Where The Wild Things Were
It is no shock to learn that human activity is responsible for the extinction of innumerable species, particularly in vulnerable habitats like islands. But what factors, in particular, make a species or habitat more vulnerable to anthropogenic extinction? The authors of this study looked at the islands of the Caribbean Sea, which are believed by scientists to have the world’s highest level of mammalian extinctions during the Holocene (our current epoch), as well as post-1500 European colonization.
The researchers examined 219 mammal extinctions and survivals across 118 Caribbean islands, excluding islands in the South American continental shelf (Curaçao, Aruba, and Trinidad and Tobago) as well as data for flying species and marine mammals. The factors included in the analysis were: average body mass of the animal, body mass squared, island area, mean and maximum island elevation, forest cover in the year 2000, forest loss from 2000 to 2014, Human Footprint Index, presence of active volcanoes, hurricane frequency, presence of mongooses – small carnivorous mammals introduced to control pests on plantations – and date of first human arrival.
This analysis was done at the population level, rather than the species level. Say an animal lives on the islands of Cuba and Hispaniola, but is extirpated on Cuba after human arrival. A species-level analysis would count this animal as not extinct, despite the fact that a population of the species did in fact go extinct. In this way, populations of the same species on different islands can function similarly to treatment and control groups in a laboratory, allowing researchers to isolate the impact of a variable.
With regards to body mass, the researchers found evidence supporting what they call the “Goldilocks Hypothesis” – that there is a “sweet spot” for animal size that is correlated with survivability, and animals that are too large or too small are at greater risk of extinction. Larger animals are more vulnerable to human hunters, and require more space and resources to survive, while smaller animals are often reliant on more niche ecosystems that are destroyed by human involvement.
The researchers also found that mammals living on islands colonized later in history, as well as those living on islands with higher mean elevation, were more likely to go extinct. Animals living on islands facing more hurricanes had a greater likelihood of extinction as well. The presence of mongooses did not have an effect on extinction probability when controlling for island elevation. This analysis contradicts some global analyses, which hand found animal populations being more enduring in higher-elevation areas, which are generally less-developed. The authors attribute this to the extinction of many populations on high-elevation Hispaniola, and the survival of populations in low-lying areas of Cuba and the Bahamas. Many satellite islands of larger, developed islands are lower in elevation and have not been colonized to the same extent, providing a refuge for species that were extirpated on the main islands. Interestingly, however, Human Footprint Index and forest cover/loss were not found to have large effects on extinction probability. The authors speculate that current forest cover and human distribution may not match historical patterns, during which many of the populations went extinct.
The authors close by suggesting that the role of body mass in extinction probability should be examined further, and recommend that efforts be focused on controlling invasive species. They also warn that the effects of climate change could be catastrophic for many small surviving enclaves of species on low-lying islands. The correlation of Hurricane frequency with extinction is a serious concern, as these events will likely get stronger and more frequent with an increase in global temperature. Efforts should be made to protect existing refuges, and to establish protected areas for new populations in prior habitats that may be more resilient to climate change and extreme weather. Island ecosystems are incredibly delicate, and much of the damage has already been done — but there is still time to prevent further catastrophe and repair some of the damage.