span 19-year periods and include 15 sampling years (with no data set having fewer than 8 sampling years).
Figure 1. Figure adapted from Haase et al., 2023 (The recovery of European freshwater biodiversity has come to a halt).
This unprecedented resolution in terms of sampled areas and time periods allows for detailed quantification of the trajectories of biodiversity trends at a continental scale. We used trait data (characteristics or attributes of an organism) to investigate changes in groups of organisms that have similar characteristics and perform the same ecological roles – also known as “functional guilds”, providing insight into where, when, and why ecosystem function is altered. Finally, we performed a thorough state-of-the-art quantification of various features of watersheds - areas of land where all water drains to a single point (such as a river or lake). Using this quantification, we investigated how freshwater biodiversity changes over time with respect to climate change, dams, and land use. Dams, rising temperatures, and upstream urban and agricultural land use all had significant negative effects on freshwater communities, providing an important piece of evidence and a foundation to build upon when seeking to develop or implement improvement measures. We showed that mitigation measures seeking to improve water quality did pay-off, leading to a partial recovery of freshwater biodiversity. This is encouraging and highlights the value of collective action. Unfortunately, improvements in freshwater communities levelled off around 2010 and remain below EU ecological quality targets. Through our collective work, we continue to call for additional measures such as legislative regulation to reduce pesticide application and water consumption, further improve wastewater treatment, and restore free-flowing rivers with connected floodplains.
Water is an essential shared resource by all and the implementation of solutions to protect freshwater systems are pressing for both industry and civil society. For example, improvements to wastewater treatment plants to better withhold chemicals (including pharmaceuticals) can be implemented by both industry and municipalities, impacting large-scale downstream areas. Technologies to enact these changes are already available and their use would significantly reduce point source pollutants. To decrease diffuse pollutions, additional oversight and reductions of fertilizers and pesticides inputs are required in farmlands, and larger buffer zones along rivers and lakes must be established. To restore heterogeneous river morphology (physical structure) and hydrology (water distribution, and movement) that many freshwater species depend on, our continent-wide network calls for additional investment in freshwater improvement initiatives conducted by authorities, industry and NGOs. Combining appropriate morphological and hydrological measures will not only improve biodiversity but also lower flood and drought risks, both of which are expected to accelerate in frequency and magnitude in the upcoming decades. These measures require sufficient resources to implement but are ultimately less costly in comparison to the damage caused by further degrading freshwaters and the many ecosystem functions they provide.
Freshwater ecosystems comprise the lowest areas within landscapes, resulting in the accumulation of multiple stressors and disproportionately high loss of species and functionality. Freshwater biodiversity is particularly affected by land system change, altered biogeochemical cycles, novel entities, and climate change, all of which have already exceeded planetary boundaries, often with further synergistic impacts from co-occurring stressors. These stressors reduce essential ecosystem services such as the provision of drinking water, food, energy, and flood and drought mitigation. Key growing areas of focus in freshwater ecology include disentangling the effects of multiple stressors and their interactions and identifying complex trajectories of biodiversity change, particularly regarding reference communities to define baselines, community assembly during recovery and emerging novel communities. An additional focus is on transformation research, particularly how measures needed to reduce the human footprint could be implemented and supported by societies.
New legislation such as the Kunming-Montreal Global Biodiversity Framework (www.cbd.int/gbf) and the EU restoration law (Hering et al. 2023) has stimulated research identifying biodiversity responses to protected areas and ecological restoration. Knowing the diversity and value of ecosystems, how these have changed over time, and why these changes have occurred are fundamental steps towards both increasing awareness of the planetary crisis in science and society, and returning to a safe operating space for our only home planet. Collective efforts must work towards a holistic approach to enact simultaneous measures to reverse the effects of components that have already crossed planetary boundaries. Our work shows the power of such approaches in the past, with significant improvements in Europe’s freshwater ecosystems between the 1980s to c.a. 2010 following the adaptation of EU initiatives. But we need to intensify our efforts in the future to return to a safe operating space.
Photo 1. Co-leading author, Ellen Welti.
