Climate Change & Anthropocene Extinction 9: Cascading extinctions speed up biodiversity loss – and warming

Yes, we found yet another publication that is critical of one-dimensional biodiversity loss projections of climate change – and again it is a component of interspecies interaction that may be underestimated. We’ve previously looked at species competition and at evolutionary responses, and today we add species’ interdependence – the ecosystem perspective:

Extinction feedback creates cascading biodiversity loss and amplified climate change
Extinction feedback creates cascading biodiversity loss and amplified climate change. “In this general example, climate change reduces top predators, leading to an increase in herbivores, and a decrease in plants. As a result, the community experiences an overall decrease in both species diversity and stability.”

The above image comes from a paper that was written by ecologists Phoebe Zarnetske and David Skelly of Yale University and Mark Urban of the University of Connecticut – and published in 2012 as a perspective article in the commentary section of Science.

Extinctions create a positive feedback, to extinctions – AND climate change

Species extinctions have a cascading effect, increasing the extinction risk for others, speeding up biodiversity loss. What’s more, climate change-induced extinctions may also create a positive climate feedback – speeding up the initial warming.

This is a phenomenon that has many analogues, both in modern times and the paleoclimatological record. For instance during the cascading End-Permian Mass Extinction (that also had a driving climate change component) at some point plagues set in that killed the forests, releasing yet more CO2 to an already warming atmosphere. Now switch to a smaller scale and think of the pine beetle plague in the boreal forests (caused both by climate warming and by decreasing forest biodiversity) that also acts as a positive climate feedback, releasing millions of tonnes of CO2 – much like a climate change-induced dry-out of in the Amazon goes hand in with increased biodiversity loss – and gigatonnes of extra CO2.

The authors of the Science paper state it’s important to look at ‘trophic interactors’ as being both strong candidates of becoming ‘biotic multipliers of climate change’ – while also being susceptible to climate change itself. They state that evidence shows that top consumers (both large herbivores and large predators) are at a higher risk of becoming extinct due to climate change, which ‘affects top consumers more strongly, disrupting vertical interactions and thereby affecting many species across trophic levels’ – presenting a situation of thinning diversity and ultimately overgrazing in a forest ecosystem, which is of course a carbon store. Also, paradoxically as it may seem, an artificial warming plus top herbivore exclusions experiment in Arctic Greenland shows that removing large tundra grazers (muskoxen and caribou) leads to a decrease of plant species diversity.

The cascading biodiversity decline can start with the local extinction of both a herbivore or a carnivore, the authors write – presenting additional North American examples:

“These destabilizing outcomes are also seen in studies of top predators. On Isle Royale (an island in Lake Superior, USA), rising winter temperatures combined with canine parvovirus produced a trophic cascade: Declining wolf populations caused moose populations to surge and balsam fir to decline. In the rocky intertidal of the North American Pacific Coast, higher temperatures led to range contractions in mussel species, exacerbating keystone predation by seastars, which resulted in the decline and local extinction of certain mussel species.”

In mass extinctions the biggest species tend to go first. That often includes large predators – creating a trophic cascade, speeding up biodiversity loss and carbon release

Another clear real world example is the biodiversity loss (including forest damage) to Yellowstone National Park as wolves died out – and increased biodiversity and forest rejuvenation as they were reintroduced. There are many analogues, again both in paleoclimatology and modern observation, that show that climate change unfavourably selects larger species.

The disappearance of the ice age megafauna (sometimes called the ‘Pleistocene-Holocene Extinction’) as another powerful example – while elephants and rhinos (crucially important to forest ecosystems) may also go that route, on our watch…

© Rolf Schuttenhelm |

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