Young forests can help heal tropical aquatic ecosystems: Study

  • Microbial communities are important indicators of ecological degradation in the tropics, often reflecting levels of disturbance and contamination in rivers and streams.
  • In an attempt to monitor the ecological condition of various streams in central Panama, researchers at the Smithsonian Tropical Research Institute (STRI) assessed the impact of various land usages, such as cattle pasture and secondary forest, on microbial diversity and community structure.
  • In less than a decade, researchers say, reforested land allows bacterial communities to recover, highlighting the importance of reforestation for overall ecosystem recovery.

Scientists have long identified a link between livestock production and deforestation, with reams of research showing the detrimental effect of agricultural activities on natural resources. Little is known, however, about how much these practices impact water microbial communities — the tiny organisms that maintain water quality by cycling nutrients and energy.

A new study published in Scientific Reports by researchers from the Smithsonian Tropical Research Institute (STRI) focused on just that: the specific impact of different land uses, such as cattle pastures and secondary forests, on bacterial communities in the water column of four streams in central Panama.

The findings reveal key insights into nature’s ability to recover from harsh environmental changes, such as land degradation and deforestation, which are largely a consequence of human-related agricultural practices. Reforesting the land, the researchers say, can restore many aspects of water quality, allowing bacterial communities to thrive again, which directly benefits both human health and the environment.

Cattle graze along a stream in Costa Rica. Image by Rhett Butler/Mongabay.
Cattle graze along a stream in Colombia. Image by Rhett Butler/Mongabay.

“Microbes are the biological engines that transform the world,” Bob Hilderbrand, associate professor at the University of Maryland Center for Environmental Science, who was not involved in the study, told Mongabay. “They can be particularly important for removing excess nutrients, such as pollutants, and even for breaking down toxic pollutants.”

Assessing the ecological conditions of freshwater streams, specifically by identifying microbe distribution and diversity, helps researchers identify the impact of human-related activities on the environment, and can therefore aid governments and policymakers by informing policy and practice.

“Since water column microbes tend to reflect the surrounding uplands, the results suggest the land is recovering in more ways than just vegetatively, and this is encouraging from the standpoint of ecosystem recovery and restoration,” Hilderbrand said.

The project was a collaboration with Agua Salud, an initiative that aims to understand the drivers and consequences of environmental change. Data was collected at the Agua Salud project site, an area managed by the STRI.

Researchers studied streams surrounded by four types of land use: mature forest, secondary forest, silvopasture, and traditional cattle pasture. A mature forest, also referred to as primary or old-growth forest, is an untouched forest, containing little or no evidence of human activity, whereas a secondary forest is a forest that has been disturbed in some way, but has been allowed to recover and regrow. Silvopasture, on the other hand, is a practice that integrates trees into pastures that are used for agricultural production, often in a mutually beneficial manner. This differs from cattle pasture, which is land used exclusively for domesticated livestock to graze.

Stream surrounded by secondary forest at the Agua Salud study site in Panama. Image by Ana Endara.
Stream surrounded by secondary forest at the Agua Salud study site in Panama. Image by Ana Endara.

From the streams that run through each land type, the researchers collected weekly water samples for more than two years to measure different aspects of water quality, as well as extract and sequence bacterial DNA.

According to Kristin Saltonstall, study co-author and researcher at the STRI, the main objective was “to understand how different land uses and reforestation treatments impact the hydrological environment and enhance ecosystem services.”

Saltonstall and her colleagues discovered high bacterial diversity in streams surrounded by both mature and secondary forests. Streams surrounded by traditional cattle pasture, on the other hand, had much lower diversity, highlighting the negative influence that raising cattle has on water quality and bacterial communities.

“When streams become polluted or surrounding landscapes are degraded, microbial communities shift, risking their ability to help maintain natural processes and often allowing harmful bacteria to flourish,” Saltonstall said.

The study reveals huge benefits in allowing forests to regrow on abandoned agricultural land, finding that bacterial communities in streams can recover and flourish in as little as a decade when cattle are removed from the area.

“Our results indicate that the presence of cattle in the watershed can decrease bacterial diversity,” said Megan Lindmark, a researcher at the STRI and co-author of the study. However, “our results also indicate that streams in secondary, restored forests have similar bacterial communities and diversity to mature forests, indicating the importance of reforestation,” she said.

Putting the brakes on livestock production would be a huge feat requiring major international effort — an extremely complex process, as demonstrated in the negotiations by IUCN members and leaders at the U.N. COP26 climate summit in Glasgow.

Instead, the researchers argue a case for alternative solutions that allow for both the restoration and maintenance of ecosystems, while still leaving room for land to be used for food production.

“The study illustrates direct benefits for stream water quality of transitioning from conventional cattle pastures to silvopastoral systems, particularly if cattle are prevented from aggregating in the streams during the dry season,” said Robin Chazdon, a professor at the University of the Sunshine Coast, Australia, who was not affiliated with the study.

Jorge Batista collecting water samples from the cattle pasture stream, Agua Salud. Image by Kristin Saltonstall.
Jorge Batista collecting water samples from the cattle pasture stream, Agua Salud. Image by Kristin Saltonstall.

Streams surrounded by silvopasture, which consists of trees deliberately planted on traditional cattle pastures, showed significant variation depending on the season. During wet months, bacterial diversity was similar to those found in forested streams, exhibiting higher diversity than dry months in which bacterial communities closely resembled those of cattle pastures.

The STRI study’s findings further highlight the importance of natural forest regrowth and targeted management, such as maintaining and restoring riparian corridors, on aquatic microbiomes and water quality in tropical landscapes. In other words, riparian corridors, which are the tree and plant communities that grow near flows of water, are critical to stream ecology.

“The results of this research emphasise the importance of the conservation and restoration of riparian forest buffers to efficiently balance agriculture and ecosystem services,” Hilary Brumberg, an environmental scientist from the University of Colorado Boulder who was not involved in the study, told Mongabay.

“All of the streams have robust communities of bacteria in them, which are likely doing many things, such as decomposing organic matter and cycling nutrients,” Saltonstall said. “The streams bordered by mature and young secondary forests showed few changes over time and change of season, suggesting that these communities are stable and resilient to environmental change.”

In agricultural settings, however, protecting streams with forest corridors and fencing can help enhance ecosystem services and improve water quality and overall bacterial diversity, particularly in the rainy season.

“Nature’s capacity for recovery is remarkable, in that a stream community can return to something resembling the natural community in a matter of years, even after decades of impact from cattle farming,” Saltonstall said.

“This provides hope and helps show a path forward in addressing some of our man-made environmental challenges.”


Citation: Chavarria, K. A., Saltonstall, K., Vinda, J., Batista, J., Lindmark, M., Stallard, R. F., & Hall, J. S. (2021). Land use influences stream bacterial communities in lowland tropical watersheds. Scientific Reports, 11(1). doi:10.1038/s41598-021-01193-7

Banner image by Rhett Butler/Mongabay.

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