Lessons from South Africa on Invasive Trees and Water Loss

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.

Concerns about the impacts of invasive species are not new; they date back to the 19th century. The term was popularised 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, leading to a rise in publications by invasion biologists.

Today, the impact of invasive species has increased substantially, with regions containing 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 worldwide from 1970 to 2017 was USD 1.2 trillion. This figure includes expenses related to the prevention, reduction, or mitigation of the damages caused by these species.

In Africa—a continent expected to be severely affected by climate change—the challenges posed by invasive species are projected to intensify. Among the most pressing issues is the spread of invasive plants, which not only threaten native biodiversity but also impact the economy, water security, food security, and livelihoods.

One striking example is found in South Africa’s Cape Floristic Region, renowned for its remarkable plant diversity, with 70% of species unique to the area. In the catchment areas of the Western Cape Water Supply System—critical for Cape Town’s water supply—invasive trees are responsible for the loss of up to 55 million cubic metres of water annually, equivalent to about two months’ supply for the city. If unmanaged, this loss could increase to 100 million cubic metres by 2045.

It is evident that 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.

As the Science, Monitoring, Evaluation, and Learning Manager for The Nature Conservancy (TNC) in South Africa, I lead a team committed to using sound science to monitor and evaluate the progress of the Greater Cape Town Water Fund. Launched in 2018 by TNC and partners, this initiative is South Africa’s first Water Fund.

Our work includes tracking the number of hectares cleared toward our 2026 target of 59,300 hectares, while also assessing water benefits and biodiversity impacts—both in freshwater ecosystems and in 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 across 31,000 hectares to prevent regrowth and maintain progress. This effort has reclaimed approximately 34 million cubic metres of water annually—benefiting both people and the environment.

TNC is committed to accurately quantifying the impact of invasive trees on water resources in South Africa. This involves applying hydrological models and conducting field monitoring. Between October 2019 and February 2020, six catchments around Theewaterskloof Dam were equipped with streamflow and rainfall monitoring instruments. The resulting data offer a unique opportunity to demonstrate the water benefits of invasive tree removal.

However, our monitoring activities face challenges such as equipment failures, storms, and wildfires. To mitigate these risks, we upgraded the paired catchment monitoring stations in December 2024, with support from Microsoft. A new telemetry system was installed, linking each instrument via radio frequency and mobile network. This upgrade significantly reduced the risk of data loss, improved accuracy, and enhanced long-term monitoring resilience.

After nearly six years of data collection, we are often asked whether clearing invasive trees is truly increasing streamflow. Preliminary results indicate that fynbos-dominated catchments exhibit, on average, a 34% increase in annual streamflow volumes compared to invaded ones. This is a significant amount of water, supporting both nature and people in this unique landscape.

Restoring biodiversity to its original state will take time. Nevertheless, I am excited by this progress. Being able to contribute daily to an issue of global importance is, in itself, a meaningful step toward success.

The author is the Science, Monitoring, Evaluation and Learning Manager at The Nature Conservancy (TNC) South Africa.