Size matters when it comes to extinction…

New study sheds light on past mammal extinctions in Caribbean in a bid to help inform modern-day conservation effort

The biggest and smallest mammals in the Caribbean have been the most vulnerable to extinction, according to a new study published today by ZSL, Stony Brook University, Arizona State University and the University of Exeter in Proceedings of the Royal Society B. 

Photograph at night of Sam wearing thick gloves and holding the small mammal
Sam with a Hispaniolan solenodon in Dominican Republic

The paper, Where the Wild Things Were, looked at past extinction patterns across the Caribbean mammal fauna in order to help scientists understand the factors that predispose species to extinction. These findings help predict future extinction risk and inform the conservation strategies needed to prevent future biodiversity loss, with the study revealing that size does indeed matter in life. 
 
The islands of the Caribbean have long been a source of fascination for scientists and conservationists. They were once home to a diverse array of land mammals including sloths, primates, unusual insectivores, and giant rodents, but the arrival of different waves of human colonists from around 6000 years ago onwards instigated the largest series of human-caused mammal extinctions since the end of the last Ice Age. 
 
Only 11 native Caribbean rodents and two insectivores still survive today – including the two solenodons, large shrew-like mammals found on Hispaniola and Cuba, which have the unique ability to inject venom into their prey using modified grooved teeth. Both solenodon species are highly ranked on ZSL’s EDGE of Existence list, due to their unique evolutionary history – they are the only representatives of an ancient mammalian lineage that diverged from the ancestors of all other living mammals during the time of the dinosaurs, approximately 72 million years ago. 
 
Conducting a huge-scale analysis that included records of extinction patterns for 219 land mammal populations across 118 Caribbean islands, the study went beyond previous research into Caribbean mammal extinctions, which has largely focused on reconstructing last-occurrence dates for extinct species and matching them with specific historical events. The new study instead sought to identify wider ecological patterns – such as the correlation between body mass and extinction risk – that influence a mammal’s chance of survival in response to human activities. 

hispalonian hutia
 
By carrying out the study at the level of mammal populations rather than species, the innovative research was able to account for the effect of varying environmental conditions across different islands on species’ chances of survival. The findings show that medium-sized Caribbean mammals – like the solenodons – have been less sensitive to extinction compared to both their smaller and larger counterparts. This is likely to reflect the fact that larger species were more vulnerable to past human hunting, whereas smaller species were more vulnerable to predation or competition by introduced species such as mongooses and rats. These findings confirm the “Goldilocks Hypothesis” – medium-sized mammals in the Caribbean were “just right”.

Professor Samuel Turvey of ZSL’s Institute of Zoology said: “Preventing the extinction of highly endangered species requires an awareness of not only the immediate risks to their survival, but also the history of human-caused biodiversity loss – and the unique insights that the past can provide about species’ vulnerability or resilience under differing conditions.

“The Caribbean islands are home to unique mammalian biodiversity, which has tragically been almost completely wiped out by past human activities. The “last survivors” of the once-rich Caribbean mammal fauna are global priorities for conservation attention, and we must draw upon as many sources of information as possible to guide our conservation efforts.

“Our study clearly highlights the importance of learning from the past to make the future better – we must use information from the historical, archaeological and recent fossil records to inform current-day conservation, or else we risk losing these remarkable species forever.”

The analyses also showed that Caribbean mammals – of all sizes - were more likely to survive on tiny, low-elevation offshore islands, meaning that their future survival could be at risk from climate change and rising sea levels unless measures are put in place to protect these vital natural refuges. 

Professor Liliana M. Dávalos of Stony Brook University said: “To answer questions such as ‘what traits predispose species to survival?’ Or ‘what island features are associated with extinction?’, we studied each population on an island as a natural experiment. With enough of them, patterns start to emerge that have often been discussed but couldn’t previously be quantified. Without the large database of many natural experiments in the Caribbean and powerful computing approaches, there is no way to answer these questions.”


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