Garden ponds as potential sources of plant invasions

As small aquatic habitats disappear at an alarming rate, private garden ponds may help compensate for the loss of natural habitats and support biodiversity in urban areas. However, as these ponds become increasingly popular, the spread of invasive species is emerging as a pressing issue.

Ponds, small (<5 ha) standing waterbodies, are among the most common freshwater habitats worldwide, but they are vanishing rapidly due to agricultural activities, urbanisation, climate change, and other human impacts. At the same time, we are creating new, secondary habitats in the form of garden ponds. These artificial ponds provide a range of ecosystem services, offering refuge for aquatic biodiversity, including protected amphibians, and supporting terrestrial species, especially during dry periods. In some regions, the number of garden ponds even exceeds that of natural waterbodies.

Yet the growing number of garden ponds may pose a new challenge for biodiversity. A wide variety of aquatic plants and animals are available through the ornamental trade, increasing the risk of invasive species introductions. Because these ponds are located on private property, researching and monitoring the species they contain, whether native or invasive, can be difficult. Citizen science, which involves the public in data collection, offers an effective way for researchers to gain access to these otherwise hidden habitats and to document their plant and animal communities.

Researchers at the Institute of Aquatic Ecology, HUN-REN Centre for Ecological Research in Hungary, launched a citizen science project to uncover the hidden diversity of garden ponds — the MyPond project. Within this initiative, citizens provided information about the presence of invasive plants in their ponds. Out of 560 ponds surveyed, almost half contained at least one of the six targeted invasive plant species. Water hyacinth (Eichhornia crassipes), considered the world’s worst invasive aquatic plant due to its environmental and economic impacts, was frequently reported. Water lettuce (Pistia stratiotes) and submerged species such as Elodea spp., Vallisneria spp., and Myriophyllum aquaticum were also common.

Some of these species originate from tropical regions, yet they have already established populations in several European countries. In Hungary, current climatic conditions may still limit their spread, but they can survive in thermal waters, and with climate change, the environment may become increasingly suitable for them. So far, it appears that these plants have primarily spread to natural waterbodies through human activity. However, garden ponds often attract waterbirds, which may also disperse seeds or plant fragments beyond garden boundaries.

“Most of these plants are on the EU’s list of invasive species, meaning it is illegal to keep them. Yet participants willingly reported having them,” said Barbara Barta. “Therefore, the main issue seems to be that the general public is not well informed about the threat posed by invasive species. Many people simply don’t realise that the plants they purchase are banned, and that keeping, growing, or exchanging them is an offence.”

The researchers emphasise the importance of environmental education and of engaging both buyers and sellers of aquatic plants used in garden ponds. “We need to take an active role in helping citizens make better choices for managing their ponds,” they said. “Providing a list of alternative, native plant species would be a good place to start.”