An analysis of IUCN’s Red List of endangered species places 10 drivers of the Holocene-Anthropocene Mass Extinction in order of severity. It concludes that classical environmental threats like deforestation, hunting and overfishing – in 2016 – still top the list of biodiversity killers.
Anthropogenic climate change is currently affecting 19 percent of species that are listed as threatened or near-threatened – making it the 7th extinction driver. (Stating the obvious: this position will change, as temperatures continue to rise.)
Six classical killers are still bigger drivers of the Holocene-Anthropocene Mass Extinction than climate change is. Overexploitation (“the harvesting of species from the wild at rates that cannot be compensated for by reproduction or regrowth”) is still by far the biggest threat to Earth’s biodiversity – second is agricultural activity.
In order of numbers of species affected the top-10 of extinction drivers is: 1. overexploitation (including deforestation, hunting and fishing); 2. agriculture (its direct ecological effects, excluding land use change); 3. urbanisation; 4. invasive species and disease; 5. pollution (agricultural, domestic, industrial); 6. ‘system modification’ (deliberate fires, dams); 7. climate change; 8. human disturbance (tourism, war); 9. transport; 10. energy production (not to be confused with energy pollution).
Two useful exercises: comparing what’s worse – and what’s getting worse
Extinction drivers work synergistically, so therefore tackling any one of them is always useful – faced with the challenge of slowing down the global decline in biodiversity. But of course we also want to know which are the largest single culprits – and here the analysis of currently endangered species offers a really good starting point.
It was published in Nature in August 2016, as a commentary article under the title ‘Biodiversity: the ravages of guns, nets and bulldozers’ – which clearly helps to put feet on the ground and remove any romantic notion of what a mass extinction looks like in practice.
The analysis was performed by a four-person team under lead author Sean Maxwell, a PhD student in environmental management at the University of Queensland, where co-authors Richard Fuller and James Watson are both associate professors. Fourth author is Thomas Brooks, head of science at the International Union for Conversation of Nature (IUCN) – the organisation that maintains the global Red List of endangered species that was evaluated by the group:
The above image shows the taxonomic representation of the IUCN Red List [that seems to place relative weight on ‘higher’ animal biodiversity, considering the extent of plant biodiversity on Earth]. Birds have the highest number of threatened and near-threatened species, followed by amphibians. Mammal species take the third place on the list. (Malacostraca are a large class of crustaceans, Magnoliopsida are a class of flowering plants and Anthozoa at place 5 are a class of marine invertebrates that includes all species of coral.)
Results of the study clearly show that classical environmental stressors are currently still a larger threat to Earth’s biodiversity than the relative newcomers [for comparison overexploitation affects 6,241 species on the IUCN Red List, agricultural activity affects 5,407 species – while invasive species threaten 2,084 Red List species and climate changes affects 1,688].
As the two images directly above and below indicate there is (apart from indirect synergy) also direct synergy between the drivers – as the vast majority of threatened biodiversity (6,994 species out of 8,688 species – that’s 81 percent) is impacted by multiple large groups of extinction drivers simultaneously.
Another indication of the synergy between extinction drivers: the ‘interaction potential’ between major threat classes. The width of links between nodes represents how many species are simultaneously threatened by the two connected threat classes. Only links that have more simultaneously threatened species than the mean (860 species) for the network are shown.
Furthermore it is of course also important to assess the relative development of these extinction drivers:
Climate change is runner up on the extinction drivers list
The authors acknowledge that especially climate change is set to become a far-larger threat to biodiversity in the future – stating “climate change will become an increasingly dominant problem in the biodiversity crisis.”
For us that makes sense for two very important reasons. The first is climate inertia – on very many levels, from fossil lock-in emissions (decades), ocean-atmospheric temperature inertia (yet more decades), Earth system temperature inertia (centuries to millennia) to ecological climate impact inertia (impacts becoming worse over time under a constant stress) – all this to illustrate anthropogenic climate change, although already manifesting itself, is still very much an escalating problem for the future. We’ll just link to our ‘Real’ Global Temperature graph for background illustration of the intrinsically delayed onset of climate change – the warming itself, let alone its consequences. (And from there of course biodiversity loss is exponential to warming.)
The second reason is the possibility that the ecological effects of climate change (the portion that is already manifesting itself) are currently already larger than thought – something that can be masked by looking at individual species and is better assessed when looking at ecosystems or entire biomes. Climate change has the potential to kill most of the coral reefs, climate change has the potential to kill most of the Arctic marine ecosystem*, climate change has the potential to dry out most of the Amazon rainforest – individual examples of macro-scale climate-biodiversity tipping points. The associated stressors are possibly already building up, while direct extinctions still lack.
[*) For illustration – the authors present an example of the Arctic marine ecosystem collapse on individual threatened species level: "Hooded seals (Cystophora cristata) are among the 1,688 species affected. These have dropped in abundance by 90% in the northeastern Atlantic Arctic over the past few decades as a result of extensive declines in regional sea ice, and so in the availability of sites for resting and raising pups."]
This ‘extinction inertia’ is just a bit more than a hypothesis. It’s an underlying trend that can be deduced from population extinctions and intraspecific biodiversity decline we learned in part 14 of this series. That’s the general decline of ordinary species – species that are not (yet) on the Red List of IUCN.
That should worry us. Just like poaching. Just like bulldozers. Just like logging and overfishing. It really is a massive problem. The largest we have on Earth.
© Rolf Schuttenhelm | www.bitsofscience.org