Gauging the threat of invasive species to UNESCO world heritage sites relative to other anthropogenic threats

There are 230 UNESCO World Heritage Sites that were designated based on their important natural features. These represent some of the most iconic and important natural places on Earth, with immense value for biodiversity conservation, ecosystem service supply and cultural well-being. Many of these sites are degraded by anthropogenic drivers, including socioeconomic factors (population growth, poverty and tourism), extractive activities (logging, mining, and hunting), external threats (fire, climate change, land-use change, pollution), and biological threats (disease and invasive species). Of these, invasive species remain one of the most problematic for management, and once introduced, populations can grow exponentially and spread to other locations even when actively managed. Given the economic and environmental threats imposed by invasive species, we asked how they compare to the other anthropogenic threats. We reviewed the primary literature for each World Heritage Site and scored the prevalence of 12 different anthropogenic drivers. We found that invasive species rank as one of the most frequently identified threats and pose the greatest degree of concern compared to all other threats, on par with the threat from pollution and greater than logging, land-use change and climate change. We compared our scoring, based on review of the literature, with that of the UNESCO and IUCN monitoring reports. Although there was general agreement between these two assessments, the literature provided information on 55 sites not included in the monitoring reports. We further examine the invasive species examined in the articles and the degree of their perceived impacts on biodiversity, habitat, rare species, ecosystem function, tourism, and economic impacts. It is important that invasive species are well monitored and managed in WHSs to reduce their impacts and meet policy mandated targets and conservation goals.