Conserving the Ecological Diversity of Crocodylians

Phoebe Griffith, Jeffrey W. Lang, Samuel T. Turvey, Rikki Gumbs

This is a plain language summary of a Functional Ecology research article that can be found here.

Crocodylians have surprisingly diverse ecological roles, from the huge, highly aquatic and mostly fish-eating gharial to the tiny African dwarf crocodiles which can catch prey on land below the rainforest canopy. Sadly, over half of all crocodylians are threatened with extinction.  If these threatened species go extinct, we will lose the diversity of ecological roles that they represent.  Functional diversity is an important aspect of biodiversity to understand and conserve.

To understand better the functional diversity of crocodylians, we collected a database of functional traits of all species of crocodylian. These traits are measurable qualities – such as skull shape or saltwater tolerance – that allow us to understand the different ecological role of species, and how similar and different species are from one another.

A gharial basks on a sandbank alongside a mugger crocodile. These crocodylians differ in many ways, from their skull morphology to differing levels of terrestrial movement ability. Despite living in the same rivers in South Asia, differing traits have led to the gharial being Critically Endangered, whilst the more resilient mugger crocodile is Vulnerable (credit: Phoebe Griffith)

We found there were four broad functional groups of crocodylians, within which species have a similar role in their ecosystem. Some species were very functionally distinct, such as the gharial and saltwater crocodile, and therefore have an ecological role that is very different from other crocodylians. We found certain traits reduced species risk of extinction, in particular species that invest more in reproduction, are habitat generalists, and can tolerate climate extremes.

We predict 32-38% of crocodylian functional diversity is likely to be lost within the next hundred years, with Asia being a hotspot of predicted losses. We calculated values that allow us to prioritise conservation of functionally unique species at high risk of extinction (EcoDGE species). We found conserving species such as the gharial and Chinese alligator would have a positive impact on conserving global crocodylian functional diversity.

However, collecting functional trait data is challenging. It is easier to calculate conservation metrics which aim to conserve the diversity of the tree of life, such as by prioritizing threatened species with no close living relatives (evolutionarily distinct and globally endangered species or EDGE species). We investigated if the EDGE metric may be a useful surrogate for conserving functional diversity when trait data are not available.

We found that the EDGE metric was better at conserving crocodylian functional diversity relative to a metric that just considered level of threat, but not as good as the EcoDGE metric, suggesting it is a useful proxy if functional data are unavailable. This was because the most and least evolutionarily distinct species tended to be the most functionally distinctive species, so EDGE prioritises species that are both evolutionarily and functionally distinctive, but misses those functionally distinctive species that have evolutionarily close relatives.