If I mention Ibiza Biodiversity and Culture in Spain, Sichuan Giant Panda Sanctuaries in China, Ngorongoro Conservation Area in Tanzania, what do you think about? Holidays and sun; cute furry creatures; The Lion King movie? Maybe. But these places represent so much more and have a key thing in common: they are natural World Heritage sites.

These three natural World Heritage areas, like many others, are being affected by an insidious and growing threat – invasive alien species. What's the big deal? Well, according to the IUCN World Heritage Outlook 2014, these species pose the second most significant threat to natural World Heritage; the top spot is taken by poaching.

World Heritage sites are listed for their natural or cultural values (so-called Outstanding Universal Value). They are areas of exceptional beauty or areas which represent major stages of Earth's history, significant ongoing ecological processes, or significant habitats for biodiversity and threatened species.

So how do invasive alien species threaten those values? As plants, animals, pathogens or other organisms that are not native to a place, invasive alien species can cause serious damage to local ecosystems by modifying the natural balance and threatening native fauna and flora. They are usually an indirect consequence of human activity such as shipping, tourism, pollution or farming, or can be introduced deliberately.

Natural World Heritage sites are facing major threats such as illegal wildlife trade and inappropriate development. The impacts of invasive species may seem less obvious in comparison as they develop over longer periods of time, but they have been affecting natural World Heritage sites for many decades.

According to the IUCN World Heritage Outlook, a new system of assessment launched in 2014, of 229 natural World Heritage sites, 104 are affected by invasive species. Island sites are particularly exposed to them and 24 out of 36 affected island World Heritage sites are found in tropical regions.

Weeds – in particular water hyacinth and Lantana camara – emerge as the most common type of invasive alien species and appear in 55 of the 104 affected sites. Animal invasive species are also frequently found, including fish (mostly trout), cats and rodents. Unsurprisingly, due to their highly adaptive nature, rats take first place among the rodents, affecting at least 12 sites.

The consequences of these species can be many, including competition with native species or modification of ecosystems. For instance, in Cuba's Alejandro de Humboldt National Park invasive alien trees are transforming the habitat by growing much faster than native trees.

Predation is also a serious danger. Rats and mice for instance, are fierce predators of birds' eggs. Their presence in a site risks decimating native species. On Macquarie Island in Australia, burrowing seabirds were being eaten by rats and mice for years but thanks to an eight-year eradication campaign, the World Heritage site is now rid of rats, house mice and rabbits.

Effective management is key to overcoming invasive species that threaten Outstanding Universal Value. This includes well-defined plans and actions such as strict bio-security measures, including the control of materials entering the site or the eradication of problematic species. Ideally, these activities should involve local communities. Related threats such as pollution, climate change, tourism and other human activities should be taken into account. Among the natural World Heritage areas affected by invasive species, a majority (87) have developed projects, and over a third (28), such as Macquarie Island, have concretely addressed the issue.

The Aldabra Atoll in the Seychelles is another example of success. After 25 years of concerted efforts, goats were finally eradicated in 2012. Concerns over the threat posed to Aldabra due to the proximity of the red-whiskered bulbul on neighbouring Assumption Island, just 27 km from the site, were quashed earlier this year when this invasive alien bird was reported by the Seychelles Island Foundation to have been eradicated. This good news shows the positive impact that the World Heritage label can have on international awareness and funding concerning challenging conservation issues.

New obstacles can in future make the management of invasive alien species more challenging, in particular climate change, which according to the IUCN World Heritage Outlook could become the biggest threat to natural sites in the coming years. Climate change-related temperature and sea level rises at the global scale can create conditions that boost the spread of invasive species. On the ground, the challenges posed by potential climate change impacts are now starting to be considered in the management plans of some natural World Heritage sites.

So, the IUCN World Heritage Outlook 2014 identifies invasive alien species as the second most significant threat to natural World Heritage sites. Coupled with climate change, their impacts on the world's iconic places could become even more widespread and devastating in the near future. Let's hope that the growing global awareness of how to deal with these species can outstrip their spread. It's time to pull out all the stops.