Lessons from South Africa on Monitoring the Impact of Invasive Trees on Water Resources
The International Union for Conservation of Nature (IUCN) estimates that the minimum economic cost of biological invasions globally from 1970 to 2017 is USD 1.2 trillion. Credit: Jorge Luis Baños/IPS
By Richard Bugan
CAPE TOWN, South Africa, Jul 4 2025 (IPS)
Concerns about the impacts of invasive species is not new; it dates to the 19th century. The term was popularized in Charles Elton’s 1958 book “The Ecology of Invasions by Animals and Plants”. However, the concept gained significant attention in the 1990s and early 2000s as academic interest surged. This led to an increase in publications by invasion biologists.
Today, the impact of invasive species has increased significantly, with regions with fragile ecosystems being more vulnerable than others. Globally, the scale of the problem is staggering. The International Union for Conservation of Nature (IUCN) estimates that the minimum economic cost of biological invasions globally from 1970 to 2017 is USD 1.2 trillion. This figure accounts for expenses related to the prevention, reduction, or mitigation of damages caused by these species.
In Africa, a continent expected to be severely affected by climate change, the challenges posed by invasive species are expected to worsen. Among the most pressing is the spread of invasive plants, which not only endanger native biodiversity but also impact the economy, water security, food security and livelihoods.
One such example can be found in South Africa’s Cape Floristic Region, which is known for its remarkable plant diversity, where 70% of plant species are unique to this area.
In catchment areas of the Western Cape Water Supply System, critical for the water supply to Cape Town, invasive trees are responsible for the loss of up to 55 million cubic meters of water per year—equivalent to about two months’ water supply for the city. If not managed, this loss could increase to 100 million cubic meters per year by 2045.
It is evident that the Earth is approaching a tipping point in terms of biodiversity loss, and there is no time to waste. Many scientists, including myself, are deeply concerned about the impact of these water-consuming invasive trees.
Richard Bugan
As the Science, Monitoring, Evaluation, and Learning Manager for The Nature Conservancy (TNC) in South Africa, I lead a team dedicated to using sound science to monitor and evaluate the progress of the Greater Cape Town Water Fund, which was launched in 2018 by TNC and its partners as South Africa’s first Water Fund.
Our work includes tracking the hectares cleared in pursuit of our 2026 target of 59,300 hectares cleared, along with assessing the associated water benefits and biodiversity impacts (for both freshwater ecosystems and the recovery of native fynbos).
As of February 2025, the Water Fund has cleared 33,000 hectares (56% of the target) and completed follow-up clearing efforts across 31,000 hectares to prevent regrowth of invasive species and maintain the cleared areas. This effort has reclaimed approximately 34 million cubic meters of water per year to the benefit of both people and the environment.
TNC is committed to accurately quantifying the impact of invasive trees on water resources in South Africa. This is achieved through the application of hydrological models and infield monitoring activities. During October 2019 – February 2020, six catchments around Theewaterskloof Dam, were instrumented with streamflow and rainfall monitoring equipment. The results of this hydrological monitoring represent a unique opportunity to provide measured evidence of the water benefits achieved through the clearing of invasive trees.
But we are learning that our monitoring activities can hit a snag due to potential equipment failures, storms, and wildfires, which pose risks to their success. To address these challenges, we upgraded the paired catchment monitoring stations in December 2024 through the support of Microsoft. A new telemetry system was installed, linking each piece of equipment via radio frequency and the mobile network. This enhancement significantly reduced the risk of data loss, improved accuracy, and supported the long-term resilience of the monitoring.
After almost six years of monitoring, we are frequently asked whether clearing invasive trees is increasing streamflow. Preliminary results suggest that reference (fynbos dominated) catchments exhibit, on average, a 34% increase in annual streamflow volumes compared to invaded catchments. This represents a significant amount of water, benefiting both people and nature in this beautiful region. We are excited about these incontrovertible findings as they provide concrete evidence of the benefits derived from the removal of invasive trees.
Restoring biodiversity to its original state may take a long time. However, I am just as excited about this moment. The fact that we can contribute every day to a matter of international importance is a gradual step to success.
The author is the Science, Monitoring, Evaluation and Learning Manager (TNC South Africa)