Passive restoration can help former farmlands in Minnesota, USA to recover their biodiversity, but even 80 years is not enough for full recovery

This study measured plant biodiversity in once-farmed fields that had been left to nature up to 80 years earlier. The researchers show how recovery varies by the size of the plot to help guide better active restoration.

The United Nations has announced 2021–2030 as the Decade on Ecosystem Restoration. It aims to restore 350 million hectares of degraded land to strengthen biodiversity and ecosystem services, such as climate change mitigation and food security.

Many scientists have long believed that simply removing the cause of ecological damage – such as agriculture or deforestation – will let biodiversity recover, given enough time. However, many studies now suggest that this passive form of restoration, or natural recovery, takes an extremely long time. And often, even after a long time (many decades), the area won’t fully recover by passive restoration alone.

This newer evidence calls for active restoration of nature. Conservation managers could, for example, bring back lost species to speed up the process and attain rich biodiversity.

However, different studies show different rates of restoration depending on the size of the plot being assessed. Some changes in biodiversity are only apparent in small plots. They may not be obvious where a study looks at large-scale changes, and vice versa.

This new study measured plant biodiversity on 17 grassland sites in Minnesota, USA, over a 33-year period. All of the sites had once been farmed, with agricultural land-use ceasing between 1 and 80 years prior to the study.

The researchers compared biodiversity in these sites to that of 18 neighbouring savannah sites which had never been ploughed.

Importantly, they assessed biodiversity at two scales to illustrate differences in restoration rates:

• 0.5 m² plots;
• combinations of 20 plots of 0.5 m² (thus totalling 10m²).

They found that biodiversity improved at all the former farm sites. However, it was always lower than at the unploughed sites – even after 80 years.

The 0.5m² plots had, on average, 59% of species richness (i.e. the total number of species) of never-ploughed sites. The 10m² scale-plots had 55% fewer species. For sites where farming had ceased more than 80 years previously, species richness in 0.5m² plots was about 70% of never-ploughed sites. At the 10m² scale, it was about 65%. When the researchers looked at the entire field, diversity was 90% of never-ploughed sites after 80 years.

Over time, the sites where agricultural land use had ceased had more of the species that were typical of the never-ploughed sites. In total, the researchers found 63 species that only occurred in never-ploughed sites. Conversely, 34 species were only found in formerly cultivated fields.

The researchers say that passive recovery can make sense when resources are limited. However, it carries some hidden costs, such as incomplete recovery. Active restoration could bring better results but is more expensive and not entirely predictable. They, therefore, hope that they can inform targeted, more predictable active restoration measures by showing how biodiversity recovers on its own and at different scales.

Highlighting separate research from the same study area in Minnesota, the researchers suggest that sowing seeds will significantly increase biodiversity by overcoming limited natural seed dispersal. Other promising measures that can accelerate or assist the recovery include controlling invasive grasses, managed fire, soil restoration and large herbivore grazing.

Source:

Ladouceur, E., Isbell, F., Clark, A. T., Harpole, W. S., Reich, P. B., Tilman, G. D., and Chase, J. M. (2023). The recovery of plant community composition following passive restoration across spatial scales. Journal of Ecology, 111: 814–829. Available from: https://doi.org/10.1111/1365-2745.14063  

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.