Semi-natural conditions promote ecological recovery of urban streams

This study considers the potential for recovery of invertebrate communities in urban streams based on 12 years of data from a city in Lower Saxony, Germany. The researchers report that streams with natural features and good water quality tended to recover well, given suitable climatic conditions.

Freshwater ecosystems support a proportionately higher level of biodiversity than terrestrial or marine environments and face a greater threat of species extinctions. Urbanisation and agriculture are threats, and freshwater habitats in urban areas are particularly prone to degradation. Factors such as modified flow and chemical pollution commonly lead to reductions in species richness and loss of sensitive native species.

However, recent studies have suggested that freshwater invertebrate communities, especially in Europe and North America, have been recovering since the 1990s – a period in which terrestrial insect populations have continued to decline. This study considers the recent recovery trajectories of invertebrate communities in a European urban freshwater system.

The researchers gathered data from 56 freshwater sites in the city of Braunschweig, Germany, between 2009 and 2020. They conducted invertebrate sampling four times a year, to measure annual species diversity (taxon richness) – the number of different species per site and year). They determined the physical characteristics of the study sites by a visual inspection that was repeated once during the study period; while they based the chemical conditions on quarterly water-quality measurements.

The researchers report that total taxon richness increased significantly over the period in 54% of sites, decreased in 11% and showed no significant trend in the remainder. Their analysis revealed two distinct time periods of 2009–2013 and 2014–2020. This distinction coincided with a notable flood event in 2013, say the researchers, and also a city-wide reduction in bankside mowing frequency in 2012.

Their analysis also divided the second time period into two sub-periods of 2014–2017 and 2018–2020; the latter notable for a series of uncharacteristically warm and dry years. During these years, sites experienced reduced flow with some temporary sites retaining water for shorter periods and some previously permanent sites becoming temporary. The researchers report that increases in species diversity were overall less marked in the first period, most pronounced in the first half of the second period and reduced or reversed in the final sub-period.

The researchers also conducted a statistical analysis to organise the stream sites into groups or clusters. This yielded four clusters:

• Cluster 1 typically consisted of small- to medium-sized streams generally in agricultural settings, which showed mostly consistent invertebrate diversity over the first period, with significant increases after 2014 slowing or reversing after 2018.
• Cluster 2 contained suburban residential and agricultural sites with low and stable levels of taxon richness.
• Cluster 3 was the smallest group, including sites in built-up areas with low and stable, or sometimes slightly decreasing, invertebrate diversity.
• Cluster 4 was the largest group, consisting of medium-to-large streams in semi-natural areas. These were characterised by a higher level of species diversity which increased through the first period and the first half of the second period, sometimes declining and sometimes continuing to increase from 2018 onwards.

The researchers report that sites with more natural conditions – higher flow rates, more natural streambeds and better water quality – tended to have more diverse invertebrate communities, including species indicating healthy stream conditions. The results demonstrate that under suitable conditions, degraded urban freshwater habitats can undergo significant ecological recovery, they argue.

The researchers highlight the potential for climatic fluctuations to reduce or reverse such progress, however, and the need to account for such events in the context of global climate change. They advise that authorities could enhance the ecological quality of urban streams by reducing organic pollution and stream degradation. They recommend avoidance of high-intervention management regimes such as total removal of aquatic plants and streambed stabilisation; and instead, to allow/promote the development of more natural stream structures through low-intensity maintenance.

Source:

Goertzen, D., Schneider, A.-K., Eggers and T. O., Suhling, F. (2022) Temporal changes of biodiversity in urban running waters – Results of a twelve-year monitoring study. Basic and Applied Ecology 58: 74–87. Available from: https://doi.org/10.1016/j.baae.2021.11.007

To cite this article/service:

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.