Invasive species are spreading rapidly – and outdoor recreation plays a part. This guide explains the risks and how to reduce your impact.

What are invasive species – and why do they matter?

Invasive species are considered a growing threat to biodiversity. But what exactly does “invasive” mean? How do non-native species spread, and what does this have to do with outdoor sports and travel?

For tourists, lupines are a colorful sign of summer in Sweden. But for locals, they’re a problem. Lupinus polyphyllus, native to North America, now displaces native flora across northern Europe. Similarly, the flatworm Obama nungara—introduced from South America—preys on earthworms essential for healthy soil and food webs. These cases show how pervasive invasive species already are in daily life.

When does non-native become invasive?

Not all non-native species are invasive. Organisms introduced by humans, whether deliberately or accidentally, are classified as either archaeobiota (introduced before 1492) or neobiota (introduced after).

Many crops, like the potato, originated outside of Europe but became essential over time. Others, however, disrupt ecosystems, outcompete native species, or harm human health—and are deemed invasive.

Do invasive species always pose a threat?

According to a 2023 report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), there are over 37,000 non-native species worldwide, with 3,500 classified as invasive. In Germany, about 90 out of 900 non-native species are considered invasive. In Sweden, 257 species pose a high risk to biodiversity.

These species are a major driver of extinction—linked to 60 percent of observed species losses and the sole cause in 16 percent of cases. Even less dramatic impacts can still alter food webs, nutrient cycles, and ecosystem stability.

The cost of invasive species

Invasive species are not just an environmental issue—they also carry economic and health costs. Crop losses, forest degradation, and allergenic plants all affect human well-being.

Between 1980 and 2019, invasive species caused an estimated $1.2 trillion in global damages, with costs rising 700 percent since 2000. The giant hogweed and common ragweed, for example, cause severe allergic reactions and increase healthcare costs—Germany alone spends around €32 million annually to treat health issues caused by ragweed.

How invasive species spread

These species spread in many ways: through global trade, climate change, and increasingly, outdoor tourism and sports. Water sports like boating and kayaking can spread aquatic plants and animals. Even without commercial shipping, fragments can hitch a ride on recreational vessels.

Boaters are encouraged to avoid paddling through plant growth, inspect their gear for hitchhiking species, and dispose of any plant matter in trash bins, not water bodies. Thorough cleaning is also essential between different lakes or rivers.

Gear up: Best practices for recreationists

Anglers should clean and dry their gear and use only local bait. Transferring equipment between waters with different crayfish species could spread crayfish plague.

Seeds, spores, and insects can also stick to hiking boots and mountain bikes. The Julius Kühn Institute recommends thorough gear cleaning after trips to prevent accidental transport.

Case study: Tranås, Sweden

In the town of Tranås in southern Sweden, the yellow floating heart (*Nymphoides peltata*) has overrun local waters. Though protected in Germany, it’s invasive in Sweden and North America.

The plant blocks sunlight from reaching the riverbed, disrupting aquatic life. To control its spread, the local government has covered parts of the Svartån River and is educating recreationists about the issue.

Mountain ecosystems at risk

So far, invasive species in the Alps are mostly confined to valleys. Harsh conditions in alpine zones have acted as a barrier—but that’s changing.

With climate change and increasing human traffic, species are spreading to higher altitudes. Roads and trails act as corridors, carrying pollen and seeds on tires, boots, and clothing.

Since 2005, the Mountain Invasion Research Network (MIREN) has monitored this phenomenon in mountain regions worldwide.

Citizen science and awareness matter

Outdoor enthusiasts can help curb the spread of invasive species by staying informed and cleaning gear thoroughly.

Citizen science also plays a vital role. In Sweden, the University of Agricultural Sciences (SLU) encourages reporting sightings of invasive species. In Germany, each federal state has contact points, and smartphone apps help users identify and report invasive species on the spot.

Photos: Miriam Ersch-Arnolds