In the spring and summer season of the year 2008, for the first time, I went to Caohai Wetland, Lugu Lake, which is a natural reserve situated across the border of Yunnan and Sichuan Provinces, Southwestern China (Figure 1). I went there for my PhD project aiming to explore the invasion success and ecological impacts of the invasive American bullfrog on native amphibians. Literature documents that this is a global biodiversity conservation priority with some endemic species such as the Yunnan pond frog, which is historically abundant there.
Figure 1 The Lugu Lake nature reserve, a global biodiversity conservation hotspot, located in Southwestern China. Photo credit: Xuan Liu
However, when I and colleagues conducted the transect surveys at night, we found that the surroundings were unexpectedly “quiet” with few native frog songs but homogenized by the loud calling of the bullfrogs (Figure 2). During the day time, we searched their oviposition habitats by boat and found the shallow waters to be dominated by bullfrog egg masses with 10,000 eggs per clutch in average (Figure 2). We sampled the adult bullfrogs and stomach contents showed that the bullfrogs could predate on a wide range of local species and particularly prefer the Yunnan pond frog (Figure 2).
Figure 2 Caohai wetland in Lugu Lake nature reserve (Top left), where the American bullfrogs have established shown as the presence of adult (Top middle) and tadpole (Top right). Field survey for bullfrog spawning site (Lower left), bullfrog egg mass (Lower middle), and the Yunnan pond frog swallowed by the bullfrog (Lower right) in Caohai. Photo credit: Xuan Liu
Something similar to the situation at Lugu Lake was going on during our subsequent years’ surveys in other nature reserves such as Erhai Lake, Yunnan Province, Weining Caohai, Guizhou Province, etc., where the bullfrogs have successfully established feral populations. Protected areas are recognized as the cornerstone of biodiversity conservation with one primary goal being to perpetuate the natural state. But, in fact, these virgin areas are being polluted by alien species invasions. I was then curious to know the degree to which alien species have become established in global protected areas.
Literature reviews show that alien species invasion in protected areas is a topic with great concerns in the field of invasion ecology and conservation biology. This concern started about 150 years ago and increased rapidly with the accelerated alien species invasions in the era of Anthropocene under globalization. Previous taxa-specific, regional or continental studies have shed important light on the alien incursions in protected areas. However, we still lack a global picture of alien species establishment and potential drivers in protected areas covering broad taxonomic groups.
Thanks to the increasing availability of public data and close collaborations with international invasion ecologists, we have been intensively collecting spatially-explicitly occurrence data of established alien animals worldwide during the past years. After careful data checking and spatial analyses, to the best of our knowledge, we provide the first global synthesis of 894 alien animal establishment in more than 190,000 terrestrial protected areas.
We find that less than 10% of protected areas are home to any of the alien animals, predominantly on islands and mainly colonized by alien birds, and that there tend to be fewer alien animals established in protected areas with higher native species richness and designated earlier. This is encouraging as it shows that protected areas have to date generally been effective in resisting alien animal incursions. However, there are more alien species incursions in protected areas with higher human footprint for all taxonomic groups and many alien animals in our database have arrived in neighboring regions of most protected areas. More than 95% of protected areas are predicted to have suitable habitats for alien animal establishments based on species distribution modeling. Remarkably, our results indicate that the current IUCN conservation-category does not match alien animal establishment status of protected areas, with the highest average alien richness being in II-category National Parks in most ecoregions worldwide.
These findings may provide useful suggestions and foundations for the future conservation planning and prioritization management of protected areas combating alien animals, and hopefully, it may be also helpful for other-taxa studies such as plants, which are likely to be more prevalent in protected areas.